N-heterocyclic carbene-catalyzed redox reactions - investigations of umpolung and oxidation pathways.

摘要

N-Heterocyclic carbenes catalyze divergent reactions such as the generation of homoenolate equivalents and redox processes under the same set of reaction conditions. Experimental and computational evidence demonstrates that the generation of homoenolate equivalents from alpha,beta-unsaturated aldehydes versus the oxidation of aldehydes to esters is highly dependent upon the choice of solvent. Protonation of these homoenolate equivalents efficiently generates saturated esters from unsaturated aldehydes. This reactivity is extended to the generation of beta-acylvinyl anions from alkynyl aldehydes. The asymmetric protonation of a homoenolate equivalent generated from a beta,beta-disubstituted aldehyde is accomplished through the use of a chiral N-heterocyclic carbene.;Additionally, N-heterocyclic carbenes catalyze the oxidation of allylic and benzylic alcohols to esters with manganese(IV) oxide in excellent yields. The oxidation proceeds under mild conditions through an acyl-triazolium intermediate which acts as an activated acylating agent. The general scope of the reactions is investigated and extended to include saturated aldehydes through the generation of transient benzylic alcohol with the heteroaromatic catalyst.;Attempts to expand this process to form carbon-carbon or carbon-nitrogen bonds results in the discovery of the dehydrogenative coupling of indoles and alcohols. This efficient approach to indolylamides proceeds in a single flask under mild conditions, catalyzed by tetrapropylammonium perruthenate. Indolylamides can be further functionalized through an additional dehydrogenative coupling to furnish elaborated polycyclic heterocycles similar to biologically active structures previously reported.