Oxetane metal catalyst

Abstract This chapter focuses on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides or oxetanes and carbon dioxide. Emphasis is placed on the use of salen metal complexes, mainly derived from the transition metals chromium and cobalt, in the presence of onium salts as catalysts for the coupling of carbon dioxide with these cyclic ethers. Special considerations are given to the mechanistic pathways involved in these processes for the production of these important polymeric materials. 

Figure 8.13 General structure of the metal (111) salen catalysts employed in the copolymerization of oxetanes and C02.

Since the keto component reacts neither with the catalyst 5 (with or without phosphine) nor with the diazoalkane (exceptions below), it is likely to enter the catalytic cycle at the metal-carbene stage 8. If this proposal withstands future mechanistic studies, then the regioselectivity - oxygen to the oxophilic rhenium center-is reasonable by considering a metallacyclic intermediate 10 (eq. (6)). This hypothetical rhena-oxetane is assumed to eliminate the olefin. As a matter of fact, catalyst 5 has been detected from such precursors (e. g., 7a, b -1- diazoalkane f aldehyde) by gas chromatography [9, 10]. 

The rearrangement of propylene oxide on a range of metal phosphate catalysts may produce acetone, allyl alcohol, or propionaldehyde as predominant products according to the acidic and basic properties of the catalyst. The Rh complex (432) is considered to adopt the role of a weak Lewis acid in catalysing the rearrangement of vinyl epoxides to a S-unsaturated aldehydes. This catalyst was also found to be effective for oxetan cleavage. ... 

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