Organic Letters
Letter
reaction proceeded smoothly under blue LED irradiation,
which showed negligible advancements during the light-free
stages. Radical trap experiments with TEMPO led to
contrasting results with respect to similar processes previously
investigated.3d,6,22 In particular, attempts to replace the
cobaloxime 3a with stoichiometric amounts of TEMPO failed
in promoting the photoredox condensation, and no overall
inhibition was observed when the radical trap was added to
optimal conditions.
AUTHOR INFORMATION
Corresponding Author
■
Marco Bandini − Dipartimento di Chimica “Giacomo
̀
Ciamician”, Alma Mater Studiorum, Universita di Bologna,
40126 Bologna, Italy; Consorzio CINMPIS, 40126 Bologna,
Authors
Mechanistically, the schematic representation depicted in
Figure 2b is proposed. Irradiation with a blue LED (465 nm,
Yang Liu − Dipartimento di Chimica “Giacomo Ciamician”,
̀
Alma Mater Studiorum, Universita di Bologna, 40126
23 W) promotes the [Acr+-Mes ClO4 ] A into the
−
Bologna, Italy
corresponding excited state [Acr+-Mes ClO4−]* A*
(Ered[Acr•-Mes•+/Acr•-Mes] = +2.06 V vs SCE)11a that
could oxidize the olefin 1a (Eox 1a/1a•+ = +2.00 V vs
SCE)23 via an SET process and deliver the aryl cation I and the
reduced form of the PC B.24 Therefore, the reoxidation of B by
Simone Battaglioli − Dipartimento di Chimica “Giacomo
̀
Ciamician”, Alma Mater Studiorum, Universita di Bologna,
40126 Bologna, Italy
Lorenzo Lombardi − Dipartimento di Chimica “Giacomo
̀
Ciamician”, Alma Mater Studiorum, Universita di Bologna,
40126 Bologna, Italy
red
[Co(III)(dmgH)2pyCl] (E1/2 Co(III)/Co(II) = −0.67 V vs
SCE)25a would restore the [Acr+-Mes ClO4 ] A with the
−
Arianna Menichetti − Dipartimento di Chimica “Giacomo
concomitant reduction of the cobalt species. The radical cation
II could undergo anti-Markovnikov condensation with the
carboxylic acid26 releasing the α-carboxyl-benzyl radical III
upon deprotonation. Therefore, the in situ formed [Co(II)]
complex might trap the radical III to deliver a [Co(III)]−alkyl
intermediate that would rapidly evolve into the final product 5
and the corresponding [Co(III)−H] via β-H elimination.27,28
It is worth mentioning that, as the β-H elimination of alkyl−
Co species is subjected to rigid stereochemical constraints (i.e.,
syn periplanar conformations are required),29 we can speculate
that the β-CH−OCO2R cannot arrange syn periplanar with
respect to the C−Co linkage, making the formation of the
enolester 5′ unlikely. Last, protonation of [Co(III)−H] would
restore the catalytically active [Co(III)] adduct via a hydrogen
evolution reaction (HER).30
̀
Ciamician”, Alma Mater Studiorum, Universita di Bologna,
40126 Bologna, Italy
Giovanni Valenti − Dipartimento di Chimica “Giacomo
̀
Ciamician”, Alma Mater Studiorum, Universita di Bologna,
Marco Montalti − Dipartimento di Chimica “Giacomo
̀
Ciamician”, Alma Mater Studiorum, Universita di Bologna,
40126 Bologna, Italy
Complete contact information is available at:
Author Contributions
‡Y.L. and S.B. contributed equally. The manuscript was written
through contributions of all authors. All authors have given
approval to the final version of the manuscript.
Finally, a kinetic isotope effect (KIE) experiment was carried
out with deuterated phenyl-cyclohexene d3-1a (Figure 2c).31 In
the intermolecular competition experiment, a 1a/d3-1a 1:1
mixture was utilized under optimal conditions (32 h, yield =
21%). Interestingly, no isotopic effect was observed (5aa:d2-
5aa = 1:1), excluding the β-elimination from the rate-
determining step of the catalytic cycle.
In conclusion, in this study, we have documented an
unprecedented dual visible-light/cobalt catalyzed redox proto-
col for the preparation of cyclic and acyclic allylic carboxylates
via direct Csp3−H oxidation of styryl compounds with
carboxylic acids. The oxidant-free methodology showed
peculiar anti-Markovnikov regiochemistry. An intramolecular
variant was also realized, resulting in the direct preparation of
isocoumarin scaffolds in up to 82% yield . Studies toward the
extension of the present methodology to the realization of
direct allylic C−H activation protocols are underway in our
laboratories.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.B. and S.B. are grateful to the University of Bologna for
financial support and PRIN-2017 project 2017W8KNZW.
M.M. is grateful to the MIUR, project PRIN 2017
2017E44A9P (BacHound). Prof. Stefano Protti (UniPv) and
Dr. Stefano Grilli (UniBo) are kindly acknowledged for the
stimulating discussions and technical support.
REFERENCES
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(1) (a) Oestreich, M. The Mizoroki-Heck Reaction; Wiley: Hoboken,
NJ, 2009. (b) Kurandina, D.; Chuentragool, P.; Gevorgyan, V.
51, 985−1005. (c) Nakashima, Y.; Hirata, G.; Sheppard, T. D.;
491.
(2) Togo, H. Advanced Free Radical Reactions for Organic Synthesis;
Elsevier: Amsterdam, 2004.
ASSOCIATED CONTENT
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sı
* Supporting Information
(3) (a) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko,
Soc. 2004, 126, 1600−1601. (b) Romero, N. A.; Margrey, K. A.; Tay,
The Supporting Information is available free of charge at
Synthetic and catalytic procedures, analytic character-
ization of unknown compounds, mechanistic experi-
ments, and NMR spectra (PDF)
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Org. Lett. 2021, 23, 4441−4446