Isocyanate dimerization with amines

Dihydro-2//-l,3,5-thiadiazines containing a ring carbonyl or thiocarbonyl group are synthesized from reaction of thioamides with phenoxycarbonyl isocyanate, dimerization of thiocarbamoylisothiocyanates, or dimerization of carbamoyl isocyanates. The latter two [4-1-2] cycloaddition reactions complement the tabulated list of dienes and dienophiles presented in Table 2 (Section 9.09.9.2). 3,6-Disubstituted-3,4-dihydro-2//-l,3,5-thiadiazines are synthesized by treatment of N-substituted A, Wbis(l//-l,2,3-benzotriazol-l-ylmethyl)-amines with thioamides and zinc bromide (Section 9.09.9.1.3). 

The cyclic dimer of dibenzylcarbodiimide was isolated in low yield from the distillation residue . Diazetidines 4 are also obtained in the reaction of 1,2,4-triazineiminophosphoranes 3 with aryl isocyanates (yields 55-76 Reaction of the diazetidines with amines affords pentasubstituted biguanides. ... 

In the case of thiazoline-2(3//)-thiones, the mesoionic thiazolo[2,3-h][l,3,4]thiadiazoles are obtained by two different routes (Scheme 65). On the one hand, thione 166 reacts with isothiocyanate via intermediate 167 and with a second equivalent isothiocyanate to afford the mesoionic 168 on the other hand, in the presence of isocyanate, the thione preferentially dimerizes 167 with the open-chain carbodiimide 169 to give the mesoionic 170. Addition of acid with removal of an amine group converts 170 into the symmetric heteroaromatic amine (171) (88CB1495 92T1285). The related transformation of an imidazoline into 1,3,4-thiadiazoles has also been described (90T4353). 

The first examples of urea-containing dimeric capsules were discovered independently by Rebek and Bohmer [165,166]. These are based on calix[4]arene ethers, in which the lower rim ether units help to fix the calixarenes in a cone conformation via intramolecular interactions. Figure 60 shows the generic structure of such systems. Synthetically, the urea calixarenes are readily prepared. The calixarenes are first nitrated, followed by a reduction of these groups into amines. The urea derivatives are fixed to the upper rim by reaction of the p-amino calixarenes with isocyanates. Many systems were studied using differently substituted ureas which contain either short alkyl chains or simple phenyl derivatives. Studies also involved the changing of the lower rim ether substituents. 

This dimer reacts as two molecules of free 5- alkyli-soxazolyl-3-isocyanate with secondary amines or with alcohols to give the corresponding ureas or carbamates useful as agrochemicals or fine chemicals intermediates. For example, 3-amino-5-tert-butyl isoxazole is a key intermediate for 3- 5-tert-butylisoxazolyl)-1,1-dimethyl urea (common name Isouron) which is useful as a herbicide for sugar cane and other crops (Ref. 180) [Scheme 127]. 

One of the major fields of isocyanate catalysis is pol3mierization. The formation of cyclic dimers and trimers from aryl isocyanates was established over 100 years ago by Hofmann (8). This early work has been reviewed by Saunders (1). Dimers are formed by aryl isocyanates at room temperature in the presence of certain amines or phosphines. Trimeriza-tion occurs in the presence of bases such as potassium acetate. Linear polymerization has been recently reported by Shashoua and co-workers at lower temperatures in polar solvents with an anionic catalyst such as metallic sodium (9, 10). 

The presently proposed mechanism of synergism implies that ligands other than amine which can coordinate with the tin ion should also synergize its catalytic activity toward alcohol-isocyanate reactions. This apparently is the case for triphenyl phosphine-DBTDL combination. When triphenyl phosphine was added to DBTDL, it accelerated the rate of reaction of isocyanate with alcohol and water (Figure 6). Although triphenyl phosphine is known to dimerize isocyanates, (12), the dimerization... 

The rate of self-polymerization of isocyanates to dimers (uretidine diones) depends upon the electronic or steric influences of ring substituents. Ortho substitution greatly retards dimerization of the NCO groups, with the ortho NCO slower to dimerize. This dimerization is catalyzed strongly by trialkyl phosphines (38, 39) and more mildly by tertiary amines, such as pyridine (40. 41). MDI dimerizes slowly on standing at room temperature even without catalysts but is stable at low or at slightly elevated temperatures (40-50 °C). 

Figure 22.1 Basic reactions of isoq nate with different reactants (a) hydroxyl compound to give urethanes, (b) amines to give ureas, (c) water to form an unstable compound, carbamic acid, which decomposes and produces an amine, (d) amines and excess isocyanate produces a urea linkage, (e) urea linkage and one molecule of isocyanate produces biuret, (f) urethane and asocyanate produce allophanates. Secondary reactions of isoc3canate self-condensation produce (g) uretidione ring (dimer-), (h) isocyanurate (trimer-), (j) carbodiimide (reprinted with permission of Chattopadh3ray et al., 2007, Elsevier [15]).

Some isocyanates also react with themselves to form thermally reversible dimer structures, the so-called uretidinediones. Such self reaction is apparently confined to aromatic isocyanates, and is illustrated with phenyl isocyanate in Fig. 8.4. The dimerization reaction is catalyzed vigorously by trialkyl phosphines and less by tertiary amines such as pyridine. 

Isocyanates also react with each other to form dimers (metdiones) and trimers (isocyanurates). Formation of metdiones is catalyzed by phosphines. Formation of isocyanurates is catalyzed by quaternary ammonium compounds trimerization of aromatic isocyanates is catalyzed by tertiary amines. Uretdiones decompose thermally to regenerate isocyanates and are used as blocked isocyanates. Isocyanurates are stable and isocyanurates of diisocyanates are extensively used as multifunctional isocyanates. 

The thermal decomposition, combustion, and fire retardancy of PU materials have been reviewed. " Upon heating, the polymer decomposes primarily by regenerating the precursor moieties polyols, isocyanates, and amines. Complete volatilization of the resulting products is prevented by the formation of thermally stable isocyanurate rings (produced by trimerization of isocyanates) and the formation of substituted ureas (produced by the reaction of hydroxy compounds with the carbodiimides generated by dimerization of isocyanates). ... 

In the reaction of chlorocarbonyl isocyanate with secondary amines, allophanoyl chlorides are formed, which are readily dehydrochlorinated to give dialkylaminocarbonyl isocyanate, which dimerizes by a [4+2] cycloaddition reaction to give 481 ". ... 

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