Branch-selective Olefin Hydroaminoalkylation from Ti(III)–Al Bimetallic Intermediates evidenced by EPR Hyperfine Spectroscopy and DFT Calculations

Stereoselective hydroaminoalkylation of alkenes via α-C–H bond activation of alkylamines is an efficient process for the preparation of complex alkylamines minimizing stoichiometric waste. Herein, we report that a combination of Cp*TiMe3 and AlMe3 catalyzes the branch-selective hydroaminoalkylation of 1-alkenes, including styrene derivatives and 1,3-dienes, with N-methylaniline derivatives. Kinetic studies reveal that the active species are generated from in situ generated Cp*TiMe2(NMePh) and alkylaluminum. Continuous wave (CW) and pulse EPR spectroscopy show that multiple Ti(III) species, bearing amido and most probably alkyl ligands as well as an Al center, are formed, paralleling catalytic activity. Based on these findings complemented by DFT studies, we propose a reaction mechanism featuring d1 Ti(III) three-membered azatitanacycle species with amidoaluminate anions as active species. The alkene insertion into the Ti–C bond of the three-membered metallacycle intermediate is a key step that drives selectivity. This step favors the branched product through both steric and electronic effects, namely by i) minimizing steric repulsion between the styrene phenyl ring and the Cp*/Al moieties and ii) spin delocalization from metal to substrate antibonding orbital, which stabilizes the transition state resulting in facile insertion into M–C bond.