Deboronation of 9-substituted-ortho- and -meta-carboranes

A systematic study has been carried out of deboronation reactions of a series of icosahedral carborane derivatives bearing a variety of substituents on selected (generally antipodal) boron atoms. Reactions of the 9-substituted meta-carboranes, 9-R-1,7-C2B10H11 (R = OH, 4-MeC6H4, F, Cl, Br, I), with tetrabutylammonium fluoride hydrate (TBAFH) in refluxing THF gave nido-carborane salts containing a mixture of 1- and 6-R-7,9-C(2)B(9)H(<(11)over bar>) nido-anions. Regioselective deboronation occurred with the 9-halo-meta-carboranes, 9-X-1,7-C2B10H11 (X = F, Cl, Br, I), producing a 2:1 ratio mixture of nido 6-X-7,9-C(2)B(9)H(<(11)over bar>) and 1-X-7,9-C(2)B(9)H(<(11)over bar>). Calculated Mulliken charges at B2 and B3 of some 9-substituted-meta carboranes show that these vary in line with observed deboronation rates and product ratios in their reactions with TBAFH. The ortho-carborane derivative 9-I-1,2-C2B10H11 and TBAFH gave the tetrabutylammonium salt of the nido anion 5(6)-I-7,8-C(2)B(9)H(<(11)over bar>). The mercurated and thallated meta carboranes, 9-M-1,7-C2B9H11 (M = HgOCOCF3 and Tl(OCOCF3)(2)), with TBAFH underwent boron-metal bond cleavage to yield mainly the nido anion 7,9-C(2)B(9)H(<(12)over bar>). Multinuclear (B-11, C-13, H-1, F-19) NMR spectra of the carboranes have been recorded and assigned. (C) 1999 Elsevier Science S.A. All rights reserved.