Effect of benzylic methyl groups on kinetic basicity of amine ligand in o-boron substituted N,N-dimethylbenzylamines

Rates of the dissociation of the intramolecular B-N coordination bond in two series of phenylborane derivatives, the boronate and diethylborane complexes, with -CHMeNMe2 or -CMe2NMe2 group at the o-position were determined by the NMR lineshape analysis or saturation transfer method. The new organoboron compounds were synthesized from the corresponding organolithium compounds with appropriate boron reagents. Comparison of the kinetic data with those of the -CH2NMe4 compounds reveals that the barrier height to the dissociation, namely the kinetic basicity of the amine ligand, is increased as the molecule possesses more methyl groups at the benzylic position for both of the series of boron compounds. The X-ray structure of one of the boronate complexes and the NMR titration measurements of model amines indicate that the basicity of the amine ligand is not affected much by the methyl substitution in the coordinated form. Therefore, the substituent effect on the kinetic basicity is mainly ascribed to the destabilization of the transition state by the geminal dimethyl groups rather than to any inductive or steric effects at the initial state, especially for the -CMe2NMe2 compounds.