15N NMR studies of guanidines. I—hydrogen bonding and electronic effects in conformationally flexible guanidines

Abstract

AbstractHydrogen bonding and electron distribution in some heterocyclic guanidines were studied by 15N NMR spectroscopy, using other substituted guanidines as points of reference. Solvent effects were used to distinguish the 15N NMR signals for their two sp2 nitrogen atoms, since ring nitrogen forms an intramolecular hydrogen bond and so is much less sensitive to solvent than is the guanidine unit. The effect of substituent is discussed in terms of competition between through‐resonance and the more localized resonance of the π‐inductive effect. These are favoured, respectively, by oxygen (nitro, acyl) and nitrogen (cyano) acceptor groups, whereas heterocycles, also nitrogen acceptors, occupy an intermediate position. Among the latter class, there are indications that the balance between these forms of resonance may be influenced by competitive conjugation with ring π‐donor heteroatoms.