Organic Letters
Letter
Accession Codes
oxidative PET with the EY−CN adduct and that the ensuing
EY•+ then oxidizes sulfinate into the sulfonyl radical II.
Reversible addition of the latter onto the olefin affords a
nucleophilic C-centered radical III, which may evolve through
two different pathways: (1) a cyano group radical transfer to
afford product V,14 or (2) the oxidation of III into a
carbocation IV, eventually trapped by a cyanide anion. In the
latter case, EY•+ is reduced back to EY. The proposed
mechanism also explains the excess of sulfonyl cyanides (1.5
equiv) needed to obtain optimal yields. Estimation of the
oxidation potential of model β-sulfonyl radicals III in DMF,
through DFT calculations (SI), indicates that oxidation of III
into IV is slightly exergonic (ΔGET ∼ −2 kcal/mol) or favored
with for more substituted olefins (IIIc, ΔGET = −14.1 kcal/
mol, SI). The absence of elimination products, alcohols, or
ethers when water or iPrOH was used as cosolvent is, however,
in sharp contrast with literature reports on radical-polar
crossover processes.13,23,24 Our experimental evidence thus
rules out the involvement of a cation such as IV as an
intermediate and a back electron transfer (BET) from III to
EY•+ cation-radical. Direct conversion of III into product V,
through a cyano group transfer process, would then constitute
a viable alternative pathway, as shown by the light ON/OFF
experiment (Figure S4). Quantum yields for the formation of
V measured in DMF and acetone upon monochromatic
excitation at 546 nm led, respectively, to values of ΦDMF = 0.2
and ΦAcetone = 1.17 (SI). Considering the intersystem crossing
efficiency of EY (ΦISC = 0.32 in methanol),25 these values
suggest that a moderately efficient radical chain is operating.
In summary, we have shown that the photocatalyzed
alkylsulfonyl cyanation of a variety of olefins affords the
corresponding β-sulfonyl nitriles in good yield. Combined with
a safe protocol for the preparation of sulfonyl cyanides, this
methodology provides a convenient access to β-sulfonyl
nitriles, useful synthetic intermediates, as shown by the
synthesis of a metalloproteinase inhibitor. We find that the
reaction proceeds through a photoinduced electron transfer
mechanism involving a cyanated derivative of EY formed in
situ. This mechanism circumvents the energetically unfavored
direct electron transfer from the excited EY triplet state to the
alkylsulfonyl cyanide and instead relies on the oxidation of the
sulfinate anion by the ground state oxidized EY•+.23a The
present mechanistic studies provide an in-depth and more
general picture of the unique reactivity of alkyl- and
arylsulfonyl cyanides in the presence of eosin,13 which adds
to the direct ET or energy transfer mechanisms usually
proposed in EY-mediated processes. Further work will be
necessary to establish whether this mechanism is also present
in other photocatalyzed transformations involving xanthene
dyes.
crystallographic data for this paper. These data can be obtained
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
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Corresponding Author
ORCID
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
V.P. thanks the “Fondation pour le developpement de la
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Chimie des substances Naturelles et ses applications” for a
PhD Grant. G.K. thanks the University of Ankara for post-
doctoral funding. Funding from the ANR (ANR-17-CE07-
0043-02), the CNRS, and the University of Bordeaux (UBx) is
gratefully acknowledged. We thank Dr. C. Absalon (UBx) and
the CESAMO Analytical Center for help with structure
analysis and quantification.
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge on the
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Experimental procedures and spectroscopic data for all
new compounds; electrochemical, UV−vis spectrometry,
and fluorescence studies; DFT calculations (PDF)
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Org. Lett. XXXX, XXX, XXX−XXX