[3,3]-Sigmatropic Rearrangement of Low-Volatile Propargyl Thiocyanates to Allenyl Isothiocyanates Using Solution Spray Flash Vacuum Pyrolysis

Conventional flash vacuum pyrolysis is the best method for the preparation of isothiocyanate-substituted allenes by [3,3]-sigmatropic rearrangement. These synthetically useful allenes undergo a variety of successive reactions; the most prominent is thiazole ring formation after nucleophilic attack at the isothiocyanate carbon. We now present the development and application of the solution spray method in flash vacuum pyrolysis of low- or nonvolatile propargyl thiocyanates. By using model reactions, the setup was optimized for a synthetic scale approach utilizing also steel nozzles (distributed for oil-fired heating furnaces) for spray generation. Selected examples emphasize advantages such as enabling gas-phase reactions of nonvolatile compounds and improvement of challenging syntheses via highly reactive species under different operating conditions (400–600 °C, 0.01–0.05 mbar).