Solvolysis of methoxy-substituted diaryliodonium tetrafluoroborates: Attempted generation of a stabilized aryl cation

Article abstract of DOI:10.1002/poc.1141

Solvolyses of monomethoxy- and dimethoxyphenyl(phenyl)iodonium (ArI ⁺Ph) tetrafluoroborates were carried out in methanol and 2,2,2-trifluoroethanol (TFE) at 130°C. The solvolysis products include alkoxide substitution products (ArOR and PhOR) as well as iodoarenes (PhI and ArI). The ratios of ArOR/PhOR range from 8/2 to 4/6. The results are argued against formation of aryl cation. Copyright

Full text of DOI:10.1002/poc.1141

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 241–244
Published online in Wiley InterScience
(
www.interscience.wiley.com) DOI: 10.1002/poc.1141
Solvolysis of methoxy-substituted diaryliodonium
tetrafluoroborates: attempted generation
of a stabilized aryl cation
Morifumi Fujita,* Eri Mishima and Tadashi Okuyama*
Graduate School of Material Science, Himeji Institute of Technology, University of Hyogo, Kohto, Kamigori, Hyogo 678-1297, Japan
Received 15 May 2006; accepted 21 November 2006
þ
ABSTRACT: Solvolyses of monomethoxy- and dimethoxyphenyl(phenyl)iodonium (ArI Ph) tetrafluoroborates were
carried out in methanol and 2,2,2-trifluoroethanol (TFE) at 130 8C. The solvolysis products include alkoxide
substitution products (ArOR and PhOR) as well as iodoarenes (PhI and ArI). The ratios of ArOR/PhOR range from
8
/2 to 4/6. The results are argued against formation of aryl cation. Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: diaryliodonium salt; phenyl cation; ligand coupling; nucleophilic aromatic substitution
INTRODUCTION
complicated mechanism without intermediary formation
9
of aryl cation. A more stabilized 3,5-dimethoxyphenyl
Phenyl cation is one of the most unstable carbocationic
1
intermediates of mechanistic interest. Considerable
cation may have more chance to be formed from the
corresponding diaryliodonium salt. In the present investi-
gation, 3,5-dimethoxyphenyl(phenyl)iodonium tetra-
fluoroborate as well as 3- and 4-methoxyphenyl
iodonium salts has been prepared and subjected to
alcoholysis.
attempts have been undertaken for solvolytic generation
of phenyl and substituted phenyl (aryl) cations, but the
only definitive method for generation of these cations was
2,3
dediazoniation of arenediazonium salts. Hyperconju-
gatively stabilized aryl cations were successfully gener-
ated by another method, solvolysis of 2,6-bis(trimethyl-
4
silyl)phenyl triflate (Scheme 1).
Although the parent phenyl cation is quite unstable, an
RESULTS AND DISCUSSION
1,4
ortho-silyl group can stabilize it hyperconjugatively.
Electron pair-donating meta substituents are also pointed
out to stabilize the phenyl cation by conjugation taking a
Solvolysis of diaryliodonium salts
1
,5,6
non-planar partially allenic structure (Fig. 1).
type of conjugation was suggested to lower the barrier for
This
Tetrafluoroborate salts of 3,5-dimethoxyphenyl(phenyl)
iodonium (1a) and 3-methoxyphenyl derivative (1b) were
prepared from phenylboronic acid and the corresponding
methoxy-substituted (diacetoxyiodo)benzenes. The 4-
methoxyphenyl iodonium salt (1c) was prepared accord-
6
the S 1 solvolysis of 3,5-diaminophenyl triflate.
N
Iodonium salts are much better progenitors for
generation of unstable carbocationic species due to the
7
high nucleofugality of the iodonio group. Vinyl
10
ing to the literature procedure.
8
iodonium salts have been used to generate vinyl cations.
Diaryliodonium salts should similarly become good
precursors for stabilized aryl cations.
Solutions of diaryliodonium tetrafluoroborates 1a–c in
methanol and 2,2,2-trifluoroethanol (TFE) were heated in
a sealed Pyrex tube at 130 8C for 1–7 days. The products
were analyzed by gas chromatography (GC) with FID and
MS detectors in comparison with authentic samples. The
products include alkoxide substitution products 2–5 and
iodoarenes, PhI and ArI. Yields of the products were
determined by an FID detector of GC with tetra(ethylene
glycol) dimethyl ether as an internal standard, and the
results are summarized in Tables 1 and 2. On injection of
the iodonium substrate 1 as an alcoholic solution into the
GC column, 1 decomposes to give a mixture of
Hydrolysis of diaryliodonium salts was investigated
long time ago, in the hope of generating an aryl cation
by S 1-type solvolysis in a similar manner to that of
N
diazonium salts. However, the reaction including 3- and
4
-methoxy derivatives was suggested to occur by some
*
Correspondence to: M. Fujita and T. Okuyama, Graduate School of
Material Science, Himeji Institute of Technology, University of Hyogo,
Kohto, Kamigori, Hyogo 678-1297, Japan.
E-mails: fuji@sci.u-hyogo.ac.jp and okuyama@sci.u-hyogo.ac.jp
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 241–244

2
42
M. FUJITA, E. MISHIMA AND T. OKUYAMA
OTf
OCH CF3
2
Me Si
SiMe3
o
Me Si
SiMe3
3
120 C
Me Si
SiMe3
3
3
CF CH OH
3
2
Scheme 1
the aryl—iodine bond cleavage for methoxy-substituted
diphenyliodonium salts 1 can be summarized as
Ar ¼ 3,5-dimethoxyphenyl > 3-methoxyphenyl > pheny-
X
X
>
phenyl > 4-methoxyphenyl.
Figure 1. Conjugation with meta substituents.
A similar tendency of product distribution of 4/5 was
observed for the solvolysis in methanol, but the radical
reaction pathways are also apparent in this solvent as
described below. An appreciable amount of
1,3-dimethoxybenzene (6), ArH, was obtained during
the methanolysis of 1a. The corresponding radical
product ArH is anisole (5) for the reaction of 1b and
1c, and it has the same structure as the methoxide
substitution product at the unsubstituted phenyl–iodine of
the substrate. Thus, the ratios of 4/5 observed in the
methanolysis are disturbed by the homolytic reactions.
Nonetheless, the value of 4a/5 observed for 1a must
reflect the heterolytic reaction: the 4a/5 value of 8/2 in
methanol is quite similar to the 2a/3 value obtained for the
solvolysis of 1a in TFE.
iodoarenes, PhI and ArI, but no detectable alkoxide
substitution products 2–5. Thermolysis of 1a and 1b gave
a 55:45 mixture of PhI and ArI, and that of 1c gave a
3
4:66 mixture. Thus, the yields of PhI and ArI in Tables 1
and 2 are partially derived from the unreacted iodonium
substrate if it remains.
During the solvolysis of 1 in TFE, yields of substitution
products 2 and 3 slightly increased with reaction time, but
the ratio of 2 and 3 did not much change with time.
Iodoarenes, PhI and ArI, must form as counterpart
products of 2 and 3, respectively. Although yields of the
iodoarenes are affected by thermal decomposition of
the remaining iodonium substrate during the GC analysis,
the ratio of PhI/ArI is consistent with that of 2/3.
Solvolysis of 1a gave more 2a than 3 (2/3 ¼ 8/2); that is,
the carbon—iodine bond cleavage occurs more readily on
the side of the dimethoxyphenyl group than at the phenyl
group of 1a. The ratio of 2/3 (¼7/3) from 1b is only
slightly smaller than that from 1a, while the reaction of 1c
gave the ratio 2c/3 less than unity (4/6). Relative ease of
A homolytic pathway may occur via one-electron
reduction of the iodonium substrate 1 as illustrated in
Scheme 2. Homolysis of the molecular radical formed
.
leads to aryl radical (Ar ) and iodobenzene (PhI) or phenyl
.
radical (Ph ) and iodoarene (ArI). The radical intermedi-
ates must abstract hydrogen from the solvent to give ArH
(6 or 5) or benzene. To distinguish the heterolysis product
from the radical product, methanolyses of 1b and 1c were
carried out in methanol-d . The GC-MS analyses show
4
that the anisole obtained from 1b or 1c has molecular
peaks both at m/z ¼ 109 (anisole-d ) and 111 (anisole-d ),
a
Table 1. Solvolysis of 1 in 2,2,2-trifluoroethanol at 130 8C
1
3
CF CH OH
and their relative intensities are 12 and 100% (from 1b) or
BF4
Ph
3
2
Ar
I
ArOCH CF
2
3 + PhOCH2CF3
o
130 C
2
3
a
1
Table 2. Solvolysis of 1 in methanol at 130 8C
a : Ar = 3,5-(MeO) C H
2
6
3
MeOH
BF4
Ph
a-c
Ar
I
b : Ar = 3-MeOC H
ArOMe + PhOMe
6
4
4
o
130 C
c : Ar = 4-MeOC H
6
4
1
5
Yield (%)
Yield (%)
PhI
1
Time (h)
2
3
PhI
ArI
2/3
1
Time (h)
4
5
ArI
4/5
b
1
1
1
1
1
1
1
1
a
a
a
b
b
b
c
20
45
90
22
70
166
93
13
22
23
13
21
37
13
21
3
5
4
40
42
42
31
38
42
22
42
28
24
24
22
25
24
40
57
8/2
8/2
8/2
7/3
7/3
7/3
4/6
4/6
1
1
1
1
1
1
a
a
b
b
c
20
45
6
1
1
26
25
22
23
56
56
37
37
24
24
44
44
27
26
42
39
8/2
8/2
4/6
4/6
4/6
4/6
c
6
16
15
13
15
22
70
21
164
5
10
16
24
27
c
a
c
340
Yields were determined by GC.
1,3-Dimethoxybenzene (6) was obtained in 25% yield.
Dimethoxybenzene 6 was obtained in 26% yield.
b
c
a
Yields were determined by GC.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 241–244
DOI: 10.1002/poc

SOLVOLYSIS OF METHOXY-SUBSTITUTED DIARYLIODONIUM TETRAFLUOROBORATES
243
Ar
ArH
ROH
ROH
BF4
Ar+ + PhI
Ph+ + ArI
2 or 4 + PhI
e
Ar
I
Ar
I
PhI
Ph
Ph
S 1
N
Ph
C6H6
3 or 5 + ArI
1
ArI
Scheme 2. A homolytic pathway in the reaction of 1
H
+
O
R
ROH
2 or 4 + PhI
3 or 5 + ArI
+
BF4–
1
3 and 100% (from 1b). The anisole-d and anisole-d3
1
Ar
I
Ar
(Ph)
I
Ph
must arise from the homolytic and heterolytic pathways,
respectively, and the former pathway occurs to the extent
of about 10% in the reactions of 1b and 1c.
LC
Ph (Ar)
1
λ3-iodane
ROH
H + R
O
‡
In contrast to the reaction in methanol, no methoxy-
benzenes (ArH) were observed in the reaction in TFE.
Thus, the radical pathway can be excluded in the reaction
in TFE. This may be due to the poor electron-donating
ability of TFE in comparison with methanol. Direct
homolysis of 1 could be an alternative pathway for
generation of aryl radicals, but this process may not be
much dependent on the solvent.
2 or 4 + PhI
3 or 5 + ArI
S Ar2
N
I
Ar
Ph)
Ph
(Ar)
(
Scheme 4. Possible pathways of solvolysis of 1
>> 3-methoxyphenyl > 4-methoxyphenyl ꢂ phenyl from
the calculated stabilization energies. The results given
in Table 1 show rather small difference between aryl- and
phenyl—iodine bond cleavages, and are not consistent
with the S 1 mechanism. The optimized geometry of the
N
aryl cation stabilized by the meta-electron-donating
group has a non-planar structure.
deformation accompanies with formation of the stabilized
aryl cation. At the transition state of the S 1 solvolysis,
N
this deformation may not much develop, and the
conjugative stabilization may not be attained to promote
the solvolysis. In contrast, hyperconjugative stabilization
of the phenyl cation by the ortho-silyl group does not
In order to compare the selectivity of bond cleavage,
reaction of 1 with a bromide nucleophile was examined.
The reaction with tetrabutylammonium bromide was
carried out in chloroform at 55 8C to give both
bromomethoxybenzene 7 and bromobenzene (8) as well
as iodo products. The ratios of 7/8 are given in Scheme 3.
This reaction is considered to proceed via ligand coupling
1,5,6
That is, large
3
of the intermediate l -bromoiodane, and it was found that
the phenyl ring carrying more electron-withdrawing
11
group(s) tends to combine with the bromide ligand. The
present results are within this general tendency.
require such deformation. This is why 2,6-disilylphenyl
1
triflate undergoes S 1 solvolysis. 3,5-Dipyrrolidinyl-
phenyl cation was evaluated to show large stabilization
Possible mechanisms for the solvolysis
N
ꢁ ꢀ ꢀ 6b
1
Three possible pathways are conceivable for the
heterolytic reaction of the solvolysis of diaryliodonium
salts 1 as illustrated in Scheme 4. The S 1-type reaction
(51.83 kcal mol at MP2/6-31G //RHF/6-31G ) com-
pared with 2,5-disilylphenyl cation (39 kcal mol
ꢁ
1
at
ꢀ
5b
RHF/6-31G ), but the relative solvolysis rates for the
corresponding triflates are opposite to those expected
N
via aryl cation was first considered. The relative stabilities
of the aryl cations were evaluated by MO calculations at
6
from the cation stability.
The other two pathways via ligand coupling and S Ar2
ꢀ
ꢀ
the level of MP2/6-31G //RHF/6-31G , and the stabil-
ization energies by 3,5-dimethoxy, 3-methoxy, and
N
involve a solvent molecule(s) as a nucleophile in the
transition state, and they are admittedly not facile
processes in TFE. However, under the enforced con-
ditions employed, one of these processes may occur, and
the relative ease of the bond cleavages observed is not
4
4
-methoxy substitution of phenyl cation are 14.99,
ꢁ
1 6b
.62, and 0.09 kcal mol , respectively. If the transition
state for the S 1 solvolysis has a character of the aryl
N
cation, the ratio of the substitution products 2/3 should
depend on the stabilization energy: Relative ease of aryl
cation formation is expected to be 3,5-dimethoxyphenyl
inconsistent with expectation. The S Ar2 mechanism
N
should be concerted, if it occurs, due to the combination
1
of a poor nucleophile and an excellent leaving group.
2
BF4
Ph
BrNBu4
Ar
I
+
ArBr
PhBr
+
PhI + ArI
CHCl
3
o
7
8
55 C
1
EXPERIMENTAL
General
a : Ar = 3,5-(MeO) C H
6/4
5/5
1/9
4/6
5/5
1/9
2
6
3
b : Ar = 3-MeOC H4
6
c : Ar = 4-MeOC H4
6
Proton NMR spectra were measured on a JEOL ECA-600
spectrometer in CDCl3 solution and recorded using
residual CHCl as an internal reference (7.24 ppm). Mass
Scheme 3. Reaction of 1 with tetrabutylammonium bro-
mide (0.1 M) in chloroform at 55 8C
3
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 241–244
DOI: 10.1002/poc

2
44
M. FUJITA, E. MISHIMA AND T. OKUYAMA
spectrometers JEOL JMS-T100LC and JEOL Automass
System II were used for MS and GC-MS, respectively.
GC was conducted on a gas chromatograph Shimadzu
dimethyl ether (10 mmol) in a Pyrex tube. The sealed
tube was left standing in an oven at 130 ꢅ 5 8C for
20–200 h. Yields of the products were determined by GC
with tetra(ethylene glycol) dimethyl ether as an internal
standard. The mixture was analyzed by GC: the column
temperature was maintained at 100 8C during the initial
1
7A with TC-1 (i.d. 0.25 mm ꢃ 30 m). Methanol and
2,2,2-trifluoroethanol were simply distilled just before
use for reaction.
ꢁ
1
1
0 min and then raised at the rate of 10 8C min . The
retention times of PhI, 3-iodoanisole, 4-iodoanisole, and
,5-dimethoxy-3-iodobenzene were 4.8, 12.8, 13.2, and
Preparation of iodonium salts, 1a–c
1
3
,5-Dimethoxyphenyl(phenyl)iodonium tetrafluoroborate
18.2 min, respectively. The retention times of 2a, 2b, 2c,
3, 4a, 4b, 4c, 5, 7a, 7b, 7c, and 8 were 14.9, 7.2, 7.3, 3.15,
15.8, 8.0, 7.9, 3.23, 16.3, 8.8, 9.2, and 3.4 min,
respectively.
(
1a). A solution of 3-(diacetoxyiodo)-1,5-dimethoxyben-
zene (0.1 g, 0.38 mmol) in dichloromethane (11 mL) was
added dropwise to a solution containing phenylboronic
acid (0.085 g, 0.26 mmol) and BF ꢄOEt (0.06 mL,
3
2
0
.5 mmol) in dichloromethane (16 mL) at rt. The solution
was stirred for 7 min, and then quenched by NaBF aq.
4
The mixture was extracted with dichloromethane, and the
organic layer was concentrated in vacuo. Crystallization
of the crude mixture in dichloromethane-ether-hexane
Acknowledgements
The authors are grateful to Professor Takaaki Sonoda of
Kyushu University for valuable suggestions and Master
Theses of Masayuki Harada and Ken-ichiro Hosoda.
1
gave 1a (0.030 g, 27% yield). H NMR (600 MHz,
CDCl ) d 7.95 (d, J ¼ 7.6 Hz, 2H), 7.65 (t, J ¼ 7.6 Hz,
3
1
3
H), 7.48 (t, J ¼ 7.6 Hz, 2H), 7.03 (s, 2H), 6.58 (s, 1H),
.78 (s, 6H); MS (ESIþ) m/z (relative intensity, %) 341
(
M—BF , 100); HRMS (ESIþ) calcd for C H O I
4
14 14 2
(
M—BF ) 341.0039, found 340.9995.
4
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(
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7
1
7
3
1
3
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4
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3
10
(
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1
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 241–244
DOI: 10.1002/poc