Ben-Ami Feit

The effect of an electron-donating β-substituent on the configurational stability and reactivity of vinyl carbanions

The effect of an electron donating β-substituent A(A = [graphic omitted]H2)2, [graphic omitted]H2, OMe, SMe, or OPh) on the relative configurational stabilities and nucleophilic reactivities of the dimethyl esters of α-lithiated fumaric acid (17)-Li and maleic acid (18)-Li was studied in THF, by comparing their reaction products with electrophiles (MeOH, Ph2CO, aldehydes). It was confirmed that when located at a trans position to the vinyl carbanion, the substituent A had two inter-related effects: decreasing the configurational stability of the vinyl carbanion and increasing its nucleophilic reactivity. The rapidly-established (17)-Li ⇌(18)-Li equilibrium was completely on the (18)-Li side with A = OR, SR, NR2. The products formed were those derived from (18)-Li only (with MeOH) or from both (17)-Li and (18)-Li depending on both the relative nucleophilic reactivities of the intermediates and on the electrophilicity of the electrophile. The products derived from (18)-Li predominated with the more reactive electrophiles.

Reactions of trimethylsilyl-derived iodohydrins with electron-rich π-systemsElectronic supplementary information (ESI) available: Additional results and experimental data, 1H NMR, 13C NMR and mass spectral data. See http://www.rsc.org/suppdata/p1/b1/b105233k/

Reactions of trimethylsilyl-derived iodohydrins of the type R1R2CH–CH(I)OTMS, with electron-rich olefins, and the effects of certain factors on these reactions, were studied. The trimethylsilyl-derived iodohydrins were obtained in situ by reacting R1R2CH–CHO (R1 = R2 = H; R1 = H, R2 = alkyl, phenyl) with TMSI. The corresponding trimethylsilyl enol ether derivatives (R1R2CCH–OTMS), and 1,1-diarylethylenes were the olefins used. Aldehydes of the type RCH2–CHO reacted smoothly in the presence of TMSI to yield the condensation product RCH2–CHC(R)–CHO. Both RCH(–CHCAr2)2 and the cyclic acetal 5 were obtained as main products of the RCHO–TMSI–CH2CAr2 reaction system, depending on the [RCHO] ∶ [TMSI] ∶ [CH2CAr2] concentration ratio. The mechanisms of formation for the various main products and by-products are discussed. TMSI substitutes, formed by reacting Me3SiCl with each of several Lewis acids, were also used.

The configurational stability of vinyl carbanions derived from monosubstituted activated ethylenes

The configurational stability of vinyl carbanions derived from the 1 -substituted activated ethylenes, acrylonitrile and methyl acrylate, was investigated. The pure cis-isomers of the corresponding β-deuterio-olefins were treated with benzophenone in the presence of a base in several solvents (diethyl ether, tetrahydrofuran, and diethyl ether-hexane). cis–trans Mixtures of the corresponding α-[hydroxy-[(diphenyl)methyl] derivatives formed were isolated and their composition determined. The mechanism of the formation of the two isomeric products is discussed. The vinyl carbanion derived from the β-deuterioacrylonitrile was configurationally stable, while that derived from the corresponding acrylic ester was configurationally unstable.

Stereoselective addition of lithium amides to activated triple bonds

The nucleophilic addition of R″2NLi to alkyl esters of R′CCCO2R (R′= H, CO2R; R = Me, Et) in apolar solvents, in the presence and in the absence of solvating agents, has been studied. The only or the main reaction product obtained under all experimental conditions was the corresponding (E)-R″2NCR′CHCO2R. It is suggested that the vinyl-lithium intermediate, which is first formed by an anti-type addition of R″2NLi to the triple bond, undergoes a very fast isomerisation, owing to the highly effective configurationally destabilising R+ effect of the electron-donating dialkylamino trans-β-substituent.