Multigram Preparation of 2-Alkylpyrimidines in the Vapor Phase from Carboxylic Acids and 1,3-Diaminopropane over a Dual Catalyst System

Article abstract of DOI:10.1021/jo00036a031

2-Alkylpyrimidines 2 were obtained from cofeeding a carboxylic acid such as pivalic acid (3a) or propionic acid (3b) and 1,3-diaminopropane (4) over first an alumina catalyst at 250-290 deg C and second a palladium dehydrogenation catalyst at 300-340 deg C to give 2 directly in 56-68percent overall yields.On the alumina bed, initial amidation of organic acid occurs to give the monoacyltrimethylenediamine 5, followed by ring closure to the tetrahydropyrimidine intermediate 6.An equilibrium between 5, 6, and water is established on the alumina bed, with an apparent equilibrium constant of 53 +/- 7 mol/kg at 290 deg C.The high temperature of the alumina bed shifts the equilibrium in favor of 6, which is directly dehydrogenated to 2 over the palladium catalyst.The method avoids the need to isolate and purify solid intermediates.The presence of low levels of sulfur acts as a strong palladium catalyst deactivator.Gradual decline of palladium catalyst activity was observed due to carbon buildup.No decline in alumina catalyst activity was observed.The continuous process allows for the preparation of multigram quantities of 2 with a laboratory-scale reactor.