Phosphinamides as Tunable Ligand Precursors for the Titanium-Catalyzed Hydroaminoalkylation of Alkenes

Phosphinamides are reported as a new class of P-bridged κ2N,O-chelating ligand precursors for titanium-catalyzed hydroaminoalkylation reactions of alkenes and for the synthesis of new titanium complexes. To evaluate the catalytic activity in six different hydroaminoalkylation reactions, TiBn4 is used as a catalyst precursor in combination with a ligand precursor library obtained by a systematic variation of the P- and N-bound phenyl substituents of the phosphinamides. The catalyst screening reveals the beneficial role of para-methoxy- and meta-tert-butyl-substitution on the P- and N-bound benzene rings of the phosphinamides. The combination of both substitution patterns in a single ligand precursor leads to a significant synergistic effect, which, in combination with TiBn4, results in a highly active catalyst system for the intermolecular hydroaminoalkylation of alkenes. As a result, the reaction of 1-octene with N-methylaniline, performed with a catalyst loading of 5 mol % at 160 °C, delivers the branched hydroaminoalkylation product in 94% yield after 2 h. The same conditions can also be applied to other alkene and amine substrates. Furthermore, the synthesis of five new titanium complexes obtained from selected phosphinamide ligand precursors and Ti(NMe2)4 is described, and in each case, single-crystal X-ray diffraction confirms the expected κ2N,O-coordination of the ligand.