Theoretical study of the reaction of acetylene with B4H8. A proposed mechanism of carborane formation .2.

A uniform computational level ([MP4/6-311+G(d,p)]//MP2/6-31G(d)+ZPC) is used to evaluate 18 intermediates and 12 transition states in the study of the mechanism of carborane formation, beginning with the elimination of H-2 from B4H10 and ending with the formation of 1,2-C2B4H6. The calculated activation barrier (33.0 kcal/mol) for the first step (B4H10 --> B4H8 + H-2) is higher than experiment (23.7 kcal/mol) but in agreement with higher-level theory (CBS-Q, 34.4 kcal/mol). The first stable intermediate is a -CH=CH- bridged B4H8 species, which is structurally similar to the known -CH2-CH2- bridged B4H8 structure. The hydroboration pathway for insertion of C2H2 into a BH bond of B4H8 has a slightly lower activation barrier than the addition barrier of C2H2 to B4H8 (10.1 versus 13.1 kcal/mol, respectively). The hydroboration reaction leads, in a series of steps, to 2,5-mu-CH2-1-CB4H7, a known product in the reaction of methylacetylene and B4H10.