名称:
Peralkylated 1,4-dibora-2,5-cyclohexadienes-formation and rearrangement into peralkylated nido-2,3,4,5- tetracarbahexaboranes(6)
摘要:
Peralkylated 1,4-dibora-2,5-cyclohexadienes (DBCH) (3a,b) and a penta-alkylaryl derivative (3c) are formed in high yield at low temperature (-78 to -30-degrees-C) via the reaction between 1,5,7,11-tetramethyl-2,3,4,8,9,10-hexaisopropyl-3,9-dibora-6-stanna-1,4,7,10-spiro[5.5]underkatetraene (8) and organoboron dihalides (MeBBr2, (i)PrBBr2, PhBCl2). In contrast with claims in the literature for stable peralkylated DBCH derivatives, compounds 3 rearrange between -30 and +25-degrees-C into the nido-2,3,4,5-tetracarbahexaboranes(6) (5). The specific arrangement of the substituents in the 2,3,4,5-position (Me, (i)Pr, Me, (i)Pr) is noteworthy. This indicates, together with the formation of carboranes (e.g. 5a from 3a) in which substituents at the boron atoms must have been exchanged, that cleavage of the DBCH derivatives (3) into borirenes (1) is involved at least to some extent. Attempts to prepare a borirene (1-methyl-2,3-ditert-butyl-borirene, 1a) by the reported reaction between the system C8K/MeBBr2 and ditert-butylethyne have failed. All reactions were monitored by H-1, B-11, C-13, and Sn-119 NMR spectroscopy and the same methods served for the final structural assignment of the DBCH derivatives 3 and the carbaboranes 5.