Substituent and Medium Effects on Nitrogen-15 Shieldings of Compounds with >C=N Bonds (Imines, Oximes, and Phenylhydrazones)

The ^(15)N chemical shifts of ^(13)N -(arylmethylidene)alkanamines, seven N-(arylmethylidene)azanols, five 1-(aryImethylidene)- 2-phenyldiazanes, and 11 N-(arylmethylidene)arenamines have been determined at the natural abundance level of ^(15)N in several solvents. The shifts of several of the N-(phenylmethylidene)alkanamines with different alkane groups have been analyzed in terms of α-,β- and y-methyl substituent effects. For those N-(arylmethylidene) azanes substituted at the para position, linear correlations with Hammett σ parameters having negative slopes are found for the ^(15)N chemical shifts. However, the ^(15)N shifts of N-(phenylmethylidene)arenamines (substituted at the para position of the arenamine moiety) are essentially insensitive to the nature of the substituent. The slopes of the Hammett correlations become more negative with increasing proton-donating power of the solvent for most series of compounds studied. In general, the ^(15)N shifts were found to be 5-12 ppm toward higher fields in methanol compared to chloroform, and except for the alkylidenazanols (oximes), about the same in dimethyl sulfoxide as in chloroform. In contrast, the alkylidenazanol resonances move 13-16 ppm downfield for the change from chloroform to dimethyl sulfoxide