Ylides nitrile, cycloaddition

Irradiation of 2,3-diphenyl-2//-azirine in the presence of fullerene leads to the formation of mono- and oligo adducts (98,99). A monoadduct, l,9-(3,4-dihydro-2,5-diphenyl-2//-pyrrolo)fullerene-60 was isolated and characterized. Mechanistic studies showed that under conditions of direct irradiation it was formed by a classic nitrile ylide cycloaddition but in the presence of 1,4-napthalenedicarbonitrile (DCA) it resulted from reaction of the radical cation intermediate 108. Cycloaddition reactions have also been carried out with diaza-phospholes and diazaarsoles (100) to give adducts of the type 189 (A = As,P) and with cyanogen to give 190 and with aryldiazocyanides where addition to both the azo moiety and the cyano group were observed (101). 

The coupling of alkynes and iminocarbene complexes (e.g., 4, Scheme 17.2) affords pyrrole ring systems (e.g., 6) in a formal [3-1-2]-cycloaddition process [2,3]. The pyrroles obtained are analogous to those obtained from nitrile ylide cycloadditions. A mechanism was proposed involving a net metalla-[4- -2] cycloaddition to afford azametallacycle 7 followed by reductive elimination and alkene isomerization. The regiochemistry arises through a nonconcerted addition to the triple bond via... 

The ring opening of 2//-azirines to yield vinylnitrenes on thermolysis, or nitrile ylides on photolysis, also leads to pyrrole formation (B-82MI30301). Some examples proceeding via nitrile ylides are shown in Scheme 92. The consequences of attempts to carry out such reactions in an intramolecular fashion depend not only upon the spatial relationship of the double bond and the nitrile ylide, but also upon the substituents of the azirine moiety since these can determine whether the resulting ylide is linear or bent. The HOMO and second LUMO of a bent nitrile ylide bear a strong resemblance to the HOMO and LUMO of a singlet carbene so that 1,1-cycloadditions occur to carbon-carbon double bonds rather than the 1,3-cycloadditions needed for pyrrole formation. The examples in Scheme 93 provide an indication of the sensitivity of these reactions to structural variations. 

Frontier molecular orbital theory correctly rationalizes the regioselectivity of most 1,3-dipolar cycloadditions (73JA7287). When nitrile ylides are used as 1,3-dipoles, the dipole... 

The evidence obtained clearly indicates that the above photorearrangements proceed by a mechanism involving a nitrile ylide intermediate since cycloadducts could be isolated when the irradiations were carried out in the presence of trapping agents. Intramolecular cycloaddition of the nitrile ylide followed by a 1,3-sigmatropic hydrogen shift of the initially formed five-membered ring readily accounts for the formation of the final product. 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

A series of 3-imino-2-aryl-l-azetines has been prepared by cycloaddition of nitrile ylides to isocyanides. Thus generation of the nitrile ylides (215) in the presence of cyclohexyl isocyanide gives the 3-cycIohexyIimino-2-aryI-l-azetines (205). Similar 1-azetines are formed by interception of nitrile ylides with a-methylbenzyl isocyanide (72AG(E)47) cf. Section 5.09.4.2.2). 

Recently, Burger devised an improved method of carrying out mild, regiospecific cyclizations that involve an intermediate that acts as a synthon for a nitrile ylide of HCN [47 (equation 48). With this methodology, cycloadditions with activated alkenes, alkynes, and azo compounds were earned out [47] (equation 49). All such reported reactions were regiospecific and had the same orientational preference... 

The chalcogene heterocycles have been used as stable precursors for sulfur-said selenium-cantaining hetero-l,3-dienes in cycloaddition reactions 3//-l,2,4-Thiaselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-thia-3-aza-buta-1,3-dienes, and 3//-diselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-selena-3-azabuta-l,3-dienes as well as bis(tnfluoro-methyl)-substrtuted nitrile ylides [137]... 

Heterocyclic sulphoxides of general structure 185,186 and 187 have been prepared by cycloaddition of diarylsulphines 173b to nitrile oxides 188226, nitrile ylides 189227 and nitrones 190228, respectively (equation 100). 

Oxazole formation can be envisaged as proceeding by three possible pathways 1,3-dipolar cycloaddition of a free ketocarbene to the nitiile (Path A), the formation and subsequent 1,5-cyclisation of a nitrile ylide (Path B) or the formation and subsequent rearrangement of a 2-acyl-2//-azirine (Path C) (Scheme 9). 

Thus, cycloprop [c] isoquinolines 101 obtained by a stereo specific 1,1-cycloaddition of nitrile ylides 100 undergo two distinct thermal (80 °C) rearrangements depending on the substituents in the cyclopropane ring (equations 35 and 36)52. 

The 1,3-dipolar cycloadditions of benzonitrile oxides with tertiary cinnamides yield the 5-phenyl and 4-phenyl regioisomers in a reversal of the expected regioselectiv-ities shown with methyl cinnamate. Calculations have shown that steric factors are responsible for this reversal of regioselectivity." The 1,3-dipolar cycloadditions of benzonitrile oxide with electron-rich and electron-poor dipolarophiles are accelerated by sodium dodecyl sulfate micelles. Phenyl nitrile ylides react with electron-deficient alkenes to produce five-membered -heterocycles where measured rate constants are between 4 x 10 and 7 x 10 lmoP ... 

Cycloaddition of nitrile ylides (204) to nitroso compounds gives mixtures of (205) and (206), which both slowly rearrange to quinazolines (207) (Scheme 89) <77CB605>. 

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