Synthesis, structure, and dynamic behavior of symmetrical cis- and trans-alkene complexes of the chiral rhenium Lewis acid [(.eta.5-C5H5)Re(NO)(PPh3)]+: binding selectivities and isomerization processes
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Reactions of [(η 5 -C 5 H 5 )Re(NO)(PPh 3 )(ClC 6 H 5 )] + BF 4 -with cis-alkenes (a, 2-butene; b, 3-hexene; c, stilbene; d, 1,2-dichloroethylene; −45 °C to room temperature) give the adducts (Z)-[(η 5 -C 5 H 5 )Re(NO)(PPh 3 )(RHCCHR)] + BF 4 - ((Z)-1a–d) in 67–95% yields after workup. Reactions with trans-alkenes are much slower, and (E)-1a–c are isolated in 86–98% yields after 12–24 h at 85–95 °C. Complexes (Z)-1a–d are obtained as 70–84:30–16, 85:15, 93:7, and 59:41 equilibrium mixtures of diastereomers that differ by ca. 180° rotations about the [formula omitted] axes. Variable-temperature and 2D NMR experiments establish rotational barriers of >17.5–11.0 kcal/mol and exclude alternative isomerization pathways. Complexes (E)-1a–c are obtained as >99–98: < 1–2 equilibrium mixtures of diastereomers that differ in the CC enantioface bound to rhenium. Kinetic selectivities, obtained from syntheses conducted at 25–60 °C, are lower, although the diastereomers of (E)-1c slowly equilibrate in CDCl3 at 25 °C. Complexes (E)-1a–c undergo rotation about the [formula omitted] axes with barriers of 18.6–11.6 kcal/mol. The crystal structures of (Z)-1a and (E)-1a are determined, and the NMR and dynamic properties of all compounds are analyzed in detail. © 1993, American Chemical Society. All rights reserved.
author list (cited authors)
Pu, J., Peng, T. S., Mayne, C. L., Arif, A. M., & Gladysz, J. A.