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1 INFORMATION TO USERS This dissertation was produced from a microfilm copy of the original document. While the most advanced technological means to photograph and reproduce this document have been used, the quality is heavily dependent upon the quality of the original submitted The following explanation of techniques is provided to help you understand markings or patterns which may appear on this reproduction. 1. The sign or "target" for pages apparently lacking from the document photographed is "Missing Page(s)". If it was possible to obtain the missing page(s) or section, they are spliced into the film along with adjacent pages. This may have necessitated cutting thru an image and duplicating adjacent pages to insure you complete continuity. 2. When an image on the film is obliterated with a large round black mark, it is an indication that the photographer suspected that the copy may have moved during exposure and thus cause a blurred image. You will find a good image of the page in the adjacent frame. 3. When a map, drawing or chart, etc., was part of the material being photographed the photographer followed a definite method in "sectioning" the material. It is customary to begin photoing at the upper left hand corner of a large sheet and to continue photoing from left to right in equal sections with a small overlap. If necessary, sectioning is continued again - beginning below the first row and continuing on until complete. 4. The majority of users indicate that the textual content is of greatest value, however, a somewhat higher quality reproduction could be made from "photographs" if essential to the understanding of the dissertation. Silver prints of "photographs" may be ordered at additional charge by writing the Order Department, giving the catalog number, title, author and specific pages you wish reproduced. University Microfilms 300 North Zoob Road Ann Arbor, Michigan A Xerox Education Company

2 73-11,591 THOMPSON, G erald L ee, PYROLYSIS, PHOTOLYSIS, AND SOLVOLYSIS STUDIES OF UNSATURATED PROPELLANES. The Ohio S ta te U n iv e rs ity, P h.d., 1972 C hem istry, o rg an ic ( University Microfilms, A XEROX C om pany, Ann Arbor, Michigan THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED.

3 PYROLYSIS, PHOTOLYSIS, AND SOLVOLYSIS STUDIES OF UNSATURATED PROPELLANES DISSERTATION P re se n te d i n P a r t i a l F u lfillm e n t o f th e R equirem ents f o r th e D egree D octor o f P h ilo so p h y i n th e G raduate S ch o o l o f The Ohio S ta te U n iv e rs ity By G erald Lee Thompson, B.S. The Ohio S ta te U n iv e rs ity 1972 A pproved "by t\lo A d v iso r^ D epartm ent o f C hem istry

4 PLEASE NOTE: Some pages may have indistinct print. Filmed as received. University Microfilms, A Xerox Education Company

5 DEDICATION To ny w ife, my son, and n$r p a re n ts ii

6 ACKNOWLEDGMENTS I would l i k e t o ex p ress my s in c e re a p p r e c ia tio n t o P ro fe s s o r P a q u e tte f o r guidance and in s p i r a t i o n th ro u g h o u t my g ra d u a te c a re e r. My a s s o c ia tio n w ith him has "been a rew ard in g e x p e rie n c e in many ways. Thanks a re due many in d iv id u a ls, h u t e s p e c ia lly t o D r. S ta n le y A. Lang f o r s e l f l e s s l y c o n tr ib u tin g an enormous amount o f tim e i n th e d e te rm in a tio n o f 100-MHz nmr s p e c tra. Acknowledgment i s a ls o due P h i l l i p s P etro leu m C o rp o ra tio n and The Ohio S ta te U n iv e rs ity f o r f in a n c i a l su p p o rt in th e form o f fe llo w s h ip s. F in a lly, I would l i k e t o th a n k Kay H enzel f o r h e r sm ilin g fa c e each m orning. iii

7 VITA March 16, 19l* B orn - Swea C ity, Iowa B.S., Iowa S ta te U n iv e rs ity, Ames, Iowa T eaching A s s is ta n t, D epartm ent o f C hem istry, The Ohio S ta te U n iv e rs ity, Columbus, Ohio R esearch A s s is ta n t, D epartm ent o f C hem istry, The Ohio S ta te U n iv e rs ity, Columbus, Ohio P h i l l i p s P etro leum F ello w, The Ohio S ta te U n iv e rs ity, Columbus, Ohio D is s e r ta tio n F ello w, The Ohio S ta te U n iv e rs ity, Columbus, Ohio PUBLICATIONS L. A. P a q u e tte, R. E. W ingard, J r., J. C. P h i l i p s, G. L. Thompson, L. K. Read, and J. C lard y, o-h alo S u lfo n e s. XVII. D ire c tiv e E f f e c ts in th e C h lo rin a tio n o f T h ia p ro p e U a n e s, J. Amer. Chem. S o c., 93* 1*508 ( ). ' L. A. P a q u e tte and G. L. Thompson, C o m p etitiv e Bond R o ta tio n and Bond S c is s io n P ro c e sse s in S e m is ta h iliz e d 1,1* D ir a d ic a ls. The Therm al F rag m e n ta tio n o f [l*.l*.2 ]F ro p e lla -2,l* -d ie n e s," i b i d., 93* 1*920 (1971). L. A. P a q u e tte and G. L. Thompson, ^ F u n c tio n a liz a tio n R e a c tio n s o f f l*.l*.2 ] P r o p e lla - 3,ll- d ie n e, i b i d., **, 7118 (1972). L. A. P a q u e tte and G. L. Thompson, S tere o c h e m ical F e a tu re s o f th e Therm al and P h o to ch em ical F rag m entatio n s o f [l*.l*. 2 ]P rq p e lla -2,l* - d ie n e s, i b i d., gk, ( ). iv

8 R. G le ite r, E. H e ilb ro n n e r, L. A. P a q u e tte, G. L. Thompson, and R. E W ingard, J r., t h c to e le c tr o n S p e c tra o f P o ly u n sa tu ra te d [4 A. 2] P r o p e lla n e s, T e tra h e d ro n ( in p r e s s ). FIELDS OF STUDY M ajor F ie ld : O rganic C hem istry

9 TABLE OP CONTENTS Page DEDICATION... i i ACKNOWLEDGMENTS... i l l V IT A... i v. LIST OF FIGURES... LIST OF ILLUSTRATIONS... LIST OF TABLES... i x v l i v i i i PART I. PYROLYSIS AND PHOTOLYSIS STUDIES I n t r o d u c t i o n... 1 R e s u lts and D i s c u s s i o n PART I I. SOLVOLYSIS STUDIES I n t r o d u c t i o n R e s u lts and D iscu ssio n EXPERIMENTAL REFERENCES vi

10 LIST OF FIGURES F ig u re 1. I n te r a c t i o n diagram f o r l,^ - d ia z a b ic y c lo [ o ctan e ( j ) Page F ig u re 2. F ig u re 3- F ig u re k. F ig u re 3. F ig u re 6. I n te r a c t i o n diagram f o r a te tra m e th y le n e w ith 9 = 1 8 0, fi = y = 9 0, la r g e a... 7 Energy p r o f i l e alo n g th e r e a c tio n c o o rd in a te f o r d ecom p o sitio n o f cy clo h u ta n e MKz nmr s p e c tra o f lia- (a ) and 128 (b ) i n CCI4 s o lu tio n c o n ta in in g 2 2 mol E u(fod ^ P a r t i a l s p e c tr a (60 MHz) o f 0,6 6 M s o lu tio n s in carbon t e t r a c h l o r i d e c o n ta in in g mol % Eu(DPM)a; t r a c e a, ; t r a c e b, 'kra c e l ^ b ; t r a c e d, m ix tu re o f l ^ a and l ^ b as o b ta in e d from P o s s ib le n eig h b o rin g group p a r t i c i p a t i o n b y f iv e d i f f e r e n t bondin g e le c tr o n n u c le o p h ile s i n th e s o lv o ly s is o f d e r iv a tiv e s o f 7 -h y d ro x y m eth y lcy clo - h e p ta tr ie n e (l 6 6 a ) vii

11 LIST OF ILLUSTRATIONS Page Scheme I. <r>c hlorination o f s u lf id e s... jk Scheme I I. P r e p a ra tio n o f th ia [ ^.4.3 ] p i,o p e l l a n e s Scheme I I I. P re p a ra tio n o f a n t i - and Byn-h y d ro x y m eth y lb icy clo - [lf.ii-.l]u n d e c a -l,3 /5 t r i e n e s {17_2a and 17,3 a ) Scheme IV. P r e p a ra tio n o f a lc o h o ls a, lg T a, and go O a viii

12 LIST OF TABLES Page T able 1. P ro d u ct D is tr ib u tio n f o r Therm al D ecom position o f ^ and ^ T able 2. P ro d u ct D is tr ib u tio n f o r D ecom position o f ^ T able 3* Table k. Comparison o f E q u ilib riu m D ata w ith P ercen ta g e Com positions from C h lo rin a tio n -O x id a tio n Sequence f o r V arious a -c h lo ro S u lfo n es Chemical S h ifts (6 values) and M u ltip lic itie s R ecorded f o r th e More D ow nfield S h ifte d P ro to n s (exclu d in g -OH) in 135a, rggb, and 138 [0.6 6 M s o lu tio n s i n CCI4 c o n ta in in g 45 mol?teu(bpm)3 ]... 6 l Table 5* Thermally Generated Isomer D istrib u tion s of TO T able 6. T able? T able 8. T able 9. P h o to ch em ically G enerated Isom er D is tr ib u tio n s o f S o lv o ly tic R e a c tiv ity o f Mono- and B ic y c lic A lco h o l D e riv a tiv e s S o lv o ly s is o f 3, 5 -jd in itro b en zo ates in 80:20 A cetone- W ater S o lv o ly s is o f 3 > 5 -B in itro b e n z o a te s i n 75:25 THF-CH3 OH a t C T able 10. S o lv o ly s is o f 197b In 80:20 A cetone-w ater a t T able 11. S o lv o ly s is o f 19Tb in 80:20 A cetone-w ater a t T able 12. S o lv o ly s is o f 19,7b in 80:20 A cetone-w ater a t 99*5 164 T able 13. S o lv o ly s is o f 172c in 80:20 A cetone-w ater a t T able 14. S o lv o ly s is o f 172c i n 8 0:20 A cetone-w ater a t T able 15. S o lv o ly s is o f 172c i n 80:20 A cetone-w ater a t 99*5 167 ix

13 Page T able 16. S o lv o ly s is o f _175c in 80:20 A cetone-w ater a t T able 17. S o lv o ly s is o f 17J5C i n 80:20 A cetone-w ater a t T able 18. S o lv o ly s is o f 173c in 8 0:20 A cetone-w ater a t T able 19. S o lv o ly s is o f l%6b in 8 0:20 A cetone-w ater a t T able 20. S o lv o ly s is o f 196b in 80:20 A cetone-w ater a t 9 9 * T able 21. S o lv o ly s is o f 196b in 8 0 :2 0 A cetone-w ater a t T able 22. S o lv o ly s is o f 175c in 75:25 THF-CK3 OH a t T able 23. S o lv o ly s is o f 200b in 75:25 THF-CH3 OH a t x

14 INTRODUCTION The p o s tu la te d involvem ent o f 1,4 d ir a d ic a ls in a v a r ie ty o f chem ical tra n s fo rm a tio n s has s tim u la te d re s e a rc h e f f o r t i n o rg a n ic, p h y s ic a l, and t h e o r e t i c a l ch em istry. A lthough th e r e s u l t s o f t h i s re s e a rc h have p ro v id e d co n v in cin g therm odynam ic, k i n e t i c, and wave m echanical argum ents f o r th e e x is te n c e o f such h ig h en erg y s p e c ie s, 1,4 d ir a d ic a ls have n o t b een d e te c te d d i r e c t l y and rem ain th e s u b je c t o f s u b s ta n tia l c o n tro v e rsy even t o th e e x te n t t h a t th e y be b e t t e r c h a ra c te riz e d as t r a n s i t i o n s ta t e s th a n t r u e r e a c tio n in te rm e d ia te s. I n t e r e s t d a tin g t o N o r ris h 's in v e s tig a tio n o f p h o to ch em ical carbon monoxide e x tr u s io n from cyclopentanone in th e mid 's 1 c o n tin u e s a t p re s e n t w ith rin g -fo rm in g and r in g - c le a v in g tra n s fo rm a tio n s i n i t i a te d by th e rm a l and p h o to ch em ical m ethods. However, owing t o th e s u b je c t o f th e p r e s e n t d i s s e r t a t i o n, t h i s in tro d u c tio n w i l l co n cern i t s e l f c h ie f ly w ith th e r o le o f 1,4 b u ta n e d iy ls i n th e rm a l and p h o to chem ical [^ 2 + g.2 ] fra g m e n ta tio n r e a c tio n s o f c y c lo b u ta n e s. R ecent re c o g n itio n o f th e im portance o f o r b i t a l symmetry c o n tr o l in o rg a n ic ch em istry has p ro v id e d th e g r e a te s t im petus f o r th e in v e s t i g a t i o n o f 1,4 d ir a d ic a ls. T o g eth er w ith th e su rg e o f i n t e r e s t in c o n certed r e a c tio n s w hich conform t o e m p iric a l sta te m e n ts such a s th e 2 Woodward-Hoffmann r u le s th e r e has been a co rresp o n d in g in c re a s e in a t t e n t i o n t o ste p w ise ( d if u n c tio n a l) p ro c e sse s w hich do n o t conform, b u t y e t 1

15 p ro v id e a m eeting ground f o r th e two ty p e s o f r e a c tio n s. B r ie f ly s ta te d, th e o r b i t a l symmetry c o n tr o lle d ground s t a t e c y c lo a d d itio n of two m olecules o f e th y le n e m ust occur by o rth o g o n a l approach o f th e two m olecules w ith a n t a r a f a c ia l u t i l i z a t i o n o f th e p - o r b i t a l s o f one it component (th e [ rt2s + n2a] mode). T his pathw ay r e s u l t s i n in v e rs io n z o f one o f th e fo u r carbon c e n te rs in th e cy clo b u tan e p ro d u c t. The r e l a t e d [g.2 s + c 2 a ] p ro c e ss must o p e ra te (m icro sco p ic r e v e r s i b i l i t y X X c o n s id e ra tio n s ) f o r th e allow ed th e rm a l fra g m e n ta tio n s o f c y c lo b u tan es. P rom otion of one e le c tr o n t o an a n tib o n d in g o r b i t a l d u rin g photochem i c a l tra n s fo rm a tio n s r e q u ir e s th e o p p o s ite mode o f r e a c tio n, [ 2 S + 2 0] o r [2a + 2a ] Thus th e th e rm a lly allo w ed [2+2] p ro c e ss fo rb id s a l e a s t m otion pathw ay; th e p h o to ch em ical p ro c e ss does n o t. A t h e o r e t ic a l ly more rig o ro u s approach in v o lv in g a s e m ilo c a liz e d m o lecu lar o r b i t a l model has a ffo rd e d th e same r e s u l t. 3 I t sh o u ld b e m entioned t h a t c e r ta in unique c y c lo a d d itio n r e a c tio n s ( e.g. k e te n e and v in y l c a tio n a d d itio n s t o o l e f in s ) have b een r a t io n a liz e d by a [ ^ q + n2a"] co n certed mechanism in v o lv in g th e o rth o g o n a l o r b i t a l o f th e k e te n e o r v in y l c a tio n as th e a n t a r a f a c ia l component. 2 However, th e s e w i l l n o t b e

16 / 4a c o n sid e red f u r t h e r, nor w i l l th e r e t r o p ro c e ss (e.g. f3-lactam, p- la c to n e, o r cyclobutanone p y ro ly s is ) b ecau se o f th e o rth o g o n a l v acan t p o r b i t a l i n k e te n e s, e t c., w hich c o n tr ib u te two s tro n g bondin g in te r a c tio n s t h a t a re a b sen t from th e [ 2 S + rt2a ] r e a c tio n p a th f o r sim ple o le f in s. Salem h as review ed th e e le c tr o n ic p r o p e r tie s o f d ir a d ic a ls and p ro v id e d th e n e c e s s a r ily somewhat vague d e s c r ip tio n o f th e s e sp e c ie s as hav in g two odd e le c tr o n s i n d e g e n e ra te o r n e a rly -d e g e n e ra te m ole- 5 c u la r o r b i t a l s. In th e s i t u a t i o n where th e s e tw o e le c tr o n s a re s u f f i c i e n t l y i s o la te d so t h a t in t e r a c t io n b etw een them i s sm a lle r th a n h y p e rfin e i n t e r a c t io n betw een odd e le c tr o n s p in and n u c le a r s p in, an ESR spectrum can be o b ta in e d c h a r a c t e r is t i c o f tw o d o u b le t s t a t e s 6 a lth o u g h t h i s has been r e a liz e d only f o r b is g a lv in o x y l (l.) and C h ich i- 7 b a b in 's hydrocarbon (2). S in ce such p u re double r a d i c a l s a r e r a r e, Cl C l 1 0 * th e more common circu m stan ce i s one o f lo w est s i n g l e t and t r i p l e t s t a t e s w ith a sm a ll energy d if f e r e n c e. Quantum m ech an ical s tu d ie s have shown t h a t each s t a t e may p o s se s s more o r l e s s th e c h a r a c t e r is t i c s o f th e o th e r depending upon th e r e a c tio n in v o lv e d, and fu rth e rm o re t h a t

17 e i t h e r s t a t e may a c q u ire p a r t i a l io n ic c h a r a c t e r.. F in a lly, i n t e r system c ro s s in g betw een s in g l e t and t r i p l e t s t a t e s may o ccu r by a v a r ie ty o f m echanisms. 5 A lthough th e g eo m etric and e le c tr o n ic s tr u c tu r e o f a 1,U d i r a d ic a l should be t o a la r g e e x te n t a f u n c tio n o f s u b s titu e n ts and m u ltip lic i t y, i t i s f i r s t d e s ir a b le t o o b ta in in fo rm a tio n r e l a t i n g t o th e s tr u c tu r e o f th e u n s u b s titu te d s p e c ie s, te tra m e th y le n e. I n 1961 Benson b r i e f l y c h a r a c te r iz e d th e in te rm e d ia te d i r a d ic a l in th e p y ro ly s i s o f cy clo b u tan e as a s tr u c tu r e p o s s ib ly re se m b lin g a lo o se complex s o f two q u a s i-e th y le n e s p e c i e s. S e v e ra l y e a rs l a t e r Hoffmann began t o develop a more rig o ro u s t h e o r e t i c a l concept o f te tra m e th y le n e a s he c o n sid e re d th e p o s s ib le i n t e r a c t io n o f o r b ita l s s e p a ra te d by a number o f in te rv e n in g a bonds e i t h e r th ro u g h space o r th ro u g h bonds. S e v e ra l m o lecu les w ere s tu d ie d, b u t o f p a r t i c u l a r i n t e r e s t w ere th e r e s u l t s o f extended H uckel c a lc u la tio n s on l,lj- d ia z a b ic y c lo [ ] - 9 o ctan e {%) w hich made two s ig n i f i c a n t p r e d ic tio n s : f i r s t, th e two nonbonding n itro g e n o r b i t a l s should i n t e r a c t th ro u g h th r e e in te rv e n in g a bonds v ia o v e rla p w ith th e p a r a l l e l Ca -C3 bond, and second, t h i s th ro u g h -b o n d I n te r a c tio n betw een nj. and na sh o u ld dom inate any th ro u g h - space e f f e c t t o produce a 1.6 ev s p l i t w ith a low er ly in g an tisy m m etric o r b i t a l. Hoffmann f u r t h e r p r e d ic te d th rough-bond c o u p lin g i n o th e r system s t o b e in s e n s itiv e t o r o ta tio n about th e C2 -C3 bond w hich le a v e s th e o v erla p unchanged ( S ^ ~ ), b u t s e n s itiv e t o r o t a t i o n 10 around th e and C3-C4 bonds. P h o to e le c tro n sp e c tro sco p y confirm e d th e s e p r e d ic tio n s a s th e a n tisy m m etric o r b i t a l was found t o l i e 11 below th e sym m etric by 2.1 ev.

18 5 a* _S_ T \ «= < ) s F ig u re 1. I n te r a c tio n diagram f o r l,u -d la z a b ic y c lo [2.2.2 ]o c ta n e {^). An approach analogous t o t h a t used f o r 3 was a p p lie d hy Hoffmann t o a d e te rm in a tio n o f th e p o t e n t i a l s u rfa c e o f te tra m e th y le n e as g e n e ra te d th e rm a lly from cy clo b u tan e in th e nonco n certed cleav ag e t o 12 y ie ld tv o e th y le n e m o lecu les. C a lc u la tio n s w ere a g a in o f th e e x te n ded H uckel ty p e w hich does not d is tin g u is h s in g le t and t r i p l e t conf ig u r a tio n s, h u t r a t h e r p ro v id e s an averag e over th e s p in s t a t e s. B reaking th e C1-C4 bond o f cy clo b u tan e w ith an in -p la n e s tr e tc h in g m otion (in c re a s in g a) w h ile h o ld in g a l l o th e r v a r ia b le s c o n s ta n t c re a te d

19 6 d is c o n tin u ity in carb o n -carb o n bond o v e rla p p o p u la tio n s a t a ~ in d ic a tin g le v e l c ro sso v e r, a phenomenon analogous t o t h a t o b serv ed f o r diam ine i l l u s t r a t e d w e ll by th e f ig u r e s from H offm ann's p a p e r. F or a < th e sym m etric ( n i + n2 ) nonbonding le v e l i s doubly occupied 9 e due t o s ta b i l i z i n g th ro u g h -sp a c e i n te r a c tio n o f th e two o r b ita l s ( s tr e tc h e d c y c lo b u ta n e ). Through-bond c o u p lin g, w hich dom inates f o r a > l l 6, a r i s e s by i n t e r a c t io n o f th e nonbonding le v e ls w ith th e a and (T* le v e ls o f l i k e symmetry, and d e p re sse s th e an tisy m m e tric l e v e l below th e sym m etric. I t i s c le a r t h a t sw itc h in g tw o e le c tr o n s from S t o A r e s u l t s in d e s ta b iliz a t i o n o f th e C2-C3 bond and concom itant in c re a s e d 1

20 F ig u re 2. I n te r a c tio n diagram f o r a te tra m e th y le n e w ith 0 = 180, p = Y = 9 0, la r g e a. The r a d i c a l o r b ita l s a re mixed w ith th e u and <r,! le v e ls o f t h e C2 -C3 "bond. "bonding a t Cx-C2 and , th e s i t u a t i o n f o r two complexed e th y le n e s. There fo llo w e d a d e te rm in a tio n o f th e p o t e n t i a l energy s u rfa c e o f te tra m e th y le n e by in d e p e n d e n tly v a ry in g a, p, y, and 9, w hich in d ic a te d t h a t two lo c a l minima o f s im ila r e n e rg ie s e x is te d, one a t 01 = 1 1 6, 9 = 1 05 (g au ch e), and one a t a = 1 1 2, 9=0 ( tr a n s ), w ith p = y = 9 0 (maximum o v e rla p w ith th e C2 -C3 bond) f o r b o th co n fo rm atio n s.

21 R o ta tio n o f one te r m in a l m ethylene "by 90 in c re a se d energy by ev; s im ila r r o ta tio n o f th e o th e r te rm in u s caused only an a d d itio n a l ev energy in c r e a s e. I n th e t r a n s confo rm atio n, th e two nonbonding le v e ls were c a lc u la te d to be s p l i t by ev, a d iff e r e n c e i n s u f f i c ie n t t o g u aran te e a ground s t a t e s in g l e t. F in a lly, and o f p rim ary im p o rtan ce, by d e c re a sin g 0 and in c re a s in g th e C2 -C3 bond d is ta n c e, th e c o n d itio n s f o r decom position t o e th y le n e, Hoffmann found m onotonic energy d e c re a se from e i t h e r th e gauche o r tr a n s co nform ation. Thus no t r u e minimum o r in te rm e d ia te e x i s t s betw een cy clo b u tan e and two e th y le n e s, b u t r a t h e r th e p o t e n t i a l en erg y s u rfa c e i s c h a ra c te riz e d by a la r g e f l a t re g io n (a, tw ix ty l ) f o r th e te tra m e th y le n e s p e c ie s which th e a u th o rs b e lie v e ftw ill b e o p e r a tio n a lly in d is tin g u is h a b le from a t r u e minimum. Salem has co n cu rred w ith Hoffmann s c a lc u la tio n s and p roposed a r e a c tio n c o o rd in a te diagram (F ig u re 3)* 5 9?he F rench th e o r e tic ia n has R e a c tio n c o o rd in a te F ig u re 3* Energy p r o f i l e a lo n g th e r e a c tio n c o o rd in a te f o r d eco m p o sitio n o f c y clo b u tan e.

22 9 been p rim a rily concerned w ith th e sim p le r te tra m e th y le n e system, w hich he e le g a n tly an aly zed w ith in th e f u l l 2 1 -d im e n sio n a l h y p e rsu rfa c e w ith th e s e lf - c o n s is te n t f i e l d m o lecu lar o r b i t a l (SCF-MO) ap proach u sin g a m inim al b a s is s e t o f S la te r o r b ita lb. 1 3 H o p efu lly such an e x h a u stiv e in v e s tig a tio n w i l l be fo rth co m in g f o r th e te tra m e th y le n e s u rfa c e. R e c e n tly, more com plete m o lecu lar o r b i t a l c a lc u la tio n s o f s p in c o r r e la tio n e f f e c t s on r o t a t i o n a l b a r r i e r s i n 1,4 d i r a d ic a l s have appeared, b a se d on SCF-MO methods u s in g th e in co m p lete n e g le c t o f d i f - f e r e n t i a l o v e rla p (INDO) approach w hich in c lu d e s e le c tr o n i n te r a c tio n. S tephenson has shown from a m odel o f s in g le t s t a t e te tra m e th y le n e 1 4 re p re s e n te d by two w eakly coupled e th y le n e s t h a t a h ig h b a r r i e r (45 k c a l / mol) t o r o t a t i o n e x i s t s in th e s in g l e t s t a t e w ith a n e g lig ib le b a r r i e r (0.5 k c a l/m o l) f o r th e t r i p l e t s t a t e. T his was eco n o m ically p re s e n te d by c o n s id e ra tio n o f th e fo u r m o lecu lar o r b i t a l s a f t e r th ro u g h -b o n d i n t e r a c t io n in t h a t r o ta tio n o f a te r m in a l m ethylene by $0 would des t a b i l i z e b o th and ife b u t would s t a b i l i z e a n tib o n d in g o r b i t a l s ^3 and ^4. The b a r r i e r t o r o t a t i o n o f s in g l e t tyi2 ^ 2 w ould b e p a r t i a l l y removed in th e t r i p l e t s t a t e. F req u e n t o v e re stim a tio n o f bond e n e rg ie s and energy sp acin g s by t h i s method was acknow ledged. These m o lecu lar o r b i t a l c a lc u la tio n s w ere la r g e ly an e f f o r t t o e x p la in more a d e q u a te ly th a n e a r l i e r p o s tu la te s (se e below ) th e i n t e r e s tin g and somewhat co n fu sin g e x p e rim e n ta l o b s e rv a tio n s o f r e a c tio n s in v o lv in g 1,4 d i r a d ic a l in te rm e d ia te s. The th e rm a l fra g m e n ta tio n o f 15 cy clo b u tan e was th e f i r s t o f th e s e r e a c tio n s in v e s tig a te d in d e t a i l. W. D. W alters re p o rte d in 1951 t h a t gas p h ase p y r o ly s is o f cy clo b u tan e

23 10 a t 1* c le a n ly y ie ld e d two m olecules o f e th y le n e in a homogeneous and k i n e t i c a l l y f i r s t - o r d e r r e a c tio n w hich was in s e n s itiv e t o a d d itio n o f in h ib ito r s and c h a r a c te r iz e d by th e h ig h p re s s u re r a t e c o n s ta n t 16 3*5 6 k = 10 exp (-62500/R T )sec_1 I t was su g g este d t h a t te tra m e th y le n e d i r a d ic a l m ight be a r e a c tio n in te rm e d ia te ; l a t e r c a lc u la tio n s in d ic a te d an ex trem ely s h o rt l i f e - / - io. tim e f o r t h i s e n t i t y 1 0 s e c ;, and th u s a d d itio n o f r a d i c a l i n h i - b i t o r s would be ex p ected t o have no e f f e c t. Of p a r t i c u l a r re le v a n c e was W a lte rs ' claim made in w h ile d e s c r ib in g th e th e rm a l d ecom position o f 1, 2 -d im e th y lc y c lo b u ta n e s (j* and jj) t o 2 -b u te n e s (j6 and T) t h a t i f a b i r a d i c a l e x i s t s m om entarily as an in te rm e d ia te, i t decomposes b e fo re th e o r ig i n a l geom etry i s c o m p letely 19 c o n v e rte d t o th e e q u ilib riu m d i s t r i b u t i o n. 16 These r e s u l t s, to g e th e r w ith th e o b serv ed c i s - t r a n s is o m e riz a tio n o f th e two cy clo b u ta n e s, w ere an a ly z ed k i n e t i c a l l y and therm odynam ically by O 'N eal and Benson 20 who lik e w is e assumed a d i r a d i c a l mechanism. A ll r a t e p ro c e sse s in th e mechanism a r e im p o rta n t. The r a t i o j6;7, ( c i s ; t r a n s ) from 1* (cis) i s 6 4 :3 6, 1 1 :8 9 from ( t r a n s ), and 4-5:55 a t e q u ilib riu m ; th e a c tiv a tio n energy f o r a cleav ag e o f t r a n s isom er i s k c a l/m o l l e s s th a n t h a t o f 4_and 2 k c a l/m o l l e s s th a n p cleav ag e o f e i t h e r isom er. The s l i g h t l y slo w er r a t e o f is o m e riz a tio n r e s u l t s i n a 7 6 :2 4 e q u ilib riu m m ix tu re o f _4 and J5.

24 11 A / \ 2$ t) 2a 2t \ it ' 5 N / I I W a i.o 'N eal and Benson have developed methods f o r making a p r i o r i t r a n s i t i o n s t a t e e s tim a te s o f a c t i v a t i o n e n e rg ie s and A rrh en iu s A f a c - 20 t o r s f o r th e rm a l r e a c tio n s o f sm a ll r in g s, b o th m onocyclic and p o ly c y c lic, by a p p lic a tio n o f g r o u p - a d d itiv ity v a lu e s and assu m p tio n o f d i r a d ic a l p ro c e s s e s. C a lc u la te d v a lu e s a re im p re s siv e ly c lo s e t o th o se o b serv ed, ex cep t i n in s ta n c e s -where a c o n c e rte d r e a c tio n i s p r e d ic te d by o r b i t a l symmetry c o n s id e ra tio n s. I n th e s e c a s e s, b o th lo g A and Ea c t 8X6 o v e re stim a te d. Thus n o t o n ly can p ro d u c ts and r a t e c o n s ta n ts and r a t e c o n s ta n t p ara m e te rs f o r th o s e p ro d u c ts be p r e d ic te d f o r d i r a d ic a l

25 r e a c tio n s, "but w here th e t r a n s i t i o n s t a t e s f o r c o n c e rte d and d ir a d ic a l p ro c e sse s f o r a r e a c tio n a re s ig n i f i c a n t l y d i f f e r e n t, A -fa c to r e s t i - 21 m ates p ro v id e a n o th e r u s e fu l d is c rim in a tin g t e s t o f m echanism. D e sp ite th e s tim u la tin g c o n trib u tio n s th u s f a r p re s e n te d, th e r e rem ained t o be d eterm ined a t th e e x p e rim e n ta l le v e l th e ste re o c h e m ic a l f a t e o f 1,4 d ir a d ic a ls upon t h e i r g e n e ra tio n from s u ita b ly - la b e le d 12 compounds. F rey compared th e r a t e o f 1,1,3,3 -te tra m e th y lc y c lo b u ta n e (8 ) decom position t o th o se o f a s e r i e s o f a lk y lc y c lo b u ta n e s i n an e f f o r t t o o b ta in a d d itio n a l in fo rm a tio n concern in g th e t r a n s i t i o n com- p le x. 2 3 At 4-50, 1, 1 -d im eth y lcy c lo b u ta n e (j?) had a decom position r a t e o f 3*34- r e l a t i v e t o c y c lo b u ta n e, as m ight b e expected, b u t 9 e x h ib ite d a r a t e d e c re a se o f r e l a t i v e t o c y c lo b u ta n e. A c tiv a tio n e n e rg ie s ran g ed from 6 l.o t o k c a l/m o l * A lthough F rey c o n sid e re d th e c o n c e rte d [a 2s + CT2a ] p ro c e ss t o b e " i n t r i n s i c a l l y more p ro b a b le, he co u ld a ls o acco u n t f o r the. e x p e rim en tal o b s e rv a tio n s w ith a d i r a d ic a l mechanism i f tw is tin g was conc u rre n t w ith s tr e tc h in g and u ltim a te bond b re a k in g o f a carb o n -carb o n bond t o c r e a te a d e s ta b iliz i n g 1,3 i n t e r a c t io n. S te re o c h e m ic a l la b e ls s u f f i c i e n t t o d is tin g u is h th e two pathw ays w ere q u ic k ly p ro v id e d by th e same a u th o rs in a r e p o r t o f th e gas p h ase p y r o ly s is o f c i s - and t r a n s -

26 ,7 -d im e th y lb ic y c lo [3.2.0 ]h e p ta n e (10 and l l ). The r e s u l t s a re rem i- n is c e n t t o th o se o f W alters f o r th e 1,2 -d im e th y lc y c lo b u te n e s and j?) 1 9 o = b CH3 ch3 klb-b53 - o ch3 ch3 ch3" ch3 10 ch3 'c h 3 11 and im p lic a te an in te rm e d ia te 1,U h u ta n e d iy l su g g ested t o a r i s e from tw is tin g o f th e four-m em bered r in g. I t i s i n t e r e s t i n g t o compare O 'N eal and B en so n 's e s tim a te d v a lu e s of Eac1- = 60.5 k c a l/m o l and lo g A = lb.6 t o th e o b serv ed v a lu e s f o r th e p a re n t hydrocarbon o f ^- k c a l/ mol and 1^. 8^, r e s p e c tiv e ly. 21 I n th e same y e a r B aldw in d e sc rib e d th e th e rm a l chem istry o f a r e l a t e d fu se d r in g compound, 'J, 8 - c is,e x o - d id e u te r io - c is - b ic y c lo [ l ] - o ctan e (1 2 ), w ith somewhat d i f f e r e n t r e s u l t s. 2 5 I t sh o u ld be n o ted t h a t such m olecules a re id e a lly s u ite d f o r in v e s tig a tio n s o f t h i s ty p e sin c e th e p rim ary cleav ag e mode g iv es cyclohexene and e th y le n e w ith th e c y c lo - alk en e r e q u ire d t o a c t as th e s u p r a f a c ia l p a r tn e r d u rin g c o n c e rte d

27 ill fra g m e n ta tio n. O ther e f f e c t s o f th e six-m em bered r in g upon t r a n s i t i o n s t a t e s tr u c tu r e a re d i f f i c u l t t o a s s e s s. Modest predom inance o f th e S * t r a n s isom er, th e "co n c e rte d p ro d u c t, " was in te r p r e te d "by th e in v e s tig a t o r s t o he in c o m p atib le w ith e i t h e r th e te tra m e th y le n e d i r a d ic a l o r th e [a 2s + a 2a ] allow ed p a th. Hoffmann c o n sid e re d th e o b s e rv a tio n s as ev id en ce a g a in s t a c o n c e rte d p ro c e ss. The th e rm a l rearran g em en t of b ic y c lo [2.2.0 ]h e x a n e s, low er homologs i n t h i s c la s s o f b i c y d i c s, t o d i a l l y l s has r e c e n tly re c e iv e d s ig n i f i ^ c an t a tte n t i o n. The f i r s t r e p o r t t o in c lu d e s u f f i c i e n t ste re o c h e m ic a l la b e lin g was made b y P a q u e tte and employed th e th r e e p o s s ib le 2,3 - d i- ae carbom ethoxy d e r iv a tiv e s 13, jui, and 13 A lthough observed p ro d u c t C00CH3 /jc00ch3!00ch3,cooch3 COOCH3 (80?) COOCH3 vv» COOCH3.COOCH3 COOCH3 21 (9*) 18 ( 11? ) ^ COOCH3

28 15 2 COOCH3 cooch3 + L + + ( 6*) (75*) (1*) COOCH3 COOCH3 (18*) 20 r a t i o s h e a v ily fa v o red ( 8 : 1 and 1 2 : l ) th o se ex p ected of a c o n c e rte d p ro c e s s, i t was re c o g n iz e d t h a t o h te n tio n o f i d e n t i c a l p ro d u c t m ix tu res from "both c is,e x o 1 and c is,e n d o Ik was ir r e c o n c ila h le w ith any p ro c ess in v o lv in g a h ig h ly tw is te d t r a n s i t i o n s t a t e t h a t w ould r e q u ire s e rio u s s t e r i c com pression of an e s te r s u b s titu e n t w ith a m ethylene c e n te r in d i e s t e r lh (n o t p re s e n t in 1 ). A more p la u s ib le mechanism was proposed in v o lv in g an in te rm e d ia te d i r a d ic a l w hich from th e most s ta b le c h a ir co n fo rm atio n ( 2 0 ) s t a b i l i z e d by fa v o ra b le th ro u g h -b o n d co u p lin g (se e above) w ould cle av e to th e o b serv ed p ro d u c ts as e a r l i e r su g g ested by Roth (se e b elo w ). A b ro ad stu d y of r a d i c a l s t a b i l i z a t i o n e n e rg ie s in th e gas and liq u id p hase h as been i n i t i a t e d by C ain in th e p y r o ly s is o f k - s u b s ti- tu te d 1 - c h lo ro b ic y c lo [ ] hexanes (2 0 ). a ls o assumed a l, k d i r a d ic a l mechanism. The A u s tra lia n group has F or a l l n in e s u b s titu e n ts in v e s tig a te d t o d a te, d ie n e fo rm a tio n h a s shown c le a r ly f i r s t o rd e r

29 16 k i n e t i c s, no in c re a s e i n r a t e w ith in c r e a s in g s o lv e n t p o l a r i t y, and no c o r r e la tio n "between s t e r i c i n te r a c tio n s and th e r a t e o f r in g opening. The observed v a lu e s w ere in agreem ent w ith p u b lis h e d d a ta o f s e v e ra l s u b s titu e n ts. Cl C l I R 20 D e sp ite im p ressiv e therm odynam ic, k i n e t i c, and ste re o c h e m ic a l ev id en ce i n su p p o rt o f a 1,4 c y c lo h e x a n e d iy l in te rm e d ia te, G o ld s te in a p p a re n tly i m p lic itly fa v o re d c o n c e rte d [g2 g + ^ a ] th e rm a l r e a rra n g e ment o f 2 1 t o 1, 5 -h e x a d ien es 22 and 2^, t o acco u n t f o r th e h ig h (? ) s te re o - 28 s e l e c t i v i t y o b serv ed. The a u th o rs o f fe r e d e i t h e r a ste p w ise o r conc e rte d p ro c e ss t o e x p la in b o th r in g opening and a h i t h e r t o unreco g n ized d e g e n e ra te in v e rs io n re arra n g e m e n t D D

30 B erson h as in v e s tig a te d th e p y r o ly s is o f c i s - and t r a n s - 2,3 - d im e th y lb ic y c lo [2.1.0 ]p e n ta n e (25 and 2 6 ). 2 9 As th e lo w est member o f th e b i c y c lic [ n ] s e r i e s, b ic y c lo [ ]p en tan e may p o sse ss a c e n tr a l carb o n -carb o n bond w ith some c h a r a c t e r is t i c s in te rm e d ia te b e tw een th e two r e l a t e d compounds b ic y c lo [ ]hexane (n o n co n certed th e rm a l fra g m e n ta tio n ) and cy clo b u tan e (c o n c e rte d ). The a c y c lic d ie n e s, a lth o u g h form ed in sm a ll y i e l d (kfy), had s te re o c h e m is trie s cons i s t e n t w ith a c o n c e rte d p ro c e ss and w ere su g g ested to a r i s e b ecau se o f a resem b lan ce o f th e c a n te d cy clopropane o r b i t a l axes w ith cy clo b u tan e it o r b i t a l s. S hould a d i r a d ic a l in te rm e d ia te b e in v o lv ed a s p r e d ic te d by O 'K eal and B en so n 's c a lc u la tio n s, 21 th e p la n a r s p e c ie s would be u n ab le t o ad o p t a fa v o ra b le tw is te d co n fo rm atio n as proposed by P a q u e tte 2 6 f o r th e six-m em bered r in g t o g iv e a ls o th e observ ed p ro d u c ts. and w ould fragm ent w ith c o n ro ta to ry opening 2 9 CH3 or

31 18 P y r o ly s is o f a r e l a t e d hydro carb o n, a n t i - t r i c y c l o [ J h eptan e (27) when a p p r o p ria te ly s u b s titu te d, r e s u l t s in a n o v el 30 f 2 + _2 + -2,,] ty p e r e a c tio n. The h ig h ly s te r e o s e le c tiv e p ro c e ss (7 S u S (J s 28 o ccu rs e q u a lly w e ll from e i t h e r 27 o r d i r e c t l y from i t s azo p re c u rs o r 2 8, p ro b a b ly i n i t i a t e d by r u p tu r e o f th e weakened C2 -C4 bond (a ro m a tic b rid g e h e a d s u b s t i t u t i o n i s re q u ire d f o r th e r e a c tio n t o fo llo w t h i s pathw ay). S tep w ise and c o n c e rte d cleav ag es a r e in d is tin g u is h a b le in t h i s in v e s tig a tio n a lth o u g h such in te rm e d ia te s a s 2 and ^ 0 a r e u n lik e ly. SSL

32 B erso n s stu d y of p o n d e ra l and e n e r g e tic e f f e c t s on r e l a t i v e r a t e s o f d eco m p o sitio n (k ^) and is o m e riz a tio n (k^) i n 1,4 tm ta n e d iy ls has re v e a le d an in c re a s e in k,j due t o in c re a s in g s t a b i l i t y o f fragm ent o le f in s w ith in c re a s in g m ethyl s u b s titu tio n and a d e c re a se in k^ due t o th e more n e g a tiv e e n tro p y o f a c t iv a tio n a s s o c ia te d w ith more m assive r o to r s. T h is e f f e c t, p r e d ic te d e a r l i e r by O N eal and Benson, was e s tim a te d by Bergman f o r m ethyl s u b s titu tio n on th e low mass m ethylene r o to r as a te n - f o l d d e c re ase d i n t e r n a l r o ta tio n fre q u en cy f a c to r. Bona f id e c o n c e rte d th e rm a l r e a c tio n s o f c y clo b u tan es c o n s is t o f such exam ples as th e s u p r a f a c ia l 1, 3 -s ig m a tro p ic rearran g em en t o f b ic y c lo [ ]h e p t- 2 -en e L t o norbornene ^ 2, 33 b u t th e co n c erte d 32 AcO + CT2a ] fra g m e n ta tio n h as o n ly b een a d e q u a te ly d em o n strated a s a 34 dom inant pathw ay f o r b ic y c lo b u ta n e s. ch3 1,4 D ir a d ic a ls a re en c o u n tered by d ecom position o f c y c lic azo compounds, and alth o u g h th e s e v a lu e s a r e w id ely quoted in a tte m p ts to

33 20 c o r r e la te c h a r a c t e r is t i c s o f v a rio u s te tra m e th y le n e in te rm e d ia te s, Bergman h as v arn ed : A s tro n g d is c la im e r sh o u ld h e n o ted w ith re g a rd t o... th e mechanism o f d eco m p o sitio n o f c y c lic azo compounds [which 3a a p p e a r] t o he v ery d i f f i c u l t t o g e n e r a liz e. Two im p o rtan t r e p o r ts in t h i s a r e a have ap p eared. Therm al e x tru s io n o f n itro g e n from h lc y - c l i c s ^ and ^ was p o s tu la te d hy R oth t o in v o lv e an in te rm e d ia te l,lf cy clo h e x a n e d iy l w hich underw ent p a r t i a l c o n fo rm a tio n a l e q u ilib r a tio n p r i o r t o cle a v a g e. 35 CH\ j d CH- C CH; H a ch3 ch3 ch3 x : a 2 L &

34 21 Table 1. P ro d u ct D is tr ib u tio n f o r Therm al D ecom position o f ^ and Isom er & % ^ * 2 b * & * a < k < B a r t l e t t c o r r e la te d mode o f d eco m p o sitio n w ith s te re o c h e m is try o f p ro d u c t cy clo b u tan es (jk> and ju.) and amount o f cleav ag e t o 2 -m e th y l-1-36 b u te n e Q5J-?) f o r th e d l and meso isom ers o f ^ 8. The la rg e amounts o f is o m e riz a tio n and cleav ag e o b serv ed f o r s e n s itiz e d i r r a d i a t i o n compared w ith v a lu e s f o r p y r o ly s is and d i r e c t i r r a d i a t i o n were r e l a t e d t o i n a b i l i t y o f th e in c ip ie n t t r i p l e t s p e c ie s t o r in g c lo se th u s p e rm itt i n g bond r o t a t i o n ab o u t Ci -Cq, Oq-Cq, and C3 -C4, w ith r e s u l t a n t in c re a s e o f cleav ag e over c lo su re due t o n o n c iso id co n fo rm atio n s. ^9 40 J+l F o rm atio n o f cyclo b u tan es in th e rm a l [2+2] r e a c tio n s h a s been a c tiv e ly in v e s tig a te d p r im a r ily by B a r t l e t t whose c o n tr ib u tio n s a re e x e m p lified by th e c y c lo a d d itio n o f te tr a f lu o r o e th y le n e (TFE) t o c is -

35 22 T able 2. P ro d u ct D is tr ib u tio n s f o r D ecom position o f 3 8. Isom er Mode o f D ecom position % 40, % 41, f R etn. in c o n fig. in 40 and 4 l,? meso A > 98 d, l A > 98 meso d i r hv d, l d i r hv meso sen s hv 77 H d, l sens hv and t r a n s - 2 -b u te n e s i l l u s t r a t i n g th e two c h a r a c t e r is t i c f e a tu r e s o f th e s e ste p w ise r e a c tio n s w ith d i r a d ic a l in te rm e d ia te s, p a r t i a l lo s s o f c o n fig u ra tio n i n th e p ro d u c ts, and g e o m e tric a l is o m e riz a tio n o f a 37 r e a c ta n t d u rin g, and o n ly d u rin g, c y c lo a d d itio n. C om p etitio n e x p e rim ents f o r e th y le n e and c i s -2 -b u te n e c y c lo a d d itio n w ith TFE p ro v id e d k e th y le n e /kb u te n e = 10 analo g o u s t o e a r l i e r s tu d ie s on [ 2+2 ] r e a c tio n s betw een l,l - d i c h l o r o - 2, 2 - d if lu o r o e th y le n e and b u ta d ie n e, t r a n s -p ip e ry - le n e, and t r a n s, t r a n s -2,4 -h e x a d ie n e. The r e s p e c tiv e r e l a t i v e r a t e s o f 1. 0, I. 6 5, and in d ic a te d t h a t s t e r i c h in d ra n c e o f a m ethyl group a t th e f i r s t s i t e o f bond fo rm a tio n i s more im p o rtan t th a n m ethyl s u b s t i t u t i o n on th e a l l y l i c group. 33 C y c lo a d d itio n o f TFE t o e i t h e r

36 II + TFE k i <- H ^ C H 3 0.T 1 krc H H k rc F fek X ^ % a 2 Z 7 F2 C* CH3 I 3 ch3 ch3 2 3 vr c + TFE 1.5 k i ch3 w n 5-^> > ' ^.H CH3 Fa J 2.2 kr -fc F, F2 C CH. 3 H CHa c i s - o r t r a n s - 1,2 -d id e u te r lo e th y le n e lik e w is e proceeded th ro u g h a d i r a d i c a l in te rm e d ia te r e s u l t i n g in a p p ro x im a tely e q u a l m ix tu res o f th e two p o s s ib le p ro d u c ts. 39 T his stu d y p ro v id e d evid en ce f o r th e g en eral i t y o f a ste p w ise p ro c e ss even f o r u n h in d ered r e a c ta n ts. Examples w hich m ight h e a n aly z ed as c o n certed th e rm a l c y c lo a d d itio n s have b een o b serv ed, a l l in v o lv in g d im e riz a tio n o f sp e c ie s c o n ta in in g a double bond h ig h ly tw is te d about i t s a x is. 40 D im e riz a tio n o f c i s, t r a n s - 1,3 - c y c lo o c ta d ie n e (42) y ie ld s a number of p ro d u c ts th e 40b m ajor b e in g (53%); r e l a t i v e therm odynam ic s t a b i l i t i e s o f v a rio u s r in g fu s io n s in v o lv in g cy clo b u tan e a r e n o t known i n most c a s e s.

37 2k I I k2 / The p h o to ch em ical g e n e ra tio n o f 1,4 d ir a d ic a ls in "both th e s in g le t and t r i p l e t s t a t e s h as "been assum ed in numerous in v e s tig a tio n s and r e c e n t rev ie w s. 41 One o f th e m ost in te n s iv e m e c h a n istic s tu d ie s has in v o lv ed th e ty p e I I p h o to p ro c e sse s. I n th e t r i p l e t d i r a d ic a l d e riv e d Ph. HO Ri -Ha Ph TT?1 JCf* hv Ph Ph Hi V from an a p p r o p ria te ly s u b s titu te d k e to n e, bond r o ta tio n i s f a s t e r th a n th e combined r a t e s o f th e th r e e p ro c e sse s shown: 1, 5-hydrogen t r a n s f e r r e s u l t s In ra c e m iz a tio n o f th e o r ig i n a l k e to n e, cleav ag e pro d u ces

38 s u b s t a n t i a l amounts o f "both p o s s ib le o le f in s, and th e m ix tu re o f d ia s - 4 ia te re o m e ric c y c lo h u ta n o ls r e t a i n s l i t t l e o p tic a l a c t i v i t y. Of p a r t i c u la r i n t e r e s t i s th e o b s e rv a tio n t h a t or, 0, and y s u b s titu e n ts i n t r o 25 duce d i f f e r e n t s t e r i c i n te r a c tio n s in to th e t r a n s i t i o n s t a t e s f o r c y c liz a tio n and cleavage t o c r e a te s u b s ta n tia l d iffe r e n c e s in ^ Cy C/^ c le * 42 The a u th o rs b a se d t h e i r in te r p r e ta tio n s upon W agner's s u g g e stio n t h a t b o th r a d i c a l p o r b i t a l s m ust o v e rlap w ith th e bond f o r cleav ag e t o o ccu r. 43 I n sharp c o n tr a s t s in g l e t ty p e I I p h o to p ro c e sse s a re q u ite s te r e o s e le c tiv e : Yang re p o rte d t h a t Ml d id n o t undergo p h o to ra c e m iz a tio n under s in g l e t c o n d itio n s as was o b serv ed f o r th e t r i p l e t. 44 S te p h e n so n 's t r i p l e t quenched i r r a d i a t i o n o f (e r y th r o ) and k6 ( th r e o ) w ith h ig h ly s te r e o s p e c if ic N o rrish ty p e I I p h o to c le a v a g e was in te r p r e te d a s a s p in c o r r e la tio n e f f e c t. CH3 JUT kb I T o u h- >Mk 0 +I + I CH3 s in g l e t H '^^C H 3 CH3'X^ I E=C02CH3 h i 90 : 1 // t r i p l e t > 1 1

39 26 CII = X : 90 s in g le t // => 1 : 1.5 t r i p l e t I n r e l a t e d work, Casey and Boggs d e s c rib e d a s n o n s te r e o s p e e if ic th e e x c ite d s in g l e t s t a t e ty p e I I c leav ag e and b u te n e fo rm a tio n from e ry - th r o and th r e o k e to n e s 4j_ (95^) and 48 (90^ ) c a lc u la te d b y an in v o lv e d d a ta a n a ly s is. 46 From th e l a t t e r two c o n tr ib u tio n s, i t i s e v id e n t t h a t CH. N o rrish ty p e I I p h o to cleav ag e from th e s i n g l e t s t a t e o ccu rs v ia a 1,4 d i r a d i c a l and n o t by a h o t ground s t a t e c o n c e rte d ene r e a c tio n a s m ight have o th e rw ise been p o s tu la te d. T here i s however a dichotom y in r a t i o n a l i z a t i o n o f s p in c o r r e la tio n e f f e c t s. Casey a p p lie d B a r t l e t t 's h y p o th e sis o f lo n g e r t r i p l e t l if e tim e s r e q u ire d by s p in in v e r s io n a llo w in g carb o n -carb o n bond r o t a t i o n t o compete w ith bond cle a v a g e,

40 46 cy clo b u tan e fo rm a tio n, o r 1,5 hydrogen t r a n s f e r w ith t h i s th e phenomenon o f p o n d e ra l e f f e c t s. 20,31,32 and has in te g ra te d In two l a t e r 27 p a p e rs Stephenson h a s r e f u te d t h i s concept a s a dom inant f a c to r s in c e C asey 's (and B a r t l e t t 's ) d e s c r ip tio n r e q u ir e s an u n reaso n ab ly sm all l 47 s p in in v e rs io n r a t e c o n s ta n t, ~ se c, and u ltim a te ly p ro posed t h a t s p in c o r r e la tio n e f f e c ts a r i s e from th ro u g h -b o n d co u p lin g (se e above) b u t r e ta in e d in h is argum ent e f f e c t s o f ponderous r o to r s. 14 Therm al r e v e r s a l o f ty p e I I p h o to p ro c e sse s has b een r e p o rte d by F e i t in th e p y r o ly s is o f c i s -1,2 -d im e th y lc y c lo b u ta n o l (jj2 /. e f f e c t o f th e r a d i c a l s t a b i l i z i n g h y d ro x y l s u b s titu e n t was 3 k c a l/m o l 46 The r e l a t i v e t o t h a t o f c i s -1,2 -d im e th y lc y c lo b u ta n e. I n a d d itio n, th e 1,5 hydrogen t r a n s f e r observed by F e it i s s tro n g ev id en ce t h a t such a p ro c e ss in p h o to ch em ical r e a c tio n s i s a pathw ay f o r r a d ia tio n le s s r e la x a tio n o f e x c ite d carb o n y l compounds. // + * \ / ~ v 7 > 7* ho h a 136 HO 6/G 12*

41 28 The o x etan e p ro d u c ts from th e P atern o -B u ch i r e a c tio n a re a ls o b e lie v e d t o a r i s e from 1,^ d i r a d ic a l in te rm e d ia te s. However, Wagner has r e c e n tly p roposed t h a t th e ra te -d e te rm in in g s te p in th e t r i p l e t keto n e r e a c tio n in v o lv e s complex fo rm a tio n and su b seq u en t c o lla p s e t o a d i r a d ic a l ; p ro d u c ts do not c o r r e la te w ith d i r a d i c a l s t a b i l i t i e s. The s in g le t r e a c tio n o f a lk a n a ls w ith n o n te rm in a l o le f in s a p p a re n tly a ls o in v o lv es an [1n-rf}f' o le f in ] e x c ip le x i n th e ra te -d e te r m in in g s te p w ith decom position t o th e s in g l e t d i r a d ic a l D ire c t i r r a d i a t i o n o f Hi hv R3 Ri- R2 JR4 ch3 ch3 X X xx ch3 ch3 c i s - and t r a n s -2 -b u te n e in th e liq u i d p h a se r e s u l t s in s te r e o s p e c if ic s i c y c lo d im e riz a tio n a ls o p o s s ib ly b y excim er fo rm a tio n.

42 The p h o t o frag m en tat io n o f cy clo b u tan es p re d ic te d by o r b i t a l symm etry c o n s id e ra tio n s t o b e an allow ed la 2 Q + CT2S] o r CCT2a + a 2a ] p ro c e ss is e x e m p lifie d by S a l t i e l 's in v e s tig a tio n o f two t r i c y c l i c cy clo b u ten es, and 5 2 The a u th o rs concluded t h a t th e ground s t a t e p ro d u c ts, J jl J5 S i & and 2 a r i s i n g from c i s isom er jjo "by c o n c erte d p ro c e sse s and tr a n s o l e f in ^ produced from w ere form ed d i r e c t l y by r e o rg a n iz a tio n o f e x c ite d cy clo b u ten e s. The e x c ite d s t a t e ch em istry o f b ic y c lo [ ^.2.0 ] o c ta d ie n e i s n o t am enable t o such a s tra ig h tfo rw a rd a n a ly s is as ev id en ced by th e r e s u l t s o f a t l e a s t s ix r e s e a rc h groups w hich have re p o rte d th e r e a c tio n. Both p o s s ib le fra g m e n ta tio n modes a r e o p e ra tiv e in th e p h o to frag m en tat i o n o f th e t r a n s -7,8 -d ia c e to x v d e r iv a tiv e B arto n o b serv ed s o le ly te tr a e n e p ro d u c t(s ) ( 5 7 ) from d ir e c t i r r a d i a t i o n o f and p ro p o sed a 1 ^ 5 > 5 -c y c lo o c ta trie n e in te rm e d ia te ( 56, no s te re o c h e m is try i s in p lie d

43 S3 f o r jj6 o r )j C aldw ell found no tr a c e o f te tr a e n e s upon s e n s itiz e d 50 p h o to ly s is o f and re p o rte d th e fo rm a tio n o f only benzene and a m ix- t u r e of c i s - and t r a n s -1,2 -a c e to x y e th y le n e s (5 8 ). 54 S in g le t p h o to ly s is,0ac AcO' OAc b OAc SL 7. OAc OAc OAc V. y~ -52 OAc c is j endo- 7, 8 -d im e th y l a ls o le d e x c lu s iv e ly t o a te tr a e n e, c is, tr a n s The b e n z o n itr ile 2 -m e th y l-2 -b u te n e p h o to p ro d u c t ( 6 l) when

44 31 i r r a d i a t e d under s in g l e t c o n d itio n s s u ffe re d cle a v a g e t o i t s o r ig in a l components and was su g g este d by B uchi t o occur v i a a p h o to e q u ilib riu m in v o lv in g a 1,4 d i r a d ic a l in te rm e d ia te (6 2 ). 56 C h a r a c te r iz a tio n o f th e sm all amount o f te tr a e n e s form ed ( 6^ ) was p re c lu d e d by extrem e i n s t a b i l i t y. F u rth e r exam ples o f th e r e a c tio n in c lu d e d i r e c t p h o to ly s is o f 57 [ ] p r o p e lla - 2,4,7 j9 te tr a e n e (63) and S c h ro e d e r's b u llv a le n e s y n th e s is. 58 hv s in g le t + H2 C ch2 & h v

45 RESULTS AND DISCUSSION When t h i s -work was i n i t i a t e d, virtually no r e p o r ts had appeared co n cern in g th e ste re o c h e m ic a l consequences o f th e rm a l o r p h o to ch em ical cy clo b u tan e cleav ag e r e a c tio n s. Of th e c o n s id e ra b le r e c e n t e f f o r t d ir e c te d tow ard th e th e rm a l fra g m e n ta tio n p ro c e s s, th e t e s t i n g o f c e r ta in p r e d ic tio n s a r i s i n g from o r b i t a l symmetry c o n s id e ra tio n s has re c e iv e d m ajor a t t e n t i o n. S in c e th e energy b a r r i e r f o r an allo w ed [ ] o S U u* r e a c tio n in v o lv in g a tw is te d t r a n s i t i o n s t a t e i s unknown, th e p o s s i b i l i t y e x is ts t h a t s e v e ra l exam ples o f t h i s r e a c tio n mode could be observed g iv en p ro p e r ste re o c h e m ic a l la b e lin g. r e a c tio n s has co n tin u ed a t an u n ab ated p a c e. The s e a rc h f o r such More r e c e n t e f f o r t s have b een d esig n ed t o f u r t h e r d eterm in e th e ch em ical c h a r a c t e r i s t i c s o f th e in te rm e d ia te te tra m e th y le n e s and have been augm ented by m o lecu lar o r b i t a l tre a tm e n ts o f ev er in c re a s in g s o p h is tic a tio n and r e l i a b i l i t y. T h is d i s s e r t a t i o n d e t a i l s p ro g re s s made tow ard c l a r i f i c a t i o n o f p o in ts a r i s i n g from th e l a t t e r c a te g o ry. The a v a i l a b i l i t y o f [ ] p r o p e lla - 3,1 1 -d ien e (6 4 ), th e p re p a ra - t i o n o f w hich was developed in th e s e la b o r a to r ie s by P h i l i p s, 5 9 a s a p o t e n t i a l p re c u rs o r t o v a rio u s d e r iv a tiv e s o f [ ] p r o p e lla - 2,4 - d ie n e ( 6 5 ) p re s e n te d a unique o p p o rtu n ity t o stu d y 1,4 d i r a d ic a l s. The in t r i n s i c s t r u c t u r a l f e a tu r e s o f 6^ w ere ex p ected t o p ro v id e ( l ) a p r e ponderance o f fra g m e n ta tio n in a s in g l e t d ir e c tio n, ( 2 ) a s u f f i c i e n t l y 32