A mechanistic study of the [La2(OCH3)2]4+- and [(1,5,9-triazacyclodo-decane):Zn:(OCH3)]+-catalyzed methanolysis of carbonates: possible application for the recycling of bisphenol A polycarbonates

Abstract

The kinetics of the methanolysis of seven methyl aryl carbonates (3) and two methyl alkyl carbonates (4) promoted by [12[ane]N3:Zn:(OCH3)]+ and [La2(OCH3)2]4+ catalysts (1 and 2, respectively) have been studied at 25.0 °C. Brønsted plots of the [Formula: see text] values for methanolysis versus aryloxy and alkoxy leaving group (LG) [Formula: see text] or [Formula: see text] values (the pKa values of the parent ArOH or ROH in methanol) for substrates 3 and 4 show an apparent downward break at [Formula: see text] ∼16.6 and 15.2 with [12[ane]N3:Zn:(OCH3)]+ and [La2(OCH3)2]4+, respectively. The breakpoint is not due to a change in rate-limiting step in a two-step process involving metal ion delivery of a coordinated methoxide to a transiently associated substrate and the subsequent breakdown of a tetrahedral intermediate to form product. The more satisfactory explanation is that the break arises when one correlates the rate constants for two dissimilar sets of substrates, namely aryloxy- and alkoxy-substituted 3 and 4. DFT calculations for the 1-promoted reactions of methyl 4-nitrophenyl carbonate (3b), which has a good aryloxy leaving group, and methyl isopropyl carbonate (4b), which has a relatively poor alkyl one, indicate that the catalyzed processes involve two steps. Accordingly, the methanolysis of all 3 having [Formula: see text] values for the parent phenols ≤15.3 involves rate-limiting nucleophilic attack and fast breakdown. For the isopropyl alkyl derivative (4b) having a [Formula: see text] > 18.13, the rate-liming step is the metal ion promoted breakdown of a tetrahedral intermediate. The catalytic system employing 2 has utility for the catalytic decomposition of poly(bisphenol A carbonate). In a semi-optimized system where 1000 mg of poly(bisphenol A carbonate), treated at 100 °C for 30 min in 2 mL of 60:40 chloroform−methanol containing La(OTf)3:NaOMe (5:7.5 mmol L−1), the reaction gave an 84% yield of bisphenol A, corresponding to >300 turnovers per catalyst.