The preparation of specialty quat carboxylates is described in this month’s PTC Catalyst of the Month. The procedure was reported by the inventors at BASF who used the quat carboxylates to catalyze the reaction of TXMDI diisocyanate to polycarbodiimide.
It is reasonable to assume that quat carboxylates were chosen in order to enhance the solubility of the carboxylate in the isocyanate phase. Three quat carboxylate salts were prepared and used for the reaction. They were tetrabutylammonium 2-ethylhexanoate, tetramethylammonium 2-ethylhexanoate and methyl tributyl ammonium 2-ethylhexanoate. We are guessing (but don’t know), that the combination of availability and cost of methyl tributyl ammonium chloride as the starting material for the quat 2-ethylhexanoate salt plus the 13 carbon atoms of this quat versus only four carbon atoms for tetramethyl ammonium might be a good compromise for effective catalytic properties. Tetrabutylammonium chloride is a more expensive starting material, whereas tetrabutylammonium bromide would be 10 times more difficult to be used for the ion exchange to the carboxylate (see explanation in this month’s PTC Catalyst of the Month).
The carboxylate-catalyzed conversion of isocyanate to carbodiimide involves the following mechanism:
Nucleophilic Attack: The carboxylate anion acts as a nucleophile and attacks the carbon atom of the isocyanate group forming a tetrahedral intermediate. This intermediate has a negatively charged oxygen atom and a positively charged nitrogen atom.
Rearrangement and Elimination: The intermediate undergoes rearrangement. The negatively charged oxygen atom pushes back to reform the carbonyl group, leading to the elimination of a molecule of carbon dioxide and forming an isocyanate anion intermediate.
Formation of Carbodiimide: The isocyanate anion intermediate then undergoes rearrangement. The lone pair on the nitrogen attacks the carbon of the original isocyanate group, leading to the formation of carbodiimide.
In summary, the carboxylate anion acts as a catalyst by first attacking the isocyanate to form a reactive intermediate. A quat carboxylate is a better candidate to solubilize the carboxylate in the isocyanate phase than an alkali metal carboxylate. The reactive intermediate then loses carbon dioxide and rearranges to form the carbodiimide product. The catalytic cycle is completed with the regeneration of the carboxylate anion.
