Glyoxal, reactions

Polyquinoxalines (PQ) have proven to be one of the better heat-resistant polymers with regard to both stabiUty and potential appHcation. The aromatic backbones are derived from the condensation of a tetramine with a bis-glyoxal, reactions first done in 1964 (61,62). In 1967, a soluble, phenylated version of this polymer was produced (63). The chemistry and technology of polyquinoxalines has been reviewed (64). 

Unsubstituted pyrazino[2,3-d]pyridazine (14) is prepared by the reaction of 4,5-diaminopyridazine (216) with glyoxal. Reaction of this starting pyridazine with pyruval-dehyde produces the methyl substituted product (217) (66JHC512). Similarily, pyrazino-[2,3-d]pyridazin-5-one (218) can be prepared by the reaction of 4,5-diamino-3-pyridazinone with glyoxal (69JHC93). 

Figure IV-D-1. Arrhenius plot for the rate coefficients for the OH + glyoxal reaction. Feierabend et al. (2008) used two different sources of OH (a) H2O2 photolysis, (b) N2O photolysis in the presence of CH4.

Glyoxalate, the product of chemical hydrolysis of allantoin and allantoic acid, is most frequently measured after reaction with diphenylhydrazine. The most commonly used procedure to measure ALN and ALA involves add or base hydrolysis to produce glyoxalate, reaction with phenylhydrazine at neutral pH to produce the phenylhydrazone of glyoxalate, and oxidation of the phenyl-hydrazone with ferricyanide and strong add to form the diphenylformazan (Vogels and Van der Drift, 1970). The colored l,S-diphenylformazan (shown below) formed absorbs strongly between 520 and 540 nm with a molar extinction coefficient of 51,0(X) (Matsui et al, 1965). 

Oxidation. Acetaldehyde is readily oxidised with oxygen or air to acetic acid, acetic anhydride, and peracetic acid (see Acetic acid and derivatives). The principal product depends on the reaction conditions. Acetic acid [64-19-7] may be produced commercially by the Hquid-phase oxidation of acetaldehyde at 65°C using cobalt or manganese acetate dissolved in acetic acid as a catalyst (34). Liquid-phase oxidation in the presence of mixed acetates of copper and cobalt yields acetic anhydride [108-24-7] (35). Peroxyacetic acid or a perester is beheved to be the precursor in both syntheses. There are two commercial processes for the production of peracetic acid [79-21 -0]. Low temperature oxidation of acetaldehyde in the presence of metal salts, ultraviolet irradiation, or osone yields acetaldehyde monoperacetate, which can be decomposed to peracetic acid and acetaldehyde (36). Peracetic acid can also be formed directiy by Hquid-phase oxidation at 5—50°C with a cobalt salt catalyst (37) (see Peroxides and peroxy compounds). Nitric acid oxidation of acetaldehyde yields glyoxal [107-22-2] (38,39). Oxidations of /)-xylene to terephthaHc acid [100-21-0] and of ethanol to acetic acid are activated by acetaldehyde (40,41). 

Reaction with Other Aldehydes. Polyacrylamide reacts with glyoxal [107-22-2], C2H2O2, under mild alkaline conditions to yield a polymer with pendant aldehyde fiinctionahty. 

Garbostyrils. Carbostydls such as (14) [33934-60-0] are prepared by the reaction of 2-alkylainino-4-iiitrotoluene with ethyl glyoxalate in the... 

Examples include acetaldehyde, CH CHO paraldehyde, (CH CHO) glyoxal, OCH—CHO and furfural. The reaction is usually kept on the acid side to minimize aldol formation. Furfural resins, however, are prepared with alkaline catalysts because furfural self-condenses under acid conditions to form a gel. 

A less important glyoxal resin is tetramenthylolglycolutil [5395-50-6] (tetramethylolacetylenediurea) produced by the reaction of 1 mol of glyoxal with 2 mol of urea, and 4 mol of formaldehyde. 

A variation involves the reaction of benzylamines with glyoxal hemiacetal (168). Cyclization of the intermediate (35) with sulfuric acid produces the same isoquinoline as that obtained from the Schiff base derived from an aromatic aldehyde and aminoacetal. This method has proved especially useful for the synthesis of 1-substituted isoquinolines. 

Ammonium cyanide [12211-52-8] NH CN, a colorless crystalline soHd, is relatively unstable, and decomposes into ammonia and hydrogen cyanide at 36°C. Ammonium cyanide reacts with ketones (qv) to yield aminonitriles. Reaction of ammonium cyanide with glyoxal produces glycine. Because of its unstable nature, ammonium cyanide is not shipped or sold commercially. Unless it is kept cool and dry, decomposition releases vapors and forms black hydrogen cyanide polymer. 

Tetrakis (4-hydroxyphenyl)ethane is prepared by reaction of glyoxal with phenol in the presence of HCl. The tetraglycidyl ether [27043-37-4] (4), mp ca 80°C, possesses a theoretical epoxide functionaUty of four with an epoxy equivalent weight of 185—208 (4). 

Most of the compounds in this class have been prepared from preexisting crown ether units. By far, the most common approach is to use a benzo-substituted crown and an electrophilic condensation polymerization. A patent issued to Takekoshi, Scotia and Webb (General Electric) in 1974 which covered the formation of glyoxal and chloral type copolymers with dibenzo-18-crown-6. The latter were prepared by stirring the crown with an equivalent of chloral in chloroform solution. Boron trifluoride was catalyst in this reaction. The polymer which resulted was obtained in about 95% yield. The reaction is illustrated in Eq. (6.22). 

Mechanistically, the reaction of diketosulfides and glyoxal likely proceeds via an initial aldol reaction to provide 22. A second intramolecular aldol reaction and the elimination of two equivalents of water produce the thiophene 23. The timing of the elimination reactions and the ring-closing, carbonyl condensation reaction is not completely understood. However, 2,5-disubstituted thiophenes 23 are available in good yields via this process. 

The reaction of diketosulfides with 1,2-dicarbonyl compounds other than glyoxal is often not efficient for the direct preparation of thiophenes. For example, the reaction of diketothiophene 24 and benzil or biacetyl reportedly gave only glycols as products. The elimination of water from the P-hydroxy ketones was not as efficient as in the case of the glyoxal series. Fortunately, the mixture of diastereomers of compounds 25 and 26 could be converted to their corresponding thiophenes by an additional dehydration step with thionyl chloride and pyridine. 

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