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Jaska CA, Manners I: Heterogeneous or homogeneous catalysis? Mechanistic studies of the rhodium-catalyzed dehydrocoupling of amine-borane and phosphine-borane adducts. J Am Chem Soc. 2004 Aug 11;126(31):9776-85.
In depth, comparative studies on the catalytic dehydrocoupling of the amine-borane adduct Me (2) NH.BH (3) (to form [Me (2) N-BH (2)](2)) and the phosphine-borane adduct Ph (2) PH.BH (3) (to form Ph (2) PH-BH (2)-PPh (2)-BH (3)) with a variety of Rh (pre) catalysts such as [[Rh (1,5-cod)(micro-Cl)](2)], Rh/Al (2) O (3), Rh (colloid)/[Oct (4) N] Cl, and [Rh (1,5-cod)(2)] OTf have been performed in order to determine whether the dehydrocoupling proceeds by a homogeneous or heterogeneous mechanism. The results obtained suggest that the catalytic dehydrocoupling of Me (2) NH.BH (3) is heterogeneous in nature involving Rh (0) colloids, while that of Ph (2) PH.BH (3) proceeds by a homogeneous mechanism even when starting with Rh (0) precursors such as Rh/Al (2) O (3). The catalytic dehydrocoupling reactions are thought to proceed by different mechanisms due to a combination of factors such as (i) the greater reducing strength of amine-borane adducts, (ii) the increased ease of dissociation of phosphine-borane adducts, and (iii) phosphine ligation and/or poisoning of active catalytic sites on metal colloids. |
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