Novel Substitutions of 1-Alkoxy- and 1-Arylsulfonyloxy-η³-allylmolybdenum Complexes. A Case for η¹-Alkenyl Carbene Complexes as Intermediates

A series of acyclic and cyclic 1-alkoxy- and 1-arylsulfonyloxy-substituted TpMo(CO)₂(η³-allyl) complexes was synthesized and characterized, and exchange of the oxygenated substituent was investigated under a variety of reaction conditions. 1-Alkoxy-substituted η³-allyl and η³-butenyl complexes participated in direct, uncatalyzed exchange of the alkoxy substituent with benzylamine, but required a Lewis acid for exchange with alcohols. The 1-alkoxy-substituted η³-cyclohexenyl complex was unreactive toward exchange under all conditions investigated. The corresponding acyclic arylsulfonyloxy-substituted complexes underwent direct, uncatalyzed exchange with both benzylamine and alcohols, while the arylsulfonyloxy-substituted cyclohexenyl compounds participated in direct substitution with benzylamine, but not alcohols. High enantiopurity acyclic and cyclic alkoxy- and arylsulfonyloxy-substituted complexes provided exchange products with predominant, but incomplete, losses in enantiomeric excess in all cases examined. Mechanisms accounting for the observed reactivity trends and for the losses in enantiomeric excess are discussed. Reactions of alkoxy-substituted complexes through an associative mechanism and of arylsulfonyloxy-substituted compounds through a dissociative mechanism are suggested.