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 (η-C5H5)Re(NO)(PPh3)(CH2C 6H5) (1) with Ph3C+PF6- at -78°C gives the benzylidene sc-[(η-C5H5)Re(NO)(PPh3)(=CHC 6H5)]+PF6- (2k), which isomerizes upon warming (t1/2 = 17 min at 29.5°C, ΔH≠ = 20.9 ± 0.4 kcal/mol, ΔS≠ = -3.8 ± 0.2 eu) to a new Re=C geometric isomer, ac-[(η-C5H5)Re(NO)(PPh3)(=CHC 6H5)]+PF6- (2t). The 2t/2k equilibrium mixture is ≥99:1, but irradiation between -78 and -20°C establishes a (55 ± 3):(45 ± 3) photostationary state. The structures of 2t and 2k are confirmed by X-ray crystallography (2t) and extended Hückel MO calculations. Nucleophiles (Nu) Li(C2H5)3BD, CH3Li, CH3CH2MgBr, C6H5CH2MgCl, PMe3, and CH3ONa attack 2t to give diastereomerically pure adducts (η-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)-(η-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)-(η-C5H5)Re(NO)(PPh3)(CHDC 6H5) (1-α-d1-t; from Li(C2H5)3BD attack upon 2t) is treated with Ph3C+PF6-, exclusive H- abstraction occurs to give first 2k-α-d1 and then 2t-α-d1. When (SR,RS)-(η-C5H5)Re(NO)(PPh3)(CHDC 6H5) (1-α-d1-k; from Li(C2H5)3BH attack upon 2t-α-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 α-hydrogen of 1 is abstracted by Ph3C+PF6- essentially stereospecifically. Insights into this specificity are gained by 1H NMR, extended Hückel MO calculations, and the chemospecific and stereospecific abstraction of CH3O- from (η-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.
author list (cited authors)
Kiel, W. A., Lin, G. Y., Constable, A. G., McCormick, F. B., Strouse, C. E., Eisenstein, O., & Gladysz, J. A.