Oxidative kinetic resolution of P-chiral phosphines catalyzed by chiral (salen)dioxomolybdenum complexes

Abstract

Abstract Synthetic LMoO2 compounds have long been of keen interest both as structural and functional models for molybdoenzymes and in their own right as catalysts for a variety of oxygen atom-transfer (OAT) reactions. Investigations of their use as catalysts for stereoselective OAT transformations, however, are little known. In this study chiral diimine-salen molybdenum complexes, L*MoO2, are evaluated for their potential to catalyze oxidative kinetic resolution of racemic monophosphines by pyridine N-oxide. A set of six L*MoO2 complexes incorporating chiral salen-type Schiff base ligands derived from 1,2-diaminocyclohexane (7-10) and 1,1’-diaminobinapthylene (12) has been prepared and characterized. Compounds 7-10 and 12 are active catalysts for the oxidation of racemic PMePhtBu by pyridine N-oxide, affording low to moderate enantioselectivities (0 - 35 % ee) of the phosphine oxide OPMePhtBu. Key structure/reactivity features of the catalysts for these reactions include: the presence of a p-NO2 substituent on the salen-unit increases the catalyst activity; and increasing the steric bulk of the ortho-salen-substituent increases enantioselectivity. DFT computational analysis has identified a viable reaction pathway that features a stereochemically-defining O-transfer transition state involving phosphine attack on the chiral LMoO2 complex, which accounts for the experimental stereoselectivity and catalyst activity.