Alkoxy Isothiocyanates as Intermediates in the Flash Vacuum Pyrolysis of Alkoxythioureas

Methoxy isothiocyanate MeO–NCS 2b was detected by matrix isolation IR spectroscopy following flash vacuum pyrolysis (FVP) of N-methoxythioureas, N-tert-butyl-N′-methoxythiourea 1d being the best precursor. Isothiocyanates 3, amines, and aldehydes are also generated by FVP of several substituted N-alkoxythioureas 1 in the temperature range 400–800°C. The formation of these products can be explained either by secondary pyrolysis of initially formed alkoxy isothiocyanates 2, or by an initial cleavage of the O–N bond in 1 via a free-radical mechanism. N-Cyanoamines 4 and/or the tautomeric carbodiimides 5 are formed by another pathway. The pyrolyses were monitored by IR spectroscopy and online mass spectrometry or tandem mass spectrometry, and the reaction mechanisms are supported by theoretical calculations.