The rhodium-MiniPHOS complex [Rh((R,R)-MeButPCH2 PMeBut)2]+BF4- (1) is reversibly and stereoselectively hydrogenated at low temperatures to give one isomer of cis-dihydride 2a. The second possible diastereomer, 2b, is 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 H2 determined by NMR are ΔH = -7.7 ± 0.1 kcal mol-1 and ΔS = -25 ± 1 cal mol-1 K-1. Formation of the trans-dihydride 3 is observed at around -20°C. 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 substrate binding when 1 is used as a catalyst for asymmetric hydrogenation.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry