Catalytic Generation of Zinc Carbenes from Alkynes: Zinc-Catalyzed Cyclopropanation and Si—H Bond Insertion Reactions

摘要

The preparation of zinc carbenoids and their applications in alkene cyclopropanations were reported by Simmons and Smith for the first time in 1958. Even today, this synthetic methodology remains one of the most important ways to access relevant cyclopropane rings. Despite the extensive studies on this process, the number of available precursors for the generation of zinc carbenoids still remains very limited (Scheme 1 a). In fact, more than fifty years after the seminal work of Simmons and Smith, diiodoalkanes remain the most common precursors of zinc carbenoids. Regarding the zinc source, subsequent developments enabled the replacement of the originally used Zn/Cu couple by alternative stoichiometric zinc precursors, which in a number of cases improved the efficiency or selectivity of the cyclopropanation reaction. Elegant studies performed by Denmark et al. and Charette et al. support a halomethylzinc structure for these reagents. Alternatively, zinc carbenoids can be generated by making use of diazo compounds and zinc salts as disclosed by Wittig and Schwarzenbach. Moreover, Motherwell et al. described that carbonyl groups and acetals, upon treatment with zinc and chlorotrialkylsilanes, afford zinc carbenoids which can be additionally trapped. It is noteworthy that all these approaches to zinc carbenoids are stoichiometric processes, with, to our knowledge, only one remarkable example of a catalytic transformation using phenyldiazomethane as the sole carbenoid source. Consequently, the development of broadly applicable methodologies for the catalytic generation of zinc carbenoids is highly desirable.