A Crystallographic Map of Chiral Recognition in π Complexes of Aromatic Aldehydes and a Chiral Transition Metal Lewis Acid: Enantioface Binding Selectivities in Solution Correlate to Distances between Metal and Carbon Stereocenters in the Solid State

The π aromatic aldehyde complexes
[(η5-C5H5)Re(NO)(PPh3)(η2-OCHAr)]+BF4-
(1+BF4-; Ar =
a,
C6F5; b,
4-C6H4CF3; c,
4-C6H4Cl; d,
C6H5; e,
4-C6H4CH3; f,
4-C6H4CH2CH3;
g, 4-C6H4OCH3)
exist as mixtures of
configurational diastereomers
(RS,SR/RR,SS or π/π‘)
that differ in the OC enantioface bound to rhenium.
Under
standard conditions (0.000 71 M, CH2Cl2,
173 K), π/π‘ equilibrium ratios are 97:3, 89:11, 84:16, 80:20,
76:24,
79:21, and 74:26, respectively. Steric interactions between the
aryl groups and cyclopentadienyl ligands destabilize
the π‘ isomers. The crystal structures of
(RS,SR)-1a−c,f+PF6-
and
(RS,SR)-1d+SbF6-
show that the distances between
the rhenium and carbon stereocenters (Å,
a/b/c/d/f:
2.157(5)−2.161(9), 2.172(4), 2.176(4),
2.182(6)−2.188(9), 2.184(5)−2.199(6)) increase as π/π‘ ratios decrease. Stronger π
accepting aldehydes give shorter bonds and higher chiral
recognition. The aliphatic aldehyde complexes
[(η5-C5H5)Re(NO)(PPh3)(η2-OCHR)]+BF4-
exhibit higher π/π‘
ratios (R = CH3, 99.0:1.0;
CH2CH3, 99.8:0.2;
CH2CH2CH3, 99.5:0.5;
CH(CH3)2 and
C(CH3)3, >99.9:<0.1), and
possible rationales are given. The π/π‘ ratios increase at
higher concentration or lower temperature, and vary
slightly
with counteranion (BF4- >
PF6- ≥
SbF6-).