Chapter 2. POLAR ADDITION AND ELIMINATION REACTIONS

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Br KOBu -t 85% Br KOBu -t 30-40% Br CH 3 Cl 3COK + 75% 25% Ph Br Br O AcONa Ph Br O 64-73% Br NaNH 2 /NH 3 Br PhC≡CH 45-52% Ph OTs aq. OH - HC≡C HC≡C-C=CH-CH 3 ∆ CH OH - 3 CH 3 98% Ionization is favored by - electron-releasing groups - good leaving groups - solvents of high ionizing strength - stronger bases favor the E1 path over the S N 1 path The precise nature of the transition state of the E2 mechanism is a function of the leaving group, and the solvent. E1cb mechanism is not observed with simple alkyl halides or sulfonates, but with functional groups that capable of stabilizing negative charge on carbon. 2.5 ORIENTATION EFFECTS IN ELIMINATION REACTIONS The experimental evidence that has been gathered concerning direction of elimination from substrates reacting by the E1 mechanism indicates that the relative stability of the product olefins is a major factor in determining direction of elimination. H R 2 C δ+ X δ- + N(CH 3 ) 3 R 2 HC + X - A′ B′ Thermodynamically Controled product R 2 CHX A (less-substituted olefin) B (more-substituted olefin) Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 6
In the E1cb mechanism, the direction of elimination is governed by the kinetic acidity of the available protons, which, in tern, is determined by the inductive and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Preferential proton abstraction from unhindered positions leads to the formation of less-substituted alkenes, i.e. “Hofmann rule” is followed. E2 eliminations that proceed through transition states with high double-bond character give mainly the more substituted olefin because the stability of the olefin is reflected in the transition state, i.e. said to follow the “Saytzeff rule.” Base/solvent + + X X = I MeO - /MeOH 19 63 18 Cl ″ 33 50 17 F ″ 69 21 9 OTs ″ 33 44 23 I t-BuO - /t-BuOH 78 15 7 Cl ″ 91 5 4 F ″ 97 1 1 OTs ″ 83 4 14 Poorer leaving groups favor Hofmann products. Stronger bases favor formation of the less-substituted olefins. Highly hindered bases favor Hofmann products. 2.6 STEREOCHEMISTRY OF E2 ELIMINATION REACTIONS E2 elimination reaction may proceed via syn or anti mode: B: B: H X H syn anti C C C=C C C X In most cases, E2 elimination proceed via anti mode. Syn elimination is possible, and becomes dominant mode when special structural features retard anti elimination. In general trend revealed that anti mode is normally preferred for reactions involving good leaving groups such as bromide and tosylate, With poorer leaving groups, eg. fluoride, trimethylamine syn mode becomes important. D anti D H syn + NMe 3 H H Ion pair promotes syn elimination of anionic leaving group the syn mode will be diminished. H RO - C C H + X Associate Prof. Surin Laosooksathit, Ph.D. 5/31/2009 7
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Chapter 2. POLAR ADDITION AND ELIMINATION REACTIONS

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