Steric tuning of silylacetylenes and chiral phosphine ligands for rhodium-catalyzed asymmetric conjugate alkynylation of enones.

Rhodium-catalyzed asymmetric conjugate addition to R,âunsaturated ketones and related compounds is now well-recognized to be one of the most efficient methods of introducing aryl and alkenyl groups with high enantioselectivity.1 On the other hand, asymmetric conjugate addition of alkynyl groups has not been welldeveloped in spite of its high synthetic utility.2 A most straightforward and convenient reaction scheme should be the addition of terminal acetylenes to enones, but it faces an inherent problem that terminal acetylene is more reactive than enone toward the alkynylrhodium intermediate, which results in the predominant formation of acetylene dimers rather than â-alkynylketones (Scheme 1).3,4 One solution to this problem is the rhodium-catalyzed 1,3-rearrangement of an alkynyl group from alkynyl alkenyl carbinols, where the concentration of acetylene is kept minimal during the reaction, leading to high yields of the â-alkynylketones.5 In this communication, we wish to report another solution to the rhodium-catalyzed asymmetric conjugate alkynylation, which is realized by use of (triisopropylsilyl)acetylene combined with DTBM-segphos6 as a chiral phosphine ligand. The sterically bulky substituents on the silicon and phosphorus atoms should hinder the acetylene from approaching the alkynyl-rhodium intermediate (Figure 1).