Cobalt-catalyzed regioselective carbocyclization reaction of o-iodophenyl ketones and aldehydes with alkynes, acrylates, and acrylonitrile:: A facile route to indenols and indenes

An efficient cobalt-catalyzed carbocylization for the synthesis of indenols and indenes and a new method for reductive decyanation are described. 2-Iodophenyl ketones and aldehydes la-g undergo carbocyclization with various disubstituted alkynes 2a-k in the presence of Co(dppe)12 and zinc powder in acetonitrile at 80 degreesC for 3 h to afford the corresponding indenol derivatives 3a-s and 4a-m in good to excellent yields. For some unsymmetrical alkynes, the carbocyclization was remarkably regioselective, affording a single regioisomer. The cobalt-catalyzed carbocyclization reaction was successfully extended to the synthesis of indene derivatives. Thus, the reaction of 2-iodophenyl ketones and aldehydes (1) with acrylates H2C=CHCO2R (7a-d) and acrylonitrile H2C = CHCN (7e) proceeds smoothly in the presence of Co(dppe)Cl-2/dppe and zinc powder in acetonitrile at 80 degreesC for 24 h to afford the corresponding indenes 8a-k and 9a-c in moderate to good yields. Interestingly, when 7e was employed for the carbocylization, reductive decyanation also occurred to give an indene derivative without the cyano functionality. A possible mechanism for this cobalt-catalyzed carbocyclization reaction is also proposed.