Catalysts thiourea

Disubstituted benzo[fo]furans were also prepared by intramolecular cyclisation in the presence of a [Pd(thiourea)4]l2 catalyst (thiourea = H2NCS NH2). No Pd precipitation occurred with this very stable thiourea complex (Scheme 31) [120]. 

The presence of a hot spot is known to generate free radicals. Thus, reactions that are initiated by the presence of free radicals can be carried out with less or no catalyst, as has been discovered in the case of isomerization of maleic acid to fumaric acid with an 3 to 16 fold increase in the isomerization rates at reduced catalyst (thiourea) concentrations (Muzumdar, 1988). 

It has been stated that thiourea (about 20 per cent, of the weight of the palladium - barium sulphate) may also be used as a catalyst poison. 

Acetoiicetyliition Reactions. The best known and commercially most important reaction of diketene is the aceto acetylation of nucleophiles to give derivatives of acetoacetic acid (Fig. 2) (1,5,6). A wide variety of substances with acidic hydrogens can be acetoacetylated. This includes alcohols, amines, phenols, thiols, carboxyHc acids, amides, ureas, thioureas, urethanes, and sulfonamides. Where more than one functional group is present, ring closure often follows aceto acetylation, giving access to a variety of heterocycHc compounds. These reactions often require catalysts in the form of tertiary amines, acids, and mercury salts. Acetoacetate esters and acetoacetamides are the most important industrial intermediates prepared from diketene. 

Hydrogen sulfide reacts with nitriles in the presence of a basic catalyst forming thioamides. A commercial example is its addition to cyanamide with the formation of thiourea [62-56-6]. ... 

The catalyst commonly used in this method is 5 wt % palladium supported on barium sulfate inhibited with quinoline—sulfur, thiourea, or thiophene to prevent reduction of the product aldehyde. A procedure is found in the Hterature (57). Suitable solvents are toluene, benzene, and xylene used under reflux conditions. Interestingly, it is now thought that Rosenmund s method (59) originally was successful because of the presence of sulfur compounds in the xylene used, since the need for an inhibitor to reduce catalyst activity was not described until three years later (60). 

The Biginelli reaction involves an one-pot reaction between aldehyde 1, 1,3-dicarbonyl 2, and urea 3a or thiourea 3b in the presence of an acidic catalyst to afford 3,4-dihydropyrimidin-2(l//)-one (DHPM) 4. This reaction is also referred to as the Biginelli condensation and Biginelli dihydropyrimidine synthesis. It belongs to a class of transformations called multi-component reactions (MCRs). 

In addition to modification of the catalyst, several variants of the Biginelli reaction have emerged as viable alternatives however, each method requires pre-formation of intermediates that are normally formed in the one-pot Biginelli reaction. First, Atwal and coworkers reported the reaction between aldol adducts 39 with urea 40a or thiourea 40b in the presence of sodium bicarbonate in dimethylformamide at 70°C to give 1,4-dihydropyrimidines 41. DHPM 42 was then produced by deprotection of 41. 

Abstract After an overview of chiral urea and thiourea synthetic methods, this review describes the main applications of urea and thiourea complexes in asymmetric catalysis. Some recent examples of thioureas as catalysts are also presented. Coordination chemistry of ureas and thioureas is briefly discussed. 

The Pd-catalysed Heck reaction performed with thiourea as the Ugand exhibit good activities for some catalysts. As for carbene ligands [104], steric hindrance improves catalytic results. Thus, thioureas wearing bulky substituents afford the formation of air- and moisture-stable Pd complexes [105]. For example, the catalyst obtained with 2mol% Pd(dba)2 and Ar,M -dimesitylene-ethylene thiourea (Scheme 24) was still active even after 2 months in an air atmosphere. 

Some chiral mono-, acyl- and di-thioureas have been used as ligand for the Rh-catalysed asymmetric hydroformylation of styrene. Although thiourea ligands form inactive systems with [Rh(COD)Cl]2 as the catalyst precursor, in standard conditions (40 °C, 40 bar CO -l- H2 1/1), the cationic Rh complex [Rh(COD)2]Bp4 combined with monothioureas as the ligand showed moderate to good activity (Scheme 29) [114]. 

Chiral lactones were also obtained by cyclocarbonylation of chiral acetylenic alcohols with Pd and thiourea (H2NCSNH2) (Scheme 32). No loss in chirality was observed, but large amounts of Pd and thiourea were used (10 mol %) since the catalyst deactivates by forming metal particles. The catalytic precursor (Pdl2 > PdCl2) and the ratio of thiourea to Pd were very important, thiourea being necessary for this reaction. The active species was supposed to be [Pd(thiourea)3l]I, which forms in situ from [Pd(thiourea)4]l2 and [Pd(thiourea)2]l2. It had to be a partially dissociated species since [Pd(thiourea)4](Bp4)2 was inactive [121]. 

Palladium salts are able to catalyse diyne carbonylation, so the reaction can be performed at room temperature under 1 atm of carbon monoxide. Thiourea (H2NCSNH2), which is added to stabilise the Pd catalyst (Scheme 34), is described as the best ligand for the efficiency of this reaction [124]. 

The reaction was first tested with these substances as ligands but the organic molecule, in the absence of any added metal ion, proved to be the most enantioselective catalyst (library 1 19% ee vs. less than 13% ee for the best metal catalyst). The effects of selective variations of the amino acid nature and of the salicylidene moiety on the diamine structure were investigated for urea and thiourea derivatives via HTS (library 2 48 urea compounds and... 

The catalysts bearing a cyclohexylamine moiety combined with a bulky sal-icylidene compound linked via one thiourea function to a terf-leucine ben-zylamide (Scheme 38, Ri = Bn, R2 = H) was the most efficient. The test was performed in solution at - 78 °C, with HCN as the cyanide source. Excellent results were obtained 78% isolated yield with 91% ee for the optimised substrate and 70-86% ee for other imine derivatives (65-92% isolated yield) [148,152-157]. 

Similar organocatalytic species to those successfully used for the Strecker reaction were used for the asymmetric Mannich reaction. Catalyst structure/ enantioselectivity profiles for the asymmetric Strecker and Mannich reactions were compared by the Jacobsen group [160]. The efficient thiourea... 

New catalysts were prepared after optimisation of the Ugand structure. The most efficient organo catalyst for this reaction was an amido-thiourea derivative (Scheme 43). Interestingly, dissymmetrical ligands were more efficient and selective for this reaction. 

This bifunctionnal amino-thiourea organocatalyst led to high selectivity because it was activating both the nitrone and the malonate, in its enol form, due to the acidic hydrogen atoms of the thiourea. Thus, the amino-thiourea catalyst promoted the Michael reaction of malonates to various nitroolefins... 

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