Cobalt-Catalyzed Regioselective Carbocyclization Reaction of <i>o</i>-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 1a−g undergo carbocyclization with various disubstituted alkynes 2a−k in the presence of Co(dppe)I2 and zinc powder in acetonitrile at 80 °C 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)Cl2/dppe and zinc powder in acetonitrile at 80 °C 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.本工作报道了一种可用于合成茚醇（indenols）与茚类化合物（indenes）的高效钴催化碳环化反应，以及一种全新的还原脱氰反应（reductive decyanation）方法。2-碘苯基酮与醛类化合物1a−g可与多种二取代炔烃（disubstituted alkynes）2a−k发生碳环化反应：在Co(dppe)I2与锌粉存在下，于乙腈溶剂中80℃反应3小时，即可获得相应的茚醇衍生物3a−s与4a−m，产率良好至优异。对于部分不对称炔烃（unsymmetrical alkynes），该碳环化反应表现出显著的区域选择性，仅得到单一的区域异构体（regioisomer）。该钴催化碳环化反应可进一步拓展至茚类衍生物的合成：2-碘苯基酮与醛（1）可与丙烯酸酯（acrylates）H2C=CHCO2R（7a−d）以及丙烯腈（acrylonitrile）H2C=CHCN（7e）在Co(dppe)Cl2/dppe与锌粉存在下，于乙腈中80℃反应24小时，可顺利发生反应，以中等至良好的产率得到相应的茚类化合物8a−k与9a−c。值得注意的是，当使用7e进行碳环化反应时，体系同时发生还原脱氰反应，得到不含氰基官能团（cyano functionality）的茚类衍生物。本文同时提出了该钴催化碳环化反应的可能反应机理。