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
Academic Article
Overview
Identity
Additional Document Info
View All
Overview
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-).
