Angewandte
Communications
Chemie
2.5 equivalents of CsF, and 8.0 equivalents of DMPU·HF in
a carbon-centered radical intermediate in the reaction, and
10:1 (v:v) 1,4-dioxane/HFIP under N2 atmosphere at 258C,
were found to produce the highest yields. No fluorination
products were detected in control experiments where either
the fluoride ion or oxidant was omitted. The roles of 1,4-
dioxane and HFIP are unclear.[18]
it was also supported by density-functional theory (DFT)
calculations (see the Supporting Information for details). In
addition, monitoring of the reaction by 19F NMR spectrosco-
py revealed that PhIF2 (d = À176 ppm) was generated in the
reaction.[19] In the presence of PhIF2, the fluorinated product 7
was formed in 79% yield with the cesium organopentafluoro-
silicate 6’’, whereas no product was formed with the organo-
trifluorosilane 6’. These results demonstrate that the in situ
generated organopentafluorosilicate can react productively
with PhIF2.
Having established the optimum reaction conditions, we
explored the substrate scope of the transformation
(Scheme 2). First, various primary alkylsilanes bearing elec-
tron-donating and electron-withdrawing substituents on aryl
rings (1a–w) and primary alkylsilanes without aryl rings (1x–
cc) were successfully converted into the desired products with
good yields. Notably, the heteroaromatic substrates 1l, 1n,
and 1o were also successfully employed to provide the
corresponding products (2l, 2n, and 2o). A good range of
functional groups including ether, nitrile, sulfonyl, chloride,
bromide, and even iodide and alkene were well tolerated
under the mild reaction conditions. These results encouraged
the application of this method to more complex small
molecules, 1dd and 1ee, which gave the corresponding
fluorination products 2dd and 2ee in moderate yields. In
addition, we prepared the compound 2g, on gram scale under
the standard reaction conditions, in 83%, thus demonstrating
the scalability of this method. No desired fluorination product
was observed with secondary alkylsilanes, such as cyclo-
hexyltrimethoxysilane, and the major byproducts were an
alkene and a ketone. Our attempts to test more secondary or
tertiary alkyltrimethoxysilanes, especially functionalized
compounds, were hampered by their difficult preparation.
Although the mechanistic details of this transformation
are not yet clear, preliminary observations provide some
insights (Scheme 3). First, a radical clock experiment was
performed with substrate 3, and the ring-opening product 4
was obtained in 27% yield, as determined by 19F NMR
analysis of the crude reaction mixture. When 4.0 equivalents
of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) were
added, the TEMPO adduct 5 was isolated in 52% yield.
Together, these observations provide solid evidence for
On the basis of these mechanistic investigations and
associated DFT calculations (see the Supporting Information
for details), we proposed the mechanism depicted in
Scheme 4. In the presence of CsF and DMPU·HF, the
Scheme 4. Proposed mechanism.
organopentafluorosilicate A and PhIF2 are generated from
alkylsilane and PhIO, respectively. Next, single-electron
transfer from A to PhIF2 generates a carbon radical inter-
mediate and iodoarene radical (B), which subsequently form
the stable hypervalent iodine(III) species C. The formation of
this species facilitates an activation-displacement sequence by
the transition-state D with the aid of DMPU·HF to produce
the final product.[20]
In conclusion, we have developed the first transition-
metal-free hypervalent iodine(III)-mediated fluorination of
alkylsilanes with fluoride. Compared to conventional meth-
ods for the preparation of alkyl fluorides by SN2-type
reactions of alkyl (pseudo)halides or alcohols,[3] this method
enables the facile replacement of various primary aliphatic
silanes with fluoride ions under mild reaction conditions and
tolerates a wide range of functional groups, including alkyl
halides. Additionally, preliminary mechanistic studies and
DFT calculations suggest that the reaction may proceed
through a radical mechanism. Although secondary alkylsi-
lanes do not give the desired fluorinated products under these
reaction conditions, this reaction provides a complementary
method for the synthesis of primary alkyl fluorides. Further-
more, these studies provide a solution to the challenge of
fluorination of alkylsilanes by fluoride while also open a new
path for the design of intermolecular radical reactions of
alkylsilanes.
Scheme 3. Mechanism studies.
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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