Environmental, safety and health concerns are major driving forces for the development of new coating systems, which in turn require catalysts with a different performance profile. One critical area for the development of new catalysts is the replacement of organotin compounds in polyurethanes with environmentally friendly catalysts, such as bismuth, aluminum and zirconium chelates. For applications in epoxies new catalysts for the epoxy‐carboxyl reaction are also being developed. To gain the needed improved performance multiple cure mechanisms are being employed in coatings requiring dual action catalysts.
In numerous natural products and synthetic compounds, ester moieties constitute major organic functional groups. Consequently, (trans)esterification reactions are widely applied in industry, 1 e.g. in the synthesis of fatty acid esters, 2,3 of polyesters 4−9 and of macrolides. 10 More specifically, polylactones and polylactides are multipurpose, bio-compatible and -degradable polyesters, which are suitable for biomedical and pharmaceutical applications. 11−14 Tin-based Lewis acids like mono-and dialkyltin compounds 4,15−19 and tetraalkyldistannoxane derivatives 8,20−22 are very efficient catalysts for transesterification reactions under mild conditions. A considerable drawback in the use of these organotin compounds in organic synthesis is the difficulty of their quantitative removal from the reaction mixture, because some organotin compounds are toxic. These limitations to the exploitation of such reagents and/or catalysts in biomedical and pharmaceutical synthesis applications 23,24 have been recently overcome by exploring alternative and less toxic catalysts, and by the improvement of workup procedures of several types of homogeneous organotin catalysts. Otera et al. have described the synthesis and catalytic activity of perfluoroalkyl distannoxanes, which combine the advantages of efficient catalysis based on organotins with fluorous biphasic technology. 21,25 These types of organotin catalysts, being highly soluble in fluorocarbon solvents, provide solutions which become perfectly miscible with the organic reaction phase upon heating. After cooling the reaction mixture, phase separation is observed and the catalyst returns into the fluorous phase, making it easily and repeatedly recyclable. Another strategy to alleviate the aforementioned toxicity issue, involves grafting the organotin reagent onto an insoluble solid support. 26−28 In this way, other types of environmentally friendly organotin catalysts, which can easily be removed from the desired reaction products by simple filtration of the insoluble support to which they are grafted, are being designed. The forecasted deliverables of such systems include improved catalyst recycling ability and a better control
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