Classical 1,2,4-Triboracyclopentanes and Their Rearrangement into Nonclassical 2-Boryl-1,3-diboracyclobutanes: Intramolecular C-H Bond Activation by a B-B Moiety

1,2,4-Triboracyclopentanes la-c in solution spontaneously transform into their nonclassical isomers 2a-c. These have been characterised by NMR spectroscopy and X-ray structural analyses. Compounds 2a-c are strongly distorted when compared with the five-membered ring of la, the structure of which was determined for comparison. In addition, the pattern of the substituents at the boron centres in 2a,b is different from that in la,b. This can be rationalised on the basis of a rapid topomerisation in 2c as determined by analysis of its temperature dependent C-13 NMR spectra. The environment of the boron centres 131 and 133 is exchanged by transforming the two 3c-2e bonds which both involve 131 into a B2-H and a B1-C 2c-2e bond. This leads to the 2-boryl-1,3-diboracyclobutane 4c and formation of corresponding new 3c-2e bonds employing 133 gives the topomer of 2c. The low barrier of this topomerisation reveals that 2a-c may also be regarded as nonclassical isomers of the 2-boryl-1,3-diboracyclobutanes 4a-c. Ab initio computations of models at the MP4SDTQ/6-311+G**//MP2/6-311+G** levels support this interpretation. In addition, they show that the transformation of 1u to 2u occurs by an intramolecular C-H bond activation process initiated by the B-B moiety. Derivatives of the type I and 2 described here represent the first examples of 1,2-diboron heterocycles which can be isolated as classical as well as nonclassical isomers. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.