Organic Letters
Letter
different mechanisms, which involve either a single electron
transfer or a hydrogen atom transfer in the radical-initiation
step and radical-quenching step. At this stage, the thiol in this
reaction is preferred to serve as a hydrogen donor in the
radical-quenching step rather than a hydrogen abstractor of
hydrosilane in the radical-initiation step.
Scheme 5. A Proposed Mechanism for the Hydrosilylation
In conclusion, visible-light-promoted metal-free hydrosilyla-
tion of alkynes has been developed by using catalytic amounts
of Eosin Y as a photocatalyst, thiol as a radical quencher, and
potassium carbonate as a base additive. A variety of terminal
and internal alkynes with different electronic properties were
successfully transformed to the corresponding di- and
trisubstituted alkenylsilanes with good functional group
tolerance. Control experiments indicate that the hydrosilylation
is initiated preferentially by a SET between hydrosilane and
excited Eosin Y, and a photoredox mechanism is preferred.
ASSOCIATED CONTENT
* Supporting Information
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S
The Supporting Information is available free of charge on the
formation of interconvertible vinyl radicals C and D.14 After a
hydrogen atom abstraction from thiol, the Z- and E-
alkenylsilane adducts will be yielded accompanied by thiol
radical E. A single electron transfer between thiyl radical E and
reduced EY will complete the catalytic cycle, affording the
photocatalyst and a thiol anion F, the latter of which is prone to
recombine with a proton to regenerate thiol. The following
should be noted: (1) The relative reactivity of the
intermolecular HAT and thermodynamic stability of silylated
vinyl radicals C and D are pivotal factors controlling the final
distribution of regioisomers and stereoisomers. For terminal
alkynes (R1 = H), anti-Markovnikov Z-alkenylsilanes derived
from radical C were isolated as a major product, even though
radical D is thermodynamically more stable than C. The
quenching of radical D should be slower than that of C, oweing
to the bulky steric hindrance of the silyl group, which makes the
abstraction of hydrogen atom disadvantageous. The excellent
Z-slectivity was also observed in trisubstituted alkenylsilane 32.
However, in the case of trisubstituted aromatic alkenylsilanes
(R2 = Ar), poor stereoselectivities were encountered, which
indicates that the quenching of radical D is competitive.
Furthermore, the introduction of an electron-deficient group
(R1 = CO2Me) on the internal alkyne might facilitate the
quenching of radical D, providing the E-alkenylsilane as a major
product. (2) From the point of view of polar effect, the thiol
might be preferred as a major hydrogen contributor of a
nucleophilic vinyl radical rather than the silane, even though a
hydrogen atom transfer from the silane to vinyl radical is
practical, especially in the absence of a thiol additive, as
evidenced by our deuterium experiments. (3) An alternative
chain-process mechanism, in which the thiol behaves as a
polarity-reversal catalyst in both the silyl-radical-initiation step
and vinyl-radical-quenching step,6c,15 is not supported by the
Experimental procedures, characterizations, and copies of
1H and 13C NMR spectra of products (PDF)
AUTHOR INFORMATION
Corresponding Authors
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ORCID
Author Contributions
†J.Z. and W.-C.C. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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The work was financially supported by the National Natural
Science Foundation of China (21572098, 21532002,
21472087). We thank Prof. Li-Zhu Wu and Dr. Tao Lei of
the Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences for the determination of quantum yields.
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quantum-yield calculations (see SI for more details). Poor
Ox
hydrosilylation was observed when triphenyl silane (E1/2
=
Ox
0.81 V vs SCE) or dimethylphenylsilane (E1/2 = 0.91 V vs
SCE) with higher oxidation potential was used, though a HAT
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