New Low-Temperature NMR Studies Establish the Presence of a Second Equatorial-Apical Isomer of [(R,S)-Binaphos](CO)2RhH

The important enantioselective hydroformylation catalyst [(R,S)-Binaphos](CO)(2)RhH has been reexamained by low-temperature NMR spectroscopy Both H-1 and P-31 NMR spectroscopy at -90 degrees C allow direct observation of a mixture or two apical-equatorial chelates The major chelate, 1(eq,ap), was shown to have an equatorial phosphine and an apical phosphite. Its structure was unambiguously assigned using low-temperature P-31 NMR spectroscopy with selective decoupling of aromatic hydrogens but not of the rhodium hydride, which showed a 225 Hz trans phosphite to RhH coupling The equilibrium constant for [1(eq,aq)]/[1(ap,eq)] was determined over a wide temperature range