Oxadiazoles 1,2,4: carbene precursors

Cycloaddition with Vinyl Isocyanate. When a large excess of 2,5-dihydro-2,2-dimethyl-5,5-bis(propylthio)-1,3,4-oxadiazole carbene precursor is added rapidly in refluxing benzene to a vinyl isocyanate, two equivalents of carbene add to the substrate prior to ring closure, leading to a six-membered adduct (eq 3). However, no carbene insertion into the amide N-H bond occurs. This phenomenon is not observed with the dimethoxy carbene. ... 

Addition of 2,5-dihydro-2,2-dimethyl-5,5-bis(propylthio)-l,3,4-oxadiazole in refluxing benzene and an aryl isocyanate releases the bis(propylthio)carbene in situ which then adds easily to the aryl isocyanate to yield a substituted isatin with the ketone functionality protected as a thioacetal (eq 4)7 Ring closure also occurred when the 2,5-dihydro-2,2-dimethyl-5,5-bis(propylthio)-1,3,4-oxadiazole carbene precursor was added to 1-naphthyl isocyanate in refluxing acetonitrile (eq 5)7 Formation of thioacetal protected isatin products are unique to the bis(propylthio)carbene as other nucleophilic carbenes added to aryl isocyanates afforded only modest yields of hydantoin products. ... 

F 4] Cycloaddition with Vinyl Isocyanates. Upon thermolysis in refluxing benzene, 2,5-dihydro-2,2-dimethyl-5,5-bis-(propylthio)-l,3,4-oxadiazole yields the bis(propylthio)carbene precursor, which reacts as a 1,1-dipole equivalent in [1 -E 4] cycloaddition reactions with various vinyl isocyanate substrates leading to highly functionalized adducts such as hydroindolones (eq 1). Reductive desulfurization with Raney nickel yields the enamide or the fully reduced system (eq 2). Cyclohexyl isocyanide also behaves as a 1,1-dipole equivalent in [1 -E 4] cycloadditions with vinyl isocyanates, however the adducts are less functionahzed. ... 

Dimerization. Upon thermolysis of 2,5-dihydro-2,2-dimeth-yl-5,5-bis(propylthio)-l,3,4-oxadiazole in refluxing benzene and in the absence of any electrophile, the resultant bis(propylthio)carbene will dimerize (eq 6). Since the reaction is highly exothermic, care must be taken in the execution of this reaction. Use of an explosion shield and slow addition of the carbene precursor are recommended. 

C6o reacts with diazomethane to yield fulleroids [97,99,100,234], Carbene generated from the thermolysis of precursors such as diazirines, sodium trichloroac-etate, cyclopropene, oxadiazole, and tosylhydrazone [60,235] adds onto C6o leading to methanofullerenes [12,15,236], Recently, Akasaka et al. described the photochemical reaction of diazirine with C6o [237], Irradiation of a benzene solution of 2-adamantane-2,3 -[3H]-diazirine 58 and C6o with a high pressure mercury lamp (cutoff <300 nm) at 15°C in a Pyrex tube resulted in the formation of mixture of the isomers 59a and 59b in a ratio of 51/49 (Scheme 24). 

Dimethoxy carbene has been shown to behave as a carbonyl 1,1-dipole equivalent in [ 1 -E 4] cycloadditions with vinyl isocyanates also yielding functionalized adducts. However, the oxadiazole precursor requires higher thermolysis temperatures and hydrolysis of the acetals in the resultant adducts can be difficult for acid-sensitive systems. 

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