Journal of the American Chemical Society
Communication
Scheme 2. Demonstration that the Strength of the
Conjugate Base (the iminium counter-anion) Influences the
Regioselectivity and Is Therefore the System’s Active Base
AUTHOR INFORMATION
Corresponding Author
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ORCID
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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We thank MINECO (CTQ2016-75520-P), and the European
Research Council (ERC 681840-CATA-LUX) for financial
support. D.M. thanks H2020-MSCA-ITN-2016 (722591-
PHOTOTRAIN) for a predoctoral fellowship. G.E.M.C.
thanks the Marie Skłodowska-Curie Actions for a postdoctoral
fellowship (H2020-MSCA-IF-2017 795793).
conjugate base lowers the ability to discriminate between the
primary and the secondary site, which leads to the
thermodynamically more stable radical (BDEs for toluene
and ethylbenzene are 368 and 357 kJ mol−1, respectively).21
These experiments are consonant with the conjugate base of
the acid cocatalyst (i.e., the counteranion of the iminium ion)
being the active base in the system. In addition, we measured a
kinetic isotope effect of 2.0 (kH/kD) when performing the
model reaction using toluene-d8. Overall, these data are
consistent with a sequential multisite PCET mechanism of
benzylic C−H bond activation where the deprotonation of the
benzyl radical cation V is the first irreversible step.
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On the basis of these experiments and of our previous
reports,12 a plausible mechanism is depicted in Figure 3. The
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Figure 3. Mechanistic proposal.
acid cocatalyst assists the formation of the iminium ion I.
Upon light absorption, the electronically excited state I*
oxidizes toluene to form the radical cation V, which is
deprotonated by the counteranion of the iminium ion (green
circle). This irreversible step is essential to avoid an irreversible
BET and delivers the benzyl radical VI. The ensuing
stereocontrolled radical coupling with IV forges the stereo-
center within product 2.
In summary, we have documented a photochemical
organocatalytic method for the direct stereoselective C−H
functionalization of feedstock chemicals, including toluene and
xylene derivatives.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
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Experimental procedures and spectral data (PDF)
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J. Am. Chem. Soc. XXXX, XXX, XXX−XXX