Carbodiimides dimerization

Diazetidine-2,4-diimines, carbodiimide dimers, formation, reactions, 59, 133, 148 ... 

Salts of aliphatic carbodiimide dimers are also obtained in the reaction of carbodiimides with dimethyl sulfate. The cyclic dimer of dibenzylcarbodiimide, mp 102-103 °C, was isolated in low yield from the distillation residue of the monomer. The crystal structure of l,3-dicyclohexyl-2,4-bis(cyclohexylimino)-l,3-diazetidine, the cyclodimer of DCC, is... 

Salts of aliphatic carbodiimide dimers are also obtained in the reaction of carbodiimides with dimethyl sulfate. 

Treatment of UCI4 with the lithium complex obtained from dicyclohexylcar-bodiimide followed by crystallization from pyridine afforded a dinuclear uranium(rV) oxalamidinate complex in the form of dark green crystals in 94% yield (Scheme 191). The same compound could also be obtained by first reducing UCI4 to LiUCli (or UQs+LiCl) followed by reductive dimerization of di(cyclo-hexyl)carbodiimide as shown in Scheme 191. The molecular structure of this first oxalamidinato complex of an actinide element is depicted in Figure 31. ° ... 

Reaction of Ph2PLi with Pr N = C = NPr in THF proceeds via insertion of the carbodiimide into the Li-P bond, affording the lithium phosphaguanidinate salt Li[Ph2PC(NPr )2] in 72% yield. The preparation and reactivity of this new ligand are summarized in Scheme 217. An X-ray crystal structure analysis of the product obtained after removal of the solvent from the reaction mixture revealed the presence of a mono-solvated, centrosymmetric dimer in the solid state (Figure... 

The drying and weighing of triethylamine hydrochloride should be carried out only after N-sulfinylaniline has been added to the solution of benzonitrile oxide, as otherwise the latter, not I)eing very stable in the free state, would dimerize resulting in the reduction in yield of the carbodiimide. 

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 synthetic route to pyrido[l,2-a][l,3,5]triazines (307) is exemplified by reacting A -(2-pyridyl)iminophosphorane (306) and diphenylcarbodiimide. This reversible transamination to a mixed carbodiimide is shown in Scheme 109. Excess diphenylcarbodiimide function as a reaction partner, as dimerization seems to be sterically hindered (77ZC371). 

General syntheses of 1,3-diazetidines are [2 + 2] cycloadditions of C = N-containing substrates (Scheme 26). Dimerization of imines and isocyanates furnishes substituted 1,3-diazetidines. Cycloaddition of isocyanates to carbodiimides and isocyanates with imines are alternative syntheses,... 

The formation of isocyanurates in the presence of polyols occurs via intermediate allophanate formation, i.e, die urethane group acts as a cocatalyst in the dimerization reaction. By combining cyclotrimeiization with polyurethane formation, processibility is improved, and the friability of the derived foams is reduced. Modification of cellular polymers by incorporating amide, imide, oxazolidinone, or carbodiimide groups has been attempted but only the urethane-modified isocyanurate foams are produced in the 1990s. PUIR foams often do not require added fire retardants to meet most regulatory requirements. A typical PUIR foam formulation is shown in Table 2. 

The dimerization and trimerization of isocyanates are special cases of the cycloaddition reaction in that they involve reagents of the same type. The uncatalyzed carbodiimidization of isocyanates likely involves a labile 2 + 2 cydoadduct (12) which liberates carbon dioxide. 

Asymmetric aryl isocyanate dimers, in which the C=0 group of one molecule reacts with the C=N group of another, have been postulated as labile intermediates in the formation of carbodiimides (17) upon heating isocyanates. 

Carbodiimides undergo [2+2] cycloaddition reactions to furnish l,3-disubstituted-2,4-bisalkyl or arylimino-l,3-diazeti-dine. For example, dibenzyl carbodiimides undergo dimerization to yield l,3-diazetidin-2,4-diimines 296 on heating. The reaction can be catalyzed by the addition of tributylphosphine (1%) (Equation 36) <1940CB1114, 1968CB174>. 

So as to get the strongest possible basicity, these biguanides were prepared with the highest possible degree of N-alkylation. However, no satisfactory precedure was described in the literature. So original synthetic methods were devised for this purpose. These rely essentially on the addition of guanidines to carbodiimides or of amines to cationic dimeric carbodiimides.18... 

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... 

Protected neamine dimers were prepared using a Quest 210 parallel synthesizer. 5-Ethylcarboxyl-1,3,2, 6 -tetraazido-6,3, 4 -tri-6>-benzylneamine (0.0826 mmol/tube was dissolved in 1.5ml/tube containing CH2C12 distributed to each tube, which was then treated with MP-carbodiimide resin (1.15mmol/g) followed by 1,12-dodecylamine (0.0413 mmol/tube). Solutions were agitated 16 hours, filtered, concentrated, and dimers isolated as colorless foams. 

Heating of isocyanates above 150 °C slowly produces carbodiimides. For example, heating of hexamethylene diisocyanate at 189-195 °C for 20 hr produced 4-6 % of oligomeric isocyanate terminated carbodiimides, but in addition 18-20 % of isocyanate terminated isocyanurates were formed. The reaction is facilitated if a slow stream of nitrogen is passed through the boiling isocyanate. The unsymmetrical isocyanate dimer 47 was proposed as an intermediate in this transformation. 

Heating of N-2-fluorophenyltrichloroiminophosphorane dimer generates the monomer, which is an active carbodiimide metathesis catalyst. ... 

Carbodiimides undergo cyclooligomerization reactions. In this regard they are similar to isocyanates, the mono imides of carbon dioxide. For example, aliphatic carbodiimides undergo rapid dimerization catalyzed by tetrafluoroboric acid at room temperature to give salts of the cyclodimers 183. Neutralization with dilute sodium hydroxide, or better filtration through basic AI2O3, afford l,3-dialkyl-2,4-bisalkylimino-l,3-diazetidines 184. ... 

In the mono P-substituted carbodiimide 185 dimerization occurs across the alkyl substituted C=N bond to give the expected cyclodimer 186. ° ... 

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