Abstract
A highly regio and stereoselective method for the synthesis of various 2-silylallylboronates from allenes and silaborane catalyzed by palladium complexes and initiated by organic iodides is described. A variety of monosubstituted aryl and alkylallenes RCH=C=CH2 and 1,1-dimethylallene react with silaborane in the presence of Pd(dba)2 (5 mol %) and an organic iodide 3 (2-methyl-3-iodo-2-cyclohexen-1-one) (10 mol%) in EA at 80 oC to give the corresponding silaboration products in moderate to excellent yields. The product formation can be explained based on an unusual and new mechanistic pathway involving a three-component assembling reaction. This catalytic silaboration is totally regioselective with the silyl group of silaborane adding to the central carbon and the boryl group to the unsubstituted terminal carbon of allene. Furthermore, the reactions show very high stereoselectivity giving the corresponding E isomer as the major species. And this silaboration chemistry can be applied to the synthesis of homoallylic alcohols. Treatment of allenes with silaborane and aldehydes in the presence of Pd(dba)2 (5 mol%) and alkenyl iodide 3 (10 mol%) at 80 °C in ethyl acetate for 5 h afforded homoallylic acohols in good yields with excellent syn selectivity. An efficient cobalt-catalyzed carbocylization for the synthesis of indenols is described. 2-Iodophenylketones and 2-iodophenylaldehydes undergo carbocyclization with various disubstituted alkynes in the presence of CoI2(dppe), and zinc powder in acetonitrile at 80 oC for 3 h to afford the corresponding indenol derivatives in good to excellent yields. For some unsymmetrical alkynes, the carbocyclization was remarkably regioselective affording a single regioisomer. A possible mechanism for this cobalt-catalyzed carbocyclization reaction is proposed. The cobalt-catalyzed carbocyclization reaction is successfully extended to the synthesis of indenes and a new method for reductive decyanation. Thus, the reaction of 2-iodophenylketones and 2-iodophenylaldehydes with acrylates H2C=CHCO2R proceeds smoothly in the presence of CoCl2(dppe) /dppe, and zinc powder in acetonitrile at 80 oC for 24 h to afford the corresponding indenes in moderate to good yields. Interestingly, when acrylonitrile H2C=CHCN was employed for the carbocylization, reductive decyanation also occurred to give indene derivatives without the cyano functionality. A possible mechanism for this cobalt-catalyzed carbocyclization reaction is also proposed.