Chemistry:Thiourea organocatalysis

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Short description: Use of ureas and thioureas to accelerate and stereochemically alter organic transformations

Within the area of organocatalysis, (thio)urea organocatalysis describes the use of ureas and thioureas to accelerate and stereochemically alter organic transformations. The effects arise through hydrogen-bonding interactions between the substrate and the (thio)urea. Unlike classical catalysts, these organocatalysts interact by non-covalent interactions, especially hydrogen bonding ("partial protonation"). The scope of these small-molecule H-bond donors termed (thio)urea organocatalysis covers both non-stereoselective and stereoselective applications.[1]

History

Pioneering contributions were made by Kelly, Etter, Jorgensen, Hine, Curran, Göbel, and De Mendoza (see review articles cited below) on hydrogen bonding interactions of small, metal-free compounds with electron-rich binding sites. Peter R. Schreiner and co-workers identified and introduced electron-poor thiourea derivatives as hydrogen-bonding organocatalysts. Schreiner's thiourea, N,N-bis3,5-bis(trifluormethyl)phenyl thiourea, combines all structural features for double H-bonding mediated organocatalysis:

  • electron-poor
  • rigid structure
  • non-coordinating, electron withdrawing substituents in 3,4, and/or 5 position of a phenyl ring
  • the 3,5-bis(trifluoromethyl)phenyl-group is the preferred substituent

Catalyst-substrate interactions

Hydrogen-bonding between thiourea derivatives and carbonyl substrates involve two hydrogen bonds provided by coplanar amino substituents in the (thio)urea.[2][3][4]
[5] Squaramides engage in double H-bonding interactions and are often superior to thioureas.[6]

Thioureas are often found to be stronger hydrogen-bond donors (i.e., more acidic) than ureas[7] because their amino groups are more positively charged. Quantum chemical analyses revealed that this counterintuitive phenomenon, which is not explainable by the relative electronegativities of O and S, results from the effective steric size of the chalcogen atoms.[8]

Ketone complex with Schreiner's N,N'-bis[3,5-bis(trifluoromethyl)phenyl thiourea. The double hydrogen-bonding, clamp-like binding motif is evident.[4][9]

Advantages of thiourea organocatalysts

Thio) ureas are green and sustainable catalysts. When effective, they can offer these advantages:

  • absence of product inhibition due to weak enthalpic binding, but specific binding-“recognition“
  • low catalyst-loading (down to 0.001 mol%)[3]
  • high TOF (Turn-Over-Frequency) values (up to 5,700 h−1)[3]
  • simple and inexpensive synthesis from primary amine functionalized (chiral-pool) starting materials and isothiocyanates
  • easy to modulate and to handle (bench-stable), no inert gas atmosphere required
  • immobilization on a solid phase (polymer-bound organocatalysts), catalyst recovery and reusability [3]
  • catalysis under almost neutral conditions (pka thiourea 21.0) and mild conditions, acid-sensitive substrates are tolerated
  • metal-free, nontoxic (compare traditional metal-containing Lewis-acid catalysts)
  • water-tolerant, even catalytically effective in water or aqueous media.[10]

Substrates

H-bond accepting substrates include carbonyl compounds, imines, nitroalkenes. The Diels-Alder reaction is one process that can benefit from (thio)urea catalysts.

Catalysts

A broad variety of monofunctional and bifunctional (concept of bifunctionality) chiral double hydrogen-bonding (thio)urea organocatalysts have been developed to accelerate various synthetically useful organic transformations

Further reading

  • Christian M. Kleiner, Peter R. Schreiner (2006). "Hydrophobic amplification of noncovalent organocatalysis". Chem. Commun.: 4315–4017. 
  • Z. Zhang and P. R. Schreiner (2007). "Thiourea-Catalyzed Transfer Hydrogenation of Aldimines". Synlett 2007 (9): 1455–1457. doi:10.1055/s-2007-980349. 
  • Wanka, Lukas; Chiara Cabrele; Maksims Vanejews; Peter R. Schreiner (2007). "γ-Aminoadamantanecarboxylic Acids Through Direct C–H Bond Amidations". European Journal of Organic Chemistry 2007 (9): 1474–1490. doi:10.1002/ejoc.200600975. ISSN 1434-193X. 

References

  1. Kotke, Mike; Schreiner, Peter R. (October 2009). "(Thio)urea Organocatalysts". in Petri M. Pihko. Hydrogen Bonding in Organic Synthesis. pp. 141 to 251. ISBN 978-3-527-31895-7. http://eu.wiley.com/WileyCDA/WileyTitle/productCd-352731895X.html. 
  2. Alexander Wittkopp, Peter R. Schreiner, "Diels-Alder Reactions in Water and in Hydrogen-Bonding Environments", book chapter in "The Chemistry of Dienes and Polyenes" Zvi Rappoport (Ed.), Volume 2, John Wiley & Sons Inc.; Chichester, 2000, 1029-1088. ISBN:0-471-72054-2.
    Alexander Wittkopp, "Organocatalysis of Diels-Alder Reactions by Neutral Hydrogen Bond Donors in Organic and Aqueous Solvents", dissertation written in German, Universität Göttingen, 2001. English abstract/download: [1]
    Peter R. Schreiner, review: "Metal-free organocatalysis through explicit hydrogen bonding interactions", Chem. Soc. Rev. 2003, 32, 289-296. abstract/download:[2]
    M. Kotke and P. R. Schreiner (2006). "Acid-free, organocatalytic acetalization". Tetrahedron 62 (2–3): 434–439. doi:10.1016/j.tet.2005.09.079. M. P. Petri (2004). "Activation of Carbonyl Compounds by Double Hydrogen Bonding: An Emerging Tool in Asymmetric Catalysis". Angewandte Chemie International Edition 43 (16): 2062–2064. doi:10.1002/anie.200301732. PMID 15083451. 
    Yoshiji Takemoto, review: "Recognition and activation by ureas and thioureas: stereoselective reactions using ureas and thioureas as hydrogen-bonding donors", Org. Biomol. Chem. 2005, 3, 4299-4306. abstract/download: [3]Mark S. Taylor, Eric N. Jacobsen (2006). "Asymmetric Catalysis by Chiral Hydrogen-Bond Donors". Angewandte Chemie International Edition 45 (10): 1520–1543. doi:10.1002/anie.200503132. PMID 16491487. J. C. Stephen (2006). "Organocatalysis Mediated by (Thio)urea Derivatives". Chemistry 12 (21): 5418–5427. doi:10.1002/chem.200501076. PMID 16514689. 
  3. 3.0 3.1 3.2 3.3 3.4 Kotke, Mike; Peter Schreiner (2007). "Generally Applicable Organocatalytic Tetrahydropyranylation of Hydroxy Functionalities with Very Low Catalyst Loading". Synthesis 2007 (5): 779–790. doi:10.1055/s-2007-965917. ISSN 0039-7881. 
  4. 4.0 4.1 Schreiner, Peter R.; Alexander Wittkopp (2002). "H-Bonding Additives Act Like Lewis Acid Catalysts". Organic Letters 4 (2): 217–220. doi:10.1021/ol017117s. ISSN 1523-7060. PMID 11796054. 
  5. Kotke, Mike (2009). Hydrogen-Bonding (Thio)urea Organocatalysts in Organic Synthesis : State of the art and Practical Methods for Acetalization, Tetrahydropyranylation, and Cooperative Epoxide Alcoholysis (Ph.D.). University Giessen/Germany. http://geb.uni-giessen.de/geb/volltexte/2010/7835/. Retrieved 2010-11-12. 
  6. Chauhan, P.; Mahajan, S.; Kaya, U.; Hack, D.; Enders, D. (2015). "Bifunctional Amine-Squaramides: Powerful Hydrogen-Bonding Organocatalysts for Asymmetric Domino/Cascade Reactions". Adv. Synth. Catal. 357 (2–3): 253–281. doi:10.1002/adsc.201401003. 
  7. Jakab, Gergely; Tancon, Carlo; Zhang, Zhiguo; Lippert, Katharina M.; Schreiner, Peter R. (2012). "(Thio)urea Organocatalyst Equilibrium Acidities in DMSO". Organic Letters 14 (7): 1724–1727. doi:10.1021/ol300307c. 
  8. Nieuwland, Celine; Fonseca Guerra, Célia (2022). "How the Chalcogen Atom Size Dictates the Hydrogen‐Bond Donor Capability of Carboxamides, Thioamides, and Selenoamides". Chemistry – A European Journal 28 (31): e202200755. doi:10.1002/chem.202200755. 
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