Aryl isocyanates dimerization reactions

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Reportedly, simple alkyl isocyanates do not dimerize upon standing. They trimerize to isocyanurates under comparable reaction conditions (57). Aliphatic isocyanate dimers can, however, be synthesized via the phosgenation of A[,A[-disubstituted ureas to yield /V-(ch1orocarhony1)ch1oroformamidine iatermediates which are subsequendy converted by partial hydrolysis and base catalyzed cycUzation. This is also the method of choice for the synthesis of l-alkyl-3-aryl-l,3-diazetidiones (mixed dimers of aromatic and aUphatic isocyanates) (58). 

General syntheses of 1,3-diazetidines 200 are [Z + Z] cycloadditions of C=N-containing substrates (Scheme 95) . The dimerization of aryl isocyanates 199 to l,3-diarylazetidin-2,4-diones is one of the classical methods for the synthesis of 1,3-diazetidinones. Carbodiimides 201 undergo [Z + Z] cycloaddition reactions to... 

Reaction of 2-nitrobenzyl triphenylphosphonium ylide 745 with aryl isocyanates afforded 2-aryl-2//-indazoles 746 (Equation 159) <2000TL9893>. Reductive cyclization of o-nitroketoximes 747 in the presence of catalytic iron dimer in dioxane under a carbon monoxide atmosphere furnished l//-indazoles 748 (Equation 160) <2004H(63)373>. 

Arylimido)vanadium(V) complexes have been prepared from the reaction of VOCl3 with various /wa-substituted aryl isocyanates. The reactivity and structure of some of these have been investigated (78).431 The reactivities of a series of vanadium(V) chalcogens have been reported. The structure of [(Me3Si)2N]2V(Se)[SeSi(SiMe3)3] 432 (79) shows that the V=Se double bond contracts by 8% compared to the single bond in a related dimeric vanadium(IV) structure (vide infra). The... 

Aryl isocyanates readily polymerize in the presence of catalysts into dimers containing 2 moles of the monomeric isocyanate. The reaction and structure is generally considered to be ... 

Dimer (151) is opened and stabilized by pyridine and S4N4- Sulphur trioxide gives (152) with A -(sulphinyl)trifluoromethanesulphonamide. Both (151) and (152) react with benzonitrile to give (153). With aryl isocyanates, they give (154). Both 1,2,3,4-dithiadiazetidines and 1,3,2,4-dithiadiazetidines have been proposed - s reaction intermediates. 

The ketene dimer, 249, reacts with isocycanates in the presence of BF3 to give the oxazinedione 250 and the amide 251. However, reaction of 249 with aryl isocyanates, in the presence of triethylamine, affords the pyridine derivatives 252. ... 

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. ... 

N-Substituted amides can be prepared by direct attack of isocyanates on aromatic rings.The R group may be alkyl or aryl, but if the latter, dimers and trimers are also obtained. Isothiocyanates similarly give thioamides. The reaction has been carried out intramolecularly both with aralkyl isothiocyanates and acyl isothiocyanates.In the latter case, the product is easily hydrolyzable to a dicarboxylic acid this is a way of putting a carboxyl group on a ring ortho to one already there (34 is... 

Ketenes, which are even more reactive than isocyanates, afford ketenimines at or below room temperature [62CRV247 84JOC2688 89JCS(P1)2140]. At elevated temperatures, dimerization or polymerization occurs (21HCA887). Although N-aryl- and A-vinyliminophosphoranes react smoothly with ketenes, strong acceptor substituents on the nitrogen hamper the reaction thus V-acyliminophosphoranes do not react with ketenes. Vinylketenimines such as 2-aza-l,3-dienes prepared in this way from... 

The P-alkyl iminophosphoranes are considerably more reactive than the P-aryl compounds, but iminophosphoranes with alkoxy or chloro groups are also known to participate in the reaction. N-alkyl- or N-aryltrichloroiminophosphoranes are usually obtained as four membered ring dimers, which on thermolysis generate the monomers. For example, thermolysis of the dimeric N-methyltrichloroiminophosphorane 63 in o-dichlorobenzene in the presence of phenyl isocyanate affords N-methyl-N -phenylcarbodiimide 64 in 57 % yield. 

C-H bond activation was also involved in the reactions of 2-arylpyridines with isocyanates promoted by the dimeric precursor [ RuCl(p-Cl)(ri -p-cymene))2] (Scheme 36) [143]. The process was satisfactorily apphed to the synthesis of a wide range (22 examples) of pyridyl-functionalized benzamides. Arylpyridines with both electron-donating and electron-withdrawing substituents were suitable substrates, and classical organic functional groups such as ether, halogen, aldehyde, or cyano were tolerated. Notably, when the aryl ring presented an unsymmetrical substitution pattern, the C-H activation took place at the less hindered position selectively. 

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