Cycloaddition reactions isocyanates

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Isocyanates are Hquids or soHds which are highly reactive and undergo addition reactions across the C=N double bond of the NCO group. Reactions with alcohols, carboxyUc acids, and amines have been widely exploited ia developiag a variety of commercial products. Cycloaddition reactions involving both the C=N and the C=0 double bond of the NCO group have been extensively studied and used for product development (1 9). 

The dimeri2ation and trimeri2ation of isocyanates are special cases of the cycloaddition reaction ia that they iavolve reageats of the same type. The uacataly2ed carbodiiaiidi2atioa of isocyanates likely iavolves a labile 2 + 2 cycloadduct (12) which Hberates carboa dioxide. 

Aziridinones are also known to undergo cycloaddition reactions. Thus, treatment of (321) with phenyl isocyanate gave adducts (335) and (336) (76CL47). 

H-pyran synthesis from, 3, 759 bis(trimethylsiloxy) in pyrrole synthesis, 4, 333 chromene synthesis from, 3, 750 cycloaddition reactions with isocyanates, azetidin-2-ones from, 7, 261 dihydropyran synthesis from, 3, 771 fuiyl... 

The dimeric tellurium diimide 10.7 undergoes a cycloaddition reaction with BuNCO to generate the Ai,iV -ureatotellurium imide 10.11, which is converted to the corresponding telluroxide 10.12 by reaction with excess BuNCO. " By contrast, BuN=S=N Bu undergoes exchange reactions with isocyanates. 

Vinyl ethers undergo many cycloaddition reactions similar to those which take place with enamines. In general, however, these cycloaddition reactions with vinyl ethers take place less readily than those with enamines. These reactions include cycloaddition of vinyl ethers with ketene (200-205), phenyl isocyanate (206), sulfene (207,208), methyl acrylate (209), diethyl acetylenedicarboxylate (210), and diphenylnitrilimine (183). 

The tricyclic 1,3-diazepines 3 are formed by the action of two molar equivalents of aromatic isocyanates on the bis(imino-A5-phosphane) 1 in toluene at 20 C for 16 hours in a tandem aza-Wittig [2 t 2]-cycloaddition reaction . No further details were reported.167... 

Iwasawa et al. also developed a new reaction involving a three-component coupling process which affords five-membered heterocycles. This [2s+2sh-1c] cycloaddition reaction supposes the consecutive addition of an alkynyllithium derivative to a Fischer carbene complex followed by the addition of a third component which can be an aldehyde, an imine, an isocyanate, or C02 [119] (Scheme 74). 

Reactions of the cyclopentadienyl-amidinate-supported imidotitanium complexes with CO2 proceed via initial cycloaddition reactions, but depending on the imido Af-substituent go on to yield products of either isocyanate extrusion or unprecedented double CO2 insertion (Scheme 89). ... 

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. 

The pyrido[l,2-tf][l,3,5]triazine-2,4(3//)-dione derivative 89 was obtained in a cycloaddition reaction of diphenyl-methyl isocyanate 90 with 2-pyridyl isocyanate 91 derived from the corresponding acyl azide via Curtius rearrangement <2002ARK438>. Compound 89 was also synthesized by the reaction of diphenylacetyl chloride 118 and picolinyl azide 116a in the presence of triethylamine (Scheme 11) <2002ARK438>. ... 

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 nitrogen atoms in ADC compounds are highly electrophilic. Nucleophilic attack on nitrogen is easy, and as with electrophilic acetylenes, such as dimethyl acetylenedicarboxylate, it seems likely that some cycloaddition reactions of ADC compounds with unsymmetrical substrates proceed via a stepwise mechanism. PTAD is a powerful electrophile, although TCNE is more reactive, and chlorosulfonyl isocyanate is more reactive still.58... 

A most intriguing cycloaddition reaction of phosphoryl isocyanates with poly-a-haloaldehydes provides a heterocyclic phosphonate monoester/monoamide as an isolable but reactive species (Equation 3.20).353... 

These routes are dimerization to furoxans 2 proceeding at ambient and lower temperatures for all nitrile oxides excluding those, in which the fulmido group is sterically shielded, isomerization to isocyanates 3, which proceeds at elevated temperature, is practically the only reaction of sterically stabilized nitrile oxides. Dimerizations to 1,2,4-oxadiazole 4-oxides 4 in the presence of trimethylamine (4) or BF3 (1 BF3 = 2 1) (24) and to 1,4,2,5-dioxadiazines 5 in excess BF3 (1, 24) or in the presence of pyridine (4) are of lesser importance. Strong reactivity of nitrile oxides is based mainly on their ability to add nucleophiles and particularly enter 1,3-dipolar cycloaddition reactions with various dipolarophiles (see Sections 1.3 and 1.4). 

Some routes of chemical transformations of nitrile oxides connected with the problem of their stability were briefly discussed in Section 1.2. Here only two types of such reactions, proceeding in the absence of other reagents, viz., dimerization to furoxans and isomerization to isocyanates, will be considered. All other reactions of nitrile oxides demand a second reagent (in some cases the component is present in the same molecule, and the reaction takes place intramolecularly) namely, deoxygenation, addition of nucleophiles, and 1,3-dipolar cycloaddition reactions. Also, some other reactions are presented, which differ from those mentioned above. 

Closure of the oxadiazole ring is still achieved through cycloaddition between pyridine iV-oxides and isocyanates, affording adducts such as 142 (Scheme 38) <1995T6451>. Nonaromatic imine fV-oxides exhibited similar reactivities, since azasugar-derived fV-oxides as a mixture of 143 and 144 underwent cycloaddition reactions in the presence of phenyl isocyanate or trichloroacetonitrile. Compounds 145 and 146 (Scheme 39) were obtained from the aldoxime W-oxide 143 two other regioisomeric heterocycles arose from the ketoxime derivative 144 <1996T4467>. 

Isonitrile complexes, having a similar electronic structure to carbonyl complexes, can also react with nucleophiles. Amino-substituted carbene complexes can be prepared in this way (Figure 2.6) [109-112]. Complexes of acceptor-substituted isonitriles can undergo 1,3-dipolar cycloaddition reactions with aldehydes, electron-poor olefins [113], isocyanates [114,115], carbon disulfide [115], etc., to yield heterocycloalkylidene complexes (Figure 2.6). 

Diazoazoles, because of charge polarization and potential bifunctional reactivity of the derived betaine, react with dipolarophiles to give cycloaddition products. Generally all the diazoazoles react with electron-rich, unsaturated derivatives. The cycloaddition reaction with isocyanates is readily observed in the case of the reactive 3-diazopyrazoles, but it is much slower with other diazoazoles. By contrast, reaction with ylides and diazoalkanes is only observed for 3-diazopyrazoles and 3-diazoindazoles. 

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>. 

Diazadienes have been shown by various groups to be suitable precursors of imidazoline derivatives by means of [4 + 1 ] cycloaddition reactions. In 1976 Matsuda and co-workers were able to cycloadd heterodienes 303, available from AMrimethylsilyl benzophenone imine and phenyl isocyanate, with cyclohexyl isocyanide to obtain 305 in 91% yield, after methanolysis of the initial adduct 304 [76JCS(P1)1523] (Scheme 67). 

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