Palladium-Catalyzed Additions of Terminal Alkynes to Acceptor Alkynes

The development of addition reactions wherein the product is the simple sum of the reactants plus anything else (only needed catalytically) constitutes an important goal for enhanced synthetic efficiency. The C−H bond of terminal alkynes (the donor alkynes) can be added to either terminal alkynes (self-coupling) or activated internal alkynes (cross-coupling) (the acceptor alkynes) in the presence of a catalytic amount of palladium acetate and an electron rich sterically encumbered ligand, tris(2,6-dimethoxyphenyl)phosphine. The activated internal alkynes for cross-coupling (the acceptor alkyne) include alkynes bearing an ester, sulfone, and ketone. Self-coupling is completely overwhelmed by cross-coupling, even at 1:1 ratios of donor and acceptor alkynes. The reaction exhibits extraordinary chemoselectivity with free carboxaldehydes, alcohols, ketones (saturated and conjugated), esters (saturated and conjugated), sulfones (saturated and conjugated), malonates, and silyl ethers all proving to be compatible. A 1:2 donor/acceptor alkyne adduct can also be optimized. Ethyl propiolate fails as an acceptor but its C-silylated analogue serves with the proper choice of silyl substituent. The products of the latter serve as useful precursors to β-keto esters. An iterative sequence is readily performed and led to a novel conformationally rigid retinoid analogue. The mechanism of this mild method for construction of conjugated enynes, versatile building blocks, is discussed.