Bifunctional ligand-enabled gold-catalyzed cycloisomerization of 2'-ethynylbiaryl-2-yl carbonyl compounds

Abstract

Gold(I) vinylidenes are powerful yet typically fleeting electrophiles whose synthetic exploitation has, until now, relied largely on two strategies: 1,2-migration of a halide or silyl group from a gold-activated alkyne, or dual gold catalysis of diyne substrates. Here, we introduce a ligand-enabled catalytic entry to these reactive intermediates based on a prior stoichiometric approach reported by Fürstner. A bifunctional biaryl-2-ylphosphine bearing a remote N,N-dialkylamide group promotes in situ formation of a gold acetylide without an external base; the proton thereby relayed to the amide acts as an internal Brønsted acid that activates a tethered carbonyl, triggering 6-exo cyclization to a putative gold(I) vinylidene. The catalysis converts 2′-ethynylbiaryl-2-yl aldehydes and ketones into synthetically valuable phenanthrene-9-carbaldehydes under mild conditions. In contrast to the observed decarbonylation in the stoichiometric case, the bifunctional ligand is proposed to promote facile protodeauration of an acylgold intermediate, thereby preserving the valuable aldehyde moiety in the products. The transformation accommodates a range of electronic environments and is moderately tolerant of steric demand, underscoring metal–ligand cooperation as a valuable approach to accessing and harnessing gold vinylidenes.