INTRAMOLECULAR BENZANNULATION REACTIONS OF CHROMIUM SILOXYCARBENE COMPLEXES - REGIOCHEMICAL CONTROL AND THE XENOCHEMICAL EFFECT OF ALKYNE ADDITIVES

Acetylenic alcohols are attached to chromium oxycarbene fragments via dialkylsilicon linkages in convenient fashion to provide siloxycarbene complexes which undergo intramolecular benzannulation upon heating. Yields of alkynol-derived quinone products after oxidative workup increase markedly when the reactions are conducted in the presence of the ''external'' alkynes diphenylacetylene, 3-hexyne, or 1-hexyne. The action of alkyne additives, which participate in competitive intermolecular benzannulation to only a minor extent, is inhibited by donor solvent or carbon monoxide. Kinetics measurements demonstrate that the benzannulation reactions are initiated by dissociative CO loss. The alkyne additives are believed tb act by coordination to vinylcarbene intermediates produced by intramolecular alkyne insertion, consistent with previous suggestions. A carbon-tethered analogue was found to be unresponsive to the addition of external alkyne. Evidence for the reversible nature of alkyne insertion and the bimolecular decomposition of siloxycarbene complexes is discussed. The methodology provides products with complete regiospecificity regardless of the size of the tethered alkyne substituents, including those not directly accessible by intermolecular reactions of terminal alkynes.