Radical Reactions. Radical Stability!!! bond dissociation energies X Y X + Y. bond BDE (kcal/mol) bond BDE (kcal/mol) CH 3 CH 3 CH 2 95 O H R 2 C H
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1 adical eactions adical Stability!!! bond dissociation energies X Y X Y bond BDE (kcal/mol) bond BDE (kcal/mol) C C 3 C C 102 (-) 93 (C-) 92 C 3 C C 3 C 3 80
2 adical eactions eaction Types!!! Addition eactions (to C=C, C=, C C, C ) C C C C Abstraction (usually or X) Z Z Bond Cleavage or Electron Transfer e ecombination
3 adical eactions Carbon adical Addition eactions!!! general features n n - reactions proceed under mild, neutral conditions - radical intermediates well suited to formation of hindered bonds - radical addition to alkene are usually exothermic, irreversible, and have early reactant-like transition states - carbon centered radicals are inert to and 2 - carbon radicals do not undergo β-elimination of or 2 groups - radical centers do not usually retain stereochemistry
4 adical eactions eneral Principles!!! mechanism driving force: Sn Sn Br BDE = 64 kcal/mol BDE = 60 kcal/mol Br Bu 3 Sn AIB, Δ inititation: C C Δ 2 C 2 azobisisobutyronitrile (AIB) Bn 3 Sn C Bn 3 Sn C Br Bn 3 Sn Bn 3 Sn Br
5 adical eactions eneral Principles!!! mechanism Br Bu 3 Sn AIB, Δ initiation Br Bu 3 Sn Br propagation Bu 3 Sn Bu 3 Sn termination
6 adical eactions Intermolecular eactions!!! addition X Bu 3 Sn AIB, Δ rate of addition: C 6 13 to = K rel = ,000 8,500 I Bu 3 Sn C AIB, Δ C 95%
7 adical eactions Intermolecular eactions!!! regioselectivity
8 adical eactions Intermolecular eactions!!! examples Ac Ac Ac Br Ac C Bu 3 Sn, hν C Ac Ac Ac Ac addition product (55%) Ac Ac Ac Ac reduction product (20%) Bn I (10 equiv) Bu 3 Sn, AIB, Δ 53% Bn 4 steps exo-brevicomin
9 adical eactions Intermolecular eactions!!! examples Ph * C 9 19 I Et Bu 3 SnCl (0.2 equiv) ab 4 (excess), ab 4 Ph C 9 19 C 2 Et 2 Pd/C 19 C 9 70% malyngolide Et 2 C Et 2 C Cl TMS tbu Bu 3 Sn AIB, Δ Et 2 C Et 2 C TMS tbu 56%
10 adical eactions X Bu 3 Sn AIB, Δ n Intramolecular eactions!!! n n regiochemistry 100 : 0 3-exo-trig vs 4-endo-trig 100 : 0 4-exo-trig vs 5-endo-trig 98 : 2 5-exo-trig vs 6-endo-trig 85 : 15 6-exo-trig vs 7-endo-trig 100 : 0 7-exo-trig vs 8-endo-trig
11 adical eactions Intramolecular eactions!!! regiochemistry 48 : 1 >200 : 1 2 : 3 1 : 100
12 Interlude Baldwin s ules!!! 5-exo-trig size of forming ring where e- end up nature of bond to which u adds where electrons end up! exo = outside newly formed ring! endo = inside newly formed ring! nature of bond to which u adds! tet = tetrahedral (sp 3 )! trig = trigonal (sp 2 )! dig = digonal (sp)!
13 Interlude Alabugin Chem. ev. 2011, 11, Baldwin s ules!! boxed structures are favored
14 adical eactions stereochemical models Intramolecular eactions!!! stereochemistry 5-exo 6-endo 65 : : : : 73
15 adical eactions Intramolecular eactions!!! vinyl radicals - more reactive than saturated counterparts than 1000x faster - low inversion barrier 2 C C 2 2 C C 2 2 C C 2 Bu 3 Sn Bu 3 Sn Br AIB, Δ AIB, Δ Br
16 adical eactions Intramolecular eactions!!! vinyl radicals Br LDA Br Br Bu 3 Sn, AIB Ph, hν 80% 70% 2 Pd/C * 85 : 15 norseychellene seychellene Stork Tetrahedron Lett. 1985, 26, 5927.
17 adical eactions Intramolecular eactions!!! TBS examples IS Et Et TBS I C 5 11 TMS Bu 3 Sn (cat) ab 3 C, tbu Et TBS TBS Et Bu 3 Sn (cat) C C Et Et Et TBS C 1. DIBAL 2. (Et) 2 P C 5 11 TBS 3 steps C C 2. Pd(Ac) 2 58% TBS (3 steps) TMS C 5 11 TBS prostaglandin F 2α
18 adical eactions Intramolecular eactions!!! examples I Bu 3 Sn AIB, Δ 80% hirsutene 5-exo-trig 5-exo-dig
19 adical eactions Intramolecular eactions!!! examples SPh Ts SPh Ts SPh Br Ts Bu 3 Sn AIB, Δ 5-exo-trig 6-endo-trig 30% Ts SPh Ts SPh -PhS Ts (-)-morphine Parker J. Am. Chem. Soc. 1992, 114, 9688
20 adical eactions Intramolecular eactions!!! tethered reaction processes Si Br Cl imidazole Si Bu 3 Sn AIB, Δ Si Br Bu 4 F 2 S Si Tamao oxidation
21 adical eactions Intramolecular eactions!!! tethered reaction processes
22 adical eactions SmI 2 Promoted eductive Coupling!!! radical cyclization / nucleophile attack X SmI 2 Sm III SmI 2 E E then I Ph Ph SmI 2 (2 equiv) TF-MPA 89% I ipr ipr SmI 2 (2 equiv) TF-MPA 55% - other electrophiles include: I 2, PhSSPh, PhSeSePh, Bu 3 SnI, PhC=
23 adical eactions SmI 2 Promoted eductive Coupling!!! reductive addition of carbonyls ' SmI 2 (2 equiv) ipr ' SmI 2 SmI 2 ; SmIII ' III Sm ' ketyl - for intermolecular reactions, must be an electron withdrawing group Ph C 2 2 SmI 2, ipr TF-MPA 78% Ph
24 adical eactions e C - 2 C E 2 SmI 2 Promoted eductive Coupling!!! n n n E reductive addition of carbonyls 2 SmI 2, tbu 2 SmI 2, tbu TF-MPA n TF-MPA 86% (n = 1) 91% (n = 2) dr = 150 : 1 dr = 36 : 1 C 2 2 SmI 2, tbu TF-MPA then C 2 2 SmI 2, tbu TF-MPA; then 2 C= % 73% 2 SmI 2, tbu E = PhC TF-MPA E = 2 83% 69% Ph
25 adical eactions SmI 2 Promoted eductive Coupling!!! reductive addition of carbonyls Et 2 SmI 2 TF-tBu 75% C 2 Et 2 SmI 2 TF-tBu 87% Et SmI 2 (2 equiv) TF-MPA C 2 Et 79%
26 adical eactions SmI 2 Promoted eductive Coupling!!! basis for selectivity SmI 2 Sm III e- E E ' Et SmI 2 III Sm ' Et III Sm ' Et e- E E ' Et
27 adical eactions SmI 2 Promoted eductive Coupling!!! examples SmI 2 TF-MPA 69% pts acetone III Sm III Sm coriolin Curran J. Am. Chem. Soc. 1998, 110, 5064
28 adical eactions SmI 2 Promoted eductive Coupling!!! examples SmI 2, tbu TF-MPA 63% paeonilactone B Sm III Sm III Sm III Kilburn J. Chem Soc., Perkin Trans
29 adical eactions SmI 2 Promoted eductive Coupling!!! examples
30 adical eactions 1. Et 3 2. PhSeBr, PBu 3 SePh Acyl adicals!!! general features Bu 3 Sn X AIB or hν
31 adical eactions Barton McCombie eaction!!! C 3 C 3 Br BDE ~ 90 kcak/mol ~ 68 kcal/mol ' S " ' ' C 3 I ~ 53 kcal/mol " = Ph, S,, 1. a 2. S=C=S 3. I Bu 3 Sn (1 equiv) AIB (cat), Δ S S Barton J. Chem. Soc. Perkin Trans , Barton, Motherwell Pure Appl. Chem. 1981, 51, 15. Tatsuta J. Am. Chem. Soc. 1979, 101, 6110.
32 adical eactions Barton McCombie eaction!!! mechanism C C Δ 2 C S C C S ' S SnBu 3 Bu 3 Sn Bu 3 Sn ' S driving force formation of C= (190 kcal/mol) vs C=S (137 kcal/mol) ' Bu 3 Sn ' S SnBu 3 S
33 adical eactions Barton McCombie eaction!!! Im S Im S Bu 3 Sn AIB, Δ Barton J. Chem. Soc. Perkin Trans , a 2. S=C=S 3. I S S S S Bu 3 Sn AIB, Δ S Im Im Bu 3 Sn AIB, Δ S
34 adical eactions Barton McCombie eaction!!! examples 1. a, CS 2, I, imid 2. Bu 3 Sn, AIB, Δ 68%
35 adical eactions eductive Decarboxylation!!! general features 1. SCl 2 2. S S AIB, Δ or hν C 2 S radical traps: Bu 3 Sn 2 then DMS or P() 3 X CX 3 (X = Br, I PhS-SPh SPh S CCl 4 Cl
36 adical eactions eductive Decarboxylation!!! mechanism - cleavage can also be initiated thermally or with light Derek Barton: obel Prize 1969 For their contributions to the development of the concept of conformation and its application in chemistry. Shared Prize with assel
37 adical eactions eductive Decarboxylation!!! examples Boc Boc tbu 1. a 2. 2-thiopyridine--oxide, EDC then tbus, hν 52% Boc tbu S Boc tbu physostigmine S Cl Ac tbus, DMAP, Δ 82% Ac
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