4
918 J . Org. Chem., Vol. 62, No. 15, 1997
Stavber et al.
9
Sch em e 5
for the reactions of CsSO
4
F with aldehydes in the case
of secondary alcohols, while in the case of benzyl alcohol
and 2-phenyl-1-propanol the inhibition is twice as strong
4
as for the reactions of CsSO F with the corresponding
aldehydes.9 The conversion of these primary alcohols to
acid fluorides or benzyl fluorides, respectively, are also
much more influenced by the solvent than the transfor-
9
mation of the analogous aldehydes to the same products,
as is evident from the data in Table 1. Less than 25% of
CH Cl in the solvent mixture stopped the reaction in
2 2
the case of 21a ,c, and 15% was sufficient to obtain the
same effect in the case of 26.
Ta ble 1. Effect of th e Rea ction Con d ition s on th e
Tr a n sfor m a tion of Acoh ols w ith CsSO4F a
Effect of th e Str u ctu r e of th e Alcoh ol on Rea ction
Ra tes. Applying the competitive technique for determi-
nation of relative reactivities expressed by relative rate
b
substrate
C6H5CH(Me)OH, 5b
solvent
yield (%)
1
0
MeCN
95; 10b
factors (krel), we established that benzyl alcohols are
more reactive than alkyl-substituted derivatives. 1-Phe-
nylethanol was found to be twice as reactive as 2-octanol
MeCN/0.1 mmol PhNO2 80
MeCN/1 mmol PhNO2
CH2Cl2
m-CF3C6H4CH(Me)OH, 5d MeCN
55
traces
93; 10d
(krel ) 1.8) and benzyl alcohol to the same degree more
reactive than decanol (krel ) 1.7) or 2-phenyl-1-propanol
(krel ) 2.1). This observation is in contrast to that found
for the relative reactivities of the corresponding alde-
MeCN/0.1 mmol PhNO2 86
MeCN/1 mmol PhNO2
MeCN
59
98; 22a
p-MeC6H4CH2OH, 21a
MeCN/0.1 mmol PhNO2 42
4
hydes in their reactions with CsSO F, where the reactiv-
MeCN/1 mmol PhNO2
MeCN/CH2Cl2 9:1
MeCN/CH2Cl2 4:1
MeCN/CH2Cl2 3:1
MeCN
18
69
54
traces
98; 22c
ity decreased in the order alkyl > aryl > benzyl substi-
tuted aldehyde.9 Correlation analysis using Hammett’s
relations between reaction rates and substituent param-
eters afforded values of the reaction constant for the
oxidation of a set of secondary (5) and primary (21) benzyl
C6H5CH2OH, 21c
MeCN/0.1 mmol PhNO2 65
MeCN/1 mmol PhNO2
MeCN/CH2Cl2 9:1
MeCN/CH2Cl2 4:1
MeCN/CH2Cl2 3:1
n-C6H14
26
79
53
0
4
alcohols with CsSO F. The Hammett correlation plot for
the transformation of 1-phenylethanols (5a -g) to ac-
etophenones, presented in Scheme 2, showed a satisfac-
0
+
tory correlation of relative rate factors with σ substit-
C6H5CH(Me)CH2OH, 26
MeCN
91; 31
30
traces
48
39
traces
+
uent constants and gave a straight line with slope F )
MeCN/0.1 PhNO2
MeCN/1 mmol PhNO2
MeCN/CH2Cl2 9:1
MeCN/CH2Cl2 8:1
MeCN/CH2Cl2 6:1
-
0.32 and a correlation coefficient of 0.97, while the
analysis of analogous data for a set of benzyl alcohols
(21a -f, Scheme 4) gave the value of -0.54 for the reaction
constant and a correlation coefficient of 0.99. The reac-
+
a
tion constant F for the oxidation of 5 to 10 has the same
sign but lower magnitude than those reported for the
Standard reaction conditions: 1 mmol of substrate in 2 mL of
solvent; inert atmosphere; 2.4 mmol (in the case of 5b and 5d 1.2
b
1
11
mmol) of CsSO4F; T ) 35 °C; 1 h. Measured from H (10a , 10d )
same reaction using dimethyldioxirane (F ) -1.57),
1
9
or F NMR spectra (22a , 22c, 31) of crude reaction mixtures using
anisole or octafluoronaphthalene as internal standard; calculated
on starting material.
12
chromic acid (F ) -1.16), or chromium trioxide in acetic
acid solvent (F ) -1.01)13 as oxidants and comparable to
those for oxidation by chromium trioxide under basic (F
1
3
ates and resulting in the formation of benzyl fluoride
derivatives (30, 31). 2-Phenylethanol (25) was thus
converted to benzyl fluoride (30) and phenylacetyl fluo-
ride in a 3:1 relative ratio, while 2-phenyl-1-propanol (26)
was selectively transformed to 1-phenyl-1-fluoroethane
31) and carbon monoxide, as detected by MS analysis
of the evolved gas.
Effect of Ra d ica l Sca ven ger a n d Solven t. The
important role of the solvent and the presence of various
radical scavengers in the reactions of CsSO F with
4
) -0.52) or neutral (F ) -0.37) conditions or by
2
-
II
14
+
S O8 -Cu ( F ) -0.27). On the other hand, F for
2
the CsSO F-mediated conversion of 21 to 22 is consider-
4
ably lower than those for the oxidation of benzyl alcohols
1
5
to benzaldehydes by HNO (F ) -2.25) or HNO (F )
3
2
-1.70, Yukawa-Tsuno method)16 and comparable to
those found for conversion of benzaldehydes to benzoyl
(
+
9
fluorides (F ) -0.38) by CsSO F.
4
P r op osed Mech a n ism . On the basis of the presented
experimental results, the possible reaction pathways of
transformation of secondary alcohols to ketones and
primary alcohols to acid fluorides by CsSO F are shown
4
in Scheme 6. The electron-transfer process (A), resulting
different types of organic molecules, pointed out in
several previous reports1 , was also established in the
,2b
4
reactions of CsSO F with alcohols. Nitrobenzene, often
used as a radical inhibitor, when added to the reaction
mixture while keeping the other reaction parameters
constant, reduced the conversion of 1-phenylethanol (5b)
in the formation of cation radical intermediate 32,
(
(
10) Pearson, R. E.; Martin, J . J . Am. Chem. Soc. 1963, 85, 3142.
11) Kova e` , F.; Baumstark, A. L. Tetrahedron Lett. 1994, 35, 8751.
3
and its m-CF analogue (5d ) to the corresponding ac-
etophenone by up to 37% (Table 1). The transformation
of benzyl alcohol (21c) or its p-Me derivative (21a ) was
suppressed by more than 80%, while the reaction of
(12) Lee, D. G.; Downey, W. L.; Maass, R. M. Can. J . Chem. 1968,
6, 441.
4
(
(
13) Kwart, H.; Francis, P. J . Am. Chem. Soc. 1955, 77, 4907.
14) Walling, C.; El-Taliawi, G. M.; Zhao, C. J . Org. Chem. 1983,
2
-phenyl-1-propanol (26) was completely inhibited by the
48, 4914.
(15) Ogata, Y.; Sawaki, Y.; Matsunaga, F.; Tezuka, H. Tetrahedron,
presence of an equimolar amount of nitrobenzene. The
degree of inhibition of the reaction caused by the presence
of a radical scavenger is comparable to those determined
1
966, 22, 2655.
(16) Moodie, R. B.; Richards, S. N. J . Chem., Soc. Perkin Trans. 2
1986, 1833.