Influence of Curvature on the Physical Properties and Reactivity of Triplet Corannulene Nitrene

Although nitrene chemistry is promising for the light-induced modification of organic compounds, the reactivity of large polycyclic aromatic compounds and the effects of their curvature remain unexplored. Irradiation of azidocorannulene (<b>1</b>) in methanol/acetonitrile followed by HCl addition produced diastereomers <b>5</b> and <b>5'</b>. Azirine <b>2</b> is apparently trapped by methanol to form diastereomeric acetal derivatives that are hydrolyzed with HCl to yield <b>5</b> and <b>5</b>'. ESR spectroscopy in a glassy matrix at 77 K showed that irradiation of <b>1</b> yields corannulene nitrene ³<b>1N</b>, which has significant 1,3-biradical character. Irradiation of <b>1</b> in a glassy matrix resulted in a new absorption band in the region of 360-440 nm, with λ_max at 360 and 410 nm, attributed to ³<b>1N</b>, as supported by time-dependent density function theory calculations, which placed the major electronic transitions of ³<b>1N</b> at 367 nm (<i>f</i> = 0.0407) and 440 nm (<i>f</i> = 0.0353). Laser flash photolysis of <b>1</b> revealed a similar absorption spectrum. Nitrene ³<b>1N</b> had a lifetime of only a few hundred nanoseconds and was efficiently quenched by oxygen, because of its 1,3-biradical character. CASPT2(12,11)/6-311G** calculations revealed small energy gap (7.2 kcal/mol) between singlet and triplet configurations, suggesting that ³<b>1N</b> is formed by intersystem crossing of ¹<b>1N</b> to ³<b>1N</b>. Spin-density, nucleus-independent chemical shift, and anisotropy of the induced current density calculations verified that ³<b>1N</b> is a triplet vinylnitrene with unpaired electrons localized on the C═C-N moiety; decaying by intersystem crossing to <b>2</b>, which is more stable owing to its aromaticity, as supported by calculations (SA-CASSCF/QD-NEVPT2/CBS).