Reactions of Pentacarbonyl(1-methoxyethylidene)molybdenum and-tungsten with α,ω-Enynes: Comparison with the Chromium Analogue and Resulting Mechanistic Ramifications

The title reactions of the molybdenum-and tungsten-containing Fischer carbene complexes 1-Mo and 1-W with the series of enynes 2 have been examined. Significant differences are noted in comparison with previous studies with the analogous chromium Fischer carbene complex 1-Cr. Namely, selectivities for cyclopropane formation are higher and, for the first time, a trisubstituted enyne has participated in the cyclopropanation pathway. A new reaction manifold which produced cyclopentenones as the result of a Pauson-Khand-like cyclization has been identified. Simple dienes arising from reaction of nondonorsubstituted alkylidenemolybdenum and-tungsten “carrier” species have also been observed. This has led to the mechanistic suggestion that the greater ability of the larger tungsten atom to access relatively hindered species of higher coordination selectively opens reaction pathways which are less accessible to the corresponding chromium and molybdenum analogues. Within this framework the seven-coordinate metallacyclobutane 15g is a necessary intermediate in the conversion of enyne 2g to the bicyclic cyclopropane 10g. Likewise, to account for the formation of the unusual Pauson-Khand cyclization products, it is proposed that seven-coordinate species like 18 and 20 are important.