Titanocene-catalyzed dehydrocoupling of the adduct Me_2NH· BH_3 via competitive pathways: A DFT study

摘要

The molecular mechanism of the dehydrocoupling of Me_2NH· BH_3 to form the cyclic diborazane [Me_2N-BH _2]_2 catalyzed by the [Cp_2Ti] fragment has been investigated using density functional theory calculations. The reaction is likely to proceed via competitive pathways: the intermolecular and the intramolecular mechanism. For the intermolecular mechanism, firstly, the linear dimer Me_2NH-BH_2-NMe_2-BH_3 is obtained by the catalytic dehydrogenation of two Me_2NH·BH_3 adducts. Then, the species [Me_2N-BH_2]_2 is generated by the Ti-catalyzed dehydrogenative cyclization of Me _2NH-BH_2-NMe_2-BH_3. The rate-determining step is the nucleophilic substitution (S_N2) step from the hydride intermediate to the dihydride complex in both gas phase and the solution phase, with the free energies of 26.0 (gas phase) and 33.6 kcal/mol (solvent), respectively. In the intramolecular process, the aminoborane Me 2NBH_2 is formed by the interaction of Me _2NH·BH_3 with [Cp_2Ti] and then undergoes an uncatalyzed dimerization cyclization reaction to form [Me_2N- BH_2]_2. The highest point on the reaction pathway of intramolecular process is the dimerization step with the free energies of 29.8 (gas phase) and 30.2 kcal/mol (solvent), respectively.