| The dehydrocoupling of primary and secondary amine-borane adducts to afford either monomeric or dimeric aminoboranes and borazines was developed using transition metal complexes as precatalysts. Mechanistic investigations performed on the dehydrocoupling of Me2NH•BH3 suggested the operation of a heterogeneous process involving Rh colloids in which the first step may be an intermolecular dehydrocoupling reaction to give the linear Me2NH-BH2-NMe2-BH 3, followed by a subsequent intramolecular reaction to afford the observed cyclic [Me2N-BH2]2. The heterogeneity of the process was evidenced by (i) the presence of a sigmoidal kinetic curve, (ii) the observation of Rh colloids by TEM, (iii) the poisoning of the active catalyst by treatment with mercury or PPh3, (iv) a reduction in activity upon filtration and (v) the isolation of Rh metal as an active catalyst. A tandem catalytic dehydrocoupling-hydrogenation reaction was developed which involved the reaction of an amine-borane adduct and an unsaturated substrate using a common precatalyst. This reaction allows a stoichiometric quantity of hydrogen to be delivered for the hydrogenation reaction and operates at room temperature. Linear hybrid aminoborane-phosphinoborane chains were synthesized and were found to possess both inter- and intra-molecular proton-hydride bonding of moderate strengths. Remarkably, comparative mechanistic studies performed on the dehydrocoupling of the analogous phosphine-borane system Ph2PH•BH3 suggested the operation of a homogeneous process. This fundamental difference in reactivity between amine-borane and phosphine-borane adducts is thought to arise from (i) differences in the reducing strength of the adducts, (ii) differences in the extent of dissociation of the adducts and (iii) the presence of phosphine ligation/poisoning of the active catalyst. Platinum hydride complexes with primary and secondary phosphine-borane ligands, such as trans -[PtH(PhPR•BH3)(PEt3)2] and cis-[PtH(PhPR•BH3)(depe)] (where R = Ph, H), were synthesized. However, further reactivity studies of these complexes did not indicate any coupling chemistry to form new P-B bonds at the metal center. The iridium hydride complex [Cp*IrH(PPhH•BH3)(PMe3)] was also synthesized, but again displayed no further reactivity when reacted with phosphine-borane adducts. Dehydrocoupling routes for the formation of platinum-phosphorus bonds from the reaction of platinum hydrides with primary and secondary phosphines and phosphine-borane adducts has been demonstrated. |
