Tuning the Selectivity of the Oxetane and CO2 Coupling Process Catalyzed by (Salen)CrCl/n-Bu4NX: Cyclic Carbonate Formation vs Aliphatic Polycarbonate Production
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The (salen)CrIIICl complex 1, (1,2-cyclohexanediamino-N,N-bis(3, 5-ditert-butylsalicylidene)chromium(III) chloride), in the presence of n-Bu 4NX (X = Br, I, Cl, N3, NCO) as cocatalyst has been shown to be an effective catalytic system for the coupling of oxetane and CO 2 providing the corresponding aliphatic polycarbonate with small quantities of ether linkages at 110 C. The selectivity of the oxetane and CO2 coupling process has been effectively tuned for the formation of polycarbonate produced directly from the homopolymerization of preformed trimethylene carbonate (TMC), in the presence of complex 1 along with n-Bu 4NBr as cocatalyst, at temperatures lower than 80 C. Notably, under these conditions the amount of carbonate linkages obtained was remarkably high (>99%). An investigation of the initial catalytic species involved in the coupling reaction was performed by ESI mass spectral analysis, revealing a Schlenk (ligand redistribution) equilibrium of the three possible anions formed after treatment of the (salen)CrCl with two equivalents of n-Bu4NBr cocatalyst. This catalytic system afforded trimethylene carbonate exclusively from the coupling of oxetane and CO2 at mild reaction conditions (50 C and 10 bar of CO2) via a backbiting process involving mostly a metal-alkoxide intermediate. 2010 American Chemical Society.