Dimerization Self-condensation

The dimer acids [61788-89-4] 9- and 10-carboxystearic acids, and C-21 dicarboxylic acids are products resulting from three different reactions of C-18 unsaturated fatty acids. These reactions are, respectively, self-condensation, reaction with carbon monoxide followed by oxidation of the resulting 9- or 10-formylstearic acid (or, alternatively, by hydrocarboxylation of the unsaturated fatty acid), and Diels-Alder reaction with acryUc acid. The starting materials for these reactions have been almost exclusively tall oil fatty acids or, to a lesser degree, oleic acid, although other unsaturated fatty acid feedstocks can be used (see Carboxylic acids. Fatty acids from tall oil Tall oil). 

Hydroxyaminoquinoxaline self-condensed with loss of 2 x (H2O) to afford a product, initially formulated as the pentacyclic near-dimer, [ 1,2,4,5]-tetra-... 

Hydroxyaminoquinoxaline self-condensed with loss of 2 x (H20) to afford a product, initially formulated as the pentacyclic near-dimer, [l,2,4,5]-tetra-zino[l,6-a 4,5-a ]diquinoxaline (170),982,992 but subsequently (after X-ray analysis) as the isomeric 2,2 -azoquinoxaline (170a)62 838 (CoC12-6H20 as... 

Polyurethanes are useful in numerous applications such as reaction injection molding, rigid and flexible foams, coatings and adhesives. However, due to the high reactivity of the isocyanate group [96], yielding either dimers, via self-condensation or a carbamate via the reaction with an alcohol, the A,jB-monomers have to be produced in-situ in the reaction vessel. 

The self-condensation of 3,4-dichlorothiophene 1,1-dioxide (89) in refiuxing xylene yields trace amounts of 2,3,7,8-tetrachlorodibenzo-thiophene 5,5-dioxide (90). The major product (50%) is the benzothio-phene (91) formed by loss of SO2 and HCl from the Diels-Alder dimer of 89. Further treatment of 91 with 89 in refiuxing trichlorobenzene again yields 90 (23%). Compound 90 is reported to possess insecticidal properties. ... 

The first attempted silylation of nitromethane resulted in the production of a silylated dimeric nitronate due to self-condensation (17). Subsequent investigations, however, have shown that sUyl nitronates are stable compounds that can be isolated and distilled at reduced pressures, Table 2.1 (18-21). Bulkier silyl groups help increase the stability of the resulting nitronate (21). 

There are very few reactions of real synthetic significance which proceed via condensation of two 1,3-electrophile-nucleophile species. Probably the most important of this latter type of reaction is the synthesis of pyrazines by self-condensation of an a-acylamino compound to the dihydropyrazine followed by aromatization (equation 132). The a-acylamino compounds, which dimerize spontaneously, are normally generated in situ, for example by treatment of a- hydroxy carbonyl compounds with ammonium acetate or by reduction of a-azido, -nitro or -oximino carbonyl compounds. Cyclodimerization of a-amino acids gives 2,5-dioxopiperazines (equation 133), many derivatives of which occur as natural products. Two further reactions which illustrate the 1,3-electrophile-nucleophile approach are outlined in equations (134) and (135), but su i processes are of little general utility. 

Under carefully worked out reaction conditions, 2b can give the 3-bromo compound 94 (CHC13,0-5°C) or the 3,3-dibromo compound 95 (2M NaOH, 0-5°C). The reactions of 94 and 95 with N-nucleophiles proceed differently. Refluxing 94 in ethanol gives dimer 96, which is clearly distinct in structure and properties from other self-condensation products of tetramic acid (85AJC1847). Otherwise, 95 and yV-methyl-A-phenylhydrazine under refluxing in ethanol form 3-hydrazone 92a in 60% yield (90TH1). (See Fig. 42.)... 

Six-Membered Heterocycle Ring Formation. Heterocycle formation involving diketene usually involves acetoacetylation of a substrate, followed by intramolecular condensation. Diketene itself readily dimerizes through self-condensation forming mainly dehydro acetic acid [771-03-9] (DHA) (13). Dehydroacetic acid and sodium dehydro acetate [4418-26-2] are used as preservatives for foods and cosmetics. DHA is found as an unwanted by-product in many diketene reactions, but can be obtained intentionally by dimerizing diketene in the presence of pyridine [110-86-1] in benzene, diazabicyclo[2.2.2]octane [280-57-9] (DABCO), and other basic catalysts. 

Hydroxymethylation of guaiacyl acetone (VII) yielded an amorphous solid, probably a self-condensation product after being methylolated in several positions. Veratyl acetone (VIII) gave a crystalline product in good yield which had no CO group. It can be assumed that the CO group was reduced by a crossed Cannizzaro reaction, and a dimeric product was formed, whose structure is not known yet. 

The self condensation of a-carbonyl ketenes also constitutes a type (i) synthesis of 2//-pyran-2-oncs. Cyclophanes 627 are synthesized by the thermal decomposition of bis(4,6-dioxo-l,3-dioxanes) 628 and dimerization of the resulting bis(a-carbonyl ketenes) 629 (Scheme 141) <1999JA8270>. Similarly, thermal decomposition of 4,5-diarylfuran-2,3-diones 630 forms the intermediate a-carbonyl ketenes, which dimerize to afford 3,4,5-aryl-2//-pyran-2-ones (Scheme 142) <2003RJ0103>. 

Wilen has shown by PMR measurements that 2,5-dihydro-3,6-dimethylpyrazine (169) is the major product from the reaction of a-aminoacetone hydrochloride with potassium hydroxide in the absence of air. PMR measurements also indicate the presence of minor amounts of either the isomeric 1,4-dihydro (170) or 1,2-dihydro derivative (171). Compound 169 is slowly converted on standing at room temperature into a dimer, C12H20N4, the structure of which is still being investigated.376 The self-condensation of two molecules of... 

Another field of research is concerned with the one-pot synthesis of this system from readily available starting materials. In Scheme 12, reaction of phenyl azide and malononitrile in the presence of sodium ethoxide leads, via the intermediate triazole (139), to the self-condensation product (140), itself a triazolopyrimidine. In the presence of other nitriles the preferred formation of the more useful compounds (141) occurs in low to moderate yields with small amounts of dimer <87JHC997>. Direct synthesis of the 5,7-dione derivatives (143) (60-90%) is achieved by the reaction of the readily available aminopyrimidinedione (142) with an azide iminium salt in dichloromethane at moderately high dilution <87JHC1493>. 

So far we have considered only self-condensations1—dimerization reactions of a single carbonyl compound. These form only a tiny fraction of known aldol reactions. Those that occur between two different carbonyl compounds, one acting as a nucleophile in its enol or enolate form, and the other as an electrophile, are called cross-condensations. They are more interesting than self-condensations, but working out what happens needs more thought. 

Bis(p-chlorostyryl)pyrazine (321) underwent self-condensation to the dicyclobutane dimer (322) in which (as shown by X-ray analysis) the pyrazine rings lay parallel on one side of the nearly coplanar cyclobutane rings and the benzene rings lay on the other side thereof [solid substrate suspended in H20,... 

The base-catalysed intermolecular condensation of nitriles of the type RCH2CN is one of the oldest known methods for the preparation of / -enaminonitriles, and in the case of simple self-condensation it leads to aliphatic analogues of the cyclic / -ena-minonitriles formed in the classical Thorpe-Ziegler cyclization. For example, the base-catalysed dimerization of acetonitrile with sodium gives 3-amino-crotononitrile661. 

Many other examples are known of acid-catalyzed intermolecular Schiff reactions18 to give cyclic dimers. Thus, condensation of two molecules of a-aminoketones gives 2,5-dihydropyrazines (e.g., 9 from l-aminopropan-2-one), most of which can be converted into pyrazines by oxidation with mercuric chloride (i.e., the Gutknecht pyrazine synthesis19). The same type of self-condensation occurs on heating... 

A simple Michael-type addition of the methyl group of a monomer to the C=N bond of another molecule gives a dimer of type B2 e.g., dimers 72 and 73 are formed by the acid-catalyzed self-condensation of 2-hydroxy- (or 2-mercapto-)4-methyl-6-phenyl-1,6-dihydropyrimidine97... 

Chloro- and 4-bromopyridine have a great tendency to self-quater-nization, giving l,4 -bound dimers such condensation is caused by acid-catalyzed substitution.192... 

Dihydropyrazines may be prepared by the self-condensation of o -(primary amino)carbonyl compounds and some of these syntheses have been described in Sections II.lA (169, 172, 178, 186, 190), Il.lJ (294-298), and II.IL (306). The conversion of a-amino acids through piperazine-2,5-diones (2,5-dihydroxy-3,6-dihydropyrazines) to pyrazines has been described in Section II.IN (93, 95, 101, 282, 312-314a) and Section II.6 (314, 314a) and ring transformations to 2,5-dihydropyrazines in Section II.9 (302, 505a, cf. 1541). 3-Phenylazirine dimerizes on standing to 2,5-diphenyl-3,6-dihydropyrazine (1577). 

It was assumed that an a-fluorosilyl potassium species was formed initially, and that this subsequently underwent a self-condensation reaction. The eventual product (1) displayed both nucleophilic as well as electrophilic character, which was demonstrated in various derivatization reactions. Although it contains fluorine and potassium atoms in close proximity, the compound displayed a remarkable thermal stability. Even at 80 °C, potassium fluoride elimination occurred only sluggishly. Attempted transmetalation reactions with various metal halides, though, caused an immediate elimination of metal fluoride and the formation of tetrakis(trimethylsilyl)disilene. The latter can be trapped in cycloaddition reactions [5] or, in the absence of trapping reagents, it dimerizes to a cyclotetrasilane (Scheme 2) [6]. 

---