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

Intermolecular 1,3-dipolar cycloadditions of muchnones with acetylenic dipolarophiles: Sorting out the regioselectivity

A series of 1,3-dipolar cycloadditions of munchnones with acetylenic dipolarophiles was studied, wherein factors related to regioselectivity were investigated. The results from munchnones with electronically divergent thioaryl substituents compared with others bearing alkyl substituents suggest that an unsymmetrical transition state structure, rather than FMO perturbation, plays a significant role in regioselection. If eclipsing interactions preclude a highly unsymmetrical transition state however, then minimizing steric interactions becomes important. A pair of complementarily substituted munchnones, differing only in the position of isotopic labels, establishes an inherently symmetrical electronic nature of the mesoionic heterocycle.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31644/1/0000578.pd