818
Bull. Chem. Soc. Jpn. Vol. 86, No. 7 (2013)
Gas-Phase Acidities of ¡- and ¡,¡-SO2CF3-Substituted Toluenes
and R2 substituents is very small, i.e., ¹13.0 for R1 = R2 = Me
to ¹14.5 for R1 = H, R2 = CF3. The present obtained μ values
for the electron-rich system forces us to reexamine the previous
results for a series of benzylic carbocations for a comparison
with the present observation. A careful examination reveals
that the μ value for the benzylic carbocation tends to decrease
with the increasing stability of a carbocation. The μ values are
correlated linearly with the stability of the unsubstituted parent
benzylic carbocation in a similar manner to the present benzylic
carbanion system (Figure S3),27 indicating that the μ value for
the carbocation system is also dependent on the thermodynamic
stability of unsubstituted parent cabocation.
intrinsic properties arising from the structure of carbanions
or carbocations. In addition, it was found that the μ values also
decreased with increasing acidity of the GA values of the
unsubstituted parent carbon acids. This would be due to the
varying distribution of the charge in the molecuar skeleton
caused by the R1 and R2 groups.
Experimental
Melting points were measured on a YanaCO MP-J3
apparatus and are reported uncorrected. The NMR spectra
were recorded on a JEOL JNM-EX400 or ECA-500 FT-NMR
spectrometer in CDCl3 and chemical shifts are reported in
¤ value downfield from the internal standard tetramethylsilane
(TMS).
μ ¼ ꢀ0:069ꢀꢀGꢁðX=HÞ ꢀ 13:1 ðR ¼ 0:975Þ
ð12Þ
where ¦¦G°(X=H) is the thermodynamic stability of benzylic
carbocation, C6H5CH+(R1)R2, relative to that of ¡-cumyl
cation (R1 = R2 = CH3). Such variation of the μ value would
be interpreted by the charge distribution in a molecular
framework of interest. In the thermodynamically more stable
carbanions with strong electron-withdrawing groups R1 and R2,
the negative charges formed at the benzylic carbon atom by
deprotonation are located at the C(R1)R2 moiety rather than the
aromatic moiety. For example, in the conjugate anion of 2 a
total natural charge of the phenyl moiety is only ¹0.035 and
the rest of the charge is localized at the C(SO2CF3)2 moiety
while in the benzyl anion (R1 = R2 = H) the group natural
charges of the phenyl moiety is ¹0.618 as seen in Table 5. That
is, the center of the charge would be far from the substituent
on the benzene ring in the conjugate anion of 2, resulting in a
small μ value. The same situation must be held in the benzylic
carbocations.
Chemicals. Preparation of (m,p-substituted phenyl)methyl
trifluoromethyl sulfones (1).31 General procedure: A mixture of
benzylic halide (10 mmol) and CF3SO2Na (2.0 g, 13 mmol) in
propionitrile (30 mL) was heated at reflux temperature. While
disappearance of the starting halide was monitored by TLC,
the mixture was cooled, the salts were removed by filtration
and the filtrate was concentrated under reduced pressure. The
residue was purified by column chromatography using a linear
EtOAc gradient in hexane to give the targeted products 1 as
solids. Phenylmethyl trifluoromethyl sulfone: mp 81-83 °C.
1H NMR (CDCl3): ¤ 4.48 (s, 2H), 7.38-7.46 (m, 5H). 3-Fluoro-
1
phenylmethyl trifluoromethyl sulfone: mp 88-89 °C. H NMR
(CDCl3): ¤ 4.47 (s, 2H), 7.16-7.46 (m, 4H). 4-Fluorophenyl-
methyl trifluoromethyl sulfone: mp 103-105 °C. 1H NMR
(CDCl3): ¤ 4.46 (s, 2H), 7.13-7.16 (m, 2H), 7.40-7.43 (m,
2H). 3-Trifluoromethylphenylmethyl trifluoromethyl sulfone:
mp 80-81 °C. 1H NMR (CDCl3): ¤ 4.54 (s, 2H), 7.59-7.76 (m,
4H). Anal. Calcd for C9H6F6O2S: C, 36.99; H, 2.07%. Found:
C, 36.45; H, 2.21%. 4-Trifluoromethylphenylmethyl trifluoro-
methyl sulfone: mp 110-112 °C. 1H NMR (CDCl3): ¤ 4.54
(s, 2H), 7.58 (d, J = 8.0 Hz, 2H), 7.72 (d, J = 8.0 Hz, 2H).
3-Nitrophenylmethyl trifluoromethyl sulfone: mp 84-85 °C.
1H NMR (CDCl3): ¤ 4.59 (s, 2H), 7.67-7.82 (m, 2H), 8.33-
8.37 (m, 2H). Anal. Calcd for C8H6F3NO4S: C, 35.69; H,
2.25; N, 5.20%. Found: C, 36.45; H, 2.21; N, 5.47%. 4-
Nitrophenylmethyl trifluoromethyl sulfone: mp 101-102 °C.
1H NMR (CDCl3): ¤ 4.58 (s, 2H), 7.66 (d, J = 8.0 Hz, 2H),
8.32 (d, J = 8.8 Hz, 2H). Anal. Calcd for C8H6F3NO4S: C,
35.69; H, 2.23; N, 5.20%. Found: C, 35.85; H, 2.20; N, 5.37%.
3-Chlorophenylmethyl trifluoromethyl sulfone: mp 88-89 °C.
1H NMR (CDCl3): ¤ 4.45 (s, 2H), 7.23-7.47 (m, 4H). Anal.
Calcd for C8H6ClF3O2S: C, 37.14; H, 2.32%. Found: C, 37.01;
H, 2.32%. 4-Chlorophenylmethyl trifluoromethyl sulfone: mp
Finally, it should be noted that the μ value of 7.4 for 2 is
remarkably smaller than the expected value from the linear
relationship shown in Figure 5. The reason for the small μ
value is not clear at the present stage. Similar small μ values as
well as the r+ value were observed for the solvolysis of ¡,¡-
di-t-butylbenzyl p-nitrobenzoates28 and 4-methylbenzo[2.2.2]-
bicycloocten-1-yl triflates29 in which the vacant p-orbital of
carbocation intermediates is significantly twisted from the
benzene ³-orbital plane.30 The calculated geometry of the
conjugate anion of 2 shows that the S-C-S plane of the
¹
C (SO2CF3) moiety is also twisted from the benzene plane by
61.3°, suggesting that there may be some common reasons for
a reduced μ value in a highly congested system. Further study
is obviously needed to answer this question.
Conclusion
Substituent effects for acidities of a series of ArCH(R1)R2,
have been analyzed successfully in terms of the Y-T equation
using substituent parameters in gas the phase. The resultant
103-105 °C. H NMR (CDCl3): ¤ 4.45 (s, 2H), 7.39 (d, J =
1
8.4 Hz, 2H), 7.44 (d, J = 8.8 Hz, 2H). 3-Cyanophenylmethyl
trifluoromethyl sulfone: mp 75-77 °C. 1H NMR (CDCl3): ¤
4.41 (s, 2H), 7.58-7.79 (m, 4H); 13C NMR (CDCl3): ¤ 55.17,
113.85, 117.58, 120.89, 125.07, 130.23, 133.64, 134.56,
135.47. Anal. Calcd for C9H6NF3O2S: C, 43.37; H, 2.41; N,
5.62%. Found: C, 43.44; H, 2.36; N, 5.87%. 4-Cyanophenyl-
methyl trifluoromethyl sulfone: mp 123-125 °C. 1H NMR
(CDCl3): ¤ 4.53 (s, 2H), 7.57 (d, J = 8.4 Hz, 2H), 7.76
(d, J = 8.0 Hz, 2H). 3,5-Bis(triluoromethyl)phenylmethyl tri-
¹
resonance demand parameter r value decreased linearly with
increasing acidity of the GA values of the unsubstituted parent
¹
carbon acids, and the change of the r value was found to be
related with the geometric parameters and natural charges of
the conjugate carbanions calculated at B3LYP/6-311+G(d,p).
Such behavior of the resonance demand in the electron-
¹
rich system, ArC (R1)R2, is completely consistent with that
observed for the electron-deficient system, ArC+(R1)R2, sug-
gesting that the resonance demand must be determined by the
fluoromethyl sulfone: mp 86-87 °C. H NMR (CDCl3): ¤ 4.60
1
(s, 2H), 7.91 (s, 2H), 8.00 (s, 1H).