Conclusions
In summary, a metal-free asymmetric hydrogenation of 6-substituted phenanthridines was achieved by using chiral monoene derived borane catalyst. a variety of dihydrophenanthridines were obtained in 81-96% yields with up to 93% ee. The obvious difference between the two sides of the N-ring was required to give reasonable reactivity and enantioselectivity. However, the bulky substituents at the 6-position would inhibit the reaction. Further efforts on searching for more effective chiral catalyst and expanding their application in asymmetric reactions are underway in our laboratory.
Experimental
Typical procedure for the asymmetric hydrogenation of phenanthridine 1c : To a glass test tube (10 mL) in a nitrogen atmosphere glovebox, HB(C6F5)2 (2 ) (13.8 mg, 0.04 mmol), chiral alkene 5a (28.0 mg, 0.04 mmol), and dry toluene (2.0 mL) were added. The resulting mixture was stirred at room temperature for 5 min, followed by the addition of phenanthridine 1c (122.2mg, 0.4 mmol). Then the tube was moved to a stainless-steel autoclave, and the autoclave was purged three times with H2 and the final pressure of H2 was adjusted to 40 bar. The reaction was stirred at 40 °C for 16 h. The solvent was removed under reduced pressure, and the crude residue was purified by flash chromatography on silica with petroleum ether/ethyl acetate (20/1) to afford dihydrophenanthridine 4c as a white solid (112.2 mg, 91% yield, 80% ee).
Supporting Information
The supporting information for this article is available on the WWW under https://doi.org/10.1002/cjoc.2023xxxxx.
Acknowledgement
We are grateful for the financial support from the National Natural Science Foundation of China (21825108 and 22331011)
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