Transforming Triplet Vinylnitrene into Triplet Alkylnitrene at Cryogenic Temperatures

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 (1) in methanol/acetonitrile followed by HCl addition produced diastereomers 5 and 5'. Azirine 2 is apparently trapped by methanol to form diastereomeric acetal derivatives that are hydrolyzed with HCl to yield 5 and 5'. ESR spectroscopy in a glassy matrix at 77 K showed that irradiation of 1 yields corannulene nitrene 31N, which has significant 1,3-biradical character. Irradiation of 1 in a glassy matrix resulted in a new absorption band in the region of 360-440 nm, with λmax at 360 and 410 nm, attributed to 31N, as supported by time-dependent density function theory calculations, which placed the major electronic transitions of 31N at 367 nm (f = 0.0407) and 440 nm (f = 0.0353). Laser flash photolysis of 1 revealed a similar absorption spectrum. Nitrene 31N 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 31N is formed by intersystem crossing of 11N to 31N. Spin-density, nucleus-independent chemical shift, and anisotropy of the induced current density calculations verified that 31N is a triplet vinylnitrene with unpaired electrons localized on the C═C-N moiety; decaying by intersystem crossing to 2, which is more stable owing to its aromaticity, as supported by calculations (SA-CASSCF/QD-NEVPT2/CBS).

External photocatalyst-free photocycloaddition between triplet vinylnitrenes with 1,3-biradical character and activated olefins under 420 nm LEDs

Herein, we report that triplet vinylnitrenes with 1,3-biradical character can directly participate in photocycloaddition reactions with olefins to produce single diastereomers of the corresponding 1-pyrrolines under 420 nm LEDs in acetonitrile solvent. Moreover, a one-pot method has been developed to produce pyrroles directly through photocycloaddition and oxidation sequences. The excited state of the substrate olefin can sensitize vinyl azide via energy transfer, eliminating the need for an external photocatalyst or sensitizer.

Preparation and Photochemistry of Parent Triplet Vinylarsinidene

Vinyl pnictinidenes are an elusive family of molecules that have been suggested as key intermediates in multiple chemical reactions and commonly display a predisposition toward open-shell electronic ground states (as is evident from quantum chemical computations). However, owing to their expected extremely high reactivity, no vinyl pnictinidene has ever been isolated and characterized spectroscopically. Here, we report the synthesis and spectroscopic characterization of vinylarsinidene, a higher congener of vinylnitrene. As we demonstrate, triplet vinylarsinidene can be prepared through the low-temperature photolysis of diazidovinylarsine at 10 K in an argon matrix. The title compound can also be generated through high-vacuum flash pyrolysis of the diazide at 700 °C and trapped analogously. Triplet vinylarsinidene was characterized by IR and UV/vis spectroscopy and displayed remarkably rich unimolecular photochemistry. Upon selective photoirradiation, it rearranges to vinylidenearsine, 2H-arsirene, triplet ethynylarsinidene or an arsinidene (H-As) acetylene complex. The formation mechanisms of these products were rationalized with DFT and CASPT2 computations.