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low temperature 19F and 13C NMR. Initial results using
PhSCF2Br (1a) at ꢀ50 °C indicated the 1a-cation was
immediately trapped with a fluoride ion forming 5a and
Table 2. Electronic Effect on the Generation and Stability of the
Difluoro(phenylthio)methylium Cationa
Sb2F11 .
ꢀ 14 The poorly nucleophilic SbF6ꢀ anion has been
previously demonstrated to provide fluoride in the pre-
sence of extremely electrophilic species such as the trifluoro-
methyl cation (CF3þ), driven by the formation of the strong
CꢀF bond in the resulting tetrafluoromethane (CF4).15
Consistent with these findings, we propose that abstraction
substrate
Ar
n
yield (%)b
ꢀ
þ
of fluoride from SbF6 by PhSCF2 results in thermally
stable PhSCF3 with an additional driving force attributed
1a
1b
1c
1d
1e
1f
C6H5
C6H5
C6H5
0
1
2
0
0
0
0
0
0
3aa
65 (61)
4a
no reaction
no reaction
18 (16)
ꢀ 16
to the polymerization of SbF5/SbF6
.
Additional evi-
þ
dence was obtained when attempting to trap PhSCF2
with 4-dimethylaminopyridine (DMAP) resulting in the
isolation of PhSCF3 (5a) and precipitation of a DMAP/
4-FC6H4
3da
3ea
3fa
71 (60)
4d
4e
4f
20 (18)
18
4-ClC6H4
47
17
4-BrC6H4
4-CH3OC6H4
4-O2NC6H4
2,4-F2C6H4
15
1g
1h
1i
3ga
50 (48)
4g
30 (25)
Sb2F11 SbF5 complex containing protonated DMAP,
[Sb2F11] , and SbF5 moieties as confirmed by single crystal
X-ray diffraction (see SI).
3 ꢀ
no reaction
no reaction
a Reaction conditions: A solution of AgSbF6 (1 mmol) in DCM
(10 mL) was added dropwise over 5 min to a mixture of 1aꢀi (0.5 mmol)
and allyltrimethylsilane 2 (0.6 mmol) in DCM (5 mL). b Yields were
determined by 19F NMR using PhSCF2SO2Ph as an internal standard;
yields in parentheses are of isolated products.
In anticipation of stabilizing the cation, we employed
bromodifluoro(4-methoxyphenylthio)methane (1g) as a
substrate. After mixing 1g with AgSbF6 for 10 min
at ꢀ78 °C, a 19F chemical shift at δF = þ45.2 ppm
(Δ19F 68.2) and 13C chemical shift at δC = þ234.4 ppm
(Δ13C 114.6) as an apparent doublet (1JCF = 390 Hz)
were observed at ꢀ60 °C. The downfield shifts of
the fluorine and carbon signals suggested the formation
of R-fluorocarbocation intermediates, which can be the
R-difluorocarbocation [(4-CH3OC6H4S)CF2]þ, 1g-cation,
or R-monofluorocarbocation [(4-CH3OC6H4S)2CF]þ, 4g-
cation, generated by self-attack of 1g. Combined theoretical
and experimental 19F and 13C NMR chemical shifts can
give additional support for structure identification.8e,17
Thus, to identify the R-fluorocarbocation intermediate
observed in this work, we performed density functional
calculations for the 19F and 13C chemical shifts of 1a,
the 1g-cation, the 4g-cation, and several other fluoro com-
pounds with available experimental chemical shifts
(10ꢀ12) (Figure S1 and Table S2).
the 1g-cation and 4g-cation showed that the CꢀF bond
˚
distance in the 1g-cation (1.29 A) is relatively shorter than
˚
that of the 4g-cation (1.33 A) (Figure S1), suggesting a
higher extent of p(π) back bonding from fluorine to the
carbocation center.15
To estimate the stability of the 4g-cation vs the 1g-cation,
wecalculatedsolvent corrected relativefreeenergiesfor the
fluoride transfer reactions (Figure S2). The fluoride transfer
from [SbF6]ꢀ to the 1g-cation, to form 4-CH3OC6H4SCF3
(5g), is slightly exergonic (ꢀ2.29 kcal/mol) while the same
reaction to 4g-cation is unfavorable (þ17.94 kcal/mol); that
is 1g-cation, once formed, is more reactive than 4g-cation.
Thus, we concluded that the experimentally observed
R-fluorocarbocation is the 4g-cation.
In summary we have developed for the first time the direct
method for electrophilic difluoro(phenylthio)methylation
of R-difluorocarbocations with carbon nucleophiles. The
chemical and theoretical evidence for the presence of the
difluoro(phenylthio)methylium cation was also provided.
The studies also revealed that the R-difluorocarbocation
is a very reactive species which could be trapped at
low temperature. Further application of this chemistry is
under investigation.
The calculated 19F chemical shift of the 1g-cation could
be observed at an average of þ34 ppm while the calcula-
ted 19F chemical shift of the 4g-cation is at þ38.9 ppm
(Figure S1). Within the calculation error, the presence
of the 4g-cation is more likely for the experimentally
observed value at þ45.2 ppm. Moreover, the calculated
13C chemical shift of the carbocation center inthe 4g-cation
(þ222.2 ppm) is in better agreement with the experiment
(þ234.4 ppm) than that in the 1g-cation (þ169.3 ppm).
The more electron shielded 13C NMR shift of 1g-cation
indicates fluorine plays a more prominent role to stabilize
the carbocation center. Indeed, optimized geometries of
Acknowledgment. We thank the Center of Excellence
for Innovation in Chemistry (PERCH-CIC), the Office
of the Higher Education Commission, and Mahidol Uni-
versity under the National Research Universities Program
of Thailand for financial support.
(14) Culmann, J.-C.; Fauconet, M.; Jost, R.; Sommer, J. New J.
Chem. 1999, 23, 863.
(15) Prakash, G. S.; Rasul, G.; Burrichter, A.; Laali, K. K.; Olah,
G. A. J. Org. Chem. 1996, 61, 9253.
(16) Jenkins, H. D. B.; Roobottom, H.; Passmore, J. Inorg. Chem.
2003, 42, 2886.
Supporting Information Available. Experimental, crys-
tallographic, computaional details, and Cartesian coordi-
nate of all calculated structures. This material is available
(17) (a) Bagno, A.; Rastrelli, F.; Saielli, G. J. Phys. Chem. A 2003,
ꢀ
107, 9964. (b) von Rague Schleyer, P.; Koch, W.; Liu, B.; Fleischer, U.
J. Chem. Soc., Chem. Commun. 1989, 1098. (c) Naredla, R. R.; Zheng,
C.; Nilsson Lill, S. O.; Klumpp, D. A. J. Am. Chem. Soc. 2011, 133,
13169.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX