Organocatalyzed michael addition of aldehydes to nitro alkenes - Generally accepted mechanism revisited and revised

Krystyna Patora-Komisarska, Meryem Benohoud, Hayato Ishikawa, Dieter Seebach, Yujiro Hayashi

Research output: Contribution to journalArticlepeer-review

152 Citations (Scopus)

Abstract

The amine-catalyzed enantioselective Michael addition of aldehydes to nitro alkenes (Scheme 1) is known to be acid-catalyzed (Fig. 1). A mechanistic investigation of this reaction, catalyzed by diphenylprolinol trimethylsilyl ether is described. Of the 13 acids tested, 4-NO2-C6H 4OH turned out to be the most effective additive, with which the amount of catalyst could be reduced to 1 mol-% (Tables 2-5). Fast formation of an amino-nitro-cyclobutane 12 was discovered by in situ NMR analysis of a reaction mixture. Enamines, preformed from the prolinol ether and aldehydes (benzene/molecular sieves), and nitroolefins underwent a stoichiometric reaction to give single all-trans-isomers of cyclobutanes (Fig. 3) in a [2+2] cycloaddition. This reaction was shown, in one case, to be acid-catalyzed (Fig. 4) and, in another case, to be thermally reversible (Fig. 5). Treatment of benzene solutions of the isolated amino-nitro-cyclobutanes with H2O led to mixtures of 4-nitro aldehydes (the products 7 of overall Michael addition) and enamines 13 derived thereof (Figs. 6-9). From the results obtained with specific examples, the following tentative, general conclusions are drawn for the mechanism of the reaction (Schemes 2 and 3): enamine and cyclobutane formation are fast, as compared to product formation; the zwitterionic primary product 5 of C,C-bond formation is in equilibrium with the product of its collapse (the cyclobutane) and with its precursors (enamine and nitro alkene); when protonated at its nitronate anion moiety the zwitterion gives rise to an iminium ion 6, which is hydrolyzed to the desired nitro aldehyde 7 or deprotonated to an enamine 13. While the enantioselectivity of the reaction is generally very high (>97% ee), the diastereoselectivity depends upon the conditions, under which the reaction is carried out (Fig. 10 and Table 1-5). Various acid-catalyzed steps have been identified. The cyclobutanes 12 may be considered an off-cycle 'reservoir' of catalyst, and the zwitterions 5 the 'key players' of the process (bottom part of Scheme 2 and Scheme 3).

Original languageEnglish
Pages (from-to)719-745
Number of pages27
JournalHelvetica Chimica Acta
Volume94
Issue number5
DOIs
Publication statusPublished - 2011 May

Keywords

  • Catalysis
  • Cyclobutane, 1-amino-2-nitro-
  • Enamines
  • Enantioselective synthesis
  • Michael addition
  • Organocatalysis

ASJC Scopus subject areas

  • Catalysis
  • Biochemistry
  • Drug Discovery
  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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