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. Y ields of alkynol-derived quinone products after oxidative workup incre ase markedly when the reactions are conducted in the presence of the ' 'external'' alkynes diphenylacetylene, 3-hexyne, or 1-hexyne. The acti on of alkyne additives, which participate in competitive intermolecula r benzannulation to only a minor extent, is inhibited by donor solvent or carbon monoxide. Kinetics measurements demonstrate that the benzan nulation reactions are initiated by dissociative CO loss. The alkyne a dditives are believed tb act by coordination to vinylcarbene intermedi ates produced by intramolecular alkyne insertion, consistent with prev ious suggestions. A carbon-tethered analogue was found to be unrespons ive to the addition of external alkyne. Evidence for the reversible na ture of alkyne insertion and the bimolecular decomposition of siloxyca rbene complexes is discussed. The methodology provides products with c omplete regiospecificity regardless of the size of the tethered alkyne substituents, including those not directly accessible by intermolecul ar reactions of terminal alkynes.