Thiourea organocatalysis

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 reactions.^[1]

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]} Squaramide catalysts 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“
• 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
• catalysis under almost neutral conditions (pk_a 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.

History

Early 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

Schreiner's thiourea

Catalysts

A broad variety of monofunctional and bifunctional (concept of bifunctionality) chiral double hydrogen-bonding (thio)urea organocatalysts have been developed.

• 
  1998: Jacobsen's chiral (polymer-bound) Schiff base thiourea derivative for asymmetric Strecker reactions.^[11][12]
• 
  2003: Takemoto's bifunctional chiral thiourea derivative, catalysis of asymmetric Michael- and Aza-Henry reactions.^[13]
• 
  2004: Nagasawa's chiral bis-thiourea organocatalyst, catalysis of asymmetric Baylis-Hillman reactions.^[14]
• 
  2005: Nagasawa's bifunctional thiourea functionalized guanidine, asymmetric catalysis of Henry(Nitroaldol)reactions.^[15]
• 
  2005: Ricci's chiral thiourea derivative with additional hydroxy-group, enantioselective Friedel-Crafts alkylation of indols with nitroalkenes.^[16]
• 
  2005: Wei Wang's bifunctional binaphthyl-thiourea derivative, asymmetric catalysis of Morita-Baylis-Hillman reactions.^[17]
• 
  2005: Soós's, Connon and Dobson's bifunctional thiourea functionalized Cinchona alkaloid, asymmetric additions of nitroalkanes to chalcones ^[18] as well as malonates to nitroalkenes ^[19]
• 
  2006: Yong Tang's chiral bifunctional pyrrolidine-thiourea, enantioselective Michael additions of cyclohexanone to nitroolefins.^[20]
• 
  2006: Berkessel's chiral isophoronediamine-derived bisthiourea derivative, catalysis of asymmetric Morita-Baylis-Hillman reactions.^[21]
• 
  2006: Takemoto's PEG-bound chiral thiourea, asymmetric catalysis of (tandem-) Michael reactions of trans"-β-nitrostyrene, aza-Henry reactions.^[22]
• 
  2007: Kotke/Schreiner, polystyrene-bound, recoverable and reusable thiourea derivative for organocatalytic tetrahydropyranylation of alcohols.^[3]
• 
  2007: Wanka/Schreiner, chiral peptidic adamantane-based thiourea, catalysis of Morita-Baylis-Hillman reactions.^[23]
• 
  2007: Takemoto's chelating bifunctional hydroxy-thiourea for enantioselective Petasis-type reaction of quinolines.^[24]
• 
  2007: Ma Jun-An's Chiral Bifunctional Primary and tertiary Amine-thiourea Catalysts Based on Saccharides.^[25][26]

See also

• Organocatalysis
• Hydrogen-bond catalysis
• Squaramide catalysis

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 (ed.). Hydrogen Bonding in Organic Synthesis. Wiley. pp. 141 to 251. ISBN 978-3-527-31895-7.
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:
   Peter R. Schreiner, review: "Metal-free organocatalysis through explicit hydrogen bonding interactions", Chem. Soc. Rev. 2003, 32, 289-296. abstract/download:
   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: 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: A European Journal. 12 (21): 5418–5427. doi:10.1002/chem.200501076. PMID 16514689.
3. 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. 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. 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. PMID 22435999.
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. hdl:1887/3512406. PMC 9324920. PMID 35322485.
9. Wittkopp, Alexander; Peter R. Schreiner (2003). "Metal-Free, Noncovalent Catalysis of Diels–Alder Reactions by Neutral Hydrogen Bond Donors in Organic Solvents and in Water". Chemistry: A European Journal. 9 (2): 407–414. doi:10.1002/chem.200390042. ISSN 0947-6539. PMID 12532289.
10. A. Wittkopp and P. R. Schreiner (2003). "Metal-Free, Noncovalent Catalysis of Diels-Alder Reactions by Neutral Hydrogen Bond Donors in Organic Solvents and in Water". Chemistry: A European Journal. 9 (2): 407–414. doi:10.1002/chem.200390042. PMID 12532289.
11. Sigman, Matthew S.; Eric N. Jacobsen (1998). "Schiff Base Catalysts for the Asymmetric Strecker Reaction Identified and Optimized from Parallel Synthetic Libraries". Journal of the American Chemical Society. 120 (19): 4901–4902. doi:10.1021/ja980139y. ISSN 0002-7863.
12. Sigman, Matthew S.; Petr Vachal; Eric N. Jacobsen (2000). "A General Catalyst for the Asymmetric Strecker Reaction". Angewandte Chemie International Edition. 39 (7): 1279–1281. doi:10.1002/(SICI)1521-3773(20000403)39:7<1279::AID-ANIE1279>3.0.CO;2-U. ISSN 1433-7851. PMID 10767031.
13. Okino, Tomotaka; Yasutaka Hoashi; Yoshiji Takemoto (2003). "Enantioselective Michael Reaction of Malonates to Nitroolefins Catalyzed by Bifunctional Organocatalysts". Journal of the American Chemical Society. 125 (42): 12672–12673. doi:10.1021/ja036972z. ISSN 0002-7863. PMID 14558791.
14. Sohtome, Yoshihiro; Aya Tanatani; Yuichi Hashimoto; Kazuo Nagasawa (2004). "Development of bis-thiourea-type organocatalyst for asymmetric Baylis–Hillman reaction☆". Tetrahedron Letters. 45 (29): 5589–5592. doi:10.1016/j.tetlet.2004.05.137. ISSN 0040-4039.
15. Sohtome, Yoshihiro; Yuichi Hashimoto; Kazuo Nagasawa (2005). "Guanidine-Thiourea Bifunctional Organocatalyst for the Asymmetric Henry (Nitroaldol) Reaction". Advanced Synthesis & Catalysis. 347 (11–13): 1643–1648. doi:10.1002/adsc.200505148. ISSN 1615-4150.
16. Herrera, Raquel P.; Valentina Sgarzani; Luca Bernardi; Alfredo Ricci (2005). "Catalytic Enantioselective Friedel-Crafts Alkylation of Indoles with Nitroalkenes by Using a Simple Thiourea Organocatalyst". Angewandte Chemie International Edition. 44 (40): 6576–6579. doi:10.1002/anie.200500227. ISSN 1433-7851. PMID 16172992.
17. Wang, Jian; Hao Li; Xinhong Yu; Liansuo Zu; Wei Wang (2005). "Chiral Binaphthyl-Derived Amine-Thiourea Organocatalyst-Promoted Asymmetric Morita−Baylis−Hillman Reaction". Organic Letters. 7 (19): 4293–4296. doi:10.1021/ol051822+. ISSN 1523-7060. PMID 16146410.
18. Vakulya, Benedek; Szilárd Varga; Antal Csámpai; Tibor Soós (2005). "Highly Enantioselective Conjugate Addition of Nitromethane to Chalcones Using Bifunctional Cinchona Organocatalysts". Organic Letters. 7 (10): 1967–1969. doi:10.1021/ol050431s. ISSN 1523-7060. PMID 15876031.
19. McCooey, Séamus H.; Stephen J. Connon (2005). "Urea- and Thiourea-Substituted Cinchona Alkaloid Derivatives as Highly Efficient Bifunctional Organocatalysts for the Asymmetric Addition of Malonate to Nitroalkenes: Inversion of Configuration at C9 Dramatically Improves Catalyst Performance". Angewandte Chemie International Edition. 44 (39): 6367–6370. doi:10.1002/anie.200501721. ISSN 1433-7851. PMID 16136619.
20. Cao, Chun-Li; Meng-Chun Ye; Xiu-Li Sun; Yong Tang (2006). "Pyrrolidine−Thiourea as a Bifunctional Organocatalyst: Highly Enantioselective Michael Addition of Cyclohexanone to Nitroolefins". Organic Letters. 8 (14): 2901–2904. doi:10.1021/ol060481c. ISSN 1523-7060. PMID 16805512.
21. Berkessel, Albrecht; Katrin Roland; Jörg M. Neudörfl (2006). "Asymmetric Morita−Baylis−Hillman Reaction Catalyzed by Isophoronediamine-Derived Bis(thio)urea Organocatalysts". Organic Letters. 8 (19): 4195–4198. doi:10.1021/ol061298m. ISSN 1523-7060. PMID 16956185.
22. Miyabe, Hideto; Sayo Tuchida; Masashige Yamauchi; Yoshiji Takemoto (2006). "Reaction of Nitroorganic Compounds Using Thiourea Catalysts Anchored to Polymer Support". Synthesis. 2006 (19): 3295–3300. doi:10.1055/s-2006-950196. ISSN 0039-7881.
23. 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.
24. Yamaoka, Yousuke; Hideto Miyabe; Yoshiji Takemoto (2007). "Catalytic Enantioselective Petasis-Type Reaction of Quinolines Catalyzed by a Newly Designed Thiourea Catalyst". Journal of the American Chemical Society. 129 (21): 6686–6687. doi:10.1021/ja071470x. ISSN 0002-7863. PMID 17488015.
25. Liu, Kun; Han-Feng Cui; Jing Nie; Ke-Yan Dong; Xiao-Juan Li; Jun-An Ma (2007). "Highly Enantioselective Michael Addition of Aromatic Ketones to Nitroolefins Promoted by Chiral Bifunctional Primary Amine-thiourea Catalysts Based on Saccharides". Organic Letters. 9 (5): 923–925. doi:10.1021/ol0701666. ISSN 1523-7060. PMID 17288432.
26. Li, Xiao-Juan; Kun Liu; Hai Ma; Jing Nie; Jun-An Ma (2008). "Highly Enantioselective Michael Addition of Malonates to Nitroolefins Catalyzed by Chiral Bifunctional Tertiary Amine-Thioureas Based on Saccharides". Synlett. 2008 (20): 3242–3246. doi:10.1055/s-0028-1087370. ISSN 0936-5214.