IMPROVED SYNTHETIC ROUTES TO SMALL CARBORANES - BENCH-SCALE PREPARATION OF NIDO-2,3-C2B4H8 DERIVATIVES FROM B5H9 AND FROM B3H8- SALTS

A method has been developed for producing the C, C'-dimethyl and C-phenyl derivatives of 2, 3-C2B4H8 in gram quantities and in yields of >50% without the use of heat, solvent, or large-volume gas bulb reactions. This approach involves formation of a B5H9-2L adduct (L = triethylamine or dimethyl sulfide) which reacts with 2-butyne or phenylacetylene in situ, at or below room temperature, to give the corresponding RR'C2B4H6 carborane. The carborane product is easily isolated in high purity by vacuum fractionation. The most satisfactory results are obtained with triethylamine, with B5H9 maintained in excess, and with simultaneous incremental addition of the amine and alkyne to the borane. The use of dimethyl sulfide in place of triethylamine gives slightly higher carborane yields but results in a much slower reaction. The triethylamine method has been combined with a previously reported synthesis of B5H9 from B3H8- salts, to generate C2B4H8 derivatives from B3H8- in a “one-pot” synthesis, thereby eliminating the isolation or handling of B5H9 and other volatile boranes. Since B3H8- salts are readily prepared from NaBH4, this method offers a safe and relatively inexpensive route to lower carboranes. © 1979, American Chemical Society. All rights reserved.