Amidines. Part 41. Effects of substitution at the amidino carbon atom and at the iminonitrogen atom on the preferred configuration at the CN bond in the 13C NMR spectra of N1,N1-dimethyl-N2-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.