Chiral complexes of aluminum containing the salcyen ligand framework, [(R,R
)-salcyen]-AIX (X = Me, (OSiMe2Bu)-Bu-t), catalyze the asymmetric addition
of diorgano-H-phosphonates to carbonyls: the phospho-aldol reaction. Reacti
on proceeds smoothly at ambient temperature in various solvents and under a
erobic conditions, to afford alpha -hydroxyphosphonate esters (MeO)(2)P(O)C
HR(OH), with enantiomeric excesses (ee's) <50%. Although catalyst activity
is linked to the nature of X, ee's appear to be only slightly affected. Sub
stitution within the chiral salcyen ligand framework seems to affect ee pri
ncipally where there is a steric or structural change near the metal center
. Although catalysis is tolerant of small quantities of water, excess water
leads to attenuation of enantioselectivity through decomposition of cataly
st to afford hydrated aluminas which competes through achiral phospho-aldol
catalysis. Kinetic analyses reveal a second-order polynomial relationship
of the form x/([A(0)] - x)-[A(0)] = k(2)t + k(2)t(2), where [A(0)] is the i
nitial concentration of H-phosphonate and carbonyl and x the degree of reac
tion. This may suggest that the metal complex must first be converted to an
other, more active precursor prior to catalytic turnover. Hammett analyses
suggest that carbonyl binding to the metal center results in enhanced ee's,
while single-crystal analyses on four aluminum complexes support the view
that twisting of the ligand framework, as measured by the five-coordinate <
tau> parameter, from a purely meridional geometry may be advantageous to st
ereoselectivity. Strategies for future developments are discussed in light
of the results herein.