MECHANISTIC STUDIES OF CARBONATE MACROCYCLIZATION

The kinetics of phenylchloroformate (PCF) reactions have been used to model some of the key chemical events in carbonate macrocyclization. T hree reactions have been studied using stopped-flow FT-IR spectroscopy : formation of acyl ammonium salt from PCF and three different trialky lamines, the conversion of acyl ammonium salt to urethane, and the con densation reaction between acyl ammonium salt and 4-isopropylphenol. T he rate dependence was studied for triethylamine (TEA), diethylmethyla mine (DEMA) and tri-n-butylamine (TBA) at 0-degrees-C in anhydrous CH2 Cl2. The reactivity order for acyl ammonium salt formation for TBA : T EA : DEMA is 1 : 2.7 : >444. By contrast, condensation and urethane fo rmation are not sensitive to the structure of the amine. The rate of c ondensation is comparable to the rate of acyl ammonium salt formation for TEA and TBA, while the rate of urethane formation is the slowest p rocess for all three amines. These results are consistent with the vie w that the yield of macrocyclic polycarbonates is related to the conce ntration of the acyl ammonium salt. The optimum amine concentration fo r obtaining high yields of cyclics varies with the amine structure and parallels the difference in the rates of acyl ammonium salt formation . (C) 1994 John Wiley & Sons, Inc.