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

Abstract

X-ray crystal structures of four substituted N , N ʹ-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 CDCl 3 , C 6 D 6 , 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. • Four diarylformamidines were synthesized and structurally characterized. • Cyclic dimers with two H-bonded s-trans isomers as a tautomeric pair were detected in solid state. • Stereochemical exchange of the formamidines was observed in solution. • Relative abundance of the s-trans : s-cis isomers in solution varies with the position and bulkiness of the substituents. • H bonding agents also play an important role in stabilizing or destabilizing one of the isomers. Rare s-cis isomers of substituted N , N ʹ-diarylformamidines were observed in solution in which their isomeric ratios were manipulated by the concentration, the bulkiness of the substituents, and chemical properties of the solvent, whereas s-trans isomers were detected in solid state or the presence of agents with hydrogen bonding capability.