Trialkylsilyl triflate promoted conjugate addition to α,β-unsaturated carbonyl compounds

Abstract

$\beta$-Functionalization of $\alpha$,$\beta$-unsaturated ketones could be successfully carried out by the conjugate addition of various nucleophiles such as organoaluminates, organocuprates, organostannanes, Grignard reagents, and enamine derivatives. The conjugate addition reactions occurred at low temperature in the presence of trialkylsilyl triflate as a promoter. In the case of organoaluminates, the conjugate addition of alkenyl and alkynyl groups was very effective with both acyclic and cyclic enones, but transfer of alkyl groups was relatively ineffective. Alkynylcuprates did not react with $\alpha$,$\beta$-unsaturated ketones at room temperature. However, conjugate addition of the alkynylcuprates to $\alpha$,$\beta$-unsaturated ketones proceeded cleanly and rapidly in the presence of trialkylsilyl triflate at $-78^\circ C$ within 30 min. Organostannane compounds were very reactive toward carbonyl compounds activated by trialkylsilyl triflate and reacted with $\alpha$,$\beta$-unsaturated ketones such as 2-cyclohexenone to give the conjugate addition products in high yields. Also, alkylmagnesium bromides and enamine derivatives reacted with $\alpha$,$\beta$-unsaturated ketones in the presence of trialkylsilyl triflate, yielding the conjugate addition products. Furthermore, the conjugate addition of dialkyl sulfide to 2-cyclohexenone in the presence of trialkylsilyl triflate at $-78^\circ C$ afforded 3-trialkylsilyloxycyclohex-2-enylenesulfonium salt, which underwent facile nucleophilic substitution with various nucleophiles. Typically, the nucleophilic substitution reactions were carried out with 1.2 equiv. of nucleophiles at low temperature and almost instantaneously, yielding conjugate addition products of 2-cyclohexen-1-one in high yields. Reaction of $\alpha$,$\beta$-unsaturated acetals with dialkyl sulfide in the presence of trialkylsilyl triflate at $-78^\circ C$ afforded 3-alkoxyalk-2-enylenesulfonium salts, which underwent nucleophilic substitutio...