1.2.4- Triazines acid-catalyzed

The replacement of a heterocyclic hydroxyl group (generally in the 0X0 form, Section II,E, 2,e) with thioxo or chloro groups by phosphorus pentasulfide or phosphorus oxychloride presumably proceeds through nucleophilic substitution (frequently acid-catalyzed, 21 and 86) of thiophosphoryloxy and dichlorophosphoryloxy intermediates. The 4-position in pyrimidine is more reactive than the 2-position and, at low temperature, this type of thionation of pyrimidine-2,4-diones is specific for the 4-position. In as-triazine... 

Table III summarizes the parameters that affect Brrfnsted acid-catalyzed surface reactions. The range of reaction conditions investigated varies widely, from extreme dehydration at high temperatures in studies on the use of clay minerals as industrial catalysts, to fully saturated at ambient temperatures. Table IV lists reactions that have been shown or suggested to be promoted by Br nsted acidity of clay mineral surfaces along with representative examples. Studies have been concerned with the hydrolysis of organophosphate pesticides (70-72), triazines (73), or chemicals which specifically probe neutral, acid-, and base-catalyzed hydrolysis (74). Other reactions have been studied in the context of diagenesis or catagenesis of biological markers (22-24) or of chemical synthesis using clays as the catalysts (34, 36). Mechanistic interpretations of such reactions can be found in the comprehensive review by Solomon and Hawthorne (37).

When aldehydes or ketones are used instead of the carboxylic acid derivatives, the corresponding l,2-dihydro-l,3,5-triazines are formed (equation 52). The reaction is acid catalyzed (61MI22000, p. 697). 

Aryl cyanates (cyanic esters), which can be prepared in situ (67AG(E)206), form 1,3,5-triazines under either acid or base catalysis (Scheme 83) (77RCR278, 78RCR975). The acid-catalyzed reactions are not properly understood, but it seems that the mechanism is dependent on the precise nature of the Lewis acid used. Yields are usually good (Table 14). The reaction of aryl cyanates with ketoximines yields 1,3,5-triazines (146) via the intermediate (145) (Scheme 84) (66CB2361). Alkyl cyanates are very reactive, and isomerize readily therefore they are unsuitable starting materials for 1,3,5-triazine synthesis. 

Dihydro-2iT-thiazolo[2,3-c][l,2,4]triazine-3,4-dione rearranges in dilute base to give an unstable acid which decarboxylates on acidification of the sodium salt to give 5,6-dihydrothiazolo[2,3-c]-s-triazole (equation 68) (81CB1200). Kinetic evidence has been put forward in favor of covalently hydrated intermediates in the acid-catalyzed rearrangement of triazolo[4,3-a]pyrazines to 1H-imidazo[2,1 -c]-s-triazoles. The intermediate triazole has been isolated and characterized (equation 69) (72JCS(P2)4). 

An efficient synthesis of l-aryl-4,6-diamino-l,2-dihydro-l,3,5-triazines from an acid-catalyzed reaction between the corresponding arylbiguanidine and carbonyl compounds has been reported <02SC2089>. A new family of functionalized terpyridine-like ligands containing a central 1,3,5-triazine, (8a) and (8b) has been synthesized. These compounds exhibit room-temperature luminiscence as their Ru(II) complexes (9a), (9b) and (9c) <02CC1356>. 

Trepanier DL, Wagner ER, Harris G et al (1966) 1,4,5,6-Tetrahydro-as-triazines I. Sulfuric acid catalyzed condensation of nitriles and hydrazino alcohols. J Med Chem 9 881-891... 

Clay acidity catalyzes hydrolysis of the chloro-s-triazine herbicides to the nonphytotoxic 2-hydroxy-s-triazines (Russell et al., 1968b) ... 

In contrast to alkyl cyanates, alkyl thiocyanates trimerize to provide the 2,4,6-tris(alkylsul-fanyl)-l,3,5-triazines 21 without notable formation of the corresponding isothiocyanu-rates.265 -267,481 In general, the reaction is acid catalyzed to prevent isomerization of the starting alkyl thiocyanate to the corresponding alkyl iso thiocyanate. 

The acid-catalyzed chlorination or bromination of alkyl-substituted 1,3,5-triazines produces exclusively the a-haloalkyl-substituted 1,3,5-triazines. Nuclear halogenation is not observed under the applied conditions. The reactions are preferably conducted in glacial acetic acid in the presence of sodium acetate at a temperature between 55-60CC and with elementary... 

The full scope of the acid-catalyzed one-step conversion of N-tert-bu-tylmethanimine, generated in situ from 1,3,5-tri-tm-butylhexahydro-1,3,5-triazine, to symmetrical 3,5-disubstituted pyridines employing enamines has been detailed.290 The generation of 2,3,5,6-tetrasubstituted pyridines from the thermal reaction of enolizable ketones with hex-amethylphosphoric triamide has been proposed to proceed by in situ generation and subsequent [4 + 2] cycloaddition of /V-methyl-1 -aza-1,3-butadienes with intermediate dimethylamino enamines.30... 

The first method is by closure of the triazine ring. Intermolecular acid catalyzed Diels-Alder cyclization of (92) gave (93) (Equation (8)) <75CPB289l>. 1-Ureidopyrroles (94) cyclized under basic conditions to give the dioxo compounds (95) (Scheme 2), which were readily decarboxylated at 280°C to the dione (96) <79JHC833>. 

The 1,2,4-triazine homo C-nucleosides prepared so far are the 1,2,4-triazin-6-yl type. Acid-catalyzed dehydrative cyclization products of the polyhydroxyalkyl chains of 6-(alditol-l-yl)l,2,4-triazines (1065) (Section XXX,D) were first assigned the corresponding 6-()3-glycofuranosyl)-l,2,4-... 

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

Base-catalyzed Dimroth rearrangements of l-aryl-4,6-diamino-1,3,5-triazines or triflic acid-induced decomposition of 6-anilino-1,3,5-triazines led to the formation of the corresponding substituted 1,2-dihydro-l,3,5-triazines, whose structure was established on the basis of H NMR spectra (93JHC849). 

Surface-catalyzed degradation of pesticides has been examined in the context of research on contaminant-clay interactions. Such interactions were observed initially when clay minerals were used as carriers and diluents in the crop protection industry (Fowker et al. 1960). Later specific studies on the persistence of potential organic contaminants in the subsurface defined the mechanism of clay-induced transformation of organophosphate insecticides (Saltzman et al. 1974 Mingelgrin and Saltzman 1977) and s-triazine herbicides (Brown and White 1969). In both cases, contaminant degradation was attributed to the surface acidity of clay minerals, controlled by the hydration status of the system. 

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