Stereochemical dependence of 13C shieldings and 13C–31P couplings in phosphonates of known geometry. Is there a Karplus-type relationship for P–C–C–C coupling?

Abstract

13C nmr chemical shifts and 13C–31P couplings through one to five bonds are reported for seven dimethylphosphono compounds of known geometry. Vicinal couplings are maximal for a dihedral angle of 180° and are severely attenuated by OH substitution, particularly when the OH is trans-coplanar to the carbon terminus of the coupling path. When a cyclopropyl system is part of the C–C–C–P path the J values are much less than those predicted on the basis of dinedral angle. A highly asymmetric dihedral dependence of vicinal J's is suggested. Some 'non-W' long range C–P couplings through saturated networks are found. The 'gauche-γ' shift of the —P(O)(OCH3)2 group is about 2 ppm.