Multivalent polyglycerol supported imidazolidin-4-one organocatalysts for enantioselective Friedel–Crafts alkylations

Abstract

SummaryThe first immobilization of a MacMillan’s first generation organocatalyst onto dendritic support is described. A modified tyrosine-based imidazolidin-4-one was grafted to a soluble high-loading hyperbranched polyglycerol via a copper-catalyzed alkyne–azide cycloaddition (CuAAC) reaction and readily purified by dialysis. The efficiency of differently functionalized multivalent organocatalysts 4a–c was tested in the asymmetric Friedel–Crafts alkylation of N-methylpyrrole with α,β-unsaturated aldehydes. A variety of substituted enals was investigated to explore the activity of the catalytic system which was also compared with monovalent analogues. The catalyst 4b showed excellent turnover rates and no loss of activity due to immobilization, albeit moderate enantioselectivities were observed. Moreover, easy recovery by selective precipitation allowed the reuse of the catalyst for three cycles.

Organocatalyzed Asymmetric Friedel‐Crafts Reactions: An Update

“…Electron‐deficient aromatic enals 219 d,e provided greater selectivities and yields, approving the strong effect of the substituent. Defiantly, aliphatic enals 219 a,b were well‐reacted and the results have been not affected considerably …”

“…In 2015, the initial immobilization of a MacMillan's first generation organocatalyst onto dendritic support was demonstrated . An improved tyrosine‐based imidazolidin‐4‐one was attached to a soluble high‐loading hyperbranched polyglycerol through a Cu‐mediated alkyne‐azide cycloaddition (CuAAC) reaction and easily purified by dialysis.…”

“…The catalyst 239 presented high turnover rates and no loss of activity because of immobilization, although good ee have been detected. Furthermore, simple recyclability via selective precipitation permitted the reapply of the catalyst for three runs …”

“…[113] In 2015, the initial immobilization of a MacMillan's first generation organocatalyst onto dendritic support was demonstrated. [114] An improved tyrosine-based imidazolidin-4-one was attached to a soluble high-loading hyperbranched polyglycerol through a Cu-mediated alkyne-azide cycloaddition (CuAAC) reaction and easily purified by dialysis. The productivity of differently multivalent organocatalyst 239 [115] has been examined in the stereoselective AF-CA transformation of a,b-unsaturated aldehydes and N-methylpyrrole.…”

A High Loading and Recyclable Pentaerythritol Supported Imidazolidin-4-one Catalyst for Enantioselective Diels–Alder Reactions

“…However, its practical use in reactions is still hindered due to the need for high catalyst loading and difficulties in recovering the catalyst from the reaction mixture. One obvious way to overcome these problems is to immobilizing or attaching the catalyst unit on a recyclable support , such as PEG [11], dendrimer [12,13], siliceous and polymer-coated mesocellular foam (MCF) [14], fluorous tag [15], poly(-methyl-hydrosiloxane) (PHMS) [16], ionic liquid [17][18][19][20].…”