Isocyanates reaction with cyclic anhydrides

Condensations. Reaction of tertiary alcohols with cyclic anhydrides, Baylis-Hillman reaction, aldol reaction involving bis(trimethylsilyl) ketene acetals, reaction of R(XIH(CN)2 with activated imines, and the [2+2]cycloaddition between isocyanates and enol ethers are assisted by high pressure. 

The preparation of imides from reaction of isocyanates with anhydrides dates back to the early days of organic chemistry [96]. With the advent of polyimide chemistry in the early 1960s, this chemistry was soon explored for the synthesis of polyimides. However, in contrast to the preparation of polyimides via thejr poly(amic acid) intermediate, the reaction of aromatic dianhydrides with aromatic diisocyanates is much less understood. The reaction of aromatic dianhydrides with aliphatic or aromatic diisocyanates is believed to form a cyclic seven-membered intermediate which then splits out C02 to form the polyimide [97], see Scheme 27. The addition of water, which has been reported to accelerate the anydride/isocyanate reaction, can result in several transformations of either the anhydride or the isocyanate reagent, see Scheme 28... 

Isocyanates have some important reactions without the participation of active hydrogen compounds. These reactions, of real importance in polyurethane chemistry are dimerisation, trimerisation, formation of carbodiimides and reaction with epoxides and cyclic anhydrides. 

Independent conHimation of the plausibility of this reaction was demonstrated by the addition of an equal molar amount of the dianim of o-sulfobenzoic acid to the anhydride giving the same linear benzoic anhydride. This result points out that attack of the caiboxylate is competitive with carbamate attack on the cyclic anhydride. The generation of linear anhydride does not appear to affect the production of isocyanate. 

Resins obtained using alternative means to phenyl isocyanate for blocking the hydroxyl functionality of a sucrose "penta" ester, e.g. reaction with aluminium diisopropoxide monoacetylacetonate or cyclic anhydride/ epoxide also show promise. 

Initially, water can cause the hydrolysis of the anhydride or the isocyanate, Scheme 28 (reaction 1 and 2), although the isocyanate hydrolysis has been reported to occur much more rapidly [99]. The hydrolyzed isocyanate (car-bamic acid) may then react further with another isocyanate to yield a urea derivative, see Scheme 28 (reaction 3). Either hydrolysis product, carbamic acid or diacid, can then react with isocyanate to form a mixed carbamic carboxylic anhydride, see Scheme 28 (reactions 4 and 5, respectively). The mixed anhydride is believed to represent the major reaction intermediate in addition to the seven-mem bered cyclic intermediate, which upon heating lose C02 to form the desired imide. The formation of the urea derivative, Scheme 28 (reaction 3), does not constitute a molecular weight limiting side-reaction, since it too has been reported to react with anhydride to form imide [100], These reactions, as a whole, would explain the reported reactivity of isocyanates with diesters of tetracarboxylic acids and with mixtures of anhydride as well as tetracarboxylic acid and tetracarboxylic acid diesters [101, 102]. In these cases, tertiary amines are also utilized to catalyze the reaction. Based on these reports, the overall reaction schematic of diisocyanates with tetracarboxylic acid derivatives can thus be illustrated in an idealized fashion as shown in Scheme 29. 

Succinimides and maleimides are accessible via processes analogous to those described for the corresponding anhydrides (see Section 9.4.3.3). Reactions of (COD)2Ni° with either alkenes or alkynes and isocyanates (i.e. heterocumulene analogs of CO2) give azametallacycles that upon carbonylation yield cyclic imides. With alkenes high yields of metallacycles are formed m both simple isocyanates as well as a,(o-diisocyanates. Carbonylation yields are variable, however (Scheme 18). Alkynes give lower metallacycle yields but high yields of die final imide (Scheme 19). ... 

Cyclic carbonyl ylides, formed from diazo amides or diazo anhydrides through intramolecular carbene addition to the carbonyl group, react with the triple bond of a dipolarophile to produce bicyclic adducts. The latter undergo a retrodiene reaction, splitting off an alkyl isocyanate or carbon dioxide to give furan derivatives. 

The dienophilic properties of vinylene carbonate (1,3-dioxol-2-one, 6) have been repeatedly exploited for the synthesis of cyclic polyols. Recently a number of ribofuranose derivatives have been synthesized starting from 6. The known adduct 7, obtained from 6 and furan, was c/r-hydroxylated, and after acetonation was hydrolyzed with base and cleaved with permanganate to give the dicarboxylic acid 8. The corresponding anhydride 9 gave, on treatment with trimethylsilyl azide, the isocyanate 10, which in turn was converted into the carbonate 11 on addition of methanol. The monoester 12, obtained in the reaction of anhydride 9 with isopropanol, was effectively resolved into enantiomers by the use of brucine or (/ )-l-(2-naphthyl)ethylamine. ... 

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