Use of Highly Cross-Linked Polystyrene Supports for Acylation and Alkylation of an Ester Enolate at Room Temperature

Article abstract of DOI:10.1021/jo00339a022

The enolate of polystyrene-bound 3-phenylpropanoic ester is prepared with (triphenylmethyl)lithium in tetrahydrofuran at room temperature and is trapped with carboxylic acid chlorides and alkyl bromides.Gas chromatographic yields exceed 90percent, and isolated yields are 73-87percent with a partially esterified 10percent divinylbenzene-cross-linked gel copolymer containing 0.67 mmol of ester/g of copolymer.Yields of p-nitrobenzoylation and self-condensation depend upon the degree of cross-linking of the polystyrene support, the gel or macroporous morphology of the polymer, the degree of functionalization of the polymer, the percent conversion of the chloromethyl polystyrene to ester, the structure of the base used to form the enolate, the time between enolate generation and trapping with p-nitrobenzoyl chloride, and the temperature.Yields of acylation product increase, and self-condensation of the ester decreases as percent cross-linking increases from 2percent to 20percent and as the concentration of ester in fully esterified polymers decreases from 1.08 to 0.18 mmol/g of polymer.When the original esterification of the polymer is carried to only partial conversion, the subsequent enolate acylation yields increase, conversion, the subsequent enolate acylation yields increase, self-condensation yields decrease, and unreacted ester yields decrease.Macroporous 20percent cross-linked polymers gave better site isolation than 10percent cross-linked gel polymers but lacked the physical stability necessary for recycling.One sample of 10percent cross-linked gel polymer in three esterification/enolate acylation/hydrolysis cycles showed no decrease in acylation yield but a decrease in the degree of functionalization.The higher degrees of cross-linking and functionalization of the polystyrene in this work than in similar experiments with a conventional 2percent cross-linked polystyrene enable ester enolate acylation in 77percent yield at room temperature on a 0.1-mol scale in just 1400 mL of reaction mixture.