Pericyclic Reactions page ELECTROCYCLIC REACTIONS and SIGMATROPIC HYDROGEN SHIFTS

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1 Pericyclic ections pge 3 ELECTCCLIC EACTINS nd SIGMATPIC GEN SIFTS. Selection rules for electrocyclic ring-opening nd ring-closure The Woodwrd-offmnn nlysis (illustrted in section.3) provided the selection rules shown elow for electrocyclic ring-opening. The electron count (n+2 or n) determines the oritl symmetry properties nd hence the stereochemistry of ring-opening. Electrocyclic rections of ions re common nd ring-closure my e the thermodynmiclly fvoured result. ELECTCCLIC EACTINS ection Electrons Therml Photochemicl Exmples n conrottion disrottion 00, 00, 02 n+2 disrottion conrottion 00, 0, 02 8 n conrottion disrottion 07 n conrottion disrottion n+2 disrottion conrottion 2 n+2 disrottion conrottion 033 n conrottion disrottion Vrious π systems were studied when this theory emerged. The π therml cyclistion proceeds y disrottion s predicted, ring closure eing the energeticlly fvoured process. ecting ends of the π system ψ 3 triene M when process is therml disrottion het (E,Z,E)-triene groups cis 00 7 C E,Z,E cis

2 Pericyclic ections pge 3 0 UV light E,Z,E ether 2 nm trns (compre with 00) 033 SF SF 00 C hlf-life 0 min t 0 C hlf-life 0 min t +3 C not detected Some exmples of oritl-controlled stereoselective electrocyclic ring opening nd closure re shown ove. The rections in 033 re cyclopropyl-llyl rerrngements. In principle the results could e interpreted s proceeding vi cyclopropyl ctions, formed y loss of chloride. owever, no evidence for the presence of such ctions ws found. Nevertheless the results show tht the process is stereospecific: the sustituents on the sme side of the cyclopropne ring s the leving group cn e expected to disrotte towrds one nother. Electrocyclic ring-opening of chlorocyclopropne disrottion The opertion of only one of the two thermlly llowed disrottion modes is consistent with the concerted S N 2-like displcement of the leving group y the electron density from the reking σ ond. In the ove digrm this electron density develops on the correct fce of the cyclopropne ring, i.e. wy from the chlorine tom, only if the groups disrotte 'inwrds' nd the groups disrotte 'outwrds' (use models to confirm this).

3 Pericyclic ections pge 37.2 Vlence tutomeristion through electrocyclic rections Smll rings tend to undergo electrocyclic ring opening ecuse of the ccompnying relese of strin, ut lrger π systems often exist in genuine equilirium, referred to s vlence tutomeristion. Some of these processes cn e viewed spectroscopiclly; for others the rection hs een reveled y isotopic lelling experiments. ing strin cn e useful predictor for ring opening nd closure rections; for exmple, trns doule ond cnnot e contined within 7-memered or smller ring, so ny conrottion or disrottion tht would produce such ond cn e expected to fil (or else e extremely difficult). For exmple, the cycloutene ring in 03 requires forcing conditions to induce therml conrottion, ecuse this genertes trns doule ond in n 8-memered ring (write out the mechnism!) C +20 C C 03 N 2 het or light 03 UV 9 C 3 C

4 Pericyclic ections pge 38 Br N % Br 2 C C 2 IENE (not detected) C 2 het C 2 ttempted [ + 2] cycloreversion NC 039 NC dioxn 00 C The photochemicl version of this triene rection is importnt in vitmin formtion in humns (lck of 3 cuses rickets). Previtmin 3 is then converted into vitmin 3 y nother pericyclic rection, this time [,7] sigmtropic shift. We will nlyse this clss of pericyclic processes in the next section. 0 sunlight conrottory electrocyclic ring-opening 7-dehydrocholesterol [,7] shift previtmin 3 = ()--methylhept-2-yl vitmin 3

5 Pericyclic ections pge 39.3 Sigmtropic rerrngements Sigmtropic rerrngements re pericyclic rections in which σ ond ppers to move cross conjugted system to new site. If the nme '[n,m] sigmtropic rerrngement' strts with '', then one end of the ond tht moves remins ttched to its originl tom the n refers to the fct tht the other end of the ond migrtes cross n toms. To egin with, we will nlyse this type of rection in the context of migrting C onds (hydrogen shifts). [n,m] SIGMATPIC EAANGEMENT σ n m n σ m. Therml [,] nd [,7] sigmtropic hydrogen shifts The exmple 020 elow illustrtes some of the fetures nd nomenclture of sigmtropic shifts. Notice tht if we lel the p oritls of the π system nd the σ oritl of the C ond s either ('ove') or ('elow'), the C ond strts ove nd ends up ove. It migrtes cross the upper fce of the π system this is descried s suprfcil rerrngement het [,] shift 3 2 migrtion is suprfcil = ove = elow If we dd one more C=C π ond (two more p electrons), the oserved therml rection is ntrfcil the C strts 'ove' ut ends up 'elow' nd the ppers to migrte from one fce of the π system to the other. This cn only hppen when the moleculr structure is le to dopt conformtion tht llows such rerrngement. 7 3 het [,7] shift 2 migrtion is ntrfcil = ove = elow

6 Pericyclic ections pge 0