SYNTHESIS, CHARACTERIZATION, AND CONFIGURATIONAL LABILITY OF 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 ydrotris(1-pyrazolyl)borato](et a-allyl)-molybdenum complexes were prepared and characterized by IR an d H-1 and C-13 NMR spectroscopy and, in some cases, by X-ray crystallo graphy. 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 cyclohex yl, but only syn-products were formed when R = aryl. The corresponding anti isomers of the i-ethyl- and 1-phenylallyl complexes were efficie ntly generated from the (Z)-stereoisomers of 2-pentenyl acetate and ci nnamyl acetate, respectively, demonstrating the importance of double b ond stereochemistry in establishing the allyl substituent kinetic ster eochemistry. Monosubstituted allyls bearing alkyl, aryl, methoxy, acet oxy, and carbomethoxy substituents were configurationally stable at ro om temperature. However, at elevated temperature (110-125 degrees C) b oth syn and anti isomers of monosubstituted allyls equilibrated to mix tures in which the anti isomer predominated when the substituent was a n 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 s olution at room temperature to mixtures in which the syn/anti isomer p redominated over the syn/syn isomer for all cases studied except the 1 ,3-diphenyl-substituted allyl. Again, aryl groups showed a strong pref erence for the syn, configuration. Fight of the complexes were charact erized by X-ray crystallography.