Catalysis with Organotin Compounds

Isocyanates are very versatile chemicals, which can undergo many reactions. Table 6.2.2 shows the commercially nsed reactions of isocyanates. 

WateH Organotin, f-amine Urea, carbon dioxide 

Isocyanate Quat. ammonium salt, K-salt Isocyanurate 

Stronger acids or more strongly bonded tin compounds substantially reduce the reaction rate of phenyl isocyanate with methanol. 

This study compares the effect of catalysts on aliphatic and aromatic isocyanates. With the exception of di-n-butyltin dithiocarbonate, all the di-n-butyltin catalysts perform similarly. The DABCO catalyst shows excellent catalysis for aromatic isocyanates and is less effective for aliphatic isocyanates. Combining this amine catalyst with DBTDL gives excellent catalytic activity for both aliphatic and aromatic isocyanates. Stannous, zirconium, and zinc octanoate show reduced activity in comparison to organotin. 

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DBTDL is soluble in a wide range of solvents, comparatively low in cost, colorless, and, in general, highly effective at levels of the order of 0.05 wt%. DBTDL promotes urethane formation without promoting allophanate formation (2) or trimerization (3). While aromatic isocyanates are more reactive than aliphatic isocyanates in uncatalyzed reactions with alcohols, the reactivity of aliphatics can be roughly equal with DBTDL. On the other hand, amine catalysts are more effective with aromatic than aliphatic isocyanates. Carboxylic acids inhibit catalysis by organotin compounds. 

In parallel with these developments, organotin compounds have found a variety of applications in industry, agriculture, and medicine, though in recent years these have been circumscribed by environmental considerations. In industry they are used for the stabilization of poly(vinyl chloride), the catalysis of the formation of the polyurethanes, and the cold vulcanisation of silicone polymers, and also as transesterification catalysts. 

When an organotin compound is the nucleophile, the coupling is called Stille coupling after J. K. Stille of Colorado State University, the inventor.26 In chapter 15 we saw how organo-tin compounds are sources of radicals, but with palladium catalysis the reaction changes. We shall start with an example which may look trivial but is very important for this chapter. Reaction of readily prepared 1,1-dibromoalkenes 184 with tributyltin hydride under Pd(0) catalysis gives Z-l-bromoalkenes 185 with high stereoselectivity.27... 

Two general classes are very well known. The first is composed of tertiary amines, of which DABCO, tertiary alkylamines, substituted morpholines, piperazines, guanidines, and substituted hydroxy amines are representative types. A wide range of activities is represented in this class, but stronger catalysts are sometimes needed to promote the reaction of secondary hydroxyl groups with isocyanates. Stronger catalysis is supplied by catalysts of the second class, organotin compounds like dibutyltin dilaurate... 

Thermosetting acrylic binder systems utilize copolymers of functional and nonfunctional acrylic (or similar) monomers. The functional monomers are incorporated for reactivity with crosslinkers. The most common functional monomer for reactions is the hydroxyl group. The hydroxyl groups on the acrylic copolymers react with melamine and urea resins (amino resins) and with polyisocyanates. These reactions are shown in Figure 11. The reaction of hydroxy functional polymers with amino resins require acid catalysis and heat. The reaction with polyisocyanates can occur at room temperature as well as at higher temperatures. A number of materials will catalyze the hydroxyl/isocyanate reaction (organotin compounds, acids, amines, metal salts, etc.)(9). 

Organotin 221290 and organoboron 222291 compounds and iodonium salts 223292 couple with diorganotellurium dichlorides under palladium catalysis to give tellurium-free products in good yields. In Scheme 120, representative examples of such transformations are given. 

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