Nido-Carborane Building-Block Reagents. 4. Regiospecific Substitution at Boron in 2,3-R2C2B4H6 Cages: Evidence for Intramolecular C-H---H Bridge Interactions in 2,3-R2C2B4H4H5-4-R' Derivatives

摘要

Reactions of nido-2,3-R2C2B4H5-ions (R = ethyl, benzyl) with organic halides (R'X) generate B(4)-substituted derivates together with the B(6)-substituted enantiomers. The formation of the R2C2B4H5-4-R' products (R' = Me, Et, CH2Ph, (CH2)3Ph,CH2C6H4Me) is proposed to occur via R2C2B4H5-mu(4,5)-R' bridged intermediates, which rapidly rearrange to the 4-substituted species. The reaction occurs cleanly, with < or = 1% contamination by other geometric isomers, and thus furnishes a useful regiospecific synthetic route to B-monosubstituted derivatives. In all cases examined, the products are air-stable liquids of low volatility that can be employed as building-block units in the synthesis of organometallic complexes and oligomers. Proton NMR spectra of several of the 2,3,4-trisubstituted products, supported by NOE experiments, indicate spin coupling between a C-CH2 methylene proton and a B-H-B bridging hydrogen. The triethyl and tribenzyl derivatives were bridge-deprotonated by NaH, complexed to Fe2+, and oxidatively fused to produce the hexasubstituted R6C4B8H6 carboranes, although the reaction was sluggish compared to those of the parent C,C'-disubstituted carboranes. The fused compounds are nonfluxional in solution, as determined from NMR spectra. Iron, Carboranes, Reprints. (mjm)