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

Reactions of [(eta5-C5H5)Re(NO)(PPh3)(ClC6H5)]+BF4- with cis-alkenes (a, 2-butene; b, 3-hexene; c, stilbene; d, 1,2-dichloroethylene; -45-degrees-C to room temperature) give the adducts (Z)-[(eta5-C5H5)Re(NO)(PPh3)(RHC=CHR)]+BF4- ((Z)-1a-d) in 67-95% yields after workup. Reactions with trans-alkenes are much slower, and (E)-1a-d are isolated in 86-98% yields after 12-24 h at 85-95-degrees-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-degrees rotations about the Re-(C-C) 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 C=C enantioface bound to rhenium. Kinetic selectivities, obtained from syntheses conducted at 25-60-degrees-C, are lower, although the diastereomers of (E)-1c slowly equilibrate in CDCl3 at 25-degrees-C. Complexes (E)-1a-c undergo rotation about the Re-(C-C) 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.