Synthesis of 1,6-dihalogeno-2,3,4,5-tetracarba- nido-hexaborane(6) derivatives

Abstract

1,4,6,9-Tetraalkyl-3,8-diethyl-2,7-bis(diethylboryl)-5-stannaspiro[4.4]nona-1,3,6,8-tetraenes (alkyl = Et ( 2a), Pr ( 2b), 1Pr ( 2c), nBu ( 2d)) react with four equivalents of boron tribromide to give 2,3,4,5-tetraalkyl-1,6-dibromo-2,3,4,5-tetracarba- nido-hexaboranes(6) ( 3a-d) in high yield. By monitoring the progress of the reactions using 13C NMR spectroscopy, 2,5-diboryl-substituted 3-borolenes ( 10) were identified as intermediates that rearrange to the carboranes via elimination of ethylboron dibromide. Treatment of 2 with boron triiodide affords the analogous 1,6-diodo-substituted carboranes 4. The reaction of 2 with an excess of boron trichloride proceeds rather Slowly, again with a 3-borolene ( 9) as intermediate, and leads finally to a mixture of the carboranes 5 and 6 as a result of elimination of EtBCl 2 or BCl 3. Treatment of 2 with BF 3 leads to decomposition without any defined products. The carboranes 3 react stepwise with Li[Et 3BH] to give first the monohydride with a B(6)-H bond ( 17) and the dihydride with B(6)-H and B(1)-H bonds ( 18).