SELECTIVE BINDING AND ACTIVATION OF ONE ALDEHYDE ENANTIOFACE BY A CHIRAL TRANSITION-METAL LEWIS ACID - SYNTHESIS, STRUCTURE, AND REACTIVITY OF RHENIUM ALDEHYDE COMPLEXES [(ETA-5-C5H5)RE(NO)(PPH3)(ETA-2-O=CHR)]+X-
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Reaction of dichloromethane complex [(5-C5H5)Re(NO)(PPh3)(ClCH2Cl)]+X- (2+PF6- or 2+BF4-) and RCHO (R = (a) CH3, (b) C2H5, (c) n-C3H7, (d) i-C3H7, (e) C6H5, (f) CH2C6H5) gives -aldehyde complexes (RS, SR)-[(5-C5H5)Re(NO)(PPh3)(2-OCHR)]+X- ((RS, SR)-3a-f+X-, 98-77%), in which one RCH0 enantioface is bound with high specificity. Crystal structures of (RS, SR)-3b+BF4- and (RS, SR)-3f+PF6- show the RCH0 carbon to be anti to the PPh3 ligand and the RCHO group to be syn to the NO ligand. Formyl complex (5-C5H5)Re(NO)(PPh3)(CHO) (5) and (RS, SR)-3a-f+X- react at-80 C to give [(5-C5H5)Re(NO)(PPh3)(CO)]+X- and alkoxide complexes (5-C5H5)Re(NO)(PPh3)(OCH2R) (6a-f, 95-80%). Reaction of deuterioformyl complex 5-d1 and (RS, SR)-3a-f+X- gives deuterioalkoxide complexes (5-C5H5)Re(NO)(PPh3)(OCHDR)(6a-f-d1) as 97-92:3-8 mixtures of (RR, SS)/(RS, SR) diastereomers. Analogous reactions starting with optically active 2+X- give optically active aldehyde and alkoxide complexes with retention of configuration at rhenium. The latter react with electrophiles EX (EX = HI, (CH3)3SiI, CH3COI) to give cleavage products EOCH2R and (5-C5H5)Re(NO)(PPh3)(X), generally with retention at rhenium. When (+)-(RS)-3a-f+BF4- are treated with (+)-(S)-5-d1 and (-)-(R)-5-d1 (+)-6a-f-d1, form as 99-94:1-6 and 91-84:9-16 mixtures of (RR)/(RS) diastereomers, respectively. Hence, enantiomers of the chiral reductant 5-d1 give different stereoselectivities. Reaction of (+)-6a-f-d1 with (-)-(R)-C6H5CH-(OAc)COOH/DCC/4-(dimethylamino)pyridine gives esters C6H5CH(OAc)COOCHDR that are comparable mixtures of (RR)/(RS) diastereomers and carboxylate (+)-(RR)-(5-C5H5)Re(NO)(PPh3)(O(CO)CH(OAc)C6H5) of >99% de. 1990, American Chemical Society. All rights reserved.
