Au(I) as a π-Lewis Base Catalyst: Controlled Synthesis of Sterically Congested Bis(triflyl)enals from α-Allenols

Abstract

A gold-catalyzed bis­[(trifluoromethyl)­sulfonyl]­ethylation of α-allenols, which allows the preparation of sterically congested bis­(triflyl)­enals bearing quaternary carbon centers, is presented. This sequence differs from the conventional reaction pathway of α-allenols under π-acid catalysis, because in our case Au­(I) functions as a π-Lewis base catalyst rather than a π-Lewis acid to activate the allene, facilitating the unusual addition of a carbon nucleophile of the gold complex belonging to the allenol moiety. Density functional theory calculations were carried out to understand this uncommon pathway.

Palladium(0) π-Lewis Base Catalysis: Concept and Development

“…The rational assemblies of polyunsaturated systems, electrophiles, and nucleophiles, especially in a multicomponent manner, would significantly enrich the structural diversity and complexity of the products. Importantly, except for activating diverse polyconjugated systems in a vinylogous version, the potential expansion of π-Lewis base catalysis to more types of polyunsaturated substances, such as allenes , and alkynes, , still remains to be explored and mechanistically illuminated.…”

“…Apart from Pd(0) catalysis, very few examples involving other metals as potential π-Lewis base catalysts have been reported. Recently, Almendros et al uncovered a gold-catalyzed bis­[(trifluoromethyl)­sulfonyl]-ethylation reaction between α-allenols 88 and 2-fluoropyridinium ethanide 89 , which represents a novel and efficient access to sterically congested bis­(triflyl)­enals 90 . Mechanism study indicated that the Au­(I) complex functioned as an uncommon π-Lewis base catalyst to increase the natural population analysis (NPA) charge of the central C3 atom of allenols 88 via regioselective η 2 -ligation, thus facilitating nucleophilic addition to Tf 2 CCH 2 generated in situ from Yanai’s betaine 89 .…”