Chiral Atropisomeric 2-Iodo-4,4′,6,6′-tetramethyl-2′-(diphenylphosphoryl)-1,1′-biphenyl-enantiodifferentiation by Rh2[MTPA]4-Adduct Formation and Conformational Analysis

Abstract

Enantiodifferentiation of the chiral 2-iodo-4,4′,6,6′-tetramethyl-2′-(diphenylphosphoryl)-1,1′-biphenyl (2) can be accomplished easily by adding one mole equivalent of the enantiopure dirhodium complex Rh (II) 2 [(R)-(+)-MTPA]4 (Rh*). The internal competition of the two bindings sites in 2, the Ph 2 P═O group, and the iodine atom was identified by variable-temperature 31 P NMR. Energy optimization (PM3) and ROESY spectroscopy of 2 in the absence and presence of Rh* reveal that 2 prefers a conformation in the adducts, which is the least stable one in the free molecule, i.e., adduct formation is accompanied by a rotation of the Ph 2 P═O group about the C-2′─P bond.