engleski

Photochemical elimination of nitrogen from diazirines and diazo compounds

Elimination of nitrogen from diazo compounds and diazirines that delivers carbenes is a well known reaction, which can be initiated thermally or photochemically [1]. Prompted by our interest in the synthesis of polycyclic strained molecules [2], we studied photoelimination of nitrogen from adamantane 1 and pentacycloundecane (PCU) diazirine derivative 2. Combined preparative irradiations, spectroscopic study and computations for 1 and 2 indicated that photoproducts were formed via singlet and triplet carbenes, even though singlet carbenes are more stable then the triplets. Complexation of diazirines with macrocyclic host molecules changed the distribution of singlet and triplet photoproducts [3]. Furthermore, upon irradiation of 2 in CH3OH we isolated some products with rearranged PCU skeleton, which indicated that carbenes undergo protonation in CH3OH, and not O-H bond insertion, changing the paradigm for the mechanism of the reaction of carbenes with alcohols. Combined experimental and theoretical investigation of photoelimination of nitrogen from diazo compounds 3 and 4 unraveled hitherto undisclosed pathway involving higher excited singlet states. Fluorescence takes place from S2, whereas the photoelimination of nitrogen upon excitation to S2 proceeds more efficiently then upon excitation to S1, due to efficient deactivation to S0 via a conical intersection [4]. The discovery of anti-Kasha photochemical pathways is interesting due to its fundamental value, as well as for potential applications, since the photoreaction can be tuned by appropriate choice of light wavelength.

carbenes ; diazirines ; diazo compounds ; photochemistry

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