Fluorinated biphenyls from aromatic arylations with pentafluorobenzenediazonium and related cations. Competition between arylation and azo coupling

Article abstract of DOI:10.1039/a701745f

High yields of the mixed perfluorinated biaryls (C₆F₅-Ar) are obtained by the catalytic dediazonlatlon of the pentafluorobenzenediazonium salt (C₆F₅N₂⁺BF₄^-) in acetonitrile solutions containing various aromatic substrates (ArH) together with small amounts of iodide salts. Activated (electron-rich) as well as deactivated (electron-poor) arenes are successfully pentafluorophenylated by this method. The arylation is distinct from the azo coupling of the same substrates, which takes place in the absence of the iodide catalyst and yields the corresponding diazene (C₆F₅N=N-Ar) as product. The catalytic role of iodide, and the isomeric product distributions obtained with this procedure indicate that the arylation proceeds via the pentafluorophenyl radical in a efficient homolytic chain process. Since azo coupling involves electrophilic aromatic substitution of electron-rich ArH by C₆F₅N₂⁺, the two competing pathways are distinct and do not have reactive intermediates in common.

Full text of DOI:10.1039/a701745f

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Fluorinated biphenyls from aromatic arylations with
pentafluorobenzenediazonium and related cations. Competition
between arylation and azo coupling
Dmitry Kosynkin, T. Michael Bockman and Jay K. Kochi*
Department of Chemistry, University of Houston, Houston, TX 77204-5641, USA
H igh yields of the mixed perfluorinated biaryls (C6F 5᎐Ar) are obtained by the catalytic dediazoniation
of the pentafluorobenzenediazonium salt (C6F 5N 2ϩBF 4Ϫ) in acetonitrile solutions containing various
aromatic substrates (ArH ) together with small amounts of iodide salts. Activated (electron-rich) as well as
deactivated (electron-poor) arenes are successfully pentafluorophenylated by this method. The arylation is
distinct from the azo coupling of the same substrates, which takes place in the absence of the iodide
catalyst and yields the corresponding diazene (C F N ᎐N ᎐Ar) as product. The catalytic role of iodide, and
᎐
6
5
the isomeric product distributions obtained with this procedure indicate that the arylation proceeds via the
pentafluorophenyl radical in a efficient homolytic chain process. Since azo coupling involves electrophilic
aromatic substitution of electron-rich ArH by C6F 5N 2ϩ, the two competing pathways are distinct and do
not have reactive intermediates in common.
heretofore unsuccessful (although it can be generated in
Introduction
strongly acidic HF solutions).17 Accordingly, we will primarily
Unsymmetrical fluorinated biaryls (Ar᎐ArF ) are promising new
materials for nonlinear optics, asymmetric catalysis, and a var-
iety of other physical applications.1–3 Accordingly, newer
methods for their synthesis via the direct arylation of aromatic
substrates (ArH) under mild conditions are particularly desir-
able. Fluorinated diazonium cations (ArF N2ϩ) are potentially
useful reagents to effect this transformation, since they repre-
sent the fluoroaryl synthon (ArF ) in a highly activated form in
which the ready loss of dinitrogen drives the arylation to com-
pletion, i.e. eqn. (1). The synthetic transformation in eqn. (1) is
Ϫ
focus on the synthesis of C6F5N2ϩBF4 and related salts suit-
able for use in nonaqueous solvents, and the development of
methods for efficiently realizing the biaryl synthesis according
to eqn. (1).
Results
Preparation of crystalline pentafluorobenzenediazonium
fluoroborate
The diazonium salt C6F5N2ϩBF4Ϫ could not be prepared by the
conventional diazotization of pentafluoroaniline with sodium
nitrite in aqueous acidic media.17 Accordingly, the pentafluoro-
benzenediazonium salt was isolated from a nonaqueous solvent
(acetonitrile) by the use of specially prepared nitrosonium
tetrafluoroborate as the diazotizing reagent.18 (See the Experi-
mental section for details of the procedure). The colorless crys-
talline salt was stable if stored at low temperature, but it rapidly
decomposed upon even a short exposure to atmospheric mois-
ture. Since the fluoroborate salts of other diazonium cations are
known generally to be stable in moist air, the rapid decom-
ArF N2ϩ ϩ ArH → Ar᎐ArF ϩ N2 ϩ Hϩ
(1)
usually realized in aqueous media via the classic Gomberg–
Bachmann reaction 4,5 and its intramolecular analogue, the
Pschorr synthesis. 6,7 These reactions are conventionally carried
out in the presence of base, but they are often characterized by
low yields and extensive formation of colored side products.8
Since these drawbacks derive from the wide variety of
undesired reactions that diazonium cations undergo in aqueous
base,9 we felt that the new developments in the use of non-
aqueous solvents and specific salt catalysis would mitigate these
undesirable features and could revive the biaryl coupling in eqn.
(1) as a useful synthetic method. In this report, we concentrate
on the transfer of the pentafluorophenyl group [ArF = C6F5 in
eqn. (1)] to represent a prototype for the introduction of
electron-withdrawing substitutents into aromatic systems.
Indeed, the product ‘push–pull’ biaryls that contain donor
moieties in conjunction with an electron-withdrawing C6F5
substituent are useful in nonlinear laser applications and have
been exploited for liquid-crystal displays.10,11
Ϫ
position of C6F5N2ϩBF4 indicated that it was especially sus-
ceptible to nucleophilic attack by water, e.g. eqn. (2). Indeed, the
+
+
N2
N2
F
F
F
F
F
F
F
F
H2O
–HF
(2)
,
etc.
F
OH
Pentafluorophenylation has been carried out under aprotic
conditions by the thermal or photochemical activation of a
C6F5-transfer agent in the presence of the arene ArH.12 Thus,
the thermolysis of perfluorobenzoyl peroxide13 or the xenon
derivative C6F5Xeϩ,14 the oxidation of pentafluorophenyl-
hydrazine15 and the photolysis of C6F5I 16 have all been
exploited to deliver the pentafluorophenyl moiety in non-
aqueous media. However, these methods suffer from low yields
and many side products, or they require the use of rather exotic
starting materials. Although C6F5N2ϩ represents an alternative
reagent for pentafluorophenylation, its synthesis by con-
ventional diazotization methods in aqueous solution has been
strongly electrophilic nature of this diazonium cation was fur-
ther exemplified by the facile replacement of the ortho- and
para-fluorine atoms with chlorine to afford the 2,4,6-trichloro
analogue merely upon its exposure to a solution of hydrogen
chloride in nitromethane at 24 ЊC (see Experimental section).
+
+
N2
N2
F
F
F
F
Cl
Cl
F
HCl
(3)
CH3NO2
F
F
Cl
J. Chem. Soc., Perkin Trans. 2, 1997
2003

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Table 1 Pentafluorophenylation of benzene induced by sodium iodide in solution a
Products (%)d
NaI consumed/
mmolb
Turnover
number c
[PhH]/
C6F5Ph
C6F5I
(C6H4P2)e
C6F5H
I2
0.17
0.34
0.69
1.38
2.75
5.52
11.0
0.41
0.26
0.12
0.07
0.03
0.02
0.01
2.8
6.9
16
31
58
36
56
73
83
92
93
94
31
14
13
13
10
6.5
6.5
3.3
1.6
3.7
2.6
1.7
0.9
0.9
0.3
0
15
7.8
5.7
2.7
2.9
0.6
0.5
5.6
3.8
3.8
0.6
0
74
85
a
Ϫ
In acetonitrile at 24 ЊC with 1.0 mmol of C6F5N2ϩBF4 (data taken from ref. 19). b Added in 0.01–0.05 mmol aliquots until nitrogen evolution
ceased. c Defined as mmol N2 evolved per mmol sodium iodide added. d Yields based on diazonium salt. e C6H4P2 = bis(pentafluorophenyl)benzene as
a mixture of o-, m-, p-isomers (see Experimental section).
Table 2 Inhibition of pentafluorophenylation by various additivesa
NaI
Products (%)
consumed/
mmol
Turnover
number
Additive/(mmol)
None
C6F5Ph
C6F5I
I2
3.8
0.07
0.24
0.38
0.47
0.22
0.33
0.44
0.83
0.24
14
83
74
71
67
76
61
55
19
67
2.7
5.3
7.9
9.7
O2
(0.2)
(0.4)
(0.8)
(0.1)
(0.5)
(1.0)
(8.0)
(8.0)
4.2
2.6
2.1
4.5
3.0
2.3
1.2
4.2
12
20
30
—
—
—
10
10
I2
11
26
38
70
CH2I2
BrCCl3
5.6
5.0 (C6F5Br)
a
See footnotes in Table 1.
Pentafluorophenylation of aromatic hydrocarbons with catalytic
amounts of iodide salts
its enhanced solubility in the reaction medium. The results in
Table 1 show that progressive increases in the initial concen-
tration of benzene led to increased yields of the arylation prod-
uct (C6F5Ph) and decreased yields of the iodination product
(C6F5I).19 Concomitant with these trends, the amount of NaI
needed to effect complete reaction decreased. In the presence of
a large excess of benzene (entry 7 in Table 1), less than 1 mol%
of NaI, was sufficient to promote the complete arylation, and
the yield of pentafluorobiphenyl was close to quantitative. The
overall trend is presented in column 3 of Table 1 in which the
turnover number (i.e. the extent of reaction induced by each
equivalent of NaI) is tabulated.
The results in Table 2 show that dioxygen drastically reduced
the efficiency of the reaction (as reflected in the turnover num-
ber in column 3), but had only a minor effect on the distribution
of products between C6F5Ph and C6F5I. Added I2 had a similar
effect on the catalytic efficiency, but was more effective in divert-
ing the course of the reaction from arylation to iodination
(compare columns 4 and 5 in Table 2). The polyhalomethanes,
CH2I2 and BrCCl3 also inhibited the reaction (note the lower
turnover number in entries 8 and 9). However, methylene iodide
was more effective in changing the reaction pathway, as shown
by a comparison of the relatively low yield of C6F5Br in entry 9
with the high yield of C6F5I in entry 8.
The addition of 1 equiv. of sodium iodide to an acetonitrile
solution of C6F5N2ϩBF4Ϫ under an argon atmosphere at 24 ЊC
immediately led to the evolution of 1 equiv. of dinitrogen.
Workup of the reaction mixture afforded the iodination prod-
uct pentafluoroiodobenzene in 85% yield, i.e. eqn. (4). In the
C6F5N2ϩ ϩ IϪ → C6F5I ϩ N2
(4)
presence of an arene such as benzene, the reaction took a differ-
ent course. For example, the addition of only 7 mol% NaI to a
solution of C6F5N2ϩBF4Ϫ in the presence of an eightfold excess
of benzene led to complete reaction of the diazonium salt and
the evolution of a full equivalent of N2 within a few minutes.19
The major product of the reaction was 2,3,4,5,6-pentafluoro-
biphenyl, which was formed in 83% yield, together with minor
amounts of pentafluoroiodobenzene and bis(pentafluoro-
phenyl)benzenes.20 The reaction was thus designated as an
iodide-promoted arylation of benzene, i.e. eqn. (5). The minor
[IϪ
]
C6F5N2ϩ ϩ PhH
C6F5Ph ϩ N2 ϩ Hϩ
(5)
product, C6F5I, together with a small amount of I2, accounted
for all of the added iodide salt.
The effect of increasing concentrations of benzene in the
heterogeneously catalysed reactions with KI was substantially
the same as that for the homogeneously catalysed reaction
promoted by NaI (vide supra). The comparison is illustrated in
Fig. 1 in which the yields of C6F5Ph and C6F5I are plotted
against the initial concentration of benzene. At high concen-
trations of benzene, the yield of pentafluorobiphenyl was
nearly quantitative.
The highest yields of pentafluorophenylated arenes were
obtained for those arenes substituted with electron-donating
groups (see Table 3). Nitrobenzene and the chlorinated ben-
zenes yielded reduced amounts of the biaryls, and increased
amounts of pentafluoroiodobenzene were obtained. Anisole
was arylated in high yield, but rather unselectively (entry 12).
The catalytic role of iodide in pentafluorophenylation reac-
tions was also emphasised by the effective promotion of aryl-
ation by potassium iodide which was insoluble in the reaction
medium. Thus, the addition of solid KI to an acetonitrile solu-
tion of C6F5N2ϩBF4Ϫ and benzene at 24 ЊC immediately led to a
vigorous evolution of gas. Dinitrogen (1 equiv.) was evolved
within 10 min, and the added KI was largely recovered as an
insoluble powder. Workup of the reaction mixture afforded
pentafluorobiphenyl in 84% yield.
Since the presence of benzene was sufficient to divert the
reaction from iodination to arylation, the effect of added ben-
zene was systematically surveyed with sodium iodide owing to
2004
J. Chem. Soc., Perkin Trans. 2, 1997

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Table 3 Pentafluorophenylation of aromatic compounds induced by potassium iodide (solid)a
Products (%)c
Isomer distribution of
ArC6F5
Arene (ArH)
Ei/eVb
C6F5I
ArC6F5
Benzene
Toluene
Isopropylbenzene
tert-Butylbenzene
o-Xylene
p-Xylene
Mesitylene
Chlorobenzene
p-Dichlorobenzene
1,3.5-Trichlorobenzene 9.30
9.23
8.82
8.71
8.65
8.56
8.44
8.42
9.10
8.94
4.1
4.8
3.6
5.0
3.9
4.1
1.2
11
46
44
44
8.0
5.0
2.2
4
84
80
66
74
76
75
62
74
45
12
25
81
69
74
81
—
o:m:p 48:30:22
o:m:p 34:38:28
o:m:p 31:36:33
3:4
54:46
—
—
o:m:p 41:29:30
Nitrobenzene
9.92
8.39
—
o:m:p 21:60:19
o:m:p 37:13:39
Anisoled
o-Dimethoxybenzenee
p-Dimethoxybenzenee 7.90
Naphthalene
8.12
α:β
62:38
a
In acetonitrile at 24 ЊC, with 1.0 mmol C6F5N2ϩBF4Ϫ, 8.0 mmol arene and 0.2 mmol solid potassium iodide, unless otherwise noted. b Ionization
potentials from ref. 44. c Yields based on diazonium salt. d At Ϫ40 ЊC. e At Ϫ48 ЊC.
Table 4 Iodide-promoted arylations with related diazonium cationsa
Products (%)
Diazonium cation
ϩ
ArЈN2
Arene (ArH)
Catalyst (mmol)
Ar᎐ArЈ
ArЈ᎐I
ArЈH
3,5-Difluoro-2,4,6-trichlorobenzenediazonium
3,5-Difluoro-2,4,6-trichlorobenzenediazonium
3,5-Difluoro-2,4,6-trichlorobenzenediazonium
4-Nitrobenzenediazonium
Benzene
NaI (0.94)
NaI (0.56)
KI b
0.2
12
66
18
64
86
45
45
79
20
12
Naphthalene
Naphthalene
Benzene
2.1
8.3
0.5
0.1
KI b
3,5-Dinitrobenzenediazonium
Benzene
KI b
a
In acetonitrile at 24 ЊC, using 1.0 mmol diazonium hexafluorophosphate salt and 3.0 mmol arene. b Not quantified (insoluble).
iodobenzene, to indicate competition from the iodination
reaction [eqn. (4)].
Iodide promotion of aromatic arylations with related diazonium
salts
The applicability of iodide catalysis in aromatic arylation was
also examined with other electrophilic benzenediazonium
ϩ
ArЈN2 salts, including those with electron-withdrawing sub-
stituents such as the 3,5-difluoro-2,4,6-trichloro, 3,5-dinitro
and 4-nitro derivatives. The results in Table 4 show that the
yields of the biaryl (ArЈ-Ph) products were generally lower
than those obtained with the highly electrophilic pentafluoro
analogue (Table 3). In each case, the deficit in the aryl moiety
(ArЈ) derived from the diazonium cation was primarily
accounted for as the corresponding aryl iodide (ArЈI in column
5) together with smaller amounts of reduced arene (ArЈH in
column 6).
Fig. 1 Catalytic efficiency of iodide in the pentafluorophenylation of
Ϫ
benzene with C6F5N2ϩBF4 in acetonitrile by either added KI (hetero-
geneous) or NaI (homogeneous), showing the increasing yield of
C6F5Ph (squares) at the expense of C6F5I (circles). (Filled data points
are for NaI and open data points are for KI).
Spontaneous (unpromoted) reactions of C6F5N2ϩ with aromatic
donors leading to azo coupling
In the absence of added iodide salts, the mixture of pentafluoro-
benzenediazonium (C6F5N2ϩ) salt and benzene (ArH) in
acetonitrile was unchanged for prolonged periods (in the dark),
and no nitrogen was evolved. Although similar solutions of
C6F5N2ϩ with a more reactive arene (ArH) such as toluene also
remained quiescent, those that contained m-xylene slowly
turned red upon standing (14 d). Analysis of the residual reac-
tion mixture indicated a 54% conversion of C6F5N2ϩ, and the
Moreover, ortho- and para-dimethoxybenzenes afforded the
products of pentafluorophenylation in reasonable yields. The
isomer distributions of the pentafluorophenylated arenes in
Table 3 indicated that substitution was quite unselective. For
example, toluene was arylated at all three positions (ortho, meta
and para) with only a slight preference being shown for reaction
at the ortho and para positions. Moreover, the arylation was
only slightly sensitive to steric effects, as shown by the slightly
lower yields of the ortho-arylated products of the reactions of
cumene and tert-butylbenzene (entries 3 and 4). Nitrobenzene
yielded a preponderance of the meta-pentafluorophenylated
nitrobenzene, but, again, the preference was not strong.
Naphthalene reacted preferentially at the α-position. The lower
yields were accompanied by increased yields of pentafluoro-
azo-coupling product,21,22 C F N᎐NC H (CH ) was isolated
᎐
6
5
6
3
3 2
in essentially quantitative yield (see Experimental section). The
electron-rich mesitylene and pentamethylbenzene reacted pro-
gressively faster (1 h and <30 s, respectively), and the corre-
sponding azo coupling products were similarly isolated in high
yields, as listed in Table 5. Neither the arylated products C6F5-
Ar nor reduced arene (C6F5H) could be observed.
J. Chem. Soc., Perkin Trans. 2, 1997
2005

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Table 5 Azo coupling of arenes with pentafluorobenzenediazonium tetrafluoroboratea
Arene (ArH)
Ea/eVb
Reaction time
Product
Yield (%)c
100d
N=NC6F5
8.71
14 d
N=NC6F5
8.88
8.88
9.07
1 h
1 h
95
92
98
N=NC6F5
OCH3
OCH3
N=NC6F5
<30 s
N=NC6F5
60 s
94
91
OCH3
OCH3
OCH3
OCH3
N=NC6F5
CH3O
CH3O
<30 se
OCH3
OCH3
a
ϩ
In acetonitrile at 24 ЊC with 1.01 mmol C6F5N2ϩBF4 and 8.00 mmol arene, unless otherwise noted. b Proton affinity from ref. 45. c Based on
diazonium salt. d 54% conversion of C6F5N2ϩBF4
.
e at Ϫ40 ЊC.
Ϫ
The methoxylated arenes anisole, o- and m-dimethoxy-
benzenes reacted rapidly with C6F5N2 in acetonitrile even at
coupling products with 3,5-dinitrobenzenediazonium (Table 6,
entry 4). A similar mixture of arylated and azo-coupling prod-
ucts was obtained with mesitylene and 3,5-difluoro-2,4,6-
trichlorobenzenediazonium (Table 6, entry 9). In the absence
of iodide, naphthalene was unreactive to both pentafluoro-
and difluorotrichloro-benzenediazonium in arylation as well as
azo-coupling.
ϩ
rather low temperatures (see Table 5) to afford the azo-coupling
products in high (isolated) yields. In contrast to the iodide-
promoted arylation of anisole in Table 3 (entry 12), only a
single (para) isomer was formed in the azo-coupling (Table 5,
entry 3). Similarly o- and m-dimethoxybenzene afforded only
one diazene isomer, e.g. eqn. (6). It is particularly noteworthy
Discussion
NC6F5
N
Pentafluorobenzenediazonium tetrafluoroborate is an effective
reagent for the introduction of the C6F5 moiety into aromatic
systems, and it represents a new and attractive alternative to
previously developed perfluoroarylation procedures. Earlier
CH3O
CH3O
CH3CN
H+
(6)
+
+
C6F5N2
+
ϩ
attempts to obtain C6F5N2 by the diazotization of penta-
OCH3
OCH3
fluoroaniline in aqueous media yielded only the triazene,
decafluorodiazoaminobenzene C F N᎐NNHC F (presumably
᎐
6
5
6
5
that the isomeric p-dimethoxybenzene under the same (dark)
conditions afforded high yields of the corresponding biaryl, i.e.
eqn. (7). Moreover, the isomeric 1,4-/1,3-dimethoxybenzenes
from the rapid addition of C6F5N2ϩ onto the unreacted penta-
fluoroaniline).17 On the other hand, we find that the gradual
addition of pentafluoroaniline to a cooled (Ϫ30 ЊC) suspension
of finely divided nitrosonium tetrafluoroborate in acetonitrile
OCH3
OCH3
Ϫ
affords C6F5N2ϩBF4 which can be isolated in 81% yield. The
C6F5
–N2
white crystalline salt is rapidly hydrolysed by water and must be
stored under strictly anhydrous conditions.
+
C6F5N2
+
+
H+
(7)
CH3CN
The diazonium salt C6F5N2ϩBF4Ϫ reacts readily with 1 equiv.
of sodium iodide in acetonitrile to yield the iodination product,
a reaction that is general for many types of diazonium cat-
ions,23,24 irrespective of the medium. In the presence of aro-
matic substrates, however, less than 1 equiv. of added iodide
initiates the pentafluorophenylation of the arene, i.e. eqn. (8).
OCH3
OCH3
showed the same dichotomy between arylation–azo-coupling
with 3,5-dinitrobenzenediazonium (Table 6, entries 2/3). Inter-
estingly, 3-methylanisole afforded a mixture of biaryl–azo-
2006
J. Chem. Soc., Perkin Trans. 2, 1997

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Table 6 Competition between arylation and azo coupling of ArЈN2ϩ with electron-rich aromatic donorsa
Products (%)
Substituted
Aromatic donor
benzenediazonium
t/h
Conv (%)
Ar᎐ArЈ
ArЈ᎐H
Ar᎐N᎐N ArЈ
᎐
b
1,4-Dimethoxybenzene
1,4-Dimethoxybenzene
1,3-Dimethoxybenzene
3-Methylanisole
m-Xylene
Pentafluoro
3,5-Dinitro
3,5-Dinitro
3,5-Dinitro
3,5-Dinitro
Pentafluoro
0.2
8
8
8
8
100
88
100
100
2
72
71
0
18
0
0
0
100
78
0
8
0
b
0
0
Naphthalene
8
3
0
0
p-Xylene
Naphthalene
Mesitylene
Pentafluoro
3,5-Difluoro-2,4,6-trichloro
3,5-Difluoro-2,4,6-trichloro
8
8
8
78
8
100
43
4
5
0
0
0
93
b
1
a
In acetonitrile solutions containing 1.0 mmol diazonium salt (ArЈN2ϩ) and 3.0 mmol arene (ArH) in the dark at 24 ЊC (without added iodide). b Not
determined.
[IϪ
CH3CN
]
from the known mechanism for iodination of diazonium
ϩ
ArH ϩ C6F5N2
ArC6F5 ϩ N2 ϩ Hϩ
(8)
ϩ
cations with iodide.34–36 Thus the redox initiation of C6F5N2
with iodide proceeds via a two-step process involving the rate-
limiting electron transfer (kET)
37 followed by the rapid dediazo-
The same transformation can be induced with the insoluble KI
salt (heterogeneously), and the yields and isomer distributions
of the products of the two reactions are essentially identical.
Unlike the stoichiometrically equivalent Gomberg–Bachmann
reactions,4,5 the yields of the biaryls are high, and there are no
significant side products. The primary difference can be attrib-
uted to the use of a nonaqueous solvent (which obviates the
hydrolysis of the diazonium starting material25) and to the use
of iodide as a promoter.
niation of the aryldiazenyl radical,38 i.e. eqn. (11).
kET
C6F5N2ϩ ϩ IϪ
C6F5N2ؒ ϩ
Iؒ
fast
C6F5ؒ ϩ N2 ϩ Iؒ
(11)
ϩ
Electrophilic aromatic substitution with C6F5N2
The direct (azo-coupling) reaction of C6F5N2ϩ with the various
The iodide-catalysed pentafluorophenylation of arenes is
particularly suitable for the synthesis of biphenyls in Table 3
and 4 with electron-donating groups on one ring and electron-
withdrawing fluorine substituents on the other. ‘Push-pull’ π-
conjugated systems of this type are promising materials for
nonlinear optical (NLO) studies, owing to their large molecular
hyperpolarizabilities.26,27 Moreover, the synthetic system,
arenes to afford the push–pull diazenes, i.e. eqn. (12), in the
ArF N2ϩ ϩ ArH
Ar N᎐NAr ϩ Hϩ
᎐
F
(12)
CH3CN
absence of added iodide (Table 5) is tantamount to other well-
known electrophilic aromatic substitutions that have been iden-
tified with cationic electrophiles.39 As such, the reactivity trend
in Table 5 (column 2) generally follows the electron-donor abil-
ity of the aromatic substitute, as measured by the ionization
ϩ
C6F5N2 plus arene, can also be utilized (in the absence of
iodide catalysis) to synthesize ‘push-pull’ diazenes, in Table 5
and 6 which have been previously identified 28 as potentially
powerful NLO materials.
potential.40,41 Although C6F5N2 is among the best diazonium
ϩ
electrophiles, it still requires a relatively electron-rich aromatic
donor such as mesitylene for efficient azo-coupling. In this
regard, the mechanism of azo coupling with C6F5N2 in
Radical-chain mechanism for iodide catalysis of pentafluoro-
phenylation
ϩ
The role of iodide as the catalytic promoter of the pentafluoro-
Scheme 2 is somewhat comparable to electrophilic nitrosation
ϩ
with NOϩ, as presented recently,42 in which the multistep
phenylation with C6F5N2 points to the involvement of a
chain reaction,29 the free-radical character of which is sup-
ported by the effect of inhibitors30,31 such as oxygen, iodine and
the halomethanes in Table 2. The intermediacy of a relatively
unselective, and thus highly reactive aryl radical (C6F5ؒ) is fur-
ther suggested by the nearly statistical isomer distribution
obtained from the reaction of substituted benzenoid substrates
in Table 3. The pentafluorophenylation of aromatic substitutes
(ArH) is a homolytic substitution,32 as judged by the similarity
of the isomer ratios obtained in related homolytic processes
such as the photolysis of pentafluorophenyl iodide16 and the
oxidation of pentafluorophenylhydrazine,15 which are known to
generate free pentafluorophenyl radicals. On this basis, the
radical-chain mechanism shown in Scheme 1 can be proposed.19
kA
ArF N2ϩ ϩ ArH
ArF N᎐N᎐ArHϩ
(13)
(14)
᎐
ArF N᎐N᎐ArHϩ → ArF N᎐NAr ϩ Hϩ
᎐
᎐
Scheme 2
substitution occurs via the reversible formation of the Wheland
intermediate.22
Comments on the competition between arylation and azo
coupling
The ambivalent behavior of the electrophilic pentafluoroben-
zenediazonium cation toward aromatic donors (ArH) is mani-
fested by the efficient dediazoniative arylations in Table 3 on
one hand and the exclusive azo couplings in Table 5 on the
other. The question thus arises as to how these are mechanistic-
ally related. Let us consider the reactivity of three aromatic
donors, viz. mesitylene, anisole and dimethoxybenzene, which
are subject to both dediazoniative arylation (Table 3) as well as
azo coupling (Table 5). In addition to the obvious difference in
stoichiometry [eqn. (6) and (7)], the two processes differ in the
following ways.
C6F5ؒ ϩ ArH → C6F5᎐ArHؒ
(9)
ϪHϩ
ϩ
C6F5᎐ArHؒ ϩ C6F5N2
C6F5᎐Ar ϩ C6F5ؒ, etc. (10)
ϪN2
Scheme 1
According to this formulation the well-known homolytic add-
ition of C6F5ؒ onto ArH [eqn. (9)]32 is followed by the rapid
oxidation of the cyclohexadienylic (radical) adduct by the
diazonium cation to regenerate the chain-carrying species
(C6F5ؒ) in eqn. (10).33 In addition, the initiation step derives
(i) Azo coupling requires a relatively activated aromatic sub-
strate. For example, m-xylene (entry 1 in Table 5) reacts slowly
J. Chem. Soc., Perkin Trans. 2, 1997
2007

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with C6F5N2ϩ over a course of 14 d for only partial conversion
to the diazene; and substrates such as naphthalene and toluene
are inert. By contrast, biaryl coupling could be effectively
carried out on benzene and toluene, and even on deactivated
arenes such as nitrobenzene.
(ii) Azo coupling yields only a single diazene isomer with
anisole, m-xylene and o-dimethoxybenzene. By contrast, the
biaryls are formed as mixtures of isomers, and the isomeric
distributions are rather unselective.
(iii) Diazoniative arylation is rather insensitive to tem-
perature, and it occurs relatively readily at even Ϫ40 ЊC. By
contrast, azo coupling is severely retarded at these low
temperatures.
(iv) Diazoniative arylation is subject to strong catalysis by
added iodide salts, whereas azo coupling is singularly
unaffected by the presence of such a reducing agent.
Most notably, facets (iii) and (iv) allow the reactivity of
Fig. 2 Correlation of the relative (negative) ionization potentials and
ϩ
proton affinities for various aromatic hydrocarbons. The dashed line is
arbitrarily drawn to connect benzene and hexamethylbenzene in order
to emphasise the positive lie (above the line) of m-xylene and mesitylene
relative to p-xylene.
C6F5N2 to be selectively controlled. For example, the aryl-
ations of ArH = mesitylene, anisole and o-dimethoxybenzene
can be effected under an argon atmosphere to afford high yields
of the mixed biaryls C6F5᎐Ar merely by adding catalytic
amounts of iodide salt to the reaction mixture at <Ϫ40 ЊC. The
alternative azo coupling of the same aromatic substrates is
readily achieved by merely allowing the reaction mixture to
equilibrate in air at room temperature (in the absence of iod-
ide). Indeed, such a clean separation of arylation from azo
coupling follows from the mechanisms of these processes, as
presented in Scheme 1 and Scheme 2, respectively. Thus, the
free-radical intermediates inherent to homolytic arylation
(Scheme 1) are distinct from the cationic intermediates import-
ant to electrophilic azo coupling (Scheme 2). Furthermore, the
relatively low activation energies of the homolytic steps43 in the
radical-chain sequence (Scheme 1) ensures that they are rapid
relative to the sequential electrophilic steps (Scheme 2), espe-
cially at low temperatures. Having said this, we are particularly
intrigued by the behaviour of mesitylene and 3-methylanisole in
Table 6 (entries 4 and 9), in which arylation competes with azo
coupling in the absence of an added iodide catalyst. The latter
suggests that thermal initiation of arylation can proceed with
these relatively electron-rich aromatic donors in a manner
analogous to that presented in eqn. (11), i.e. eqn. (15), where
On the basis of the comparisons, we deduce that m-dimethoxy-
benzene has a substantially greater proton affinity than the
isomeric p-dimethoxybenzene.46 Such a conclusion would thus
ϩ
account for their different behavior toward C6F5N2 in aryl-
ation and azo coupling (Table 6). We hope that further com-
parative studies of electron transfer and electrophilic reactions
of aromatic donors will shed additional light on this interesting
dichotomy.
Summary and conclusions
Pentafluorophenylation of aromatic substrates is achieved by
ϩ
their iodide-promoted coupling with C6F5N2 in acetonitrile
solution. The reaction can be promoted either homogeneously
(with NaI) or heterogeneously (with KI), and the yields and
distributions of the isomeric products are essentially the same
for both procedures. The catalysed reactions proceed rapidly at
room temperature or below (Ϫ40 ЊC) and are applicable to
arenes (ArH) with both activating and deactivating substitu-
ents. The role of the iodide catalyst and the lack of selectivity in
the isomer distribution of arylation products points to a
radical-chain process, with the pentafluorophenyl radical
(C6F5ؒ) as the active arylating reagent. Arylation (with the loss
of N2) is to be contrasted with the uncatalysed azo coupling of
some of the same ArH substrates which yields the correspond-
ing 1-pentafluorophenyl-2-aryl diazenes. Azo coupling of
C6F5N2ϩ occurs with only the most electron-rich arenes, typical
of other electrophilic aromatic substitutions. As such, arylation
and azo coupling do not share reactive intermediates in com-
mon, and quite distinctive (competing) pathways are observed
in the interaction of some of the ArЈN2ϩ/ArH pairs in Table 6.
kET
C6F5N2ϩ ϩ ArH
C6F5N2ؒ ϩ
ArHϩ
fast
C6F5ؒ, etc.
(15)
᝽
ϪN2
ArH = mesitylene, dimethoxybenzene, etc. Homolytic initiation
by electron transfer [kET in eqn. (15)] also directly relates to the
electron-donor properties of the aromatic substrate, as meas-
ured by the ionization potentials in Table 3 (column 2).44 As
such, the conversions to biaryls in Table 6 are the greatest with
the most electron-rich donors such as p-dimethoxybenzene
(entries 1 and 2). The question then arises as to why the iso-
meric m-dimethoxybenzene (with essentially the same ioniz-
ation potential) is quantitatively converted to the product of
azo coupling (entry 3). Clearly, the answer to this problem
depends on the bimolecular rate of electron transfer [kET in eqn.
(15)] relative to the rate of electrophilic addition [kA in eqn.
(13)]. Although the rate of electrophilic addition generally cor-
relates with the ionization potential (Ei) of the aromatic
donor,40,41 perhaps a more appropriate measure is the proton
affinity (Ea).45 Indeed, Fig. 2 shows that the proton affinities of
various aromatic hydrocarbons generally track the (negative)
ionization potentials (The dashed line is arbitrarily drawn to
connect the points for benzene and hexamethylbenzene at the
extrema). Particularly noteworthy is the detailed comparison of
points for mesitylene and p-xylene which have essentially the
same value of Ei, but whose proton affinities are widely differ-
ent; and a similar divergence pertains to the o-/m-xylene pair.
Experimental section
Materials
Nitrosonium tetrafluoroborate from
a commercial source
(Strem) was triturated with acetic anhydride (10%) in acetic
acid. This was followed by filtration under an argon atmos-
phere, washing with dry dichloromethane and vacuum dry-
ing to provide a colorless free-flowing crystalline powder. The
aromatic compounds were reagent grade commercial samples
that were purified by distillation or recrystallization from
alcohol followed by sublimation in vacuo: benzene, toluene, o-
and m-xylenes (EM Science), isopropylbenzene, tert-
butylbenzene, mesitylene, chlorobenzene, p-dichlorobenzene,
anisole, o-, m- and p-dimethoxybenzenes (Aldrich), and naph-
thalene (Matheson). The haloarenes, 2-iodotoluene, 3-iodo-
toluene, 4-iodotoluene, 2-iodonitrobenzene, 3-iodonitro-
benzene, p-diiodobenzene, 2-iodoisopropylbenzene, 4-iodoiso-
2008
J. Chem. Soc., Perkin Trans. 2, 1997

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propylbenzene, 2,3-dimethyliodobenzene, 3,4-dimethyliodo-
benzene, 2,5-dimethyliodobenzene, 2-chloroiodobenzene, 3-
chloroiodobenzene, 4-chloroiodobenzene, 1-iodonaphthalene,
2-iodonaphthalene, hexafluorobenzene and pentafluoro-
benzene were obtained from Aldrich and used as received. The
aminoarenes, 4-nitroaniline, 4-(trifluoromethyl)aniline, ethyl 4-
aminobenzoate, 4-aminobenzonitrile, 3,5-bis(trifluoromethyl)-
aniline and 2,3,4,5,6-pentafluoroaniline (Aldrich) were used
without further purification. Sodium and potassium iodides
(Aldrich) were dried in vacuo for 2 h at 150 ЊC and stored under
an atmosphere of argon. Acetonitrile (OmniSolv, EM Science)
and nitromethane (Spectrophotometric grade, Aldrich) were
stored in Schlenk flasks under an argon atmosphere. Dichloro-
methane (HPLC grade, EM Science) was initially stirred with
concentrated sulfuric acid; the separated layer was neutralized,
dried over anhydrous Na2CO3 and finally distilled from
anhydrous P2O5 under an argon atmosphere. Absolute ether,
methanol, ethyl acetate, chloroform, carbon tetrachloride
(Reagent grade, EM Science) and ethanol (USP grade,
McCormick) were used without further purification. Deuter-
ated solvents for the NMR experiments (Aldrich) were stored
over molecular sieves prior to use. UV–VIS absorption spectra
were measured on a Hewlett-Packard 8450A diode-array spec-
956, 943; δC(CD3CN) 139.87 (dm, J 260), 149.08 (dm, J 280),
154.43 (dm, J 278); δF (CD3CN) Ϫ40.09 (m, 1F ), Ϫ43.70 (m,
2F ), Ϫ71.82 (s, 4F ), Ϫ73.02 (m, 2F ). Anal. Calc. for C6BF9N2:
C, 25.57; H, 0.00; N, 9.95. Found: C, 25.37; H, 0.11; N, 9.90%.
The diazonium salt was stable overnight at room temp. in an
argon atmosphere, but further storage at 24 ЊC led to appre-
ciable decomposition. Exposure of the diazonium salt to the
atmospheric moisture caused yellow discoloration after 2᎐3 h
and total degradation after 2 d.
3,5-Difluoro-2,4,6-trichlorobenzenediazonium hexafluoro-
phosphate
Pentafluorobenzenediazonium tetrafluoroborate (9.15 g, 0.05
mol) was dissolved in nitromethane (50 ml) at Ϫ20 ЊC, and a
stream of dry HCl was passed through the solution until it was
saturated. The yellow residue was dissolved in concentrated
hydrochloric acid (50 ml) and treated with 60% aqueous HPF6
(8 ml, 0.06 mol) to form a yellowish precipitate, which was
collected by filtration, dried in vacuo at room temp. and repre-
cipitated from acetonitrile with absolute ether three times to
give yellow needles (12.8 g, 77%): δC(CD3CN) 156.3 (dd, J1 257,
J2 5), 131.1 (t, J 20), 125.1 (d, J 30), 121.2. Anal. Calc. for
C6F8Cl3N2P: C, 18.51; H, 0.00; N, 7.20. Found: C, 18.55; H;
0.09; N, 7.23%.
1
trometer with 2 cmϪ1 resolution. The H, 13C and 19F NMR
spectra were obtained on a General Electric QE-300 FT-NMR
spectrometer with internal (CH3)4Si or external CF3COOH
standards. J values are given in Hz. The infrared spectra were
recorded on either a Nicolet 10DX, Nicolet 560 FTIR or a
Mattson Genesis spectrophotometer. Gas chromatography
was performed on a Hewlett-Packard 5790A series FID gas
chromatography equipped with a 3392 integrator, the fused
silica capillary column (WCOT, 0.25 mm i.d. × 12 m, coating
CF-Sil-5-CB, Chrompak). All inert atmosphere manipulations
were carried out in a Vacuum Atmospheres MO-41 dry box
filled with high purity argon. GC–MS analyses were carried out
on a Hewlett-Packard 5890 chromatograph interfaced to a HP
5970 quadrupole mass selective detector (EI, 70 eV).
Preparation of the other diazonium salts
A solution of the aniline (0.06 mol) in hot concentrated hydro-
chloric acid (50 ml, 0.6 mol) was prepared in a 250 ml beaker
and cooled in a dry ice–ethanol bath until it began to freeze. A
cold solution of NaNO2 (5.6 g, 0.08 mol) in water (10 ml) was
added, and the reaction mixture was stirred at 0 ЊC until clear.
It was then treated with activated charcoal (0.5 g) and filtered
through a Celite pad. The clear yellowish filtrate was treated
48% aqueous HBF4 (13 ml) or 60% aqueous HPF6 (11 ml) to
give a colorless crystalline precipitate. The precipitate was fil-
tered off under an argon atmosphere, washed with either
aqueous HBF4 or HPF6 (3 × 10 ml), dried at 0.3 Torr and
reprecipitated from acetonitrile with absolute ether.
Pentafluorobenzenediazonium tetrafluoroborate
3,5-Dinitrobenzenediazonium tetrafluoroborate. Colorless
flakes (13.5 g, 80%) from 3,5-dinitroaniline. ν/cmϪ1(KBr) 3102,
3090, 3079, 3042, 2319, 1616, 1605, 1557, 1352, 1105, 1082,
1035, 1024, 998, 925, 899; δH (CD3CN) 9.619 (d, 2H, J 1.8),
9.506 (t, 1H, J 1.8); δC(CD3CN) 150.1, 133.6, 131.6, 119.7.
CAUTION should be used in handling this material due to its
potentially explosive nature.
3-Nitrobenzenediazonium hexafluorophosphate. Colorless
needles (13.0 g, 74%) from 3-nitroanilines. δH (CD3CN) 9.256 (t,
1H, J 2.1), 8.955 (dd, 1H, J1 9.3, J2 2.1), 8.791 (dd, 1H, J1 9.3, J2
2.0), 8.172 (t, 1H, J 9.3).
An aqueous solution of fluoroboric acid (130 ml, 1 mol) was
placed in a 1 l three-necked round-bottomed flask equipped
with a mechanical stirrer and immersed in an ice bath. Acetic
anhydride (595 ml, 6.28 mol) was added through a dropping
funnel at such a rate as to keep the temperature of the reaction
mixture below 10 ЊC. To this solution of HBF4 in acetic acid
and acetic anhydride, liquid N2O3 (76 g, 1 mol) was added at
once to cause the immediate formation of a voluminous pre-
cipitate. The precipitate was filtered off under an argon atmos-
phere, washed with an acetic acid–dichloromethane (1:1 v/v,
3 × 50 ml) followed by dichloromethane (3 × 50 ml) and dried
in vacuo overnight to yield a free-flowing white powder (69.4 g,
59%) having an IR spectrum identical to a purified commercial
sample of nitrosonium tetrafluoroborate from Strem (vide
supra). A 250 ml Schlenk flask equipped with a magnetic stir-
ring bar and a rubber septum was charged with finely dispersed
nitrosonium tetrafluoroborate (11.8 g, 0.10 mol) under argon.
Dry acetonitrile (20 ml) was then injected with the aid of a
hypodermic syringe and the flask was immersed in an aceto-
nitrile–dry ice bath. After the solution had cooled to Ϫ30 ЊC,
2,3,4,5,6-pentafluoroaniline (18.3 g, 0.10 mol) in acetonitrile
(20 ml) was added over 30 min. The yellow reaction mixture
with an abundant heavy precipitate was stirred for 1 h at
Ϫ30 ЊC, treated with dry dichloromethane (150 ml) and filtered.
The white crystalline precipitate was washed with dry dichloro-
methane (3 × 50 ml), dissolved in a minimum volume of dry
acetonitrile (ca. 60 ml) and reprecipitated by the slow addition
of dichloromethane (200 ml) to yield shiny clear octahedra of
the diazonium salt (22.9 g, 81%) mp 124–126 ЊC (decomp.); ν/
cmϪ1(Nujol) 2314, 1655, 1648, 1578, 1571, 1561, 1538, 1524,
1504, 1438, 1326, 1158, 1112, 1070, 1052, 1037, 1020, 1005, 980,
4-Nitrobenzenediazonium
hexafluorophosphate.
Yellow
needles (13.6 g, 78%) from 4-nitroaniline. δH (CD3CN) 8.760 (m,
2H), 8.645 (m, 2H); δC(CD3CN) 155.4, 135.4, 127.7, 121.4.
4-(Ethoxycarbonyl)benzenediazonium hexafluorophosphate.
Colorless crystals (16.6 g, 86%) from ethyl 4-aminobenzoate.
δH (CD3CN) 8.575 (m, 2H), 8.415 (m, 2H), 4.420 (q, 2H, J 7.2),
1.380 (t, 3H, J 7.2); δC(CD3CN) 164.2, 142.7, 133.8, 132.9,
119.2, 63.9, 14.3.
3,5-Bis(ethoxycarbonyl)benzenediazonium
hexafluorophos-
phate. Colorless crystals (18.0 g, 76%) from diethyl 5-amino-
isophthalate. δH (CD3CN) 9.249 (d, 2H, J 1.2), 9.133 (t, 1H, J
1.2), 4.472 (q, 2H, J 7.1), 1.419 (t, 3H, J 7.1); δC(CD3CN) 162.8,
142.0, 137.0, 135.6, 64.2, 14.3.
3,5-Bis(trifluoromethyl)benzenediazonium
hexafluorophos-
phate. Colorless crystals (16.7 g, 72%) from 3,5-bis(trifluoro-
methyl)aniline. δH (CD3CN) 9.07 (br s, 1H), 8.86 (br s, 2H);
δC(CD3CN) 136.9 (t, J 4.7), 135.3 (q, J 36), 134.6 (t, J 4.7), 12.3
(q, J 274), 116.9.
Pentafluoroarenes (ArC6F5)
A 50 ml Schlenk flask equipped with a magnetic stirring bar
J. Chem. Soc., Perkin Trans. 2, 1997
2009

View Article Online
and a rubber septum was charged with the appropriate iodoarene
(3.0 mmol) and absolute ether (10 ml) under an argon atmos-
phere. A solution of n-butyllithium in hexane (1.2 ml, 3.0
mmol) was injected with the aid of a syringe. After stirring at
room temp. for 30 min, the mixture was transferred into a 50 ml
Schlenk flask containing a magnetically stirred solution of
hexafluorobenzene (1.116 g, 6.0 mmol) in diethyl ether (10 ml).
The opaque solution was stirred for 30 min. Aqueous work-up
and threefold extractions with diethyl ether afforded the
pentafluorophenyl-substituted arene, ArC6F5, as a yellow crys-
talline residue that was further purified by sublimation in vacuo
followed by recrystallization from ethanol.
Reaction of other diazonium salts induced by potassium iodide
In a typical procedure, a 50 ml Schlenk flask was charged with
the diazonium salt (1.00 mmol), equipped with a magnetic stir-
ring bar and sealed with a rubber septum in a dry box.
Acetonitrile (5.8 ml) and arene (8 mmol) were injected via
hypodermic syringes and a crystalline granule of potassium
iodide (166 mg, 1.0 mmol) was added in the counter stream of
argon. The reaction mixture was stirred for 1 h, quenched with
water (30 ml) and worked up as usual and subjected to GC
analysis after the addition of p-xylene (100 µl, 86.4 mg) as an
internal standard. The data for each reaction follows in serial
order as follows: Diazonium salt: Arene. Products (mmol).
3,5-Dinitrobenzenediazonium tetrafluoroborate: Benzene. m-
Dinitrobenzene (0.001), 5-iodo-3,5-dinitrobenzene (0.195), 3,5-
dinitrobiphenyl (0.730). Evaporation of the dichloromethane
extracts followed by column chromatography on silica with
CH2Cl2–hexane, recrystallization from ethanol and sublimation
(0.3 Torr) afforded 3,5-dinitrobiphenyl (155 mg, 64%) as color-
less flakes, mp 147–148 ЊC.
2,3,4,5,6-Pentafluorobiphenyl,47
4Ј-methyl-2,3,4,5,6-penta-
fluorobiphenyl,16 2Ј-methyl-2,3,4,5,6-pentafluorobiphenyl,16 3Ј-
methyl-2,3,4,5,6-pentafluorobiphenyl,16 2Ј,3Ј-dimethyl-2,3,4,5,6-
pentafluorobiphenyl,48
3Ј,4Ј-dimethyl-2,3,4,5,6-pentafluoro-
biphenyl,48 2Ј,5Ј-dimethyl-2,3,4,5,6-pentafluorobiphenyl,48 2Ј-
isopropyl-2,3,4,5,6-pentafluorobiphenyl,48 4Ј-isopropyl-2,3,4,5, 6-
pentafluorobiphenyl,48
4Ј-tert-butyl-2,3,4,5,6-pentafluoro-
biphenyl,49 2Ј,4Ј,6Ј-trimethyl-2,3,4,5,6-pentafluorobiphenyl,48 3Ј-
chloro-2,3,4,5,6-pentafluorobiphenyl,49 4Ј-chloro-2,3,4,5,6-penta-
fluorobiphenyl,49 2Ј,5Ј-dichloro-2,3,4,5,6-pentafluorobiphenyl,49
2,4,6-Trichloro-3,5-difluorobenzenediazonium
hexafluoro-
phosphate. Benzene. 2,4,6-Trichloro-3,5-difluorobenzene
(0.083), 2,4,6-trichloro-3,5-difluoroiodobenzene (0.242), 2,4,6-
trichloro-3,5-difluorobiphenyl (0.65). Evaporation of the
dichloromethane extracts followed by column chromatography
on silica with CH2Cl2–hexane, recrystallization from ethanol
and sublimation (0.3 Torr) afforded 2,4,6-trichloro-3,5-
difluorobiphenyl (153 mg, 52%) as colorless needles, mp 61–
62 ЊC.
4-Nitrobenzenediazonium hexafluorophosphate. Benzene.
Nitrobenzene (0.005), 4-iodonitrobenzene (0.788), 4-nitro-
biphenyl (0.181).
1-pentafluorophenylnaphthalene,15
2-pentafluorophenyl-
naphthalene,15 2Ј-methoxy-2,3,4,5,6-pentafluorobiphenyl,49 3Ј-
methoxy-2,3,4,5,6-pentafluorobiphenyl,49 4Ј-methoxy-2,3,4,5,6-
pentafluorobiphenyl,50
biphenyl,49
2Ј,3Ј-dimethoxy-2,3,4,5,6-pentafluoro-
3Ј,4Ј-dimethoxy-2,3,4,5,6-pentafluorobiphenyl,49
2Ј,5Ј-dimethoxy-2,3,4,5,6-pentafluorobiphenyl,49 1,4-bis(penta-
fluorophenyl)benzene,15 3Ј-nitro-2,3,4,5,6-pentafluorobiphenyl.51
The isomeric 1,2- and 1,3-bis(pentafluorophenyl)benzenes were
not separately synthesized from o- and m-diiodobenzene. How-
ever, GC–MS analysis of the (C6F5)2C6H4 fractions of the reac-
tion mixture from C6F5N2ϩ and benzene (especially at low ben-
zene concentrations) showed three separate peaks (with the
same MS cracking patterns) in a roughly 1:2:0.5 (area) ratio
for the ortho, meta and para isomers respectively. The last was
identical to the authentic sample of 1,4-bis(pentafluoro-
phenyl)benzene prepared above.
Aryl iodides. Pentafluoroiodobenzene, ethyl 4-iodobenzoate,
diethyl 4-iodoisophthalate, and 4-iodobromobenzene obtained
from Aldrich were used as received. 2,4,6-Trichloro-3,5-
difluoroiodobenzene49 and 3,5-dinitroiodobenzene49 were syn-
thesized by iododediazoniation of the corresponding diazo-
nium tetrafluoroborate salts.
Azo-coupling of pentafluorobenzenediazonium tetrafluoroborate
with aromatics
In a typical procedure, a 100 ml two-neck Schlenk flask was
charged with pentafluorobenzenediazonium tetrafluoroborate
(282 mg, 1.00 mmol), equipped with a magnetic stirring bar,
thermometer and a rubber septum in a dry box. Acetonitrile
(5.8 ml) was injected with a hypodermic syringe, and the flask
was carefully degassed three times and immersed in an
acetonitrile–dry-ice bath. After the temperature of the reaction
mixture had fallen to Ϫ40 ЊC, the solution of m-dimethoxy-
benzene (1.10 g, 8 mmol) in acetonitrile (2 ml) was slowly added
with intense stirring. The fast transformation was manifested
by the appearance of dark red coloration. The resulting dark
red reaction mixture was quenched with water (30 ml) after 10
min and extracted with dichloromethane (3 × 25 ml). The com-
bined dichloromethane extracts were washed with water (3 × 50
ml), dried over magnesium sulfate and subjected to GC analysis
after the addition of p-xylene (100 µl, 86.4 mg) as an internal
standard. Evaporation of the solvent and twofold recrystalliz-
ation from methanol afforded pure 1-pentafluorophenyl-2-(2,4-
dimethoxyphenyl)diazene49 as red needles with metallic lustre
(302 mg, 91%).
m-Xylene. The reaction mixture was kept at 24 ЊC in the
dark for 14 d. An aliquot (0.5 ml) of the resulting orange
solution was withdrawn, incubated with m-dimethoxy-
benzene (138 mg, 1 mmol) for 10 min, quenched with water
and extracted with dichloromethane (3 × 5 ml). A GC analysis
of the combined extracts with p-xylene as an internal stand-
ard showed that 0.46 mmol of the diazonium salt remained
unreacted. The rest of the original reaction mixture was
poured into 20% hydrochloric acid (50 ml) and extracted with
dichloromethane (3 × 20 ml). The combined dichloromethane
extracts were washed with aqueous sodium hydrogencarbonate
and water and evaporated to dryness. Twofold recrystallization
of the orange residue thus obtained afforded pure 1-
pentafluorophenyl-2-(2,4-dimethylphenyl)diazene49 as light
orange needles (110 mg, 68% based on the conversion of the
diazonium salt).
Pentafluorophenylation of aromatic compounds catalysed by
iodide
In a typical procedure, a 50 ml Schlenk flask was charged with
pentafluorobenzenediazonium tetrafluoroborate (282 mg, 1.00
mmol), equipped with a magnetic stirring bar and sealed with a
rubber septum in a dry box. Acetonitrile (5.8 ml) and benzene
(0.71 ml, 8 mmol) were injected via hypodermic syringes and a
crystalline granule of potassium iodide (33 mg, 0.2 mmol) was
added under a counter stream of argon. A vigorous gas evolu-
tion was observed immediately. The reaction mixture was
stirred for 10 min and quenched with water (30 ml) containing a
few drops of a starch solution, titrated with 0.020  sodium
thiosulfate and extracted with dichloromethane (3 × 25 ml).
The combined dichloromethane extracts were washed with
water (3 × 50 ml), dried over magnesium sulfate and subjected
to GC analysis after the addition of p-xylene (100 µl, 86.4 mg)
as an internal standard. Evaporation of the solvent and
repeated sublimation (45 ЊC, 0.3 Torr) of the yellow crystalline
residue afforded pure colorless 2,3,4,5,6-pentafluorobiphenyl
(191 mg, 79.9%, purity 99.8% by GC). The same procedure was
followed for the reactions with the other aromatic donors. The
products were detected by GC and GC–MS analysis.49 The
arylation reactions of the arenes with pentafluorobenzene-
diazonium catalysed by sodium iodide in Tables 1 and 2 were
taken from ref. 19.
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Mesitylene. 1-Pentafluorophenyl-2-(2,4,6-trimethylphenyl)-
diazene52 was obtained as orange needles (297 mg, 95%).
Pentamethylbenzene. The reaction was complete within 30 s
at room temp. A voluminous orange precipitate was collected
and orange needles of 1-pentafluorophenyl-2-(2,3,4,5,6-penta-
methylphenyl)diazene49 were obtained after a twofold recrystal-
lization (334 mg, 98%).
Anisole. The reaction was complete after 1 h at room temp.,
and 1-pentafluorophenyl-2-(4-methoxyphenyl)diazene52 (278
mg, 92%) was obtained as orange scales.
was quenched and worked up after 30 min. 2,4,6-Trichloro-3,5-
difluorobenzene (0.27), 2,5-dimethoxyphenylacetonitrile
(0.05), 2Ј,5Ј-dimethoxy-2,4,6-trichloro-3,5-difluorobiphenyl
(0.67).49 No unreacted diazonium salt was found. Solvent
removal yielded
a
colorless crystalline residue. 2Ј,5Ј-
Dimethoxy-2,4,6-trichloro-3,5-difluorobiphenyl49 was isolated
from the residue after passing it through a column of silica
with diethyl ether–hexane, fractional vacuum sublimation and
recrystallization from methanol as colorless needles (114 mg,
32%).
1,2-Dimethoxybenzene. The coupling was complete within 1
min at room temp. The usual aqueous work up and recrystal-
lization yielded 1-pentafluorophenyl-2-(3,4-dimethoxyphenyl)-
diazene49 as orange–brown needles (311 mg, 95%).
Acknowledgements
We thank the National Science Foundation and the R. A.
Welch Foundation for financial assistance.
Thermal (dark) reactions of arenes with diazonium salts
In a typical procedure, a solution of the diazonium salt (1.00
mmol) and the arene (3.00 mmol) in acetonitrile (20 ml) was
prepared in a Schlenk test tube and maintained under an argon
atmosphere for 8 h in the dark. The reaction mixture quenched
with water (10 ml) and extracted with chloroform (3 × 20 ml).
The chloroform extracts were analysed by GC and GC–MS,
and the products are listed in serial order: Diazonium salt.
Arene. Products (mmol).
3,5-Dinitrobenzenediazonium tetrafluoroborate. p-Dimethoxy-
benzene. m-Dinitrobenzene (0.08), 2,5-dimethoxyphenylaceto-
nitrile (0.03), 2,5-dimethoxy-3Ј,5Ј-dinitrobiphenyl (0.83). The
amount of untreated diazonium salt was 0.10 mmol, 10%. m-
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J. Chem. Soc., Perkin Trans. 2, 1997
2011

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Paper 7/01745F
Received 12th March 1997
Accepted 16th May 1997
2012
J. Chem. Soc., Perkin Trans. 2, 1997