Imidazoles 2,4,5-triaryl

Triaryl-4H-l,3,5-thiadiazine reagieren in Benzol bei 20° mit Triethylamin als Katalysator in Ausbeuten > 80% zu 2,4,5-Triaryl-imidazolen, wobei aus dem Substituenten in 4-Stel-lung des 4H-1,3,5-Thiadiazins der Rest in 2-Position des Imidazols entsteht442. 

Siddiqui SA, Narkhede UC, Palimkar SS et al (2005) Room temperature ionic liquid promoted improved and rapid synthesis of 2, 4, 5-triaryl imidazoles from aryl aldehydes and 1, 2-diketones or a-hydroxyketone. Tetrahedron 61 3539-3546... 

In the absence of hydrogen sources the thermolysis of 2,4,4-triaryl-5-methylthio-4//-imidazoles (41) produces l,l -biimidazolyls (42), probably by way of a free radical pathway (Scheme 12). 

There are a number of thermal reactions which transform isoimidazoles into the fully aromatic compounds, e.g. the rearrangement of 2,2-dialkyl-2H-imidazoles (16S) to the 1,2-dialkyl isomers proceeds by a concerted process. In the absence of hydrogen sources thermolysis of 2,4,4-triaryl-5-methylthio-4H-imidazoles gives l,T-biimidazolyl products... 

When urea (or thiourea) reacts with a-hydroxy ketones or a-diketones the products are imidazolin-2-ones (or -thiones) (70AHC(12)103,66RCR122). The reaction is limited to the preparation of 4,5-alkyl(or aryl)- or l,4,5-trialkyl(or triaryl)-imidazoles since an oxygen or sulfur function appears at C-2. Benzoin condenses with iV-phenylthiourea in hexanol in the presence of catalytic quantities of HCl to give l,4,5-triphenylimidazoline-2-thione (131) in 50-60% yield (Scheme 69). While 1-methylurea can also take part in the reaction. 

It is convenient to combine these two synthetic approaches because they are formally similar. Both condense an a-functionalized ketone or aldehyde (C-4-C-5 synthon) with an amine or ammonia (N-1, N-3) and an aldehyde (C-2). "Die alternative Bredereck modification uses formamide as the source of the C-2-N-3 bond and of N-1. The older (Radziszewski or Weidenhagen) methods give 4-mono-, 4,5-di- and 2,4,5-trialkyl or -triaryl imidazoles the Bredereck formamide synthesis is largely restricted to the preparation of imidazoles with no 2 substituent. 

Many of the classical methods grew out of the earliest synthesis of imidazole, which was achieved in 1858 by Debus [1] when he allowed glyoxal, formaldehyde and ammonia to react together. Although the earliest modifications of this method used a-diketones or a-ketoaldehydes as substrates [2, by the 1930s it was well established that a-hydroxycarbonyl compounds could serve equally well, provided that a mild oxidizer (e.g. ammoniacal copper(ll) acetate, citrate or sulfate) was incorporated [3. A further improvement was to use ammonium acetate in acetic acid as the nitrogen source. All of these early methods have deficiencies. There are problems associated with the synthesis of a wide range of a-hydroxyketones or a-dicarbonyls, yields are invariably rather poor, and more often than not mixtures of products are formed. There are, nevertheless, still applications to the preparation of simple 4-alkyl-, 4,5-dialkyl(diaryl)- and 2,4,5-trialkyl(triaryl)imidazoles. For example, pymvaldehyde can be converted quite conveniently into 4-methylimidazole or 2,4-dimethylimidazole. However, reversed aldol reactions of pyruvaldehyde in ammoniacal solution lead to other imidazoles (e.g. 2-acetyl-4-methylimidazole) as minor products [4]. Such... 

Giordano and Belli have formed imidazoles from 2,4,6-triaryl-4/f-l,3,5-thiadiazines by a base-catalyzed extrusion of sulfur [77], Schmidt et al. also refer to an analogous extrusion of oxygen from the 4/f-l,3-oxazine [78]. Scheme 4 depicts two extrusion reactions used to form imidazoles. 

Rasmussen et al. [205] described the use of resin-bound triaryl bismuth diacetates as starting material for O, N and C arylations [206]. In their approach one of the non-polymer-bound aryl groups is transferred onto imides, amides, carbamates, imidazoles, ahphatic amines, anilines, and phenols to yield the corresponding N- and O-arylated compounds. In the case of p-naphthol the a-C-arylated naph-thol was obtained in 49-68% yield. The authors used 1.5 equiv. of P-naphthol, and... 

Fig. 12.74 Proposed mechanism for the preparation of 2,4,5-triaryl imidazoles in TBAB...

A simple, efficient, and eco-friendly procedure has been developed using tetrabutylanunonium bromide ((TBAB), 10 mol%) as a novel neutral ionic liquid catalyst for the synthesis of 2,4,5-triaryl imidazoles by a one-pot three-component condensation of benzil, aryl aldehydes, and ammonium acetate in refluxing isopropanol (Fig. 12.73) [44]. 

From the up-to-date literature and patent review of catalysts used In anhydride and phenolic cured epoxy molding compounds, It Is evident that Imidazoles and their derivatives predominate (Table I). Metal complex, trialkyl or triaryl phosphines and their complexes, Lewis acids such as zinc or stannous octoate are used to a much lesser extent (Table II). There are a few examples of tertiary amines and urea derivatives used. 

Some 2,4,6-triaryl-l,3,5-triazincs 1 can be converted to triaryl-substituted imidazoles 2 by reduction with nascent hydrogen generated from zinc in acetic acid or zinc in potassium hydroxide (see Houben-Weyl, Vol. E8c, p 93). In this reaction one nitrogen is lost in the form of ammonia. The reaction times required arc between 20 minutes and two hours in hot acetic acid15 or refluxing aqueous potassium hydroxide 16 the yields are reported to be nearly quantitative.13 17... 

Maleki, B., Keshvari-Shirvan, H., Taimazi, R, Akbarzadeh, E. 2012. Sulfuric acid immobilized on silica gel as highly efficient and heterogeneous catalyst for the one-pot synthesis of 2,4,5-triaryl-lH-imidazoles. International Journal of Organic Chemistry 2 93-101. 

Triaryl-substituted imidazoles were synthesized from benzil or benzoin, aldehyde, and ammonium acetate. A literature survey reveals several reports for the synthesis of triarylimidazoles using SSA as a catalyst. Shaabani et al. (2007) reported the synthesis of triarylimidazoles using SSA under conventional heating as well as microwave irradiation (Scheme 5.15). Microwave irradiation afforded the corresponding products in shorter reaction times (5-10 min). Catalytic reusability... 

Shelke et al. (2009) reported the synthesis of 2,4,5-triaryl-l//-imidazoles (70) from the three-component one-pot condensation of benzil (67)/benzoin (68), aldehydes (69), and ammonium acetate in aqueous media under ultrasound at room temperature (Scheme 8.22). BO3H3 (5 mol%) was used as the catalyst. The reaction, performed under conventional stirring without ultrasound, required a reaction time (180 min) clearly longer than that needed when ultrasound was used (30-95 min). 

Shelke, K. R, Sapkal, S. B., Sonar, S. S., Madje, B. R., Shingate, B. B. and Shingare, M. S. 2009. An efficient synthesis of 2,4,5-triaryl-lH-imidazole derivatives catalyzed by boric acid in aqueous media under ultrasound-irradiation. Bull. Korean Chem. Soc. 30 1057-1060. 

Reza, M., Shafiee, M., Cheraghipoor, M. and Ghashang, M. 2011. ZnO nanopowder An efficient eatalyst for the preparation of 2,4,5-triaryl imidazoles under solvent-free condition. Int. Cortf. Nanotech. Biosens. IPCBEE. 25 90-93. 

V. S. V. Satyanarayana, A. Sivakumar, Chem. Pap. 2011, 65, 519-526. An efficient and novel one-pot synthesis of 2,4,5-triaryl-l//-imidazoles catalyzed by U0j(N03)j-6H20 under heterogeneous conditions. 

J. Sangshetti, N. Kokare, S. Kotharkara, D. J. Shinde, Synthesis 2007, 2829-2834. One-pot efficient synthesis of 2-aryl-l-aryhnethyl-l//-benzimidazoles and 2,4,5-triaryl-1//-imidazoles using oxalic acid catalyst. 

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