SYNTHETIC APPROACHES TO THE CHIRAL, PYRAMIDAL, TRANSITION-METAL LEWIS ACID [(ETA-5-C5H5)RE(NO)(PPH3)]+X- - GENERATION, CHARACTERIZATION, AND REACTIONS OF A DICHLOROMETHANE ADDUCT
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Reaction of (5-C5H5)Re(NO)(PPh3)(CH3) (2) with HBF4.0(C2H5)2and HPF6.0(C2H5)2(CH2C12, -78 C) gives reactive intermediates (1+BF4, 1+PF6) that are functional equivalents of the chiral, pyramidal, 16-valence-electron, transition-metal Lewis acid [(5-C5H5)Re(NO)(PPh3)]+. Low-temperature13C NMR spectra show that 1+X contains coordinated CH2C12. Reactions of 1+X with Lewis bases (RCN, CO, H2C=CH2, 0=PPh3, -CN) give adducts [(5-C5H5)Re(NO)(PPh3)(L)p+(65-86%). Optically active (S)-l+X is generated from optically active methyl complex (+)-(S)-2, as assayed by the formation of Lewis base adducts in 98-99% ee and with overall retention of configuration. Low-temperature NMR data suggest methyl hydride complex [(5-C5H5)Re(NO)(PPh3)(CH3)(H)]+X to be a precursor of 1 +xlThis is supported by the characterization of more stable analogues [(5-C5H5)Re(NO)(PPh3)(R)(H)]+BF4 (R = H, CH2C6H5). While 1+PF6 decomposes to many products, 1+BF4 decomposes (24 h, -20 C) chiefly to bridging chloride complex (SS, RR)-[(5-C5H5)Re(NO)(PPh3)]2Cl+BF4~ ((SS,iLR)-13+BF4_, 64%). This structural assignment is confirmed by an independent synthesis of (+)-(SS)-13+BF4~ from (5')-l+BF4 and chloride complex (+).(S)-(5-C5H5)Re(NO)(PPh3)(Cl) ((+)-(S)-9). When (SS^R)-13+BF4~ is refluxed in CH3CN or acetone, [(r?5-C5H5)Re(NO)(PPh3)(L)]+BF4- (L = CH3CN, acetone; 92-96%) and 9 (96-98%) form. 1989, American Chemical Society. All rights reserved.
