Conformational and tautomeric studies of acylguanidines. Part 2. Vibrational and carbon-13 nuclear magnetic resonance spectroscopy

Abstract

The i.r. and Raman spectra are described and discussed for molecules which embody in fixed form six of the possible conformers of an acylguanidine. Four of these correspond to structural units common in heterocyclic chemistry, and it is shown that all are readily distinguished by a variety of criteria which depend on the frequencies and intensities of the ν(CO) and ν(CN) modes. It is shown that the above are strongly coupled in all cases but two, the nature of the coupling depending on the conformation and being responsible in large part for the characteristic differences between the structural types. Where CO and CN are conjugated an analogy is drawn with the aminoenones, and the analysis is carried out on similar lines. An important difference is that, here, the s-cis(E) is preferred to the s-trans(A) conformer because of lone-pair repulsion in the latter; the former is better conjugated and has much lower double-bond frequencies. Evidence is presented that neither form is as electron-rich as the aminoenones. The sp²-carbon signals show a clear distinction between through-conjugated and cross-conjugated types, which is sharper for carbonyl since uncomplicated by hybridisation changes. These affect the imine ¹³C signals considerably and, as a result, some subsidiary criteria emerge. An unexpected bonus exists in the distinction between the N-methyl signals of 5- and 6-membered rings, a distinction which appears to be of general applicability. The i.r. and ¹³C n.m.r. results are critically compared in several respects, one being an instructive contrast between the effects of through (i.r.) and localised (¹³C) electrical forces.