Imine or Enamine? Insights and Predictive Guidelines from the Electronic Effect of Substituents in H-Bonded Salicylimines

Abstract

Imine and enamine bonds decorate the skeleton of numerous reagents, catalysts, and organic materials. However, it is difficult to isolate at will a single tautomer, as dynamic equilibria occur easily, even in the solid state, and are sensitive to electronic and steric effect, including π-conjugation and H-bonding. Here, using as model Schiff bases generated from salicylaldehydes and TRIS in a set of linear free energy relationships (LFER), we disclose how the formation of either imines or enamines can be controlled and provide a comprehensive framework that captures the structural underpinning of this prediction. This work highlights the potentiality of tailor-made designs en route to compounds with desirable functionality.

A synthetically benign one-pot construction of enamino-xanthene dyes

“…Negative values for the energy barrier (Δ H ‡ and Δ G ‡ ) have been described in other intramolecular hydrogen transfers in non-symmetric systems. 28…”

A Spectroscopic Overview of Intramolecular Hydrogen Bonds of NH…O,S,N Type

“…A plot of 1 J(N,H) vs. dNH for DNA and RNA demonstrated that the N gen bonds are stronger in dsRNA A:U than in dsDNA A:T bases pairs [70]. 1 H-31 P and three bond 15 N-31 P couplings have also been seen from a histidi phate group of DNA in a zink finger (see Figure 17) [71]. A very large N…N coupling of 40 Hz through a hydrogen bond is se pound in Figure 15E.…”

“…However, recently a large number of Schiff bases based on salicylaldehyde and TRIS have been shown to be on the enamine form in the solid state. This is true for the following salicylaldehydes with substituents as follows: 5-nitro, 5-methylcarboxylate, 4-fluor, 4-choro, 4-bromo, 4-methoxy, 4-amino and 5-phenylazo [ 1 ]. A few other examples of compounds entirely on the NH form are Schiff bases of 1,3,5-triacyl-2,4,6-trihydroxybenzene [ 19 , 20 , 21 ], 1,3,5-triformyl-2,4-6-trihydroxybenzene [ 19 ], of gossypol [ 22 , 23 ] or more recently of primarily on the NH form (2-(anilinemethylidenen)cyclohexane-1,3-dione) [ 24 ].…”

“…As the title indicates, this review is dealing with intramolecular hydrogen bonding. Recent reviews cover this subject [ 1 , 2 ]. Review [ 1 ] concentrates on Schiff bases made from salicylaldehyde and TRIS.…”

“…Recent reviews cover this subject [ 1 , 2 ]. Review [ 1 ] concentrates on Schiff bases made from salicylaldehyde and TRIS. Rules are set up to predict the predominant tautomer based on linear free energy relationships.…”