The use of B-haloboranes in solution and on alumina in organic chemistry

A series of boronated aluminas were prepared by the reaction of BX₃ (X = F, Cl, Br) with alumina or by a two step synthesis involving the reaction of BH₃*Me₂S with alumina and then with I₂. The modified aluminas, BX_N/Al₂O₃, were found to have n = 3 (F), n = 1.5 (Cl), n = 1 (Br) and n = 1 (I). The acidity of the solids (indicator dyes) were large and decreased in the order: I~Br>Cl>F. Isomerizations of several dienes, including dimethyl maleate to dimethyl fumarate, were completed within minutes with the iodo- and bromoboronated aluminas. The solids catalyzed the Diels-Alder reactions of methyl acrylate with cyclopentadiene, isoprene with methyl acrylate, and dimethyl maleate with cyclopentadiene, all of which occurred in high yield and with high selectivity. The regioselectivity of the isoprene/methyl acrylate reaction in fact is higher than seen previously. The results of the heterogeneous reactions were compared to those catalyzed by B-bromo- and B-chlorocatecholborane, which serve as models for the boronated aluminas. To compare the acidity with the boronated aluminas, (E)-2-butenal was complexed with B-bromo- and B-chlorocatecholborane in CD₂CI₂ at -20°C. The proton and carbon NMR chemical shifts of the aldehyde showed two species that unexpectedly moved upfield which indicated pi-complexed s-cis and s-trans conformations. The hypothesis was confirmed with COSY, HETCOR, NOESY, and ROESY 2D NMR experiments. The s-cis and s-trans conformations of the aldehyde exist in equal amounts which interconvert (EXSY 2D NMR). Infrared spectrum of the complexes validated the existence of the carbonyl vibration. Reaction of the complexes with cyclopentadiene afforded the Diels-Alder endo and exo adducts cleanly. Quenching the complex with water generated catechol, boric acid, (E-2-butenal, and 3-bromobutanal, as expected.