11+2 Cycloaddition, with nitriles photoinduced

Scheme 4 Plausible mechanistic pathways for the generation of nitrile imine and its reactions in the chloride-containing aqueous medium. The photoinduced tetrazole ring rupture followed by cycloaddition with aUcenes are colored in blue...

In a further example, a genetically encoded alkene-containing protein in Escherichia coli was modified with a library of 2,5-diaryl tetrazoles under similar conditions (Song et al., 2008). Although the photoinduced formation of the nitrile-imine was very fast (2 min), the subsequent cycloaddition with the aUyl phenyl ether functional protein was considerably slower (the reaction was allowed to proceed overnight at 4 °C). By comparison, the second-order rate constant of the cycloaddition with allyl phenyl ether was determined to be 0.00202 0.00007 M s, whereas that for acrylamide (as was used in the previous example) was determined to be 0.15s. A reasonable explanation for this observation is the lower LUMO energy of acrylamide than that of allyl phenyl ether, which offers better orbital overlap with the HOMO of the nitrile imine. 

A series of 3-amino (or 3-N-substituted amino)-l,2,4-oxadiazoles 108 can be obtained in moderate to high yields by the photoinduced rearrangement of 3-acylamino-l,2,5-oxadiazoles (furazans) 106 irradiated in the presence of an excess of ammonia or aliphatic amines [64—66]. The reaction follows a fragmentation-cycloaddition route, with the initial formation of a nitrile and a nitrile-oxide the latter is attacked by a nitrogen nucleophilic reagent (ZH in Scheme 12.29), and the open-chain interme-... 

Various other [3 + 2] cycloadditions, affording chiral, anellated C6o derivatives with stereogenic centers in the addends are reported in literature. The products were generally obtained as racemates and resulted from reaction of buckminsterfullerene with species like 2,3-disubstituted 2//-azirincs (via nitrile ylides [under direct irradiation] or via 2-azaallenyl radical cations [sensitization by photoinduced electron transfer]),365 1-substituted 5-diazopentane-1,4-diones (via cyclic carbonyl ylides),366 7-alkylidene-2,3-diazabicyclo[2.2.1] hept-2-ene (via a diradicaloid trimethylenemethane derivative),367 1-benzylpy-razolidine-3-ones in the presence of aldehydes (via pyrazolidinium ylides),368 2-trifluoromethyl-2,5-dihydro-l,3-oxazol-5-ones (via nitrile ylides),369 nitro-alkanes in the presence of triethylamine and trimethylsilyl chloride (via N-silyloxynitrones),370 or dv-HOCH2 CH=C H C H 2 OCO 2 H( in the presence of... 

Topics reviewed during the year include the photochemistry of indoles, sulfoxides, pyrazoles and isothiazoles, (S-hetero)cyclic unsaturated carbonyl compounds, photoinduced single electron transfer (SET) reactions of amines and of azo compounds, SET reactions of organosilanes and organostannanes with Qo and ketones, photochromic polypeptides and di(hetero)arylethenes, processes in chromophore sequences on a-helical polypeptides,aryl-aryl coupling in furans, thiophenes and pyrroles," [3+2]cycloaddition of aromatic nitriles (and esters) with alkenes, and reactions of benzylsilane derivatives. ... 

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