Zinc Acetates as Efficient Catalysts for the Synthesis of Bis-isocyanate Precursors

Anhydrous and hydrated zinc acetates were found to efficiently catalyze alkoxycarbonylation of the aromatic diamines 2,4-toluenediamine (2,4-TDA) and 4,4'-diphenylmethane diamine (4,4'-MDA) with diethyl and dimethyl carbonate, providing the desired bis-carbamates in yields from 93 to 98%. Driving these reactions to completion required temperatures of ca. 190 degrees C for 2,4-TDA and ca. 180 degrees C for 4,4'-MDA, and alkyl carbonate-to-substrate molar ratios of 25:1 to 30:1. Higher catalyst loadings were required for 2,4-TDA (2.5 mol %) than for 4,4'-MDA (1 mol %).