Mechanism of the Reaction of Amines with 5‐[(Aryl‐ or Alkylamino)hydroxymethylene]‐2,2‐dimethyl‐1,3‐dioxane‐4,6‐diones in the Presence of Chlorotrimethylsilane (Me₃SiCl)

Abstract

Addition of chlorotrimethylsilane (Me 3 SiCl) to the mixture of a carbamoyl-substituted Meldrums acid, i.e., a 5-[(arylamino)hydroxymethylene]-2,2-dimethyl-1,3-dioxane-4,6-dione of type 1 and a secondary amine as nucleophile strongly accelerated the rate of their reaction. The reason for this phenomenon observed, during our previous research, remained, however, unclear. To elucidate the mechanism of this reaction, we assumed and verified three possible pathways for the action of Me 3 SiCl (cf. Scheme 2): The acceleration of the reaction is caused i) by formation of a O-trimethylsilylated Meldrums acid of type 2, ii) by the silylated amine 3, or iii) by the presence of HCl liberated from Me 3 SiCl. The performed experiments revealed that the faster course of reaction is caused by the formation of N-trimethylsilylated amines of type 3.

Thermal Decomposition of Carbamoyl Meldrum's Acids: A Starting Point for the Preparation of 1,3‐Oxazine Derivatives

“…The reaction of ordinary ketenes with imines is properly described by a stepwise mechanism leading to an azetidone ring as the only product , whereas thermolysis of acyl Meldrum's acids produce 3‐oxo‐ketenes or even 3‐oxo‐2‐carboxyketenes , which allow six‐membered heterocycles to be formed through [4 + 2] cycloaddition. Moreover, our own research results seem to indicate that the reaction of acyl Meldrum's acid with nucleophile may take place even before a loss of acetone and CO 2 .…”

“…It should be added that reaction between 4 and 2 may lead by [4 + 2] concerted mechanism, omitting an ionic intermediate (path B′). In the light of our previous studies on the mechanism reaction of carbamoyl Meldrums acids , path A seems to be unlikely because of the use of high boiling solvent that causes the complete decomposition of 1 within approximately 3 h as well as a low nucleophilicity of aldimines. Most probably, the observed reaction occurs through path B with 4 as an intermediate which under moderately basic conditions is relatively stable allowing a high yield of [4 + 2] cycloaddition.…”