DIRECT PREPARATION OF DIFFERENTIALLY PROTECTED HYDROQUINONE-CHROMIUM(TRICARBONYL) COMPLEXES FROM THE BENZANNULATION REACTION OF FISCHER CARBENE COMPLEXES

Methods for the preparation of differentially protected hydroquinone-chromium(tricarbonyl) complexes directly from alkenyl Fischer carbene complexes and alkynes are described. The benzannulation reaction of methoxy carbene complexes with alkynes produces hydroquinone chromium tricarbonyl complexes as their mono-methyl ethers which may be protected at the free aryl hydroxyl with either sequential or concurrent reaction with a variety of electrophiles, including silyl chlorides and triflates, acyl and alkyl halides, and anhydrides. The yields obtained for the arene complexes equal or exceed the yields for the phenol or quinone products isolated from the same benzannulation reactions in which the chromium tricarbonyl group has been oxidatively liberated from the arene. Differentially protected complexes can also be obtained directly from the reaction of beta-silyl-alkenyl complexes with alkynes. These methods are not applicable to the preparation of differentially protected naphthohydroquinone chromium tricarbonyl complexes from the reaction of aryl carbene complexes with alkynes.