Generation and reactivity of the chiral rhenium chlorobenzene complex [(η5-C5H5)Re(NO)(PPh3)(ClC6H5)]+BF−4: an improved functional equivalent of the chiral Lewis acid [(η5-C5H5)Re(NO)(PPh3)]+BF−4

Abstract

Reaction of (η5-C5H5)Re(NO)(PPh3)(CH3) (1) and HBF4 · OEt2 in C6H5Cl at −45°C gives the chlorobenzene complex [(η5-C5H5)Re(NO)(PPh3)(ClC6H5)]+BF−4 (5). The rhenium is bound chiefly to chlorine in 5, but other species, possibly η2-arene isomers, are apparent by 31P NMR. The relative amounts change upon warming, and new species appear. Complex 5 reacts with neutral donor ligands L (CH3CN, C6H5CH2CH=CH2, THF) to give the corresponding adducts [(η5-C5H5)Re(NO)(PPh3)(L)]+BF−4, halide ions X− to give halide complexes (η5-C5H5)Re(NO)(PPh3)(X), and HSi(OEt)3 to give hydride complex (η5-C5H5)Re(NO)(PPh3)(H) (74–91%). When 5 is generated from optically active 1, analogous reactions with L ((−)-(S)-2-phenylbutyronitrile, benzaldehyde) give adducts in high optical yields and with overall retention of configuration at rhenium. For example, carboxylate complex (+)-(RR)-(η5-C5H5)Re(NO)(PPh3)(O(C=O)-CH(OAc)C6H5) forms in > 99% d.e. upon treatment of 5 derived from (+)-(S)-1 with (−)-(R)-C6H5CH(OAc)COOH and then base. Thus, 5 serves as a functional equivalent of the chiral Lewis acid [(η5-C5H5)Re(NO)(PPh3)]+—superior in many aspects to the analogous dichloromethane complex reported earlier.