N,N′-diarylformamidines: Solid state structural analysis and steric-induced stereochemical exchange in solution

X-ray crystal structures of four substituted N,N'-diarylformamidines demonstrated that cyclic dimers form in solid state from two formamidines as a tautomeric pair in s-trans configuration facilitated by two hydrogen bonds. NMR studies in various deuterated solvents along with subsequent acid addition indicate that the introduction of sterically demanding substituents at the ortho position seems to destabilize the formamidine dimer formation in solution resulting in the appearance of proton signals indicative of the rare s-cis isomers. When molecules feature significant steric hindrance from the substituents, s-trans isomers coexist with the s-cis isomers in CDCl3, C6D6, and DMSO-d(6). The distribution of the isomers in solution depends on the choice of the solvent, concentration, and also the bulkiness of the alkyl substituents. Addition of acid or coordinating solvent stabilizes the s-trans isomers. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

X-ray crystal structures of four substituted N,N'-diarylformamidines reveal the formation of cyclic dimers in solid state, while steric hindrance from substituents affects the coexistence of s-trans and s-cis isomers in solution. The distribution of isomers in solution depends on solvent choice, concentration, and addition of acid or coordinating solvent.