Calcium catalyzed enantioselective intramolecular alkene hydroamination with chiral C-2-symmetric bis-amide ligands

摘要

The chiral building block (R)-(+)-2,2 '-diamino-1,1 '-binaphthyl, (R)-BINAM, which is often used as backbone in privileged enantioselective catalysts, was converted to a series of N-substituted proligands (R)1-H-2 (R = CH(2)tBu, C(H)Ph-2, PPh2, dibenzosuberane, 8-quinoline). After double deprotonation with strong Mg or Ca bases, a series of alkaline earth (Ae) metal catalysts (R)1-Ae.(THF)(n) was obtained. Crystal structures of these C-2-symmetric catalysts have been analyzed by quadrant models which show that the ligands with C(H)Ph-2, dibenzosuberane and 8-quinoline substituents should give the best steric discrimination for the enantioselective intramolecular alkene hydroamination (IAH) of the aminoalkenes H2C = CHCH2CR ' 2CH2NH2 (CR '(2) = CPh2, CCy or CMe2). The dianionic (R)1(2-) ligand in (R)1-Ae center dot(THF)(n) functions as reagent that deprotonates the aminoalkene substrate, while the monoanionic ((R)1-H)(-) ligand formed in this reaction functions as a chiral spectator ligand that controls the enantioselectivity of the ring closure reaction. Depending on the substituent R in the BINAM ligand, full cyclization of aminoalkenes to chiral pyrrolidine products as fast as 5 minutes was observed. Product analysis furnished enantioselectivities up to 57% ee, which marks the highest enantioselectivity reported for Ca catalyzed IAH. Higher selectivities are impeded by double protonation of the (R)1(2-) ligand leading to complete loss of chiral information in the catalytically active species.