Mechanism of equilibration of diasteromeric chiral rhenium alkene complexes of the formula [{eta}{sup 5}-C{sub 5}H{sub 5}Re(NO)(PPh{sub 3})(H{sub 2}C = CHR)]{sup +}BF{sub 4-}. The metal traverses between alkene enantiofaces without dissociation
The (RS,SR)/(RR,SS) diastereomers of [({eta}{sup 5}-C{sub 5}H{sub 5})Re(NO)(PPh{sub 3})(H{sub 2}C=CHR)]{sup +}BF{sub 4}{sup -} (1: R = (a) CH{sub 3}, (b) CH{sub 2}CH{sub 2}CH{sub 3}, (c) CH{sub 2}C{sub 6}H{sub 5}, (d) C{sub 6}H{sub 5}, (e) (CH)(CH{sub 3}){sub 2}, (g) Si(CH{sub 3}){sub 3}) differ in the alkene enantioface bound to rhenium, and interconvert in chlorocarbons at 95-100 {degrees}C. Isomerization is nondissociative (no incorporation of deuterated alkenes or PPh{sub 3}) and occurs with retention of configuation at rhenium and without scrambling of E/Z deuterium labels. The latter excludes mechanisms that involve intermediate carbocations ReCH{sub 2}CHR{sup +} and alkylidene complexes, and nucleophilic addition to the alkene. The isomerization of (RR,SS)-1d to (RS,SR)-1d proceeds (96.5 {degrees}C) with k(H)/k(=CHD{sub E}) = 1.64, k(H)/k(=CHD{sub Z}) = 1.07, and k(H)/k(=CDC{sub 6}H{sub 5}) = 1.15. Triethylamine promotes the isomerization of substrates that bear allylic protons via {sigma}-allyl complexes ({eta}{sup 5}C{sub 5}H{sub 5})Re(NO)(PPh{sub 3})(CH{sub 2}CH=CHR{prime}). However, rate data suggest that {open_quotes}conducted tour{close_quotes} mechanisms involving transient binding to RC=C substitutents are unlikely. These results are best accomodated by a mechanism in which the rhenium moves though {pi} nodal plane of the alkene via a carbon-hydrogen {open_quotes}{sigma} bond complex{close_quotes} involving H{sub E} and/or a vinyl hydride oxidative addition product.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 232171
- Journal Information:
- Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 11 Vol. 114; ISSN JACSAT; ISSN 0002-7863
- Country of Publication:
- United States
- Language:
- English
Similar Records
Configurational equilibria in amido and lithioamido complexes of formulas ({eta}{sup 5}-C{sub 5}H{sub 5})Re(NO)(PAr{sub 3})(NHCHRR`) and ({eta}{sup 5}-C{sub 5}H{sub 5})Re(NO)(PAr{sub 3})(NLiR``): Epimerization occurs at rhenium via phosphine dissociation
Unprecedented regiospecificity and stereospecificity in reactions of Ph/sub 3/C/sup +/PF/sub 6/- with rhenium alkyls of the formula (eta-C/sub 5/H/sub 5/)Re(NO)(PPh/sub 3/)(CH/sub 2/R)