Silaesterification reaction

Chauhan BPS, Rathore JS, Chauhan M, Krawicz A (2003) Synthesis of polysiloxane stabilized palladium colloids and evidence of their participation in silaesterification reactions. 1 Am Chem Soc 125 2876-2877... 

Direct processes are the most common synthetic routes available for the preparation of sUyl esters. A brief survey of the literature indicates that the conventional synthetic routes available for silaesterification reactions require harsh reaction conditions and most often are accompanied by side reactions. " For example, triethylacetoxysilane was prepared by the cleavage of acetic anhydride and triethylethoxysUane (Scheme 3.1). The reaction involves nucleophilic attack of anhydride groups on the silicon center, resulting in the elimination of triethylacetoxysilane. 

Mercury (Hg) is a well-known poison for nanoclusters because of either amalgamation formation with the metal nanoparticles or physicoabsorbtion on the nanocluster surface. To test the poisoning with Hg during silaesterification reactions (Scheme 3.8), typically, in a Schlenk tube, Pd(OAc)2 (0.004 g, 0.02 mmol) and acetic acid (0.06 mL, 1.0 mmol) are dissolved in 4 mL of benzene and the solution is examined by UV-visible spectroscopy. A peak at 400 nm suggestive of Pd(OAc)2 is noted. PMHS (0.06 mL, 1.0 mmol, 33-35 Si-H units) was then added to the above solution. 

Figure 3.4. (a) TEM images of the reaction mixture during the silaesterification reactions, (b) Particle size plot showing size distribution of Pd nanoparticles. 

TABLE 3.1. Comparison of the Catalytic Activity Toward Silaesterification Reactions... 

Scheme 3.10. Proposed reaction mechanism during silaesterification reaction.

The catalytic silaesterification activity of the isolable Pd nanoclusters was explored with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) and PMHS as substrates. Substitution of both the substrates was performed under similar reaction conditions as used for Pd(OAc)2- Indeed, the substitution of both the substrates proceeded with ease. A comparative study was performed with isolated Pd nanoclusters and Pd(OAc)2 as catalyst and results are summarized in Table 3.1. As is clear from the table, comparable results of reactivity and selectivity were observed. Moreover, isolated Pd nanoclusters were reused for catalysis for three cycles without significantly losing the activity. 

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