Organocatalytic Decarboxylative Doebner-Knoevenagel Reactions between Arylaldehydes and Monoethyl Malonate Mediated by a Bifunctional Polymeric Catalyst

Abstract

A bifunctional polystyrene bearing both DMAP and piperidine groups has been prepared and used as an organocatalyst for decarboxylative Doebner-Knoevenagel reactions of arylaldehydes and monoethyl malonate. Isolated yields of the resulting cinnamates were very high, and in all cases only the E-isomer was detected. When a polystyrene catalyst functionalized with only DMAP or piperidine groups was used in these reactions, catalysis was much less efficient. Furthermore, catalysis using a combination of the monofunctional polymers was also less efficient than with the bifunctional polystyrene. Thus, it appears that there is a synergistic effect obtained by co-locating the two different catalytic amine groups on the same polymer backbone.

Doebner-Knoevenagel reaction - organocatalysis - polystyrene - DMAP - cinnamates

References and Notes

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See Supporting Information for details.

19

Increasing the catalyst loading did not significantly affect the isolated yield or stereoselectivity of the reaction.

20

General Procedure for Doebner-Knoevenagel Reactions
Commercially available arylaldehydes 6a-s (0.5 mmol), 7 (0.75 mmol), and catalyst 3 (0.025 mmol) were dissolved in DMF (0.5 mL). The mixture was stirred at 50 ˚C for 15-18 h, and then the reaction mixture was purified directly by column chromatography (EtOAc-hexane) to afford the desired product 8a-s. In all cases only the E-stereoisomer was observed by ^¹H NMR spectroscopy.