Amidines. Part 41. Effects of substitution at the amidino carbon atom and at the imino nitrogen atom on the preferred configuration at the CN bond in the 13C NMR spectra of N 1,N 1-dimethyl-N 2-alkylamidines

Abstract

A new approach, based on the correlation between the chemical shifts of carbon atoms directly bonded to the two nitrogen atoms, enabled the determination of the preferred configuration of the CN bond in N1,N1-dimethylamidines containing a CH2 group at the imino nitrogen atom. The 13C NMR chemical shifts of 40 N1,N1-dimethyl-N2-alkylamidines and 35 N1,N1-dimethyl-N2-benzylamidines of general formula Me2N–CR1NR2, each divided into seven series depending on the substituent R1, have been determined in CDCl3 solutions. The V-shaped relationship between the chemical shifts of carbon atoms bonded directly to the amino and the imino nitrogen atoms reveals that, in the case of N2-alkyl and N2-benzylamidines, the volume of the substituent (R1) at the amidino carbon atom determines the preferred isomer. For amidines where R1 is isopropyl, isobutyl, tert-butyl or cyclohexyl the Z configuration is the preferred one.