Thiourea organic catalysis | Thiourea halogen bond donors

Thiourea organocatalysis involves the use of ureas and thioureas to accelerate and stereochemically alter organic transformations. Unlike classical catalysts, these organocatalysts interact through non-covalent interactions, particularly hydrogen bonding. Here are some key points:

Hydrogen Bonding Interaction: Thiourea derivatives form hydrogen bonds with carbonyl substrates. These interactions arise from coplanar amino substituents in the (thio)urea. Squaramides, which engage in double hydrogen bonding, are often superior to thioureas in this context.

Strength of Hydrogen-Bond Donors: Thioureas are often stronger hydrogen-bond donors (more acidic) than ureas due to their positively charged amino groups. This counterintuitive phenomenon results from the effective steric size of the chalcogen atoms (sulfur in this case).

Advantages of Thiourea Organocatalysts:

Green and Sustainable: Thioureas are environmentally friendly catalysts.

Weak Enthalpic Binding: They avoid product inhibition due to weak enthalpic binding.

Low Catalyst Loading: Effective at low catalyst concentrations (down to 0.001 mol%).

High Turn-Over-Frequency (TOF): Achieve high reaction rates (up to 5,700 h⁻¹).

Simple Synthesis: Easily synthesized from primary amine functionalized starting materials and isothiocyanates.

Neutral Conditions: Catalysis occurs under almost neutral conditions (pKa of thiourea is 21.0).

Halogen Bonding: Thiourea moieties are also well-suited for forming halogen bonds, which can be used to create larger assemblies through intermolecular weak interactions. These interactions have been explored in various contexts, including reversible capsules.

Recently, researchers have synthesized a series of thiourea-based tritopic receptor molecules as building blocks for halogen-bonded assemblies. These receptors exhibit two to four accepted C-I···S halogen bonds, demonstrating their directional and flexible acceptor properties. The alkyl substituents in the benzene ring play a crucial role in isothiocyanate group formation.

In summary, thiourea-based compounds offer exciting possibilities in both organocatalysis and halogen bonding, making them valuable tools for chemical transformations and molecular recognition.

For more in-depth information, you can explore the references provided below:

Thiourea Organocatalysis

Thiourea-Based Tritopic Halogen-Bonding Acceptors

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