A crystallographic map of chiral recognition in pi 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
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abstract
The aromatic aldehyde complexes [(5-C5H5)Re(NO)(PPh 3)(2-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 /) that differ in the O=C enantioface bound to rhenium. Under standard conditions (0.000 71 M, CH2Cl2, 173 K), / 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 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 (, 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 / ratios decrease. Stronger accepting aldehydes give shorter bonds and higher chiral recognition. The aliphatic aldehyde complexes [(5-C5H5)Re(NO)(PPh 3)(2-O=CHR)]+BF4- exhibit higher / 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 / ratios increase at higher concentration or lower temperature, and vary slightly with counteranion (BF4- > PF6- SbF6-).
