Thiourea hydrogen bonding

Reductive amination of ketones using p-anisidine and the Hantzsch ester for transfer hydrogenation is a low-yielding reaction in toluene at room temperature, but thiourea is an efficient catalyst, and yields of up to 94% are reported at 50 °C.334 A mechanism involving thiourea hydrogen bonding to the intermediate imine is supported by ab initio calculations. 

The results of our studies on urea/thiourea/selenourea-anion inclusion compounds have demonstrated that the classical urea or thiourea hydrogen-bonded host lattice can be modified in interesting ways by the incorporation of various anionic moieties, with or without cocrystallized water or other uncharged molecules, and that novel host frameworks bearing different urea, thiourea, or selenourea/guest molar ratios are generated by variation in size of the hydrophobic, pseudo-spherical R4N guest species. The stoichiometric formulas of 46 inclusion compounds and their structural details are listed in Table 10. For convenient description of stoichiometric ratios, the letters u, a, and c are used to denote the urea/thiourea/se-lenourea molecule, the anion, and the cocrystallized neutral molecule, respectively. 

A new motif mimicking the thiourea hydrogen bonding has been developed. Here, the chiral scaffold is provided by tra i -l,2-diaminocyclohexane with additional benzimidazole units. Its catalytic action (together with CF3CO2H) in the Michael addition... 

Association Complexes. The unshared electron pairs of the ether oxygens, which give the polymer strong hydrogen bonding affinity, can also take part in association reactions with a variety of monomeric and polymeric electron acceptors (40,41). These include poly(acryhc acid), poly(methacryhc acid), copolymers of maleic and acryflc acids, tannic acid, naphthoHc and phenoHc compounds, as well as urea and thiourea (42—47). 

Platinum-thiourea complexes have been extensively studied because of their biological activity [54], but few have been used in catalysis. Neutral thioureas are able to coordinate to metal centres through their sulfur atom (Scheme 9) [55,56] monomeric (I) and oligomeric (II) species are known for Rh [57], and an X-ray structure has also been determined for the chiral complex III [58]. In many complexes hydrogen bonding has been observed... 

Fig. 9. Schematic representations of the canal structures of (a) urea and (b) thiourea, drawn to emphasise the similarities and differences between them, Each triangular host molecule is denoted by a stippled triangle, with the NH2 functions at the ends of the vertical edge. Oxygen or sulfur atoms lie in layers, outlined by the dotted hexagons. The thick inter-host lines are hydrogen bonds each O or S atom is involved in four hydrogen bonds, and each NH2 function is involved in two...

Thioureas are most commonly prepared from reaction of isothiocyanates with amines. Also there are some reports on reactions of anime or carbodiimides with several thionating reagents. AA -Disubstituted thioureas bearing double NH groups have been developed as sensors, and as catalysts because of their hydrogen bonding ability. A,A -Disubstituted thiourea-linked sugar chains have been prepared by the reaction of isothiocyanate with amine. 

Thioureas-binding site can be introduced to molecules like anthraquinone and is for use in various kinds of anion or fluoride sensors. Thiourea-based sensor are generally designed to recognize anion through an intermolecular hydrogen bonding.167 171... 

Chiral fe-thiourea-type catalysts effectively provide the Baylis Hillman reaction with cyclohexenone and aldehydes.181 In several reactions, thiourea derivatives have been used as significant and specific catalyst because of their intermolecular hydrogen bonding ability (Scheme 74).182 186... 

R, Thaimattam, D. S. Reddy, F. Xue, T. C. W. Mak, A Nangia, G. R. Desiraju, Interplay of strong and weak hydrogen bonding in molecular complexes of some 4,4 -disubstituted biphenyls with urea, thiourea and water , J. Chem Soc, Perkin Trans. 2,1998,1783-1789. 

Hydrogen-bond donors have the ability to enhance the selectivities and rates of organic reactions. Examples of catalytic active hydrogen-bond donor additives are urea derivatives, thiourea derivatives (Scheme 10, Tables 12 and 13) as well as diols (Table 14). The urea derivative 7 (Scheme 9) increases the stereoselectivity in radical allylation reactions of several sulphoxides (Scheme 10)171. The modest increase in selectivity was comparable to the effects exerted by protic solvents (such as CF3CH2OH) or traditional Lewis acids like ZnBr2172. It was mentioned that the major component of the catalytic effect may be the steric shielding of one face of the intermediate radical by the complex-bound urea derivative. 

Most hydrogen-bond-containing crystal structures are homomolecular, i.e. both the hydrogen-bond accepting and donating functionality exist in the same molecule. Examples of such co-crystals are urea and thiourea derivatives as well as non-cyclic imides156,179,180. 

As expected, the reaction is fastest in water due to its hydrogen-bonding ability and high dielectric constant. Addition of 1 mol% of the thiourea catalyst 10 increases the yields after 1 h in cyclohexane and chloroform by about 60% a 40 mol% catalyst doubles the yield. A sizeable catalytic effect of the m-trifluoromethyl-substi luted thiourea was also found in water. Explanations for the surprising fact that this hydrogen-bond donor is catalytically active even in a highly competitive solvent such as water will be given in Section III.D.3. 

Electron donation by potential carcinogens, such as 2-acetylaminodi-benzothiophene, has been estimated from the strength of their charge-transfer complexes with chloranil in acetonitrile. - In this context it should be noted that the hydrogen bonding of phenol to the 77-electrons of dibenzothiophene has been studied and that a thiourea adduct has proved useful in the removal of dibenzothiophene from oil. - ... 

The acyl-Pictet-Spengler reaction is also catalyzed by chiral thiourea derivative 6 to provide M-acetyl p-carbolines in high enantioselectivities. Notably, thiourea derivatives can activate not only electronically distinct imine derivatives such as N-alkyl and N-Boc imines but also a weakly Lewis basic N-acyhminium ion with high enantioselectivity using a chiral hydrogen bond donor (Scheme 12.4). 

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