SYNTHESES AND STRUCTURES OF COMPLEXES OF ALPHA,BETA-UNSATURATED CARBONYL-COMPOUNDS AND THE CHIRAL RHENIUM FRAGMENT [(ETA(5)-C5H5)RE(NO)(PPH3)]+ - DIVERGENT KINETIC AND THERMODYNAMIC O=C/C=C BINDING SELECTIVITIES

Reactions of the substitution-labile dichloromethane complex [(eta5-C5H5)Re(NO)(PPh3)(ClCH2Cl)]+BF4-with alpha,beta-unsaturated aldehydes and ketones are described. Acrolein gives a pi O=C complex, which cleanly isomerizes in the solid state (100-degrees-C) to a pi C=C complex. Crotonaldehyde gives both pi and sigma O=C complexes (52:48, CH2Cl2, room temperature), which convert at 80-degrees-C to a pi C=C complex. Methyl vinyl ketone gives a sigma O=C complex, which is characterized by NMR at -25-degrees-C. At room temperature, a pi C=C complex forms. Other acyclic vinyl ketones behave similarly. Cyclopentenone and cyclohexenone give sigma O=C complexes that only partially isomerize to pi C=C complexes at 60-90-degrees-C. The acetylenic ketone 4-phenyl-3-butyn-2-one gives a sigma O=C complex at -25-degrees-C. At room temperature, a pi C=C complex forms. The crystal structure of a pi C=C complex of trans-4-hexen-3-one is determined. The NMR, configurational, and conformational properties of the preceding complexes, all of which can exist in several isomeric forms, are analyzed in detail. These data show that, with respect to the rhenium fragment [(eta5-C5H5)Re(NO)(PPh3)]+, the O=C groups of alpha,beta-unsaturated aldehydes and ketones are the kinetically preferred binding sites, but the C=C or C=C groups are generally the thermodynamically preferred binding sites.