Structure of the Tin Catalyst

the notion of an unsolvated tetrahedral tin compound in solutions containing both alcohols and isocyanates is an over-simplification and such ligand association and exchange processes form the basis for any proposed mechanism of catalysis. 

There are at least three distinct mechanisms proposed thus far for tin catalysts in the isocyanate/hydroxyl reaction. Thiele and Becker have categorized tin compounds as either insertion or Lewis Acid catalysts (Table 6.2.1). The authors report that one can easily determine the mechanism by which any catalyst works by observing their behavior in the presence of isocyanate alone. Catalysts that undergo an exothermic reaction with the isocyanate to generate isocyanurates are insertion catalysts. 

The major evidence in favor of this mechanism was provided by Davies, who demonstrated that tri-n-butyltin methoxide nndergoes clean insertion of phenyl isocyanate to generate the tri-n-butyltin carbamates (Fignre 6.2.5) J 

This mechanism was based on two pieces of evidence. First, a kinetic study showed that the order of the reaction with respect to both the catalyst and alcohol was 0.5. 

Secondly, the reaction was inhibited by both strong and weak acids. Strong acids, such as HBF4, completely stopped the reaction. Weaker acids, snch as acetic acid, had a much less pronounced and concentration-dependent effect. It has been snggested that the concept of the ionic mechanism mnst be viewed with some degree of caution, since the reaction proceeded faster in non-polar solvents, snch as cyclohexane, compared with a dipolar aprotic solvent, snch as dimethylformamide, whereas one wonld expect that the polarity of the solvent wonld significantly stabilize the ionic catalyst intermediates. However, Urban et al. have demonstrated that an ionic mechanism is likely operative in the reaction of hexamethylene diisocyanate with an acrylic polyol, nsing DBTDL as catalyst. 

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