Cross-metathesis of terminal alkynes.

Terminal acetylenes are amongst the most problematic substrates for alkyne metathesis because they tend to undergo rapid polymerization on contact with a metal alkylidyne. The molybdenum complex 3 endowed with triphenylsilanolate ligands, however, is capable of inducing surprisingly effective cross-metathesis reactions of terminal alkyl acetylenes with propynyl(trimethyl)silane to give products of type R(1)-C≡CSiMe. This unconventional way of introducing a silyl substituent onto an alkyne terminus complements the conventional tactics of deprotonation/silylation and excels as an orthogonal way of alkyne protecting group chemistry for substrates bearing base-sensitive functionalities. Moreover, it is shown that even terminal aryl acetylenes can be cross-metathesized with internal alkyne partners. These unprecedented transformations are compatible with various functional groups. The need to suppress acetylene formation, which seems to be a particularly effective catalyst poison, is also discussed.