Uretidinediones

Uretidinedione — see l,3-Diazetidine-2,4-dione 2-Uretidinone, 1,3-diphenyl-4-(phenylimino)-synthesis, 7, 474 Urgothianine, 5, 498 Urie acid... 

Aqueous acid treatment of uretidinediones leads to formation of ureas (B-67MI51500) labile groups on nitrogen can be removed with anhydrous HC1 with subsequent ring opening (Scheme 26) (66CB3103). 

Simple uretidinediones, as expected, are attacked at the carbonyl with ultimate formation of urea derivatives (Scheme 37) (71JOC3056). More complex derivatives react similarly, although subsequent reaction on labile substituents also occurs (69M1860, 66LA(692)151, 62CJC935, 53JA5439). 

Dialkylcarbodiimides undergo [2 + 2] additions with chlorosulfonylisocyanate at least to some extent. The other product is a triazinedione (2 eq. of CSI with 1 eq. of carbodiimide) and is the exclusive product with diphenylcarbodiimide (77JCS(P1)47). The S02C1 group can be removed with KI, and if careful hydrolysis of the imine could be effected, it might be an entry to monoalkylated uretidinediones (Scheme 52). 

The conversion of carbodiimides to allophanyl chlorides as illustrated in Scheme 68 appears to be a general synthesis of uretidinediones and the method of choice for dialkyl derivatives (78JOC4530). 

Another procedure for preparation of allophanyl chlorides from isocyanates and subsequent cyclization in 35-95% yield to uretidinediones has also been reported (70AG(E)372). 

Cyclohexylamine is reported to react with formaldehyde giving a 60% yield of 1,3-diazetidine. One additional example reports a photochemical closure of iV-cyclohexylben-zaldehydeimine (68JA1666). An excellent review covers much of the earlier work on dimerizations of isocyanates (69ACR186). Aromatic isocyanates readily dimerize to uretidinediones (l,3-diazetidine-2,4-diones) when catalyzed by trialkyl phosphites or pyridine presumably by a dipolar intermediate AH = 2.1 to 4.2 kJ mol-1, AS = -251 to -293 JK-1 mol- (Scheme 86) (66JA3582, 76JGU799). 

Triaryl phosphites are not effective as catalysts and alkyl isocyanates usually are not cyclized by phosphites at all, although benzyl isocyanates are reported to yield the corresponding uretidinediones with 1,2-dimethylimidazole as a catalyst (75S463). 

Isocyanate groups also will react with themselves to form a variety of compounds. Two isocyanate groups can react to form a dimer or uretidinedione. 

The uretidinediones, which are the pyclic dimers of isocyanates, form the most extensive series of four-membered heterooydic compounds with two nitrogen atoms. Their chemistry closely resembles that the isoi nate monomera with which they are in equilibrium at high temperatures. The parent compound, the dimeric fcyanic acid, is unknown, and all hut one member of the series are 1,3-diaiyl derivatives. 

Diisocyanatotoluene (272) was treated with DBU in xylene to give l,3-bis(4 -methyl-3 -isocyanatophenyl)uretidinedione (273) in 93.3% yield (71JAP(K)37503). 

Isocyanates can also react with other isocyanate molecules to form oligomers (Fig. 4). This polymerization is more likely to occur in the presence of basic catalysts.Isocyanate dimers, also called uretidine-diones, can only be formed by aromatic isocyanates, and uretidinedione formation is inhibited by ortho substituents. Hence, only 4,4-diphenylmethanediiso-cyanate (MDI) dimerizes at room temperature, and its rate of formation is quite low. At higher temperatures, MDI would form an insoluble polymeric mate-rial. Trimers of isocyanates are also possible these structures are called isocyanurates. Isocyanurates are formed by both aliphatic and aromatic isocyanates, and the resulting structure is highly stable to a temperature of approximately Isocyanurates give... 

Isocyanates give two types of dimerisation reactions formation of uretidinediones and of carbodiimides [1, 3, 12, 13, 15, 23-25] ... 

Some data are available about catalysis in 1,2-cycloadditions. Tributyl phosphine catalyses dimerisation of phenyl isocyanate to uretidinedione in toluene . The reaction is kinetically of first order with respect to catalyst and overall third order the reverse process is first order with respect to catalyst and overall second order. The mechanism is complex, as revealed by the value of the apparent activation energy of the forward reaction (E= l.l 0.7 kcal.mole" ), which presumably results from the combined temperature dependence of two or more steps, including formation of an isocyanate-phosphine complex (see eqn. (13), p. 113). 

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