REACTIONS OF KETONES WITH URANIUM TETRABOROHYDRIDE - MECHANISM AND STEREOSELECTIVITY - SYNTHESIS AND STRUCTURE OF URANIUM(IV) TETRAHYDROBORATO ALKOXIDE COMPLEXES

The mono- and bisalkoxide uranium complexes U(BH4)3(OCHR1R2)(THF)2 1 a nd U(BH4)2(OCHR1R2)2(THF)2 2 were prepared successively in tetrahydrof uran (THF) by treatment of U(BH4)4 with the corresponding ketone R1R2C = O (acetone, benzophenone, cyclohexanone, 2-methylcyclohexanone, 4-t ert-butylcyclohexanone, and norcamphor). The borane BH3.THF was libera ted during the formation of 1 and 2, and reacted with the ketone to gi ve the alkoxyborane species B(OCHR1R2)nH3-n. Formation of 1 also resul ted from: a) reaction of U(BH4)4 with the alkoxyborane species; b) rea ction of U(BH4)4 with 2; and c) reaction of 2 with BH3-THF. The alkoxi de groups of 1 and 2 rapidly exchange with those of the alkoxyborane s pecies. The uranium complexes 1 and alkoxyborane species resulting fro m the reaction of U(BH4)4 with the substituted cyclohexanones or norca mphor have been separated and their hydrolysis afforded the correspond ing epimeric alcohols in different proportions. The monoalkoxide compo unds 1 were alternatively prepared by reaction of the ketones R1R2C=O with UCl4 in the presence of LiBH4 or by treatment of U(BH4)4 with the alcohols R1R2CHOH. The octahedral crystal structures of U(BH4)3(OCHPh 2)(THF)2 1b and U(BH4)2(OCHPh2)2(THF)2 2b show that in lb, the two equ atorial THF ligands (and the two equatorial BH4 groups) are in relativ e cis positions whereas they are trans in 2b.