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that performing a reaction for the synthesis of 3b in a batch
reactor for 43 min under the optimized reaction conditions
(Table 1) gave 3b in 36% yield. The improved product yield
(65% in batch vs 81% in flow) and decreased reaction time (24 h
in batch vs 43 min in flow) can be explained by considering the
short length scale and increased illumination homogeneity,
which led to a high photon flux in the microreactor.
In conclusion, we have developed an unprecedented visible
light-driven photocascade catalysis that utilizes dual photo-
catalysis to synthesize structurally novel, strained β-carbolines. It
involves highly regio- and diastereoselective [3 + 2] cyclo-
addition reactions of in situ generated 2H-azirines and
azomethine ylides from visible light-mediated photosensitized
decomposition of vinyl azides and photoredox oxidation of
1,2,3,4-tetrahydro-β-carbolines, respectively. The scope of the
reaction was studied by performing visible light-driven coupling
of several 1,2,3,4-tetrahydro-β-carbolines and vinyl azides in
batch and continuous flow microreactors using Ru(bpy)3(PF6)2
as a photocatalyst and TBHP as an oxidant. This successful
demonstration of photocascade catalysis opens the door for
exploring the possibilities of generating structurally novel
molecular libraries of biological importance and provides a
potential tool for the direct utilization of visible light for
constructing otherwise difficult molecular architectures.
ASSOCIATED CONTENT
* Supporting Information
■
S
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G.; Maurya, R. A. Org. Chem. Front. 2015, 2, 1308.
The Supporting Information is available free of charge on the
Full experimental details, characterization data for all
products, copies of 1H and 13C spectra, and single-crystal
(8) Hu, B.; DiMagno, S. G. Org. Biomol. Chem. 2015, 13, 3844.
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AUTHOR INFORMATION
Corresponding Author
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
R.A.M. is thankful to the Science and Engineering Research
Board, Department of Science & Technology, Ministry of
Science and Technology India, for financial support. S.B. is
thankful to the University Grants Commission for his fellowship.
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