Amine functionalized polymeric catalysts and reagents

Abstract

Polymer-supported reagents and catalysts, which allow for simple product

separation and easy recycling, have been widely studied in the context of organic

synthesis. The past decade has witnessed a number of new variations of

polymeric materials, and among the most frequently immobilized functionalities

are amines that possess versatile synthetic utilities.

Polymers with new structures and improved properties for use in synthesis

have been continuously developed since the support may impact the chemical

reactions in which they are used in various ways. A new heterogeneous

polystyrene-based amine, rasta resin-DMAP, has been synthesized and used in

addition reactions of carbon dioxide to epoxides to afford cyclic carbonate

products. This new material was found to be a more efficient catalyst than

divinylbenzene cross-linked polystyrene supported DMAP, and was readily

recycled without significant loss of catalytic activity.

Compared to polymers bearing a single functionality, polymers possessing

multiple different functional groups attached to a single polymer backbone would

have greater potential utility, especially in reactions requiring multiple catalysts or

reagents. As an example of this concept, a bifunctional polystyrene bearing both

DMAP and piperazine groups has been prepared and applied as an organocatalyst

for decarboxylative Doebner-Knoevenagel reactions of arylaldehydes and

mono-ethyl malonate to produce (E)-,-unsaturated esters in high yields.

Additionally, both non-cross-linked and cross-linked bifunctional polystyrenes

featuring amine and thiourea groups have been developed, and their catalytic

performance were evaluated in reactions of nitroalkenes with either nitroalkanes

or sulfur ylides. Both polymers proved to be efficient catalysts in these reactions

and the insoluble polymer demonstrated high recyclability. Control experiments

using monofunctional polymers indicated that both catalytic groups of these

bifunctional polymers are essential and they could work cooperatively to achieve

efficient catalysis.

Finally, a second generation bifunctional phosphine-amine polymer, rasta

resin-PPh3-NBniPr2, was prepared and examined in tandem Wittig-reductive aldol

reactions. In these reaction cascades, the phosphine oxide groups generated

from the Wittig reaction served as the catalyst for the reductive aldol reaction, and

moderate yields of structurally diverse -hydroxy ketones could be obtained from

one-pot processes involving 5 sequential reactions.