SYNTHESIS, STRUCTURE, AND DYNAMIC BEHAVIOR OF SYMMETRICAL CIS-ALKENE 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 [( 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)-[( 5 -C 5 H 5 )Re(NO)(PPh 3 )(RHCCHR)] + BF 4 - ((Z)-1ad) in 6795% yields after workup. Reactions with trans-alkenes are much slower, and (E)-1ac are isolated in 8698% yields after 1224 h at 8595 C. Complexes (Z)-1ad are obtained as 7084:3016, 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 >17.511.0 kcal/mol and exclude alternative isomerization pathways. Complexes (E)-1ac are obtained as >9998: < 12 equilibrium mixtures of diastereomers that differ in the CC enantioface bound to rhenium. Kinetic selectivities, obtained from syntheses conducted at 2560 C, are lower, although the diastereomers of (E)-1c slowly equilibrate in CDCl3 at 25 C. Complexes (E)-1ac undergo rotation about the [formula omitted] axes with barriers of 18.611.6 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.
