Reaction of a rhodium-miniPHOS complex with dihydrogen: NMR and computational study

The rhodium-MiniPHOS complex [Rh((R,R)-(MeBuPCH2PMeBut)-P-t)(2)](+) BF4- (1 ) is reversibly and stereoselectively hydrogenated at low temperatures to g ive one isomer of cis-dihydride 2a. The second possible diastereomer, 2b, i s not observed experimentally. DFT calculations suggest that 2b is 3.5 kcal mol(-1) less stable than 2a. The parameters of the equilibrium between 2a, 1, and H-2 determined by NMR are DeltaH = -7.7 +/- 0.1 kcal mol(-1) and De ltaS = -25 +/- 1 cal mol(-1) K-1. Formation of the trans-dihydride 3 is obs erved at around -20 degreesC. Most probably 3 is formed by isomerization of 2a or 2b. This isomerization involves reversible dissociation of the Rh-P bond. Similar dissociation of the Rh-P bond in 2a might make possible the s ubstrate binding when 1 is used as a catalyst for asymmetric hydrogenation.