Abstract
Visible-light-excited 9,10-phenanthrenequinone (PQ*) was used as a photocatalyst for the synthesis of polysubstituted quinolines via the electrocyclization of 2-vinylarylimines. Up to quantitative yields of 2,4-disubstituted quinolines were received after 1 h of excitation with blue LEDs at room temperature when MgCO3 was used as an additive in DCM. On the basis of experimental and DFT studies, we propose that PQ* induces one-electron oxidation of the imine substrate that triggers the electrocyclization mechanism.
Highlights
Visible-light-excited 9,10-phenanthrenequinone (PQ*) was used as a photocatalyst for the synthesis of polysubstituted quinolines via the electrocyclization of 2-vinylarylimines
Quinolines are an important class of N-heterocyclic aromatic compounds as they are ubiquitous structures in natural products and pharmaceuticals.1a The selective functionalization of different positions of the quinoline ring is often hard to achieve and substituted quinolines are usually synthesized from precursors carrying functional groups placed at the desired positions.[1]
Recent developments in methods to generate organic radicals have led to the rise of novel synthetic methodologies to perform organic transformations under mild reaction conditions.[5]
Summary
K2CO3 as additived,e mg/mL MgCO3d,e mg/mL MgCO3d,e mol % PQd mol % PQd mmol scale, 21 h reaction time. With the optimized reaction conditions in hand, we moved on to study the cyclization scope by varying the substitution on the R1−R3 positions, probing electronic effects (R1−R3) and the aromatic or aliphatic substituent effect (R1 and R2) Both 4-methylquinolines 2a−2h and 4-phenylquinolines 2t−2z were obtained in excellent yields in 1−3 h (Scheme 2). Computed pKas of the iminium radical cation INT1 and the cyclized intermediate INT2 (Scheme 4) resulted in values of 45−51 and 17−20 in DCM, respectively (Table S10) This indicates the dramatically increased acidity for the proton laying on the 2-position of INT2, favoring the C−C coupling step before the deprotonation event in the mechanistic route.[23]. The performed preliminary mechanistic studies suggest that excited-state PQ oxidizes imines to iminium cation radicals that trigger the cyclization step, leading to the subsequent deprotonation and dehydrogenation steps and the final formation of quinolines. Experimental procedures, full reaction optimization, 1H NMR reaction monitoring, mechanistic studies, computational procedures and details, and 1H and 13C NMR spectra for all compounds (PDF)
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