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 the presence of tris(trimethylsilyl)silane (TTMSS), 2 is formed selectively. Transient absorption and ESR spectroscopy verify that the irradiation of 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 the selectivity. Thus, triplet alkylnitrenes can undergo selective reductive cyclization via H atom abstraction from TTMSS.