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

Regioselective 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 key reaction intermediates, where alkene insertion into the Ti-C bond of the three-membered metallacycle intermediate drives selectivity. This step favors the branched over the linear product, which may stem from differences in the spin delocalization from the metal to the alkene antibonding orbital for the corresponding transition states.