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 tw o series of phenylborane derivatives, the boronate and diethylborane comple xes, with -CHMeNMe2 or -CMe2NMe2 group at the o-position were determined by the NMR lineshape analysis or saturation transfer method. The new organobo ron compounds were synthesized from the corresponding organolithium compoun ds with appropriate boron reagents. Comparison of the kinetic data with tho se of the -CH2NMe4 compounds reveals that the barrier height to the dissoci ation, 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 substit ution in the coordinated form. Therefore, the substituent effect on the kin etic basicity is mainly ascribed to the destabilization of the transition s tate by the geminal dimethyl groups rather than to any inductive or steric effects at the initial state, especially for the -CMe2NMe2 compounds.