Dearomative [2 + 2] Cycloaddition and Formal C-H Insertion Reaction of o-Carboryne with Indoles: Synthesis of Carborane-Functionalized Heterocycles.

o-Carboryne (1,2-dehydro-o-carborane) is a very useful synthon for the synthesis of a variety of carborane-functionalized heterocycles. Reaction of o-carboryne with N-protected indoles gave carborane-fused indolines if the protecting group was TMS via dearomative [2 + 2] cycloaddition or carboranyl indoles for N-alkyl ones through formal C-H insertion reaction. For N-aryl indoles, both reactions were observed, giving two products, in which the product ratio was dependent upon the nature of the substituents on the aryl rings. In general, electron-withdrawing substituents favor [2 + 2] cycloaddition, whereas electron-donating substituents promote a formal C-H insertion pathway. This reaction is also compatible with other heteroaromatics. Thus, a stepwise reaction mechanism was proposed to account for the experimental observations. These protocols offer general and efficient methods for the preparation of carborane-functionalized indoles and indolines as well as other heterocycles. The observed dearomative [2 + 2] cycloaddition represents the first example of indoles to undergo such reaction in the absence of transition metals or without UV irradiation. All new compounds were fully characterized by (1)H, (13)C, and (11)B NMR spectroscopy as well as HRMS spectrometry. Some were further confirmed by single-crystal X-ray analyses.