Substituent Effects on 13c Nmr Chemical Shifts in Dialkylaminophenylchlorophosphines

The relative chemical shifts and 2J(PC) coupling constants in the low-temperature limiting spectra of a series of Ph(R,N)PCl compounds [R = Me, Et, PhCH2, ‘Pr and c-Hex] differ for R = primary or secondary. For primary alkyl substituents, the more downfield signal exhibits a large, positive coupling and the more upfield resonance shows a small, negative coupling. These observations are reversed for secondary alkyl substituents. Calculated minimum-energy molecular structures indicate that the source of this reversal does not lie in differences in conformation about the P-N bond. Analysis of the highand low-temperature limiting spectra of a series of Ph(RR’N)PCl compounds [R, R’ = Me, Et, Bz, ‘Pr and c-Hex] suggests that the N-C carbon syn to the phosphorus lone-pair is subject to a relatively constant deshielding effect from the phosphorus-lone pair and a shielding contribution from the anti substituent that increases with increasing bulk of that anti substituent. Conversely, the chemical shift of the carbon anti to the phosphorus lone-pair is relatively insensitive to changes in the syn substituent, giving rise to the observed chemical shift reversal. The barrier to rotation about the P-N bond in aminophosphines has been known for some time. ’ Measurements of that barrier have been made in a wide variety of tricoordinate and tetracoordinate phosphines via variable-temperature NMR coalescence techniques.2 The preferred conformation for these compounds has the nitrogen and phosphorus lone-pairs orthogonal, thus holding one C-N bond syn-periplanar and one C-N bond anti-periplanar to the phosphorus lone-pair. At low temperature, this results in a pair of doublets in the ’ 'C NMR with different coupling constants. The magnitude and sign of the 2J(PNC) coupling constants in the 13C NMR spectra of these compounds, observed at sufficiently low temperature to exhibit slow rotation, are dependent upon the dihedral angle between the phosphorus lone-pair and the N-C bond ; coupling constants are large and positive * Authors to whom correspondence should he addressed. when the angle is near O”, and small and negative when the angle is near 180”.3,4 We recently described a correlation between the barrier to rotation about the P-N bond in phenyl (dialkylamino)chlorophosphines, ph(R,N)PCl], and the products of the reaction of the corresponding bis(dialkylamino)phosphine oxides, [(R,N),P(O)H], with molybdenum hexacarbonyl.’ At that time, we noted an unexplained difference in the relative positions of the large and small coupled resonances between compounds bearing methyl or primary alkyl substituents and those having secondary alkyl substituents on the amino group. For small alkyl groups (Me, Et, Bz), the downfield signal exhibited the large coupling constant, while for secondary alkyl groups (‘Pr, c-hex) it was the more upfield signal that showed the large coupling constant. The same reversal had’ been observed earlier.2f*g This raised some concerns as to whether the ground state conformations of these molecules