Trimerization of imidates

Trimerization of imidates is a valuable route to 1,3,5-triazines. Imidates can be considered as activated nitriles and cyclotrimerize more readily. Most symmetrical 2,4,6-trialkyl-1,3,5-triazines are easily formed, although large alkyl substituents may give rise to steric hindrance (61JOC2778). Symmetrical isocyanurates (525) are readily available from isocyanates, RNCO catalysts include tertiary amines, phosphines and sodium methoxide. Aldehydes RCHO and ammonia give hexahydro-1,3,5-triazines (526), known as aldehyde ammonias (73JOC3288). 

Probably the cyclotrimerization of nitriles is the best known route to 1,3,5-triazines. The reaction has the obvious limitation that it is of value for preparing the symmetrical derivatives only. Nevertheless, many important triazines, such as cyanuric chloride, are made in this way. There are a number of other cyclotrimerization reactions which are also useful, in particular the trimerization of imidates. An easy route to 1,3,5-triazine from ammonium acetate has been developed. 

Substituted amidines are of limited use in the synthesis of symmetric 1,3,5-triazines. Only formamidine or amidines bearing strongly electron withdrawing groups react readily. The proposed mechanism (59JA1466) resembles that of the trimerization of imidates (Scheme 87). 

This reaction has been extended to the trimerization of imidates, and the reaction between amidine salt and imidate. In addition, the reaction between amidrazone and Q, -dicarbonyl compounds has been used for the preparation of a -triazines or 1,2,4-... 

The key to the success of this process is a combination of specific catalysts and optimum reaction conditions. Effective catalysts for trimerization of aromatic nitriles are listed in Table I (4 ). Optimum reaction conditions for processing the aromatic nitrile-modified imide precursors depend on the chemical structure and characteristic property of the individual precursor of concern. In general, yield of the polymeric products increases with the increase of reaction temperature, pressure, time, and concentration of catalyst within the range of practical experimental limits (5 ). 

Aromatic nitriles can be thermally selectively polymerized in pyrrolidin-2-one at temperatures above 200 C. Between 200 and 350 °C trimerization to 2,4,6-triaryl-l,3,5-triazines occurs, while at temperatures above 300 °C imidization occurs resulting in nonmelting, linear polyaryl-carbimine. Mixtures of trimers and polymeric imines are separable by sublimation or extraction with aromatic hydrocarbons.180 Since aryltriazines and polymeric aryltriazines have various industrial applications, several catalytic systems have been developed for the trimerization of aromatic nitriles and dinitriles, e.g. metal chlorides,181 iron(II) or iron(lll) cyanide,182 cop-per(II) carbonate,183 and molten zinc(II) chloride.184... 

Trimerization of nitriles, isocyanates, isothiocyanates, imidates, and carbodiimides all lead to symmetrical 2,4,6-trisubstituted 1,3,5-triazines (see Section 6.12.9.5). The use of lanthanide trifluoromethanesulfonate and ammonia as cocatalysts is claimed as a big improvement. The trisaminal of 2,4,6-triformyl-l,3,5-triazine is also useful for further derivatization to unusual structures (see Section 6.12.7.1). Treatment of a 1 1 pyridine/conc. ammonia solution of an aromatic aldehyde with excess Fremy s salt is another development. Separation of the amide coproduct was claimed to be easy. The synthesis fails with aliphatic aldehydes (see Section 6.12.9.5.4). Aminolysis of 2,4,6-triaryl-1,3,5-oxadiazinium salts gives symmetrical 1,3,5-triazines but the reactions are limited in that electron-withdrawing groups in the aromatic rings lead to instability and difficulty in separation of products (see Section 6.12.10.4). 

The imide from which the salt (LVI) is derived is the oxygen analog of the imide corresponding to the silver salt (XLVII). Its probable mode of formation entails initially the loss of ammonia and HOSN from two molecules of HN (SONH2)2. The resulting trimeric thionyl imide then tends to form its anhydride... 

Moreover, it has been shown (Imamura et al. 1995a) that such rare-earth imide or imide-like species exhibit oligomerization activity of alkynes. Selective cyclic dimerization and trimerization of propyne and ethyne to cyclohexadiene and benzene occur at 453 K during the oligomerization, respectively, in which the active catalysts are characterized as rare-earth imides induced by the thermal treatment of R/C. 

The purpose of the present investigation was to study the trimerization of aromatic nitriles under the conventional resin/fiber composite fabrication conditions using p-toluenesulfonic acid as a catalyst. Trimerization parameters investigated included reaction temperature, pressure, time, and concentration of catalyst. The influence of the nature of aromatic nitriles on trimerization was also studied. Also presented are preliminary results on the use of the catalytic trimerization of the nitrile-terminated imide oligomers to fabricate graphite fiber reinforced composites. 

Phenyl azide reacts with 2//-l,2,3-diazaphospholes with loss of nitrogen to give a cyclic trimer (85) of the diazaphosphole P-imide which is suggested as an intermediate (Scheme 27) <81IZV1600, 90ZOB35>. 

Fig. 9. (a) An isolated, flat (iminoLi)3 trimeric ring (one imide ligand shown) (b) a representation of how two such rings associate, giving a hexamer. 

Base-catalyzed trimerizations are facile also for example, trifluoromethyl cyanide trimer-izes in the presence of ammonia, presumably through the formation of the amidine intermediate (139 Scheme 77) (67JOC231). Similarly, perfluoro-n-propyl cyanide forms the 1,3,5-triazine in the presence of sodium methoxide, probably via the imidate (140 Scheme 78) (52JA5633). 

Methyl isocyanate and all isocyanic acid esters are an interesting and highly reactive class of organic compounds, since the isocyanate group (-NC0) reacts readily with a wide variety of compounds as well as with itself to form dimers, trimers, ureas, and carbodi-imides. Methyl isocyanate (MIC) is an intermediate in the preparation of carbamate pesticides and conceivably could be applied to the production of special heterocyclic polymers and derivatives. 

The dimers have a four-membered A12N2 ring (6-XV) which may have isomers if the groups are different. The aluminum imides [RA1(NR )] usually have cubane or larger cage structures (6-XVI) trimers (e.g., the alumazene 6-XVII) and heptamers have also been found, depending on the size of R and R and the reaction conditions.29... 

Considerable effort has been expended to synthesize trialkyl-substituted 1,3,5-triazines by cyclotrimerization of alkyl cyanides,119-127137 unfortunately with little success. The reactions require high pressures and high temperatures, and in many cases alcohols are used as solvents and catalysts.l3s 141 An intermediate aliphatic imidate 2 is postulated, which trimerizes to the substituted 1,3,5-triazine 3. 

Especially in the case of unstable free imidates, the corresponding hydrochlorides can be utilized for the trimerization reaction. Additional bases, e.g. sodium hydroxide,6 sodium acetate,280 A.A-diethylaniline,281 or tributylamine,282 can be used to neutralize the hydrogen chloride. 

Apparently less basic amidines favor this interchange. The newly generated imidate competes with the original imidate in the trimerization reaction and thus results in the formation of alternative unsymmetrically substituted 1,3,5-triazines. 

Cyanuric chloride can be considered as a trimeric imide chloride, the chlorine atoms of which can easily be substituted by nucleophilic reactants, such as e.g. alcohols, phenols, mercaptans, thiophenols and amines. 

---