Triplet aryl carbenes

Nitrenes, the nitrogen analogs of carbenes, are too reactive for isolation under ordinary conditions. The ground state of NH, and of most nitrenes, is a triplet. Aryl nitrenes have been trapped at 77 K,186 the more reactive alkyl nitrenes in matrices at 4 K.187... 

C in THF, which cleanly led to the formation of (amino)(aryl)carbene Xllla (Scheme 8.8). ° A NMR signal at 8 = 314.2 ppm leaves no doubt of the formation of Xllla. Carbene Xllla is stable for days in solution at —50 °C but undergoes a C—H insertion reaction at room temperature within a few hours, giving rise to the 4,6-di-ferf-butyl-l,l-dimethyl-3-(methyl-rcrt-butylamino)mdane as the major product. It is interesting to note that this reaction, typical of transient singlet and triplet carbenes, has never been observed for diamino carbenes. This striking difference demonstrates the less perturbed character of carbene Xllla. 

The prototypical carbene, CH2, and other simple alkyl and aryl carbenes have triplet ground states, with two unpaired electrons. In striking contrast, all silylenes known to date have singlet ground states, with the two nonbonding electrons paired. This difference is of the greatest importance for the properties, spectra, and chemical behavior of silylenes. 

Particular attention has been devoted to the characterization and to the reactions of aryl carbenes. Energy differences between singlet and triplet ground state carbenes are usually... 

In the case of singlet phenylchlorocarbene and triplet diphenylcarbene, the abstraction of chlorine has been shown to be accelerated by the presence of electron-donating substituents on the aryl carbene, by increasing ease of reduction of the chlorine donor and... 

C-Labelling showed that the thermal conversion of the phenyl trimethylsilyl diazo-compound (168) into (169) involved isomerization of a phenyl carbene into a cycloheptatrienylidene, silacyclopropanes were not involved. It was shown that the P-naphthyl carbene obtained by rearrangement of 4,5-benzocycloheptatrienylidene enters the singlet-triplet manifold as a singlet. Evidence has been reported that aryl carbenes can show both electrophilic and nucleophilic properties in their intramolecular ring expansion. Low-temperature photolysis of the sodium salt of the toluene-p-sulphonyl hydrazone (170) gave, by intramolecular carbene addition, the dibenzobicyclo[4,l,0]heptatriene (171) which was trapped by buta-l,3-diene. ... 

Itakura, H. and Tomioka, H., Generation and characterization of triplet anthryl (aryl)carbenes,... 

Three types of photoextrusion reactions have been identified in the irradiation of aryl-substituted 1,3,2-dioxathiolane 2-oxides [5 1-1-2 4-2] cycloelimination to produce a carbonyl compound, a carbene and sulfur dioxide extrusion of sulfur dioxide accompanied by a pinacol-like rearrangement to yield an aldehyde or ketone and extrusion of sulfur trioxide to give an alkene <72JOC2589>. Sensitization and quenching experiments indicate that a singlet state is responsible for the cycloelimination reaction, whereas the rearrangement and sulfur trioxide extrusion reactions arise from a triplet state <82JCR(S)175>. 

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

Nitrenes,239 R—N, are the nitrogen analogs of carbenes, and most of what we have said about carbenes also applies to them. Nitrenes are too reactive for isolation under ordinary conditions. Alkyl nitrenes have been isolated by trapping in matrices at 4 K,24n while aryl nitrenes, which are less reactive, can be trapped at 77 K.241 The ground state of NH, and probably of most nitrenes,242 is a triplet, though nitrenes can be generated in both triplet and singlet states. In additions of EtOOC—N to C=C double bonds two species are involved,... 

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