Catalysis of the formation of urethanes

Abstract

In an effort to further elucidate the mode of catalysis of metal salts in the urethane reaction, monofunctional compounds of various types and polyfunctional alcohols were allowed to react with either phenyl isocyanate or tolylene diisocyanate (80/20 mixture of 2,4- and 2,6-isomers) under catalysis by a tertiary amine or one of two metal salts. With the monofunctional compounds, the relative activity of the catalysts was cobaltous stearate > stannous 2-ethylhexoate > triethylenediamine. With polyfunctional compounds, however, the positions of the salts in this activity series are reversed. Evidence is presented based upon reaction kinetics and infrared spectra for the presence of a ternary complex as the reactive intermediate in the reaction catalyzed by metals salts. It is postulated that the high activity of some metal salts, such as stannous 2-ethylhexoate, can be attributed to the formation of a ternary complex which holds the functional groups in the optimum positions for reaction. Less optimum positions resulting from a different geometry of the coordination complex, lead to lower activity as observed with cobaltous stearate.