sumably is the result of the competing electron transfer mech-
anism. The results described above suggest that for a given sub-
strate the ET mechanism mainly will be controlled by the nature
of the solvent. HFP represents one extreme: it increases
strongly the oxidation potential of ICl—presumably to a
large extent by blocking the formation of ICl᎐halide ion
complexes—and therefore extends the upper redox potential
limit of substrates which will undergo ET oxidation. The other
extreme should involve complexation of ICl with a donor mole-
cule which reduces its redox potential. The favourable iodinat-
ing properties of the pyridine–ICl complex29 might originate
from this effect. In any given solvent, the iodination reaction
will also be favoured by choice of a substrate which has as high
In general, the spectra were resolved to the same extent as or
better than published spectra in HFP or in other solvents. In
most cases, except for ArH with the highest EЊ(ArH ϩ/ArH)
values, the spectral intensity was high and the persistency of the
radical cations high enough for spectra of very good quality to
be obtained. The following compounds either produced EPR
spectra identical to those previously recorded by generation
from TlIII reagents and ArH in HFP or their EPR spectra were
analysed and shown to give hfs constants in satisfactory agree-
ment with previous observations: 4,4Ј-di-tert-butylbiphenyl,8b
1,2-dimethoxybenzene (2),8b 1,4-dimethoxybenzene (1),8d pen-
tamethylanisole,8b 1-methoxynaphthalene (forms the radical
cation of 4,4Ј-dimethoxy-1,1Ј-binaphthalene8d), 2,2Ј-dimeth-
oxy-5,5Ј-di-tert-butylbiphenyl,32 2,2Ј-dimethoxy-5,5Ј-dimethyl-
biphenyl,8b anthracene,13 3,3Ј,4,4Ј-tetramethyl-1,1Ј-binaph-
thalene [aH 0.590 (6 H), 0.209 (2 H), 0.105 (6 H), 0.0352 (2 H),
0.0176 (4 H) mT; the spectrum was identical to that published
from the 4-tolyl-TlIII trifluoroacetate oxidation of this sub-
strate8b,33], 1,4,5,8-tetramethylnaphthalene [aH 0.782 (12 H),
0.176 (4 H); lit.34 in TFA: aH 0.781 (12 H), 0.182 (4 H) mT],
thianthrene [HFP, a2,3,6,7-H 0.128 (4 H), a33S 0.892 mT; the coup-
ling of 0.012 mT to 1,4,5,8-H was not seen due to line-
broadening; lit.35 in nitromethane: a2,3,6,7-H 0.128 (4 H), a33S
0.892 mT], 1,4-dimethoxy-2-methylbenzene [aH 0.45 (1 H), 0.334
(6 H), 0.305 (3 H), 0.111 (1 H), 0.076 (1 H) mT], 1,4-dimethoxy-
2,3-dimethylbenzene (4) [aH 0.298 (6 H), 0.290 (2 H), 0.139 (6
H); lit.36 in nitromethane at Ϫ25 ЊC: aH 0.307 (6 H), 0.290 (2 H),
0.148 (6 H) mT], 1,4-dimethoxy-2,5-dimethylbenzene (3) [aH
0.450 (6 H), 0.044 (2 H), 0.312 (6 H); lit.36 in nitromethane at
Ϫ25 ЊC: aH 0.421 (6 H), 0.057 (2 H), 0.315 (6 H) mT],
9,10-dimethylanthracene [aH 0.791 (6 H), 0.252 (4 H), 0.117 (4
H); lit.37: aH 0.800 (6 H), 0.254 (4 H), 0.119 (4 H) mT], perylene
[aH 0.408 (4 H), 0.308 (4 H), 0.044 (4 H); lit.37: aH 0.404 (4 H),
0.303 (4 H), 0.044 (4 H) mT], 1,4-dimethoxy-2,5-di-tert-
butylbenzene [aH 0.308 (6 H), 0.076 (2 H), 0.013 (18 H); lit.36 aH
0.324 (6 H), 0.103 (2 H), 0.0103 (18 H) mT], hexamethoxy-
triphenylene [aH 0.069 (6 H), 0.018 (18 H) mT], 4-tert-butyl-
N,N-dimethylaniline [aH 1.10 (6 H), 0.501 (2 H), 0.156 (2 H),
0.039 (9 H), aN 1.12; lit.38 in acetonitrile: aH 1.22 (6 H), 0.520
(2 H), 0.130 (2 H), 0.040 (9 H), aN 1.13 mT], pyrene [aH 0.618
(4 H), 0.246 (4 H), 0.130 (2 H); lit.39: aH 0.539 (4 H), 0.217
(4 H), 0.120 (2 H) mT], 1,2,4,5,6,8-hexamethylanthracene [aH
0.537 (2 H), 0.341 (12 H), 0.226 (6 H), 0.079 (2 H); lit.40: 0.543
(2 H), 0.348 (12 H), 0.237 (6 H), 0.079 (2 H) mT], 1,2,4,5-
tetramethoxybenzene [aH 0.220 (12 H), 0.088 (2 H); lit.41 in
nitromethane: aH 0.225 (12 H), 0.088 (2 H) mT], 1,2,3,4,5,6,7,8-
octamethylanthracene [0.540 (2 H), 0.330 (12 H), 0.165 (12 H);
lit.42 in TFA: 0.545 (2 H), 0.334 (12 H), 0.164 (12 H) mT], N-
methylphenothiazine [aH 0.723 (6 H), 0.210 (2 H), 0.082 (4 H),
aN 0.750, the smallest aH of 0.022 mT was not seen with ICl, I2
or Br2 as the oxidant (it was seen in a spectrum from a sample
made by 4-tolyl-TlIII trifluoroacetate oxidation); lit.43: aH 0.723
(6 H), 0.220 (2 H), 0.076 (4 H), 0.029 (2 H), aN 0.750 mT]; bis[4-
(dimethylamino)phenyl]squaraine [aH 0.451 (12 H), 0.223 (4 H),
0.050 (4 H), aN 0.451 (2 N); lit.44 in dichloromethane: aH 0.454
(12 H), 0.227 (4 H), 0.047 (4 H), aN 0.454 (2 N) mT], 4,4Ј-
dimethoxystilbene [shortlived, not resolved: multiplet of ca. 15
lines with an hfs splitting of 0.10–0.11 mT; a resolved spectrum
was obtained by oxidation with 4-tolyl-TlIII trifluoroacetate in
HFP–trifluoroacetic acid (2%): aH 0.38 (2 H), 0.169 (4 H), 0.114
(6 H), 0.0175 (4 H) mT; with high modulation amplitude, the
simulated spectrum appears as a multiplet with a splitting of
0.10–0.11 mT].
an EЊ(ArH ϩ/ArH) as possible.
Conclusions
The results presented above show that halogenation of ArH by
common halogenating agents in HFP follows an ET mechan-
ism, where an initial fast ET step gives a mixture of ArH ϩ and
halide ion which react slowly to give halogenated product(s). It
is also shown that ArH ϩ can abstract a halogen atom from ICl
or Br2, a reaction earlier demonstrated only for iodine.7c The
role of HFP in favouring the ET mechanism appears to be at
least two-fold: it increases significantly the oxidative power of
halogens and deactivates nucleophiles drastically, thus provid-
ing conditions for convenient observation of the intermediate
ArH ϩ of a wide range of substrates. The initial ET step should
also be amenable for study, for example by stopped-flow
techniques.
Experimental
Methods
NMR spectra were recorded on a Varian XL-300 or Bruker 400
spectrometer. Mass spectrometry was performed on a JEOL
JMS SX-102 instrument. GLC analyses (HP5892 series II) were
made on a fused silica column (OV-1701, 25 m).
EPR spectra were recorded by the Upgrade Version ESP
3220-200SH of a Bruker ER-200D spectrometer. Spectral simu-
lations were carried out by either the public domain programme
WINSIM30 or Simfonia (Bruker AG). Cyclic voltammetry
was performed by the BAS-100 instrument, using Bu4NPF6
(0.15 mol dmϪ3) as the supporting electrolyte and an Ag/AgCl
electrode as the reference, with iR compensation. All potentials
were calibrated against the internal ferricinium/ferrocene
couple (0.43–0.44 and 0.05 V vs. Ag/AgCl in acetonitrile and
HFP, respectively). UV spectroscopy was performed by the HP-
8452A UV–VIS diode array spectrophotometer, the kinetics
being monitored by the HP89532K software package. All
evaluations of rate constants were made by the SigmaPlot
programme.
Materials
1,1,1,3,3,3-Hexafluoropropan-2-ol (HFP) and acetonitrile were
of Uvasol quality from Merck AG31 while dichloromethane
was of Suprasolv quality from the same source. Most com-
pounds used were either purchased in highest quality available
or available from earlier work. 1,4-Dimethoxy-2,5-dimethyl-
benzene (3) and 1,4-dimethoxy-2,3-dimethylbenzene (4), were
available from earlier work.5
EPR spectra
Reactions with hexamethylbenzene were performed either as
above or by injecting 10–50 µl of a fresh, saturated solution of
chlorine in HFP via a plastic tube into 0.70 ml of a solution of
hexamethylbenzene in HFP kept in the sample tube with the
cavity tuned. With this arrangement, the time delay between
mixing and start of spectral recording was 4–5 s. Reactions in
acetic acid were performed in the same way.
The substrate ArH was dissolved in HFP in a concentration of
2–40 mmol dmϪ3 in a sample tube and the solution bubbled by
Ar, all operations being performed under dimmed light con-
ditions. A deficit of the halogenating agent, dissolved in HFP or
dichloromethane, was added. After bubbling with Ar for 100–
150 s, the sample tube was sealed and transferred to the cavity.
68
J. Chem. Soc., Perkin Trans. 2, 1998