Aryl carbodiimides 3+2 cycloaddition reactions

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>. 

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... 

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. 

Because of their predictable behavior and reactivity, thioacyl isocyanates comprise the bulk of this work, and extensive studies of their [4 + 2] reactions with olefins,83 enamines,84 enol ethers,843 thioacyl isocyanates,85 imines,85 1 86 carbodiimides,84387 isocyanates,843 azirines,88 /3-enaminoke-tones,89 dianils,86d azines,90 hydrazones,91 imidazoline-4,5-diones,92 aryl cyanates,93 disubstituted cyanamides,93 aldehydes,94 ketones,94 ketenes,94 alkyl or aryl iminodithiocarbonates,95 and the carbon-carbon double bond of ketenimines96 have been detailed. In an extensive comparative study of the [4 + 2] cycloaddition reactions of thioacyl isocyanates, the heterocu-mulenes bearing strong electron-withdrawing substituents were found to be more stable and less prone to participate in cycloaddition reactions.84 Representative examples are summarized in Scheme 9-IV. 

A successful method for preparing 1,3-thiazetidines is the cycloaddition of carbodiimides with isocyanates. The cycloadduct 151 was derived by the interaction of carbodiimide 152 and tosyl isothiocyanate . Reaction of 152 with methyl or phenyl isothiocyanate preferentially leads to iminotriazine-thiones. Cycloaddition between aryl isothiocyanates and dicyclohexylcar-bodiimide produced 153. ... 

Also, the one pot reaction of heterocyclic bis-iminophosphoranes with one or two equivalents of aryl isocyanates leads to selective formation of fused heterocycles. For example, from the pyrazole derivative 17 either [l,3]-diazepines 18 or tricyclic ring systems 19 are formed. The latter reaction involves an intramolecular [2-1-2] cycloaddition of both carbodiimide substituents. ... 

Carbonyl diisocyanate (46) was shown to undergo a Diels-Alder type of cycloaddition with azomethines to give 2,3,6,7-tetrahydro-4/7,8/7-[l,3,5]triazino[2,l-Z>][l,3,5]oxadiazin-4,8-diones (16) (Scheme 5). A large variety of the triazino[2,l-ft][l,3,5]oxadiazines were realized by the suitable selection of dienophiles. Thus, l,3,5-triazino[2,l-Z>][l,3,5]oxadiazines (17), (18 R = NMe), and (19) and were synthesized by the reaction of carbonyl diisocyanate with alkyl or aryl isocyanates, dimethyl cyanamide, or aliphatic carbodiimides, respectively <86CB1133>. Due to the high reactivity of the cumulated double bonds, carbonyl diisocyanate (46) was also found to undergo [4 + 2] cycloadditions with cyclohexanone to yield cyclohexan-l-spiro-9 -[l,3,5]-oxadiazino-[3,4-e][l,3,5j-dioxazin-5 -spiro-l"-cyclohexane-2, 7 -dione (47) (Scheme 6) <76LA1634>. 

Hoberg and Burkhardt discovered that alkynes and carbodiimide can be coupled to afford N-analogs of pyridones [24], A brief investigation of the substrate scope revealed that symmetric alkyl-alkyl (93) and aryl-aryl (94) alkynes can be coupled with diphenyl carbodiimide (Scheme 2.28). Regioselective formation of iminopyri-dine 95 was observed when phenylmethylacetylene was subjeeted to standard reaction conditions. To our knowledge, no further development in this area of nickel-catalyzed cycloaddition has been made. Also, a successful version of Ni-catalyzed cycloaddition of diyne and carbodiimides is yet to be reported. 

Density functional theory (DPT) calculations were performed to elucidate the relative energies of the intermediates and products of this reaction. There are two reaction pathways that could generate the bis(imido) product 9 and the oxo-imido species 8 from U(N Bu)2(I)2(OPPh3)2 (Paths 1 and 2, Scheme 14.3). See color plate section. The first pathway involves the [2 - - 2] cycloaddition of the C = N bond of the aryl isocyanate to form an AQV-bound ureato intermediate (13), which can isomerize to form species 14 with the -NPh group trans to the terf-butyl imido moiety. Compound 14 can then eliminate BuNCO to generate the unsymmetrical bis(imido) complex 9 (top line in Scheme 14.2). Alternatively, AO-bound carbamate intermediates 15 and 16 could form that result from the [2 + 2] cycloaddition of the C = O bond of the aryl isocyanate across the U = N imido bond. Elimination of a substituted carbodiimide would generate the oxo-imido complex 8 (bottom line in Scheme 14.2). 

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