Lithium diamidobinaphthyl catalysts for asymmetric hydroamination

Lithium catalyzed intramolecular hydroamination of unactivated alkenes has been known for over 20 years, although growth in this field has been limited. Deduction of structure activity relationships for these systems is a challenge because of their complex solution behavior. A series of axially chiral C 2-symmetric diamidobinaphtyl dilithium salts given by the formula [{C 10H6N(CH2CH2X)}2Li 2]n where n = 2, 6 for X = NMe2, n = 2 for X = OMe, and n = 1 for X = NEt2 or N(CH2)5 were synthesized. Solid state characterization of these dilithium salts free of outside donor solvents allowed ligands to direct aggregation based on their steric hindrance and flexibility. All complexes showed pi interactions between lithium and the naphthyl rings playing a significant role in the stabilization of the metal center. The complexes based on the sterically least demanding N-dimethyl-substituted ligand [{C10H6N(CH2CH 2NMe2)}2Li2]n crystallize as a dimer in enantiopure form and as a hexameric cycle in racemic form.;When used in combination with excess LiCH2SiMe3 various gem-dialkyl substituted aminopentenes were cyclized in up to 67% ee at 22°C. Cyclization of aminopentene at 40°C was also performed producing 2-methylpyrrolidine in up to 64% ee. The precatalysts were studied using DOSY, 7Li and variable temperature NMR spectroscopy. DOSY NMR spectroscopy suggests that solutions of (rac)-64 and (R)-64 exist as a mixture of monomers and dimers at 25°C, while (R)-65 and (R)-66 are monomeric. Addition of one equivalent LiCH2SiMe3 to (R)-64 resulted in a sharpening of 1H NMR spectra and produced a single peak in the 7Li NMR spectra, suggesting the formation of a new mixed aggregate, (R)-69.;Kinetic studies for the cyclization of C-(1-allyl-cyclohexyl)-methylamine demonstrated a first order rate dependence on substrate and catalyst concentrations. A first order rate dependence on LiCH2SiMe3 was also observed, with enantioselectivity decreasing linearly as the concentration of LiCH2SiMe3 was increased. (R)-70 is a general representation of components found in the rate limiting transition state for the hydroamination/cyclization of C-(1-allyl-cyclohexyl)-methylamine. Solid state characterization of (R)-64 with two equivalents of 3-methyl-2-aza-spiro[4.5]decane bound to it revealed the formation of a monomeric species, (R)-68.