Carbodiimides cycloaddition reactions with

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... 

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

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

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 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 ... 

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). 

Cyclic carbodiimides also undergo a facile [2+2] cycloaddition reaction with alkyl- and aryl isocyanates to give the cycloadducts 44. With aryl isocyanates the reaction is exothermic and is completed within several minutes. 

Carbodiimide groups undergo a [2-1-2] cycloaddition reaction with isocyanates to form four-membered ring cycloadducts. This reaction is used to convert the low melting MDI into a liquid commercial product (Isonate 143-L) (85) The liquid MDI is used in RIM applications. 

Often the initially formed bonds at low temperature are not the ones that are isolated at room temperature. Also the electronic configurations play a part in product formation. For example, in the [2+2] cycloaddition reaction involving two carbodiimides the more nucleophilic carbodiimide attacks the more electrophilic carbodiimide giving rise to the formation of only one reaction product. The latter reactions proceed stepwise, while sometimes concerted reactions are observed. Sterical hindrance also plays an important role in product formation. We have utilized A-methyl-A -t-butylcarbodiimide as a probe in determining the structure of the derived cycloadducts, because the reaction always proceeds via addition across the C=N bond with the methyl substituent. For example, in the [2+2] cycloaddition reaction with benzoyl isocyanate the reaction proceeds across the C=0 bond of the isocyanate, because t-butyl isocyanate is the only product generated in the retro reaction... 

In the [3+2] cycloaddition reaction with carbodiimides, the initially formed cycloadducts 14 rearrange to give the heterocycles 15. ... 

Carbodiimides also undergo [3-1-2] cycloaddition reactions with l-aza-2-azoniaallene salts to give 4,5-dihydro-5-imino-li/-l,2,4-triazolium salts 56. The reaction proceeds stepwise, but the linear intermediate cannot be detected. 

The carbodiimide 57 undergoes a [2+2+2] cycloaddition reaction with aliphatic isothiocyanates to form six-membered ring triazine derivatives 58. ... 

Iminophosphoranes, such as 94, also undergo [2+2] cycloaddition reactions with carbodiimides. For example, reaction of 94 with diisopropylcarbodiimide gives the [2+2]... 

The metallacycle undergoes a carbodiimide exchange reaction with a different carbodi-imide . Also, imino zirconocene complexes 103 undergo the [2-1-2] cycloaddition reaction... 

Vinyloxetanes 153 also undergo the palladium catalyzed cycloaddition reaction with carbodiimides to give 4-vinyl-l,3-oxazin-2-imine derivatives 154 " . ... 

The 1,3-dipolar character of triazaallenium salts, from now on referred to as l,3-diaza-2-azoniaallene salts, is evidenced by the many [3+2] cycloaddition reactions these types of compounds can participate in. The l,3-diaza-2-azoniaallene salts are generated in situ and trapped with suitable dipolarophiles. For example, l,3-diaza-2-azoniaallene salts undergo stereospecific [3+2] cycloaddition reactions with alkynes and olefins. However, they fail to react with isocyanates, isothiocyanates and azo compounds. Also, [3+2] cycloadditions to carbodiimides and cyanamides are observed. In contrast, nitriles fail to react. 1,3-Diaza-2-azoniaallene salts are obtained in the oxidation of 1,3-disubstituted triazenes with t-butyl hypochlorite. The resultant Al-chlorotriazenes react with antimony pentachloride to form the salts as reactive intermediates. Above —25°C, l,3-diaza-2-azoniaaUene salts disproportionate into diazonium salts and azo compounds. 

Azides can undergo cycloaddition reactions with sulfonyl isocyanates as well. L Abbe et al. showed the formation of 4-sulfonyl-tetrazolin-5-ones that underwent thermolylically-induced decomposition and produced carbodiimides with the formation ofNa. ... 

Cycloadditions of isocyanates and their derivatives with vinyiaziridines were first reported by Alper and coworkers. From their previous studies of cydoadditions to vinylepoxides or alkylaziridines, they investigated cydoadditions to vinyiaziridines and found that such reactions with isocyanates, carbodiimides, or isothiocyanates in the presence of catalytic amounts of Pd(OAc)2 (2 mol%) and PPh3 (10 mol%) at room temperature afforded five-membered ring products 249 in 34—97% yields (Scheme 2.61) [91]. When an aziridine 247 possessing an alkyl substituent at the... 

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>. 

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... 

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>. 

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). 

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>. 

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. 

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