1H and 13C nuclear magnetic resonance studies of the conformational equilibrium of 2-(diphenylphosphino)benzaldehyde in polar and nonpolar solutions. Signs of the proximate coupling constants, 4J(CHO, 31P) and 3(13CHO, 31P)

Abstract

The analyses of the 1H nuclear magnetic resonance spectra of 2-(diphenylphosphino)benzaldehyde in CS2/C6D12 and acetone-d6 solutions yield stereospecific coupling constants from which the populations of the O-cis and O-trans conformers are derived. The free energy differences favouring the O-trans conformer at 300 K are 2.7 and 0.9 kJ/mol, in the polar and nonpolar solutions, respectively; in the crystal only the O-cis conformer exists. The coupling constant, 4J(CHO, P), is estimated as −7.1(2) Hz in the O-trans confomer and 3J(CHO, P) as +29.4(1.3) Hz. Their magnitudes depend on the proximity of the C—H bond to the lone pair on phosphorus. nJ(C, P) are reported for triphenylphosphine and for the benzaldehyde derivative as dilute solutions in the two solvents, demonstrating a significant solvent dependence for some of these coupling constants. Some simple relationships are proposed between nJ(C, P) and the torsion angle about the C—P bond, estimates of the latter coming from AM1 and STO 3G MO computations. nJ(C, P) are also sensitive to intrinsic ring substituent perturbations, as are the nJ(H, P); for example, 5J(H, P) is negative in the disubstituted ring of 2-(diphenylphosphino)benzaldehyde but positive in the phenyl groups. The nJ(H, P) are also discussed with respect to their dependence on the torsion angles about the C—P bonds. It appears that the conformational properties of the aromatic rings in triphenylphosphine and its formyl derivative are very similar. Further, the phosphorus atom is polarized such that the carbonyl bond is attracted towards the positive region near phosphorus, and the C—H bond of the formyl group more towards the lone-pair region; the actual torsion angles represent a compromise between these attractive forces and the repulsive forces between bonds on neighbouring aromatic moieties. CNDO/2 MO and INDO MO FPT computations of nJ(C, P) and nJ(H, P) are of mixed utility, although the former bear out the idea that the proximate [Formula: see text]lone-pair interaction dominates 3J(CHO,P) and 4J(CHO,P).