Zirconocene-Based Methods for the Preparation of BN-Indenes: Application to the Synthesis of 1,5-Dibora-4a,8a-diaza-1,2,3,5,6,7-hexaaryl-4,8-dimethyl‑s‑indacenes

A method for the preparation of 3-bora-9-azaindene heterocycles based on zirconocene-mediated functionalization of the o-CH bonds of pyridines has been developed and used to make two such compounds. Unlike other methods, the boron center in these heterocycles remains functionalized with a chloride ligand, and so the compounds can be further elaborated through halide abstraction and reduction. The utility of the method was further demonstrated by applying it toward the preparation of 1,5-dibora-4a,8a-diaza BN analogues of the intriguing hydrocarbon s-indacene starting from 2,5-dimethylpyrazine. Gram quantities of one such compound were prepared and fully characterized, and both experimental and computational data are presented to compare its properties to those of the parent hydrocarbon s-indacene. These data indicate that the BN-substituted derivative exhibits lowered aromaticity in relation to the hydrocarbon.