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Open AccessArticle

Comparison of Cinchona Catalysts Containing Ethyl or Vinyl or Ethynyl Group at Their Quinuclidine Ring

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Department of Organic Chemistry & Technology, Budapest University of Technology & Economics, Szent Gellért tér 4, H-1111 Budapest, Hungary
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Chemical Department, Chemical Works of Gedeon Richter Plc., P.O. Box 27, H-1103 Budapest, Hungary
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Department of Inorganic & Analytical Chemistry, Budapest University of Technology & Economics, Szent Gellért tér 4, H-1111 Budapest, Hungary
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Furukawa Electric Institute of Technology, Késmárk utca 28/A, H-1158 Budapest, Hungary
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Department of Chemical & Environmental Process Engineering, Budapest University of Technology and Economics, Műegyetem rkp. 3, H-1111 Budapest, Hungary
*
Authors to whom correspondence should be addressed.
Materials 2019, 12(18), 3034; https://doi.org/10.3390/ma12183034
Received: 22 August 2019 / Revised: 11 September 2019 / Accepted: 12 September 2019 / Published: 18 September 2019
(This article belongs to the Special Issue Advances in Organocatalysts: Synthesis and Applications )
Numerous cinchona organocatalysts with different substituents at their quinuclidine unit have been described and tested, but the effect of those saturation has not been examined before. This work presents the synthesis of four widely used cinchona-based organocatalyst classes (hydroxy, amino, squaramide, and thiourea) with different saturation on the quinuclidine unit (ethyl, vinyl, ethynyl) started from quinine, the most easily available cinchona derivative. Big differences were found in basicity of the quinuclidine unit by measuring the pKa values of twelve catalysts in six solvents. The effect of differences was examined by testing the catalysts in Michael addition reaction of pentane-2,4-dione to trans-β-nitrostyrene. The 1.6–1.7 pKa deviation in basicity of the quinuclidine unit did not result in significant differences in yields and enantiomeric excesses. Quantum chemical calculations confirmed that the ethyl, ethynyl, and vinyl substituents affect the acid-base properties of the cinchona-thiourea catalysts only slightly, and the most active neutral thione forms are the most stable tautomers in all cases. Due to the fact that cinchonas with differently saturated quinuclidine substituents have similar catalytic activity in asymmetric Michael addition application of quinine-based catalysts is recommended. Its vinyl group allows further modifications, for instance, recycling the catalyst by immobilization. View Full-Text
Keywords: organocatalysis; cinchona; squaramide; thiourea; pKa; Michael addition organocatalysis; cinchona; squaramide; thiourea; pKa; Michael addition
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MDPI and ACS Style

Nagy, S.; Fehér, Z.; Dargó, G.; Barabás, J.; Garádi, Z.; Mátravölgyi, B.; Kisszékelyi, P.; Dargó, G.; Huszthy, P.; Höltzl, T.; Balogh, G.T.; Kupai, J. Comparison of Cinchona Catalysts Containing Ethyl or Vinyl or Ethynyl Group at Their Quinuclidine Ring. Materials 2019, 12, 3034.

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