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-C5 H5)Re(NO)(PPh3)(ClCH2Cl)]+BF4-with alpha,beta-unsaturated aldehydes an d ketones are described. Acrolein gives a pi O=C complex, which cleanl y isomerizes in the solid state (100-degrees-C) to a pi C=C complex. C rotonaldehyde gives both pi and sigma O=C complexes (52:48, CH2Cl2, ro om temperature), which convert at 80-degrees-C to a pi C=C complex. Me thyl 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. Ot her acyclic vinyl ketones behave similarly. Cyclopentenone and cyclohe xenone 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 conf ormational properties of the preceding complexes, all of which can exi st 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 th e kinetically preferred binding sites, but the C=C or C=C groups are g enerally the thermodynamically preferred binding sites.