SYNTHESIS, CHARACTERIZATION, AND CONFIGURATIONAL LABILITY OF (ETA(3)-ALLYL)DICARBONYL[HYDROTRIS(1-PYRAZOLYL)BORATO]MOLYBDENUM COMPLEXES BEARING SUBSTITUENTS AT THE TERMINI - THERMODYNAMIC PREFERENCE FOR THE ANTI STEREOISOMER

1-, 1,3-, 1,1,3- and 1,2,3-substituted dicarbonyl[hydrotris(1-pyrazolyl)borato](eta-allyl)-molybdenum complexes were prepared and characterized by IR and H-1 and C-13 NMR spectroscopy and, in some cases, by X-ray crystallography. From the (E)-stereoisomers of 1 degrees allylic acetates, syn-monosubstituted 1-ethyl- and 1-phenylallylic complexes were generated in high yield. The isomeric 1,3-transposed 2 degrees allylic acetates, CH2=CHCHR(OAc), gave syn/anti mixtures for R = Et, i-Pr, and cyclohexyl, but only syn-products were formed when R = aryl. The corresponding anti isomers of the i-ethyl- and 1-phenylallyl complexes were efficiently generated from the (Z)-stereoisomers of 2-pentenyl acetate and cinnamyl acetate, respectively, demonstrating the importance of double bond stereochemistry in establishing the allyl substituent kinetic stereochemistry. Monosubstituted allyls bearing alkyl, aryl, methoxy, acetoxy, and carbomethoxy substituents were configurationally stable at room temperature. However, at elevated temperature (110-125 degrees C) both syn and anti isomers of monosubstituted allyls equilibrated to mixtures in which the anti isomer predominated when the substituent was an alkyl, acetoxy, or carbomethoxy group. The syn isomer of 1-aryl- and l-methoxy substituted allyls was strongly favored at equilibrium. 1,3-Di-, 1,1,3-tri-, and 1,2,3-trisubstituted complexes equilibrated in solution at room temperature to mixtures in which the syn/anti isomer predominated over the syn/syn isomer for all cases studied except the 1,3-diphenyl-substituted allyl. Again, aryl groups showed a strong preference for the syn, configuration. Fight of the complexes were characterized by X-ray crystallography.