Cooperative Reduction: Selective redox transformation remains a general challenge in chemical synthesis. All too often, the most readily available precursor to a compound must be over-reduced (or over-oxidized) and then carefully coaxed back to a desired intermediate state. Such is the case with the synthesis of cyclohexanone, which is mass-produced for use in the preparation of nylon: Access by direct reduction of phenol is plagued by the rapid addition of too many hydrogen atoms to the substrate, producing an alcohol (cyclohexanol) in place of the ketone. Liu et al. (p. 1250 ) have discovered that the unexpected cooperation of supported palladium and a Lewis acid such as aluminum trichloride—two catalysts widely used alone but rarely in concert—facilitates highly selective conversion of phenol to cyclohexanone near room temperature. The key appears to be inhibition of the undesired ketone-to-alcohol reduction step by the Lewis acid.

The development of efficient heterogeneous catalysts for the cycloaddition of CO₂ with epoxides to produce five-membered cyclic carbonates under mild reaction conditions is of great importance. In this work, the coupling reaction of CO₂ with propylene oxide (PO) to produce propylene carbonate (PC) catalyzed by MOF-5 (metal-organic frameworks) in the presence of quaternary ammonium salts (Me₄NCl, Me₄NBr, Et₄NBr, n-Pr₄NBr, n-Bu₄NBr) was studied in different conditions. It was discovered that MOF-5 and quaternary ammonium salts had excellent synergetic effect in promoting the reaction, and the MOF-5/n-Bu₄NBr catalytic system was the most efficient among them. The optimal temperature for the reaction was around 50 °C. The reaction could be completed in 6 h at low CO₂ pressure with very high selectivity. A decrease of the yield of PC was not noticeable after MOF-5 was reused three times, indicating that the MOF-5 was stable. The MOF-5/n-Bu₄NBr catalytic system was also very active and selective for the cycloaddition of CO₂ with other epoxides, such as glycidyl phenyl ether, epichlorohydrin and styrene oxide. The mechanism for the coupling reaction is also discussed.