1,2-Diboracyclopentanes without Strong Donor Substituents: Synthesis, Reactions, and Computational Analysis

The 1,2-diduryl-1,2-diboracyclopentane 1a was obtained in good yield from the recently described 1,2-dichloro-1,2-diboracyclopentane 1c. Thermolysis of la led to a mixture of compounds from which degradation products 5a and 6a as well as isomer 7a could be isolated. Sterically less-hindered Ib spontaneously transformed into its isomer 7b. Attempts to prepare 4a by deprotonation of 1a led to ring enlarged anions 8a and 8b. The structures of all new compounds were definitively proven by X-ray structural analyses. Computations at the B3LYP/6-311+G(center dot center dot) level of theory suggest that derivatives of the kinetically preferred isomer 2u of In are the key intermediates on the route to 5a and 6a, which are the main products of the thermolysis of 1a. In contrast to expectation, derivatives of thermodynamically preferred isomer 3u could neither be observed nor shown by computations to be involved in the formation of the products obtained from 1a and 1b. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)