Aromatic iodination: A new investigation on the nature of the mechanism

Article abstract of DOI:10.1039/b103758g

Following a suggestion by the late Lennart Eberson, we have employed the ICl-HFP (HFP being hexafluoropropan-2-ol) system in iodination reactions, and found unambiguous evidence for the occurrence of an ET-mechanism of halogenation. The evidence is based on the use of 'intelligent' substrates, which make it possible to fix the boundaries between the occurrence of an ET-mechanism and of a conventional polar mechanism. In an 'intelligent' substrate, in fact, the nature of the product(s) changes significantly depending on the operating mechanism. The ICl-HFP combination is instrumental to the onset of a one-electron oxidation with electron-rich substrates, followed by halogenation. The most prominent example is that of the electron-rich substrate durene (1,2,4,5-tetramethylbenzene, DUR), when compared to mesitylene (1,3,5-trimethylbenzene, MES): with a 'conventional' iodination system (i.e., I₂/ Ag⁺) and in common solvents, where the polar mechanism holds, durene is less reactive (k_MES/k_DUR = 46 ± 3), but becomes more reactive (k_MES/k_DUR = 0.23) in HFP with ICl, where the ET-mechanism takes over. Other substrates also support the onset of ET-pathways in HFP. Finally, a preliminary survey of a biohalogenation reaction induced by laccase indicates the modest occurrence of a polar process of iodination with a few substrates.

Full text of DOI:10.1039/b103758g

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Aromatic iodination: a new investigation on the nature of the
mechanism†
Maura Fabbrini,a Carlo Galli,*a Patrizia Gentili,a Daniele Macchitellaa and Horia Petrideb
2
a Dipartimento di Chimica and Centro CNR Meccanismi di Reazione, Università ‘La Sapienza’,
p. le A. Moro 5, Roma 00185, Italy
b Centrul de Chimie Organica, Spl. Independentei 202-B, 76250 Bucuresti,
O. f. 15, P. O. Box 254, Romania
Received (in Cambridge, UK) 25th April 2001, Accepted 18th June 2001
First published as an Advance Article on the web 31st July 2001
Following a suggestion by the late Lennart Eberson, we have employed the ICl–HFP (HFP being hexafluoropropan-
2-ol) system in iodination reactions, and found unambiguous evidence for the occurrence of an ET-mechanism of
halogenation. The evidence is based on the use of ‘intelligent’ substrates, which make it possible to fix the boundaries
between the occurrence of an ET-mechanism and of a conventional polar mechanism. In an ‘intelligent’ substrate,
in fact, the nature of the product(s) changes significantly depending on the operating mechanism. The ICl–HFP
combination is instrumental to the onset of a one-electron oxidation with electron-rich substrates, followed by
halogenation. The most prominent example is that of the electron-rich substrate durene (1,2,4,5-tetramethylbenzene,
DUR), when compared to mesitylene (1,3,5-trimethylbenzene, MES): with a ‘conventional’ iodination system (i.e., I2/
Agϩ) and in common solvents, where the polar mechanism holds, durene is less reactive (kMES/kDUR = 46 3), but
becomes more reactive (kMES/kDUR = 0.23) in HFP with ICl, where the ET-mechanism takes over. Other substrates
also support the onset of ET-pathways in HFP. Finally, a preliminary survey of a biohalogenation reaction induced
by laccase indicates the modest occurrence of a polar process of iodination with a few substrates.
Introduction
The debate over the mechanism of electrophilic aromatic
substitution reactions, which are among the most important
processes in organic chemistry, has continued in the recent
literature. The ‘conventional’ polar route of substitution, via
rate-limiting collapse of the electrophile (Eϩ) and the aromatic
substrate (ArH) to form a σ-complex (Scheme 1),1 has been
Scheme 3
occurred (Scheme 3) between the electron-donor and the
electron-acceptor species, according to a Benesi–Hildebrand
formulation.4
Close scrutiny of many electrophilic reactions from an
electrochemical approach led to the conclusion that the ET
mechanism is not supported by the available facts,5 with the
possible exception of the case of the strongest oxidant among
the electrophiles, i.e., the nitronium ion NO2ϩ, whenever it
reacts with electron-rich aromatic substrates.2c,5,6 Operation of
the ET mechanism could also be excluded in the case of
aromatic iodination,7 and of some other electrophilic reac-
tions,8 by resorting to the determination of reactivity ratios for
appropriate pairs of substrates.9 Certainly, each electrophile
would require a mechanistic assessment on its own, because the
threshold of the oxidation potential of the substrate, beyond
which a transition from the conventional polar route to the ET
route becomes possible, will depend on the particular oxidation
potential of the electrophile involved [eqns. (1) and (2)].
Scheme 1
challenged by other formulations that originate from the
electron transfer (ET) concept.2 For example, in view of the
oxidising character of some electrophiles, an alternative mech-
anism could involve an initial electron transfer step leading to a
radical cation (ArH ) that combines with E to give the same
ϩ
ؒ
ؒ
σ-complex as the polar pathway (Scheme 2).
Eϩ (strong oxidant) ϩ ArH →
ET substitution (Scheme 2) (1)
Scheme 2
An even more subtle formulation of this point regards the
transition state of the process as a resonance hybrid between
the initial state and a radical pair, that would lead to the
σ-complex.3 This radical pair can be viewed as the excited state
of a π-complex between the substrate and the electrophile (viz.,
a charge-transfer complex, CT), where a shift of electron has
Eϩ (weak oxidant) ϩ ArH →
polar substitution (Scheme 1) (2)
A recent investigation by the late Lennart Eberson of
aromatic halogenation (mainly iodination with ICl),10 has
provided evidence for the involvement of radical cations in
1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) with substrates of
† Dedicated to the memory of Professor Lennart Eberson
1516
J. Chem. Soc., Perkin Trans. 2, 2001, 1516–1521
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103758g

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moderate-to-good electron richness. The explanation provided
for the role of HFP was that this solvent fostered the ET
mechanism by increasing both the oxidation power of the halo-
was once more obtained (Table 1, entry 1). When the reaction
was run in HFP, however, a completely different intermolecular
selectivity was obtained (entry 2): 3-iododurene prevailed over
2-iodomesitylene, with chlorodurene, chloroiododurene and
diiododurene also being detected in small amounts (Scheme 4).
genating agent and also, because of its very weak nucleophilic
ϩ
character, the persistency of the ArH species.10 This finding
ؒ
stimulated us to expand our previous investigation of the iodin-
ation reaction towards the use of HFP as the solvent. The aim
was to detect any transition between the competing mechan-
isms (i.e., polar vs. ET) in this new reaction medium. To enable
the nature of the operating mechanism to be appraised we
studied the reactivity of our previous substrate pairs,7 and
also of other ‘intelligent’ substrates. Reaction of all these
substrates with iodine will therefore be the common feature of
the experiments reported below. A preliminary survey of a
biohalogenation process, and its mechanism, is also described.
Scheme 4
The possible interference from side-chain halogenation,
resulting either from DUR ϩ (ET route),15 or from a polar ipso-
ؒ
addition, followed by side-chain functionalisation (Scheme 5),16
Results and discussion
The following pairs of substrates are ‘intelligent’ as far as the
competition between the polar and the ET mechanisms of
halogenation is concerned because, within each pair, the relative
substrate reactivity presents clearly contrasting trends depend-
ing on the mechanism.
Mesitylene vs. durene
The experimental σ-basicity (viz., log KB) of mesitylene is
higher than that of durene, the values being Ϫ0.4 and Ϫ2.2,
respectively.11 MES should therefore be more reactive than
DUR in a ‘conventional’ electrophilic aromatic substitution,
where the structure of the transition state (TS) of the rate
determining step resembles that of the σ-complex (Scheme 1).9
Conversely, since the oxidation potential (EЊ) of DUR is lower
than that of MES (2.07 and 2.35 V vs. NHE in TFA, respec-
tively12), DUR should be more reactive than MES if the TS of
the electrophilic process is structurally closer to the radical pair
(Scheme 2).9 Accordingly, when this intermolecular selectivity
was determined for the most often studied electrophilic aro-
matic substitutions, MES : DUR reactivity ratios significantly
>1 were in general obtained, both from competitive experi-
ments and from absolute rate constant determinations.9 This
result supports a σ-complex structure for the TS of the investi-
gated reactions, including the iodination reaction,7,8b and the
operation of the conventional polar mechanism with these two
particular substrates, or with substrates of comparable acti-
vation. Clearly, this finding does not preclude that the ET
mechanism may take over for substrates easier to oxidise than
the polyalkylbenzenes, such as the polymethoxybenzenes.10,13
Our interest here is bound to determine whether the reported
‘beneficial’ effect of the HFP solvent10 on the stability of the
radical cation of the substrate can be such as to induce a shift
of the mechanism of iodination, from polar to ET. In addition
to the solvent, the use of ICl as the iodinating agent, in view of
its higher oxidation potential with respect to that of I2, particu-
larly in HFP (Epc 0.98 vs. 0.39 V, respectively),10 would appear
to be instrumental to the onset of an ET iodination. It has been
found, however, that a competitive reaction of MES and DUR,
with a deficit of ICl as the iodinating agent, in a AcOH–TFA–
TFAA–CH3CN 60 : 8 : 8: 24 v/v solvent mixture, did provide a
kMES/kDUR reactivity ratio of 43 at room temperature,8b in keep-
ing with the polar mechanism. This value is very close to those
of other iodinating systems, based on molecular iodine,7 that
cluster around kMES/kDUR values of 45 5. It is also in line with
the value of 52 that can be obtained from the ratio of the abso-
lute rate constants of reaction of MES and of DUR with ICl in
AcOH,14 emphasising the consistency between results originat-
ing from competitive experiments and from direct kinetic
determinations.9b
Scheme 5
was tested by subjecting the reaction products to exhaustive
acetoxylation, to convert any benzylic haloarene (possibly co-
eluting in the GC analysis with the nuclear counterpart) into a
benzylic acetoxy-derivative: this acetoxylation had no effect on
the nature of the products nor on the kMES/kDUR reactivity
value.9b,17,18
The haloarenes were therefore truly nuclear-halogenated
derivatives, and this supported an ET pattern of nuclear aro-
matic substitution in HFP [eqns. (3)–(5)], at least with DUR;
ϩ
ؒ
Ϫ
ؒ
ArH ϩ ICl → ArH ϩ I (0.5 I2) ϩ Cl
(3)
(4)
(5)
ϩ
ؒ
ϩ
ؒ
ArH ϩ I (0.5 I2) → Ar–I ϩ H
ArH ϩ ϩ ClϪ → → Ar–Cl ϩ Hϩ ϩ eϪ
ؒ
durene, the easier to oxidise substrate, becomes accordingly
more reactive than mesitylene (kMES/kDUR = 0.23). In agreement
with the operation of the ET mechanism is also the formation
of chloro-derivatives from DUR, deriving from capture of
DUR ϩ by ClϪ [eqn. (5)].10,13
ؒ
It is remarkable that another iodination of MES and DUR in
HFP, run with I2 and with a Agϩ salt as a Lewis acid, provided
the ‘normal’ kMES/kDUR value > 1 (entry 3). This result indicates
that it is only the whole HFP–ICl system that is capable of
inducing a polar → ET shift in the mechanism of aromatic
iodination with this pair of substrates (compare Table 1, entries
1–3).
m-Dimethoxy-/p-dimethoxybenzene
The oxidation potential (E p) of p-dimethoxybenzene (p-MeO)
is lower than that of m-dimethoxybenzene (m-MeO) (1.34 and
1.55 V vs. SCE in TFA,2b respectively), both compounds being
easier to oxidise than MES and DUR. Unfortunately, no
σ-basicity data are available for these two dimethoxybenzenes,
but the meta-isomer is much more reactive than the para-isomer
in a typical electrophilic process such as the bromination reac-
tion.19 Analogous σ-basicity values can therefore be drawn with
The competition of MES and DUR for a deficit of ICl was
repeated here in acetonitrile, and a kMES/kDUR value over forty
J. Chem. Soc., Perkin Trans. 2, 2001, 1516–1521
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those of m- (Ϫ3.2) and p-xylene (Ϫ5.7).11 Based on these con-
siderations, an intermolecular selectivity m-MeO/p-MeO > 1
would support the polar mechanism, while the ET mechanism
would cause a ratio of < 1.
Iodination of these two substrates by ICl in a AcOH–
(CF3CO)2O–CH3CN mixed solvent provided
a
km-MeO/
kp-MeO = 1400 (Table 1, entry 4), in agreement with the polar route
and with a previous result.8b The iodination reaction with ICl has
also been run in HFP. Even though m-MeO was again more
reactive than p-MeO, the km-MeO/kp-MeO decreased to 210 (entry 5).
The crucial observation was, however, that a chloro-substituted
product appeared and accompanied that of iodo-substitution in
the case of p-MeO,13 but not of m-MeO (Scheme 6).
Scheme 6
Occurrence of the ET route of halogenation, delineated in
[eqns. (4) and (5)], is therefore supported for p-MeO in HFP,10,13
which accordingly gains in reactivity, while m-MeO continues
to react by the polar mechanism.
Mesitylene vs. naphthalene
The ionisation potential of naphthalene (Naph; EЊ 2.08 V)20 is
similar to that of durene, while its σ-basicity (i.e., Ϫ4)21 is much
lower. Accordingly, a competitive iodination of MES and Naph
with I2 in a AcOH–CH3CN 3 : 1 mixed solvent, with CF3CO2-
Ag as the Lewis acid promoter, gave kMES/kNaph ≅ 2000.8b The
reaction has been repeated here with ICl in HFP. Although
naphthalene was again much less reactive (kMES/kNaph ≥ 1700;
Table 1, entry 6), a small amount of 1-chloronaphthalene did
accompany 1-iodonaphthalene this time. Therefore, the oper-
ation of the ET route, and of (eqn. 5), appears to play some role
towards Naph in the ICl–HFP system.
From the foregoing it can be seen that the use of substrate
pairs as a mechanistic tool has its advantages.9 The substrates
that follow have been investigated singularly, not pairwise. They
are also mechanistically ‘intelligent’ in that the nature of the
product(s) of their reaction with iodine does change signifi-
cantly in response to a change of the reaction conditions, in a
way that suggests a shift of the mechanism.
N,N-Dimethylaniline 1
Iodination of 1 with I2 in CHCl3 (or also in CHCl3–CH3OH
3 : 1 v/v) gave only N,N-dimethyl-p-iodoaniline (2) in good yield
(Table 1, entries 7 and 8) (Scheme 7).
Repetition of the iodination of 1 with I2, but in HFP as the
Scheme 7
solvent, gave instead small amounts of N-methylaniline 3,
besides 2, and traces of benzidinic dimers (entry 9). While an
aromatic substitution on the highly activated nucleus of 1 is
expected for a polar route, formation of 3 is typical for the
fragmentation of the radical cation of a tertiary amine.22 In
fact, independent reaction of 1 with potassium 12-tungsto-
cobaltate() (CoIIIW), an outer-sphere oxidising agent (EЊ 1.49
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V vs. SCE in CH3CN),22b did yield 3 (entry 10), accompanied
by the benzidinic dimers.23 The mechanistic relevance of
N-dealkylation steps resulting from the radical cation of
tertiary amines is well documented, even in connection with
some biomimetic processes.22–24 Thus, the lack of formation of
the N-dealkylation product in the iodination experiments run in
conventional solvents (entries 7 and 8), in contrast to the
appearance of 3 and of the benzidinic dimers on changing the
solvent to HFP (entry 9), could endorse a change of the mech-
derivative 10 (entry 13). This result is consistent with the inter-
ϩ
ؒ
mediate formation of 7 and with a stereoelectronic problem
associated with it. In fact, the bulky N,N-dimethylamino group
in 7, in order to relieve the steric interference with the two C–H
bonds at the ortho positions, is likely to be tilted away from
planarity with the aromatic ring (Scheme 10). This means that in
ϩ
ؒ
anism from polar to ET, and support the oxidation of 1 to 1
by I2 in HFP, where molecular iodine becomes a stronger
oxidant.10 This would not exclude the possibility that the iodin-
ation mechanism (to 2) has shifted from the polar to the ET
route (see eqn. (4)) in HFP, with this electron-rich substrate (E p
of 1 is 0.76 V vs. SCE at 0.5 V sϪ1 in CH3CN). It would not
prevent either the dealkylation pathway (through 1 ϩ) and the
ؒ
iodination pathway (through a polar σ-complex) from being
independent and competing events.
N,N-Dimethyl-4-methylaniline 4
Scheme 10
While the redox potential of this substrate is comparable (E p
0.61 V vs. SCE at 0.5 V sϪ1 in CH3CN) with that of 1, its
iodination at the para position is prevented by the presence of
the methyl group, the reactivity of the ortho-positions being
strongly depressed sterically by the bulkiness of the dimethyl-
amino-substituent. Experimentally, no nuclear iodination
occurred for the reaction of 4 with I2 in a CDCl3–CD3OD (3 : 1,
v/v) mixed solvent (Table 1, entry 11). Rather, some N-dealkyl-
ation took place (to 5), while the main product (6) was that of
capture of the solvent by the intermediate side-chain carbo-
cation,23a,25 originating from the radical cation of the precursor
(Scheme 8).26
ϩ
ؒ
7
the radical cation is mainly localised onto the anisole-like
ring, and further stabilised there by the combined action of the
solvent HFP and of the methoxy-substituent. Attack by ClϪ on
the ring, in keeping with the operation of eqn. (5), would
accordingly have more chances to prevail over electron transfer
from the nitrogen lone-pair of the dimethylamino group, with
ensuing N-demethylation.
Conclusion
Molecular iodine is both a weak halogenating agent and an
oxidant of moderate strength.1,10 In general, and with sub-
strates of moderate-to-good activation, the polar mechanism of
iodination prevails (see the MES–DUR or m-MeO/p-MeO
probes).7–9 However, whenever the use of the non-nucleophilic
HFP solvent strongly favours the formation of the radical
cation of the substrate,10 the onset of ET pathways, and in
particular of the ET-route of halogenation (Scheme 2), can be
documented (see Schemes 4 and 6). The elucidation of the
remarkable role of solvent HFP in this mechanistic dichotomy
represents an opportunity to acknowledge the contributions of
the late Lennart Eberson to the field of physical-organic chem-
istry.2f The mechanistic usefulness of our ‘intelligent’ substrates
is also confirmed.
Scheme 8
Presumably, the interaction of I2 with 4 provokes the form-
ation of 4 ϩ even in this solvent of conventional activation. It is
ؒ
also likely that the conventional attack of ‘Iϩ’ at the para-
position of 4, where the presence of the methyl-substituent
prevents substitution, becomes reversible; this would enable
ϩ
ؒ
Biohalogenations
the concurrent oxidation of 4 to 4 by I2 to take place, with
ensuing N-dealkylation and functionalisation of the N-alkyl
group as the only observable productive routes.25
Halogenation reactions also occur in Nature: the haloperoxi-
dase enzymes are active in this respect.27,28 Another enzyme that
could seemingly induce iodination reactions is laccase. This is a
multi-copper oxidase, expressed by ligninolytic fungi, which
catalyses the oxidation of phenols,29 but has also been shown to
function as an iodide oxidase.30 In fact, in line with a reported
oxidation potential of around 0.7 V,29,31 laccase can oxidise IϪ
to I2 (EЊ 0.7 V vs. NHE),32 while the other halide ions, being
more difficult to oxidise,32 act as inhibitors.30 It appeared useful
to investigate the potential of the IϪ/laccase system as a novel
method of iodination; however, in view of the weak electro-
philicity of molecular iodine,1 this could be attempted only with
very electron-rich substrates. The possibility could then exist
that the enzyme directly oxidises the electron-rich substrate
before iodination, thereby providing a new entry into the mech-
anistic dichotomy delineated in Schemes 1 and 2. The sub-
strates investigated were: (i) p-methylphenol 11 (viz. p-cresol),
as a model of tyrosine, in view of the role of iodine in the
thyroid33 where this amino acid plays an important metabolic
role; (ii) 1,2,4,5-tetramethoxybenzene 12, in view of its low
oxidation potential (0.81 V vs. SCE),34 which is comparable to
N,N-Dimethyl-4-methoxyaniline 7
The redox potential of this substrate is even lower (E p 0.45 V vs.
SCE at 0.5 V sϪ1 in CH3CN) than that of 4, and the para-
position is once again occupied.
Treatment of 7 with ICl in HFP (Scheme 9) gave only nuclear
Scheme 9
chlorination (to 8) after 0.5 h (Table 1, entry 12); over a longer
reaction time (4 h), the N-monomethyl-chloro-derivative 9
began to appear, along with the N,N-dimethyl-dichloro-
J. Chem. Soc., Perkin Trans. 2, 2001, 1516–1521
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Table 2 Biohalogenation experiments using the IϪ/laccase systema
Materials
1,1,1,3,3,3-Hexafluoropropan-2-ol (HFP) was from Aldrich,
while the other solvents were from Carlo Erba. ICl was
from Aldrich. Most substrates were either commercial, or
available from earlier work.7,8,22 N,N-Dimethyl-4-methoxy-
aniline (7) had been obtained from methylation of commer-
cial N-methyl-4-methoxyaniline (Aldrich).24c Potassium 12-
tungstocobaltate(III) (CoIIIW) was available from previous
investigations.22 Laccase from a strain of Tremetes villosa was a
gift from Novo Nordisk Biotech. The enzyme was purified by
Halogenating
species
(60 mmol)
Entry
number
Substrate
(60 µmol)
Product/µmol
1
11
IϪ
IϪ
IϪ
IϪ
IϪ
IϪ
IϪ
phenolic dimers (<1%)
—
quinones (<1%)
—
m-MeO-I (2.4)
m-MeO-I (7.2)
m-MeO-I (6.4)
2
12
3b
4
12
13
5
m-MeO
m-MeO
m-MeO
6b
7bc
chromatography on Q-Sepharose.35 The activity (104 units mlϪ1
)
a At room temperature, for a 72 h reaction time. b In the presence of
was then determined spectrophotometrically by the standard
reaction with ABTS.37
ABTS (20 µmol). c In the presence of Cu(OAc)2 (60 µmol).
Products
that of laccase; (iii) 1,4-dimethoxy-2-methylbenzene 13 (EЊ 1.4
vs. SCE),10 for its methyl group that could enable the detection
of side-chain radical iodinations; (iv) m-dimethoxybenzene
(m-MeO), for its high reactivity in the iodination reactions seen
above.
2-Iodomesitylene, 3-iododurene, the iodo-derivatives of p- and
m-dimethoxybenzene and the N-(trideuteriomethoxymethyl)-
derivative (i.e., 6) of N,N-dimethyl-p-toluidine (4) were available
form previous investigations,7,8,26 while 1-iodonaphthalene and
N-methyl-p-toluidine (5) are commercial (Aldrich) products.
Mono- and di-methylation of commercial 3-chloro-p-anisidine
(Aldrich) with MeI provided analytical samples of 9 (m/z 171
and 173) and 8 (m/z 185 and 187), respectively. Compound 10
was characterised only by its MS spectrum (m/z 219 and 221). A
sample of 4-iodo-N,N-dimethylaniline (2) was obtained from
reaction of 1 with I2 in CHCl3,38 and crystallised from ethanol–
water, m.p. 75–77 ЊC (lit.39 m.p. 79 ЊC); 1H NMR δ 7.45 and 6.48
(dd, 4H, aromatic para protons), 2.92 (s, 6H, Me2N), m/z 247.
The iodination of these compounds, with laccase and IϪ,
was carried out in a buffered acidic medium (pH 3.5) as
required for optimal laccase activity.29,30 While the change
of the colour of the solution, subsequent to the addition
of the enzyme, did support conversion of (some) iodide ion
into molecular iodine, the results reported in Table 2 show
that conversion into iodinated products was negligible. In
particular, substrate 11 (entry 1) underwent the typical
oxidative coupling of phenols induced by laccase.29 It has been
suggested that the low reactivity of the IϪ/laccase system can be
overcome by the use of suitable mediators of the enzyme.29,30,35
For example, ABTS (2,2Ј-azinobis(3-ethylbenzthiazoline-6-
sulfonate)) has shown promise as an efficient mediator of the
oxidase activity of laccase,36 and we accordingly attempted to
foster the production of molecular iodine by reacting 12 with IϪ
and laccase in the presence of ABTS (Scheme 11; Table 2,
compare entries 2 and 3).
Determination of oxidation potentials
Cyclic voltammetry determinations were carried out with an
Amel 5000 potentiostat in a 0.1 M Bu4NBF4 acetonitrile
solution with a platinum working electrode (planar disk, ø 1
mm) and a saturated calomel electrode (SCE); ferrocene was
employed as the reference compound. Irreversible oxidation
potentials (E p) were determined for compounds 1, 4, and 7, and
the corresponding values (at 500 mV sϪ1) are reported in the
text.
Iodination reactions
The iodination of compound 7 is described in detail; the other
reactions were performed analogously. A solution of 7 (25 mg;
0.16 mmol) and ICl (30 mg; 0.18 mmol) in 2 ml HFP was stirred
at room temperature for 0.5 or 4 h. Addition of the internal
standard and conventional work-up with diethyl ether preceded
GC analysis for quantitative determination of the iodination
products 8, 9 and 10.
Scheme 11
The effect was only a small increase in the modest production
of quinonic derivatives from 12. A slightly more encouraging
result was obtained in the case of m-MeO, where the addition
of ABTS did promote more nuclear iodination than in the reac-
tion without mediator (compare entries 5 and 6). The enzyme,
though, did not appear to provide any suitable polarisation of
the molecule of iodine7 so as to push the ensuing electrophilic
attack forward. The additional use of a Lewis acid (possibly
inert toward the enzyme) such as Cu(OAc)2 was therefore
attempted (entry 7), but no significant difference was obtained.
We conclude that the efficiency of the IϪ/laccase system is not
sufficient to deserve further attention as a biohalogenation
procedure.
Reactivity studies in competition experiments
The yield of products from the competitive experiments was
determined by GC with respect to an appropriate internal
standard, suitable response factors being employed to convert
the GC areas into mmols of products. Experiments were run in
duplicate, and the conditions described in previous publications
were in general followed.7,8b The competition experiment of
MES and Naph is reported in detail as an example. A solution
of naphthalene (575 mg; 4.5 mmol) and mesitylene (156 mg; 1.3
mmol) was prepared in 4 ml of HFP; ICl (0.41 mmol) was then
added, and the solution stirred at room temperature for 3 h.
Addition of the internal standard and conventional work-up
with CH2Cl2 preceded GC analysis for quantitative determin-
ation of the iodination products. The intermolecular reactivity
was calculated from the molar amounts of the two nuclear iod-
inated products by the use of the standard equation for com-
petitive reactions40 and by taking into account the statistical
factors of the two competing substrates (3 to 4, respectively, for
MES and Naph). Minute amounts of 1-chloronaphthalene,
Experimental
1H NMR spectra were taken at 200 MHz on a Bruker AC 200
instrument, and at 300 MHz on a Varian Gemini A 300A
instrument. GLC analyses were performed on a VARIAN 3400
instrument, fitted with a 30 m × 0.25 mm methyl silicone gum
capillary column. GC-MS analyses were carried out on a HP
5892 GC, equipped with a 12 m × 0.2 mm methyl silicone gum
capillary column, and coupled with a HP 5972 MSD instru-
ment operating at 70 eV.
1520
J. Chem. Soc., Perkin Trans. 2, 2001, 1516–1521

View Article Online
16 (a) E. Baciocchi and G. Illuminati, J. Am. Chem. Soc., 1967, 89,
even smaller than those of the accompanying 1-iodo-
naphthalene, were detected.
4017; (b) E. Baciocchi and G. Illuminati, Prog. Phys. Org. Chem.,
1967, 5, 1; (c) P. B. D. de la Mare, Acc. Chem. Res., 1974, 7, 361.
17 The lack of side-chain halogenation, in combination with the
consistency of the kMES/kDUR values from our competitive
experiment and from the absolute rate measurements,14 argue
against the criticism18 that the mechanistic conclusions deriving
from use of the MES/DUR probe could be affected by a transfer of
the electrophile from a rapidly formed ipso-substituted durenium ion
to mesitylene.9b
Biohalogenation reactions
In a typical reaction with laccase,30 3 ml of a 0.1 M phosphate
buffer solution at pH 3.4 was placed in a vial; 60 µmol of sub-
strate, 60 µmol of NaI and, if present, 20 µmol of ABTS
as a laccase mediator, were added. Finally, 1.5 µl of a solution
of purified laccase (with an activity of 104 units mlϪ1) was
added. After the chosen reaction time at room temperature,
the reaction was worked up and analysed as in the previous
case.
18 T. M. Bockman and J. K. Kochi, J. Phys. Org. Chem., 1994, 7, 325.
19 P. B. D. de la Mare and C. A. Vernon, J. Chem. Soc., 1951, 1764.
20 L. Eberson and F. Radner, J. Am. Chem. Soc., 1991, 113, 5825.
21 H. H. Perkampus, Adv. Phys. Org. Chem., 1966, 4, 195.
22 (a) M. Bietti, A. Cuppoletti, C. Dagostin, C. Florea, C. Galli,
P. Gentili, H. Petride and C. Russo Caia, Eur. J. Org. Chem., 1998,
2425; (b) A. Cuppoletti, C. Dagostin, C. Florea, C. Galli, P. Gentili,
O. Lanzalunga, A. Petride and H. Petride, Chem. Eur. J., 1999, 5,
2993.
23 (a) J. R. Lindsay Smith, R. O. C. Norman and W. W. Walker,
J. Chem. Soc. (B), 1968, 269; (b) G. Galliani and B. Rindone,
Gazz. Chim. Ital., 1983, 113, 207.
24 (a) B. Meunier, Chem. Rev., 1992, 92, 1411; (b) F. P. Guengerich and
T. L. Macdonald, Acc. Chem. Res., 1984, 17, 9; (c) E. Baciocchi,
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25 K. Acosta, J. W. Cessac, P. Narasimha Rao and H. K. Kim, J. Chem.
Soc., Chem. Commun., 1994, 1985.
Acknowledgements
The authors are indebted to Novo Nordisk Biotech, Inc., for a
generous gift of laccase, and to Dr F. Xu (Novo Nordisk
Biotech, 1445 Drew Avenue, Davis, CA 95616, USA) for
suggestions and advice.
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