Catalyst direct condensation

Mesityl oxide can also be produced by the direct condensation of acetone at higher temperatures. This reaction can be operated ia the vapor phase over 2iac oxide (182), or 2iac oxide—2irconium oxide (183), or ia the Hquid phase over cation-exchange resia (184) or 2irconium phosphate (185). Other catalysts are known (186). 

More conveniently, compound (13) was directly condensed with barbituric acid (14) in acetic acid (28) or in the presence of an acid catalyst in an organic solvent (29). The same a2o dye intermediate (13) and alloxantin give riboflavin in the presence of palladium on charcoal in alcohoHc hydrochloric acid under nitrogen. This reaction may involve the reduction of the a2o group to the (9-phenylenediamine by the alloxantin, which is dehydrogenated to alloxan (see Urea) (30). 

A Japanese patent has claimed improvements in the direct condensation of menadione with phytyl chloride in the presence of a reducing metal such as 2inc or iron powder (30). Tin chloride has been reported to be a useful catalyst for this condensation (31,32). 

The scheme shows that arylamine plays the role of nucleophilic catalyst reaction of direct condensation of monosaccharide with the ureido derivatives. That is why in order to simplify the synthesis of N-glycosylureas, one can go without N-arylglycozid and instead use the catalyst additions of arylamine [13, 14],... 

The direct condensation of ethanol to form n-butanol was investigated by Dolgov and Vol nov in 1933, using titanium oxide promoted with iron-aluminum oxides on charcoal as the catalyst (71). They suggest, with insufficient proof, that ethanol is dehydrated to form ethylene which then reacts with more ethanol to form the butanol. 

Olefinic esters may be obtained directly by the Knoevenagel reaction. Alkyl hydrogen malonates are used in place of malonic acid. Decarboxylation then gives the ester directly as in the preparation of ethyl 2-heptenoate (78%) and methyl m-nitrocinnamate (87%). Alkyl hydrogen malonates are readily available by partial hydrolysis of dialkyl malonates. The use of malonic ester in the condensation leads to olefinic diesters, namely, alkylidenemalonates such as ethyl heptylidenemalonate (68%). A small amount of organic acid is added to the amine catalyst since the salts rather than the free amines have been shown to be the catalysts in condensations of this type. Various catalysts have been studied in the preparation of diethyl methylenemalonate. Increased yields are obtained in the presence of copper salts. Trimethylacetalde-hyde and malonic ester are condensed by acetic anhydride and zinc chloride. Acetic anhydride is also used for the condensation of furfural and malonic ester to furfurylidenemalonic ester (82%). ... 

A number of methods that utilize enolsilanes directly in the aldol process with either aldehydes or acetals have been developed recently. These reactions are usually performed either in the presence of Lewis acids such as titanium tetrachloride (67) or with fluoride ion (68). Recently trimethylsilyl triflate (CF3S03SiMe3) was found to be an efficient acid catalyst for condensation... 

The direct condensation of carboxylic acids with aliphatic alcohols has been investigated by the use of fluorous ammonium salts as metal-free catalysts (Equation 4.24). Esterification reactions were carried out under mild fluorous biphasic conditions, in the presence of 1 mol% of fluorous ammonium triflate and without any water removal techniques. In the case of primary and secondary aliphatic alcohols, good to excellent ester yields were obtained. The fluorous ammonium triflate salt was easily recovered by simple phase separation and reused at least three times without any significant loss of activity [45],... 

An interesting and effective synthesis of 1,2,4-thiadiazoles is the direct condensation at high temperatures of aromatic nitriles and sulfur under the influence of suitable catalysts. Benzonitrile, for example, reacts with sulfur in the presence of tri-n-octylamine in closed vessels at 250°C to afford 3,5-diphenyl-1,2,4-thiadiazole in 75% yield. The effectiveness of the catalysts increases with their chain length, tri-n-octylamine being five times as active as triethylamine.250... 

We also examined aldol condensations of the dialdehyde 17. Without the special catalysis afforded by the cyclodextrin i w-imidazoles there was an almost random reaction to form compounds 18 and 19, as either aldehyde acted as the enofizing group. However, the cyclodextrin imidazole catalysts directed the selective formation of products 19, with no selectivity among its stereoisomers. Interestingly, the least selective catalyst for 17 was the cyclodextrin mono-imidazole, the AD isomer of the fcw-imidazole was more selective, and the most selective was the AB isomer. Obviously these results indicate that the cyclodextrin imidazole catalysts promote enolization of the aldehyde group closest to the cyclodextrin, as expected, but the subtlety of preferences among the bw-imidazole isomers is not yet understood in this case. 

There are two important methods for PLA synthesis direct polycondensation of lactic acid and ROP of lactic acid cyclic dimer, known as lactide. In direct condensation, solvent is used and higher reaction times are required. The resulting polymer is a material of low to intermediate molecular weight. ROP of the lactide needs catalyst but results in PLA with controlled molecular weight. Depending on the monomer and reaction conditions, it is possible to control the ratio and sequence of d- and L-lactic add units in the final polymer [74,75],... 

As was described for HMF in Sect. 2, CMF has likewise been used as a platform for carbon chain extension for the purposes of making higher alkanes. Thus, Silks, et al. describe the direct condensation of CMF with acetone, hydroxy acetone, or dihydroxyacetone in the presence of a zinc-proline complex catalyst [146]. Similarly, Seek recruited CMF as an intermediate in a process that involves various condensations of its derivatives and ultimately hydrotreating to arrive at biofuels [147]. 

As an alternative to direct condensation with either acetone or benzaldehyde, transacetalation with 2,2-dimethoxypropane (Method 19) or a,a-dimethoxytoluene (Method 20) is popular because, with DMF as solvent and CSA as the catalyst. 

The old commercial synthesis starts with the dimethyl ester because of the insolubility and high melting point of terephthalic acid. Direct condensation with the acid presents difficulties because of the high temperatures necessary and the sublimation which then occurs. The dimethyl ester is transesterified in a first stage with excess glycol at 200°C, with acetates of alkaline earth metals as transesterifying catalysts, whereby the methanol is removed and an oligomeric polyester is formed ... 

Recently, Sharghi et al. (2011) also investigated the assembly of 2-aryl/2-heteroaryl benzimidazoles from o-phenylenediamines and aromatic aldehydes using Cu nanoparticles on activated carbon (CuNPs/C) as the reusable heterogeneous catalyst (Scheme 4.45). The substrates can directly condensate at room temperature to... 

This work represents a landmark in the area of stereoselective metal-free (i.e., aminocatalysis) alkylation of benzenes based on Michael-type condensation via covalent catalyst-substrate interaction [22]. Subsequently, asymmetric acid catalysis based on hydrogen bond catalyst-substrate recognitions has found elegant applications in 1,4-conjugated additions and direct condensation of arenes with carbonyl compounds. The following sections will be organized based on the reactivity exploited in the arene functionalization. 

---