Light-Mediated Synthesis of 2-(4-Methoxyphenyl)-1-pyrroline via Intramolecular Reductive Cyclization of a Triplet Alkylnitrene

Irradiation of p-methoxyazidobutyrophenone(1) in methanol yielded 2-(4-methoxyphenyl)-1-pyrroline(2) and several other photoproducts. However, in thepresenceof tris-(trimethylsilyl)-silane (TTMSS), 2 is formed selectively.Transient absorption and ESR spectroscopy verify that the irradiationof 1 forms triplet alkylnitrene (3) 1N through intramolecular energy transfer from the triplet ketone (T-1K). DFT calculations indicate that (3) 1N abstracts H atoms from TTMSS but not methanol, which explains theselectivity. Thus, triplet alkylnitrenes can undergo selective reductivecyclization via H atom abstraction from TTMSS.

Automated summary

Irradiation of p-methoxyazidobutyrophenone (1) in methanol led to the formation of 2-(4-methoxyphenyl)-1-pyrroline (2) and several other photoproducts. In the presence of tris-(trimethylsilyl)-silane (TTMSS), 2 was selectively formed. Transient absorption and ESR spectroscopy confirmed the formation of triplet alkylnitrene (3) 1N through intramolecular energy transfer from the triplet ketone (T-1K). DFT calculations revealed that (3) 1N abstracts H atoms from TTMSS but not methanol, explaining the observed selectivity. Therefore, triplet alkylnitrenes can undergo selective reductive cyclization via H atom abstraction from TTMSS.