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Organic & Biomolecular Chemistry
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Journal Name
esterifications that are catalytic in
visible light and readily available riboflavin derivative 3d, which
is used in a catalytic amount, being recycled with molecular
oxygen, an inexpensive and green terminal oxidant. The only
drawback is side photooxidation of Ph3P to Ph3P=O under
aerial conditions, which can be eliminated by in-situ back-
reduction. We have observed a completely new esterification
pathway being involved in our photocatalytic esterification
COMMUNICATION
1
. The method is based on
1469.
DOI: 10.1039/C6OB02770A
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13 Very recently, idea on system catalytic both from the point
of view of azo-compound and phosphine based on the above
mentioned recycling agents was introduced (ref.12) but
realization is still subject of discussion; see D. Hirose, M.
Gazvoda, J. Kosmrlj, T. Taniguchi Org Lett 2016, 18, 4036.
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15 S. Kümmel, R. Cibulka, B. König, B. In Chemical
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that occurs without any contribution from the azo compound
+
1
or hydrazine
2
. It proceeds through Ph3P∙ generated by
photoinduced electron transfer to flavin. To the best of our
knowledge, no similar synthetic application of photochemically
+
generated Ph3P∙ has hitherto been reported. As there is still
room for improvement, optimization of both concepts is
currently being pursued in our laboratories.
Acknowledgement
This project was supported by the Czech Science Foundation
(Grant No. 16-09436S). R.C. thanks the German National
Science Foundation (GRK 1626 „Chemical Photocatalysis“).
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Notes and references
‡ Notably only trace of ester was formed in stoichiometric
Mitsunobu reaction in the presence of hydrogen peroxide (1
equiv.) in acetonitrile while 90% of ester was formed after
addition of MS 4A thus demonstrating H2O2-quenching of
Mitsunobu reaction. Hydrogen peroxide was not detected by
iodometry after photocatalytic esterifications in the presence of
MS 4A.
§ This value was not affected by either light or the flavin (see
ESI).
§§ Notably analogous formation of alkoxyphosphonium and
acyloxyphosphonium species was observed when Ph3P∙+ was
generated electrochemically, see ref.26.
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