Carbodiimides 2+2 cycloaddition reactions

A full development of the rate law for the bimolecular reaction of MDI to yield carbodiimide and CO indicates that the reaction should truly be 2nd-order in MDI. This would be observed experimentally under conditions in which MDI is at limiting concentrations. This is not the case for these experimements MDI is present in considerable excess (usually 5.5-6 g of MDI (4.7-5.1 ml) are used in an 8.8 ml vessel). So at least at the early stages of reaction, the carbon dioxide evolution would be expected to display pseudo-zero order kinetics. As the amount of MDI is depleted, then 2nd-order kinetics should be observed. In fact, the asymptotic portion of the 225 C Isotherm can be fitted to a 2nd-order rate law. This kinetic analysis is consistent with a more detailed mechanism for the decomposition, in which 2 molecules of MDI form a cyclic intermediate through a thermally allowed [2+2] cycloaddition, which is formed at steady state concentrations and may then decompose to carbodiimide and carbon dioxide. Isocyanates and other related compounds have been reported to participate in [2 + 2] and [4 + 2] cycloaddition reactions (8.91. 

An aza-Wittig reaction-cycloaddition reaction sequence was utilized for the synthesis of pyridotriazines 94. Treatment of iminophosphorane 92 with phenylisocyanate leads to the formation of the corresponding carbodiimide intermediate, which with another molecule of isocyanate affords 94 in [4+2] heterocycloaddition reactions (Equation 13) <1997T16061>. 

The palladium-phosphine-catalyzed cycloaddition reactions of vinyloxetanes 530 with aryl isocyanates or diaryl-carbodiimides led to 4-vinyl-l,3-oxazin-2-ones 531 or l,3-oxazin-2-imines 532, respectively (Scheme 101). In the absence of phosphine ligands (PPhs, bis(diphenylphosphino)ethane (DPPE), l,3-bis(diphenylphosphino)propane (dppp), no conversion of heterocumulenes was observed. Starting from fused-bicyclic vinyloxetanes, both types of cycloadditions proceeded in a highly stereoselective fashion, affording only the r-isomers of alicycle-condensed 1,3-oxazine derivatives <1999JOC4152>. 

Carbodiimides undergo a [4-1-2] cycloaddition reaction to form pyrrolo[3,4-f]pyridine derivatives <1998J(P1)3065>. Dehydrogenation of the cycloadduct in the presence of manganese dioxide gives improved yields (Scheme 8). 

Studies focused on the ability of ALvinyl carbodiimides to undergo cycloaddition reactions have been carried out in recent years (Scheme 56). Thus, 2-azadiene derivatives 245 reacted with tetracyanoethylene to yield dihydropyridines 246 (86CL135), whereas treatment of 245 with... 

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. 

In a similar fashion, cycloaddition reaction of 2-vinylpyrrolidines with carbodiimides in the presence of Pd(OAc)2 and l,5-bis(diphenylphosphino)pentane affords the seven-membered ring cyclic arylguanidines in acceptable yields and conversions (Equation 16) <2004T73>. 

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>. 

The [2+2] cycloaddition reaction of the P=N double bond across the C=N double bond took part when carbodiimides were reacted with trichloroiminophosphoranes <2000CC1375, 2002JA10698>. Heating a mixture of iminophosphorane 116 and diisopropylcarbodiimide in toluene yielded diazaphosphetidine 11 (Scheme 33). 

The nitrene 28 is not produced from the azide precursor, but from heterocycles via photolysis and thermolysis as shown in Sch. 11 [20]. Iminoacyl nitrenes react intramolecularly giving benzimidazoles with good yields (Sch. 11), and, dependending on the precursor used and the reaction conditions, varying amounts of carbodiimides are obtained. The reactivity of the acyl nitrenes is influenced by the substituent connected to the acyl group (see Sch. 10), however all acyl nitrenes are quite reactive and therefore rather unselective. Apart from cycloaddition reactions with Tt-bonds, insertion reactions into a-bonds, additions to lone pair electrons of... 

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... 

Z + Z] Cycloaddition reactions are important in the construction of 1,3-thiazetidine rings . Isothiocyanates add carbodiimides across the carbonsulfur bond < 1975J(P2) 1475 >. For example, the reaction of the 2-thioxo-l,3-thiazepan-4-one derivative 208 with A,A-dicyclohexylcarbodiimide affords the 1,3-thiazetidine system 209 (Scheme 100) <1999JHC1167>. 

I would like to acknowledge the contributions of my former co-workers at the Donald S. Gilmore Research Laboratories of the Upjohn Company, especially Dr R.H. Richter and B. Tucker who were involved in the synthesis and cycloaddition reactions of carbodiimides Dr L.M. Alberino who participated in the synthesis of polycarbodiimides Dr K. Onder and Dr WJ. Farrissey, Jr, who played a major role in the development of thermoplastic polyamides based on carbodiimide chemistry Dr H.W. Temme and Dr C.P. Smith, who developed novel polymeric catalysts for the conversion of isocyanates into carbodiimides and A. Odinak, who developed the liquid MDI process. 1 would especially like to acknowledge the encouragement of the late Dr A.A.R. Sayigh. 

S,N,N-trimethyl-N -arylisothiourea 30, upon reaction with an isothiocyanate, produces a carbodiimide 31. This reaction involves a [2 + 2] cycloaddition reaction, followed by a cycloreversion to give the observed products. 

Cycloaddition Reactions Across C=N Bonds. Cycloaddition reactions of heterocumulenes are well known reactions. Like many of the other heterocumulenes carbodiimides form cyclodimers and cyclotrimers (see Section 2.4.1). The obtained dimeric... 

The [2+2] cycloaddition reaction of isothiocyanates with carbodiimides generally proceeds across the C=S bond of the isothiocyanate to give thiazetidine derivatives 218. However, in the reaction of methyl isothiocyanate with N-(4-dimethylaminophenyl)-N -methylcarbodiimide a mixture of the thiazetidine derivative 218 and the isomeric iminodi-azetidinethione derivative 219 is obtained. The structure of 219 was confirmed by X-ray crystallography. ... 

Isoselenocyanates also undergo [2 + 2] cycloaddition reactions across their C=Se bond with carbodiimides to give l,3-selenazetidine-2,4-diimines 228. ... 

Similarly, imidozirconocene complexes 233 undergo the [2+2] cycloaddition reaction with carbodiimides to give the metalacycles 234. ... 

Cycloaddition Reactions Across C—C Multiple Bonds. Cycloadducts derived from carbodiimides and olefins or allenes are not known. However, the [2+2] cycloaddition of ketenes, R2C=C=0, to carbodiimides affords 4-imino-2-azetidinones (/3-lactames) 239 in high yield. Aliphatic carbodiimides show higher reactivity in comparison to aromatic carbodiimides, and the reaction proceeds across the aliphatic C=N bond in N-alkyl-N -arylcarbodiimides. The cycloadducts obtained in this reaction are listed in Table 2.3. 

Iminophosphoranes also undergo [2-1-2] cycloaddition reactions with carbodiimides to give phosphetidines. For example, reaction of ClsF N-tolyl 278 with diisopropylcarbodiimide... 

Cycloaddition Reactions. Carbodiimides also react as dipolarophiles in [2+3] cycloaddition reactions. For example, generation of diphenylnitryl imine 292 in the presence of diphenylcarbodiimide affords a spiro compound 293 as the result of the reaction of the initially formed [2+3] cycloadduct with a second equivalent of the nitrile imine. ... 

Several azoniaallene salts, generated in situ, undergo a [2+3] cycloaddition reaction with carbodiimides. For example, generation of 300 from the chloroalkylazo compound 299 in... 

Cycloaddition Reactions. In [2+2] cycloaddition reactions with car-bodiimides sometimes [2+4] cycloadducts are produced as coproducts. Examples include the reaction of phenylcarbonyl isocyanate, phenylcarbonyl isothiocyanate and thiocarbamoyl isothiocyanate with carbodiimides to give [2+4] cycloadducts, discussed in Section 5.3.1. In the current section, mainly [2+4] cycloaddition of carbodiimides as dienophile with dienes derived from oxoketenes, generated in situ or masked oxoketenes, especially 2,3-diones investigated by Kollenz and his coworkers are discussed. 

The masked a-oxoketene 339 reacts with carbodiimides via a [2+4] cycloaddition reaction to give the cycloadduct 340. The reaction is conducted by heating the reagents at 120-130 ° C... 

Carbodiimides readily undergo reactions across their C=N bonds to form heterocycles. For example, in their di- and trimerization reactions four and six membered ring N-hetero-cycles are obtained (see Section 2.4.1). Also, numerous four, five and six membered ring heterocycles are formed in the cycloaddition reactions of carbodiimides, often in excellent yields (see Section 2.4.2). Nucleophilic reactions of carbodiimides sometimes give rise to... 

The reaction of thiosalicyclic acid 614 with two equivalents of carbodiimides may involve a thioacylketene intermediate 615, which undergoes a [2+4] cycloaddition reaction with the second equivalent of the carbodiimide to give the cycloadduct 616.5 ... 

Lactams are also obtained in the [2-1-2] cycloaddition reaction of ketenes with carbodiimides (see Section 2.4.2.1). 

Also, carbodiimides with a C=N bond in conjugation to the cumulative system undergo [2-1-4] cycloaddition reactions with suitable dienophiles (see Section 5.3.1). 

Unsaturated carbodiimides 27, generated in situ, can also be trapped with azo compounds to form 28 or with imines to give 29 via a [2+4] cycloaddition reaction. ... 

An example of a [2+2] cycloaddition reaction across two carbodiimide groups generated in a pyrazole system is shown Section 3.2.3. However, unsaturated carbodiimides often react as azadienes in [2+4] cycloaddition reactions (for example in the preceding reaction). 

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