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(R2N)PCl compounds [R = Me, Et, PhCH2, iPr 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 high- and low-temperature limiting spectra of a series of Ph(RR'N)PCl compounds [R, R' = Me, Et, Bz, iPr 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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30376/1/0000778.pd