Regioselective reactions in heteroaromatic systems. Rules for methyl migration and nucleophilic substitution in methyl cyanurates and thiocyanurates

Abstract

A comparative analysis carried out on 19 methyl cyanurates and thiocyanurates (some of which reported for the first time) allowed the following general conclusions to be drawn about their reactivity in nucleophilic substitution and thermal isomerisation reactions. The nucleophilic substitution of the methylthio-groups by methoxide anions is always possible, but requires different experimental conditions which are specific for the molecular region in which the methylthio is located ; the thermal isomerisation reaction of the methoxy-derivatives into the corresponding S-or N-methyl isomers requires temperature conditions which are also largely dependent upon the molecular site of the methoxy-group. In both reactions the reactivity is enhanced by local asymmetry of substitution at the ring carbon atom to which the XMe (X = S or 0) group is bound.WE have recently reported a synthetic procedure which by means of methanolysis and isomerisation allowed four trimethyl dithiocyanurates and four trimethyl monothiocyanurates to be obtained from (l)? and (9).t A preliminary study of the two reactions allowed us (i) to point out the selectivity of nucleophilic attack in methanolysis and (ii) to develop some rules for the methyl group migration in the thermal isomerisation reactions.

Examining structure property relationships in coatings based on substituted linear aromatic polycyanurates

“…The greatly reduced enthalpy observed in the DSC analysis of diphenoxymethoxy-s-triazine (13.6 kJ/mol) suggests that partial isomerisation may be taking place. This is consistent with the findings of Tosato and Soccorsi who have shown, using NMR, that methyl transfer takes place between the methoxy oxygen and one of the two equivalent nitrogen atoms on the triazine ring at temperatures close to 200°C in methoxy-cyanurates [27,28].…”

“…It has been known for many years that purely aliphatic cyanurates can be thermally unstable and isomerise readily to yield the corresponding isocyanurate. Trimethoxys-triazine isomerises in the vapour phase when heated above 265°C, with an enthalpy of 202.6 kJ/mol [27]; to give an reasonable estimate for the enthalpy of isomerisation of a single methoxy-cyanurate unit, this figure was divided by a third (65.5 kJ/ mol). The greatly reduced enthalpy observed in the DSC analysis of diphenoxymethoxy-s-triazine (13.6 kJ/mol) suggests that partial isomerisation may be taking place.…”

Examining thermal stability and structure property relationships in coatings based on linear aromatic poly(methoxy-thiocyanurate)s

“…The greatly reduced enthalpy observed in the DSC analysis of diphenoxymethoxy-striazine (13.6 kJ/mol) suggests that partial isomerisation may be taking place. This is consistent with the findings of Tosato and Soccorsi who have shown, using NMR, that methyl transfer takes place between the methoxy oxygen and one of the two equivalent nitrogen atoms on the triazine ring at temperatures close to 200 C in methoxy-cyanurates [24,25]. This is further supported by the loss of the triazine vibrational mode at 736 cm À1 following the introduction of an exo-carbonyl group on the isocyanurate ring.…”

Six‐Membered Heterocycles: Triazines, Tetrazines and Other Polyaza Systems

“…Thermal isomerization of cyanurates and thiocyanurates to isocyanurates and thioisocyanurates, respectively, is also known [665,666].…”