Structure−Activity Relationships for Group 4 Biaryl Amidate Complexes in Catalytic Hydroamination/Cyclization of Aminoalkenes

Synthesis of bis(carboxamide) proligands derived from (R,S)-2,2‘-diamino-6,6‘dimethylbiphenyl was readily achieved by treatment of the amine with acid chlorides. Direct reaction with homoleptic alkyls of the group 4 metals cleanly yielded amidate complexes. These complexes were shown by single-crystal X-ray diffraction to be monomeric in the case of titanium and dimeric in the case of zirconium. The complexes formed do not yield well-defined cations upon reaction with standard borate/borane activators, and although some hydroamination catalysis was observed, it was not at a rate that is useful. In-situ treatment of Zr(NMe2)4 with the proligands H2L1-4 yielded complexes of varying nuclearity depending on the steric bulk of the amide substituents, but in contrast to the metal alkyl series, mononuclear species are accessible; the mesityl derivative [(S)-L4Zr(NMe2)2] was found to be a highly enantioselective catalyst for the hydroamination/cyclization of 1-amino-2,2-dimethylpent-4-ene, with an enantiomeric excess of 91%.