The Aprotic Conjugate Addition-Reactions of the Carbanions of Octenyl Sulfides and Thiocarbamates With γ-Crotonolactone- Stereochemical and Mechanistic Aspects of Reactions Conducted in the Absence and Presence of Hexamethylphosphoric Triamide

Abstract

The lithiated carbanions of (E)- and (Z)-1-(phenylthio)oct-2-ene, (E)-1-(methylthio)oct-2-ene, S-[(E)-oct-2-enyl] N,N-dimethyl( thiocarbamate ), S-[(a-oct-2-enyl] N,N-dimethyl- ( thiocarbamate ), and (E)- and (Z)-1-(phenylsulfony1)oct-2-ene undergo conjugate addition to γ-crotonolactone (but-2-en-4-olide) in tetrahydrofuran at -70� to give as predominant products, syn and anti allylic sulfides, thiocarbamates and sulfones arising from reaction through C1 of the octenyl carbanions . Also formed are smaller amounts of syn and anti vinylic sulfides, thiocarbamates and sulfones arising by reaction through C3 of the octenyl carbanions. The carbanions of the octenyl sulfones give smaller amounts of the vinylic products than do the other carbanions. In the presence of hexamethylphosphoric triamide (hmpa), the relative amounts of the latter compounds are depressed in favour of the former. The diastereoselection of these reactions is significantly smaller than the corresponding reactions involving 4-t-butoxycyclopent-2-enone. The constitutions and stereostructures of all the products was established by analysis of 1H n.m.r. data and n.O.e. difference experiments conducted at 400 MHz. The enhancement of the amounts of the allylic at the expense of the vinylic compounds induced by the hmpa is tentatively ascribed to frontier molecular orbital effects, and suggests that the operation of chelation and charge control in the reactions of related allylic carbanions with carbonyl compounds is less important than is generally considered to be the case. The various factors which cause carbanions to undergo conjugate addition to α β-unsaturated carbonyl compounds and the role played by the hmpa in these reactions are critically discussed in terms of the model of Seyden-Penne and coworkers.