Synthesis and properties of [(.eta.-C5H5)Re(NO)(PPh3)(:CHC6H5)]+PF6-: a benzylidene complex that is formed by a stereospecific .alpha.-hydride abstraction, exists as two geometric isomers, and undergoes stereospecific nucleophilic attack
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Reaction of (-C5H5)Re(NO)(PPh3)(CH2C 6H5) (1) with Ph3C+PF6- at -78C gives the benzylidene sc-[(-C5H5)Re(NO)(PPh3)(=CHC 6H5)]+PF6- (2k), which isomerizes upon warming (t1/2 = 17 min at 29.5C, H = 20.9 0.4 kcal/mol, S = -3.8 0.2 eu) to a new Re=C geometric isomer, ac-[(-C5H5)Re(NO)(PPh3)(=CHC 6H5)]+PF6- (2t). The 2t/2k equilibrium mixture is 99:1, but irradiation between -78 and -20C establishes a (55 3):(45 3) photostationary state. The structures of 2t and 2k are confirmed by X-ray crystallography (2t) and extended Hckel MO calculations. Nucleophiles (Nu) Li(C2H5)3BD, CH3Li, CH3CH2MgBr, C6H5CH2MgCl, PMe3, and CH3ONa attack 2t to give diastereomerically pure adducts (-C5H5)Re(NO)(PPh3)(CHNuC 6H5), in which a new chiral center is stereospecifically generated. The same nucleophiles react stereoselectively with 2k, affording adducts in (generally) 92-95:8-5 diastereomer ratios. The minor diastereomers from 2k are identical with the products from 2t. Thus nucleophilic attack upon 2t and 2k occurs preferentially from the same direction. An X-ray structure of (SS,RR)-(-C5H5)Re(NO)(PPh3)(CH(CH 2C6H5)C6H5) (5t, from C6H5CH2MgCl attack upon 2t) establishes this direction to be antiperiplanar to the PPh3. When (SS,RR)-(-C5H5)Re(NO)(PPh3)(CHDC 6H5) (1--d1-t; from Li(C2H5)3BD attack upon 2t) is treated with Ph3C+PF6-, exclusive H- abstraction occurs to give first 2k--d1 and then 2t--d1. When (SR,RS)-(-C5H5)Re(NO)(PPh3)(CHDC 6H5) (1--d1-k; from Li(C2H5)3BH attack upon 2t--d1 or Li(C2H5)3BD attack upon 2k) is treated with Ph3C+PF6-, preferential (92:8) D- abstraction occurs to give undeuterated 2k and (subsequently) 2t. These data (after an isotope effect correction) indicate that the pro-R -hydrogen of 1 is abstracted by Ph3C+PF6- essentially stereospecifically. Insights into this specificity are gained by 1H NMR, extended Hckel MO calculations, and the chemospecific and stereospecific abstraction of CH3O- from (-C5H5)Re(NO)(PPh3)(CH(OCH 3)C6H5) (SR,RS: 7t; SS,RR: 7k) by Ph3C+PF6-. A mechanism is proposed in which the least stable rotamer of 1 is the most reactive toward Ph3C+PF6-. 1982 American Chemical Society.
Journal of the American Chemical Society
author list (cited authors)
Kiel, W. A., Lin, G. Y., Constable, A. G., McCormick, F. B., Strouse, C. E., Eisenstein, O., & Gladysz, J. A.
complete list of authors
Kiel, William A||Lin, Gong Yu||Constable, Anthony G||McCormick, Fred B||Strouse, Charles E||Eisenstein, Odile||Gladysz, JA