Ti═NR vs Ti−R′ Functional Group Selectivity in Titanium Imido Alkyl Cations from a DFT Perspective

Abstract

The reactions of Ti(NtBu)(Me3[9]aneN3)Me+ (1) with a series of polar and nonpolar unsaturated organic substrates give different kinds of products, resulting either from cycloaddition to the Ti═N bond (PhCCH) or from insertion into the Ti−Me bond (PhCCPh, iPrNCNiPr, ethylene polymerization). DFT(B3PW91) calculations of the reaction pathways for cycloaddition and insertion on small model systems illustrate the basic electronic properties in terms of reaction site selectivity. In all cases the cycloaddition is shown to be the preferred pathway kinetically. ONIOM(B3PW91:HF) calculations on the experimental systems allow the evaluation of the influence of the steric bulk of the ligand set on the outcome of the reaction. Introduction of the actual ligands results essentially in a destabilization of the cycloaddition pathway and leads both to a cancellation of the kinetic preference for cycloaddition and to a reduced thermodynamic stability of the cycloaddition product. Only for the terminal alkyne PhCCH does cy...