Scandium trifluoromethanesulfonate as catalyst in the hydrogen/deuterium exchange of 1,3,5-trimethoxybenzene: Evidence for a direct interaction of scandium with the aromatic ring

Article abstract of DOI:10.1016/S0040-4020(00)00723-7

Scandium trifluoromethanesulfonate in CD₃OD is a very active catalyst for hydrogen/deuterium exchange at the aromatic nucleus of 1,3,5-trimethoxybenzene. The very low proticity of the system, spectrophotometric evidence, and easy iodination of

Full text of DOI:10.1016/S0040-4020(00)00723-7

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 8161±8166
Scandium Tri¯uoromethanesulfonate as Catalyst in the
Hydrogen/Deuterium Exchange of 1,3,5-Trimethoxybenzene:
Evidence for a Direct Interaction of Scandium with the
Aromatic Ring
a
a
b
Carla Bisi Castellani,a, Angelo Perotti, Marta Scrivanti and Giovanni Vidari
*
a
Á
Dipartimento di Chimica Generale, Universita di Pavia, Via Taramelli 12, 27100 Pavia, Italy
Á
Dipartimento di Chimica Organica, Universita di Pavia, Via Taramelli 10, 27100 Pavia, Italy
b
Received 12 June 2000; revised 25 July 2000; accepted 10 August 2000
AbstractÐScandium tri¯uoromethanesulfonate in CD3OD is a very active catalyst for hydrogen/deuterium exchange at the aromatic
nucleus of 1,3,5-trimethoxybenzene. The very low proticity of the system, spectrophotometric evidence, and easy iodination of the substrate
point to the formation of a transient unprecedented arylscandium species as a plausible intermediate of the reaction. q 2000 Elsevier Science
Ltd. All rights reserved.
Introduction
Hydrogen exchange is one of the most useful reactions for
the study of the mechanism of electrophilic aromatic substi-
tutions; moreover, 1,3,5-trimethoxybenzene is a particularly
suitable substrate due to the activating effect of the methoxy
groups. Therefore, it seemed worthy to investigate this reac-
tion in detail. Our results point out to the intermediate
formation of an unprecedented arylscandium species
which may be involved in a number of ScIII-catalyzed
reactions of arenes.
The use of scandium tri¯uoromethanesulfonate (tri¯ate) in
homogeneous organic catalysis has expanded in the last
decade.1,2Although it is generally accepted that Sc31 exerts
catalytic activity thanks to the strong oxophilicity and Lewis
acid character, most papers simply deal with the preparative
aspects of the described reactions and rarely investigate the
mechanism in detail. However, a better knowledge of the
role exerted by scandium tri¯ate should be very helpful for
extending the use of this active catalyst in organic synthesis.
Results and Discussion
Among several electrophilic aromatic substitution rections,
only Friedel±Crafts acylation and deformylation of
aromatic aldehydes have been reported to be catalyzed by
Sc(OTf)3 so far. During our previous studies on the
anhydrous Sc(OTf)3 catalyzed protiodeformylation of
aromatic aldehydes in MeOD,3 we observed that the fast
deformylation of 1,3,5-trimethoxybenzaldehyde under
very mild conditions was accompanied by a rapid and
complete hydrogen/deuterium exchange of both the
substrate and the deformylation product, i.e. 1,3,5-
trimethoxybenzene (1), at all the positions of the aromatic
nucleus. By contrast, the exchange did not exceed 15% in
the absence of the Lewis acid. Sc(OTf)3 assisted
deuteriation reactions have never been described before in
the literature.
1H NMR measurements indicated that the deuteriation of 1
in the presence of anhydrous Sc(OTf)3 (substrate 1.3 M in
CD3OD; molar ratio substrate±catalystˆ6:1; 208C)
followed
a
pseudo
®rst-order
path,
with
KScˆ1.5£1023 s21. Under the same experimental condi-
tions, anhydrous lanthanum and yttrium tri¯ates also
promoted the proton±deuterium exchange, albeit much
less ef®ciently, the rate constants being KLaˆ9£1027 s21
,
²
and KYˆ3£1025 s21, respectively.
The catalytic properties of Sc(OTf)3 were then extended to
the deuteriation of other aromatic compounds such as 1,3-
dimethoxybenzene and 1,3,5-trimethylbenzene under the
reaction conditions indicated above. The former compound,
which did not give rise to any H/D exchange in the absence
Keywords: scandium and compounds; exchange reactions; substitution;
metalation.
* Corresponding author. Tel: 139-0382-507343; fax: 139-0382-528544;
e-mail: bisichem@unipv.it
²
Since unavoidable traces of water affected the measurements to some
extent, the reported values are merely indicative of the relative catalytic
effect.
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00723-7

8162
C. Bisi Castellani et al. / Tetrahedron 56 (2000) 8161±8166
Scheme 1. Accepted mechanism for the acid catalyzed H/D exchange of aromatic compounds.
of Sc(OTf)3, gave the corresponding 2,4,6-trideuterated
derivative in 77% yield after 15 days, along with minor
amounts of 4,6-dideuterated-1,3-dimethoxybenzene. By
contrast, 1,3,5-trimethylbenzene was not deuterated after
the same period.
Sc(OTf)3 in CD3OD would require knowledge of the
composition and the structure of the ®rst coordination
sphere. It is known that alcohols are good ligands to REIII
ions and such coordinated hydroxy groups behave as
relatively strong acids,4,9 but no information concerns
speci®cally Sc(OTf)3. It is reported that RE(OTf)3 in
anhydrous organic solvents display a signi®cant innersphere
interaction between the REIII cation and the counter anion;
whether they behave as 1:2 or 1:1 electrolytes depends on
the solvent and on the presence of traces of H2O. For
instance, La(OTf)3 behaves as a 1:2 electrolyte in anhydrous
MeOH, while the species [La(OTf)2]1 prevails in MeCN.
Moreover, the amount of complexation with more than one
anion per cation seems to be higher for lighter lanthanides
and the presence of traces of water has a crucial effect on the
dissociation.10
Consistent with our previous results, these data showed an
increase of the catalytic activity of rare earth(III) ions upon
decreasing the cation size and increasing the electron
density on the aromatic ring.3,4
Acidic catalysts generally promote the aromatic hydrogen±
deuterium exchange quite ef®ciently and, notwithstanding
the wide variety of reagents employed, it is generally
accepted that the reactions take place via a general A-SE2
mechanism.5 In particular, this mechanism was ascertained
for the deuteriation of compound 1 when it is catalyzed by
protic acids.6 The intermediate trimethoxybenzenonium ion
was characterized by means of electronic and NMR spectro-
scopy.7
Therefore, being impossible to have an accurate account
of the composition of the species occurring in our solution,
we assumed the simultaneous presence of solvated
21
species such as [Sc(MeOD)n]31, [Sc(OTf)(MeOD)n21
] ,
When transition or non-transition metal ions are used as
Lewis acid catalysts, aromatic radical-ions may be formed;8
however, in our case, a mechanism involving an aryl radical
was immediately excluded since the system was ESR silent.
and [Sc(OTf)2(MeOD)n22]1, which all should exhibit a
signi®cant proticity. Indeed, in the 2Log[H1] range 4±9,
potentiometric titrations of Sc(OTf)3 with NaOMe in metha-
nol (see Experimental) could be interpreted in terms of the
presence of two deprotonation steps (Fig. 1), according to
the following equilibria [Eq. (1) and (2)]
With this background, the following mechanisms were
considered for explaining the accelerating activity of
Sc(OTf)3.
Sc‰ꢀMeOH† Š Y Sc‰ꢀMeOH†n21ꢀOMe†Š21 1 H1
31
ꢀ1†
n
(i) coordination to ScIII increased the proticity of MeOD
and the H/D exchange then followed the usual arenium
ion mechanism of protic acids (Scheme 1);
Sc‰ꢀMeOH†n21ꢀOMe†Š Y Sc‰ꢀMeOH†n22ꢀOMe† Š1 1 H1
21
2
ꢀ2†
(ii) ScIII directly activated the aromatic substrate, and the
exchange occurred through the intermediate metalation
of the arene ring by ScIII, according to the mechanism
outlined in Scheme 2.
having pK1ˆ5.35 and pK2ˆ6.12, respectively.
Considering a negligible isotopic effect, the above constants
allowed us to calculate the [D1] of our system to be
2.5£1024 M.
Therefore, assuming that [DOTf] matched [D1] in a very
diluted methanolic solution, we established a value of
KDOTfˆ2.68£1025 s21 for the deuteriation rate of com-
pound 1 in the presence of 2.5£1024 M DOTf. The same
The two mechanisms were investigated as follows.
Mechanism of Scheme 1
A thorough understanding of the catalytic behaviour of
Scheme 2. Proposed mechanism for the Sc(OTf)3 catalyzed H/D exchange of aromatic compounds.

C. Bisi Castellani et al. / Tetrahedron 56 (2000) 8161±8166
8163
Figure 1. Titration curve of Sc(OTf)3 with NaOMe in MeOH. B/L is the
ratio of the moles of standard base added to the moles of Sc(OTf)3. Nega-
tive values indicate excess of acid.
Figure 3. UV spectrum of 1,3,5-trimethoxybenzene (1), 1.0£1023 M in
anhydrous HOTf.
experiment was then repeated in the presence of
2.5£1024 M DCl (the value of pKaˆ1,23 for HCl in
MeOH11 indicated a complete dissociation at the employed
molarity) in order to exclude that methanolic DOTf behaved
as a weak acid. The value determined for KDCl was
3.5£1025 s21, which matched nicely KDOTf. These rate
constants, being more than two orders of magnitude lower
than KSc, clearly excluded that the increased proticity of
MeOD could entirely account for the accelerating effect of
Sc(OTf)3 on the deuteriation of compound 1. The mechan-
ism of Scheme 1 was therefore discarded.
exchange reaction which precluded the detection of species
3 in methanol (vide infra).
The electronic spectrum of 1 in neutral organic solvents
(Fig. 2) shows a very strong band below 230 nm
(e 8.7£103) and a weak absorption band at about 260 nm
(e 530), which is characteristic of hydroxy- and alkoxy-
benzenes; in aqueous HClO4, the 260 nm band is shifted
to 250 nm (e 2£104) and a new band appears at 347 nm
(e 1.23£104) which is assigned to the arenium species 2
³
(DˆH). Protonation is almost complete in 65% HClO4.7a
As expected, we observed identical spectral modi®cations in
anhydrous tri¯uoroacetic acid, anhydrous HOTf (Fig. 3),§
5 M aqueous HOTf, 9 M tri¯uoroacetic acid in MeCN and
1 M HOTf in MeCN.k
Mechanism of Scheme 2
As mentioned before, the protonated species 2, which is
considered an intermediate of the acid-catalyzed deuteria-
tion of aromatic compounds (Scheme 1), could be identi®ed
by means of electronic and NMR spectroscopy.7
When excess Sc(OTf)3 was added to a concentrated solution
of 1 in MeCN (substrate concentration.0.1 M; substrate±
Sc31, about 1:5 molar ratio) or, conversely, increasing
amounts of compound 1 were added to a concentrated
solution of Sc(OTf)3 in MeCN, the UV spectrum was
immediately perturbed in a similar way (Fig. 4), except
that the absorption band at 350 nm was much less intense
than that at 260 nm. This feature precluded the study of the
260 nm band since at concentrations suitable to the study of
this region, no band was detectable at 350 nm.
If an arenium±scandium species 3, analogous to 2, were
formed in the Sc(OTf)3-catalyzed deuteriation of compound
1, analogous spectral evidence would be expected. Actually,
these clearly emerged from the comparative spectrophoto-
metric study of the systems 1-protic acids and 1-Sc(OTf)3 in
anhydrous acetonitrile. This aprotic solvent was used
instead of MeOD in order to prevent the very rapid
Anyhow, in our opinion, the band at 350 nm could be
attributed to a direct interaction of ScIII with the aromatic
ring, leading to chromophore 3 which is quite similar to the
protonated species 1. Indeed, detection of the weak band at
350 nm required very high concentration of the reagents,
indicating that the molar ratio arylscandium species
3:substrate 1 was very low.
For this reason, all our attempts to determine the constant of
formation of intermediate 3 failed, as well as our efforts to
detect this species in CD3CN by means of NMR spectro-
scopy.
³
Slight shifts, due to experimental conditions, occur to this band. There-
fore, it will be reported as 350 nm band.
Anhydrous HOTf and 60% HClO4 have about the same molarity.
1 M HOTf does not interfere with MeCN.
Figure 2. UV spectrum of 1,3,5-trimethoxybenzene (1), 1.0£1023 M in
§
k
MeCN.

8164
C. Bisi Castellani et al. / Tetrahedron 56 (2000) 8161±8166
a higher ionic potential than TlIII, the species Sc(OTf)12
should be more electrophilic than Tl(OTf)12 .
Species 4 has a structure similar to known isolable arylmetal
derivatives in which the aryl-metal bond is readily cleaved
by protonation, according to a mechanism involving an
intermediate metalated arenium ion of the type 3. Moreover,
a proton transfer from medium to substrate was proved to be
involved in the rate determining step of these reactions.13
In conclusion, comparing the mechanism shown in
Scheme 2 with the acid catalyzed deuteriation of 1,3,5-
trimethoxybenzene (1) (Scheme 1), one can explain the
accelerating effect of Sc(OTf)3 by assuming that equi-
librium Scheme 2 largely favours compound 1 and that
the readily formed arylscandium species 4 is more reactive
than substrate 1 towards D1 attack. This difference in
reactivity is likely due to an increased stabilization of inter-
mediate 3 compared to 2 as it has been reported for other
metalated arenium ions.13 In addition, coordination of
CD3OD to scandium, besides increasing the proticity of
the hydroxy group, might facilitate the exchange reaction
through a template effect.
Figure 4. UV spectra of the system Sc(OTf)3 ± 1,3,5-trimethoxybenzene
(1) in anhydrous MeCN. Only the signi®cant region (300±450 nm) is
shown. Spectrum a, 0.354 M Sc(OTf)3; spectrum b, molar ratio
Sc(OTf)3:1,3,5-trimethoxybenzene (1)ˆ1:10; intermediate spectra, molar
ratios 1:1; 1:2; 1:5.
In order to exclude that the weak 350 nm band could be due
to interference of traces of HOTf produced by minor
hydrolysis of Sc(OTf)3, the spectrophotometric investi-
gation was repeated in the presence of 2,6-di-tert-butyl-
pyridine (base±ScIIIˆ1:1 molar ratio). As expected, the
spectra were superimposable with those recorded in the
absence of the base. Moreover, if the 350 nm band was
caused by moisture, addition of H2O should increase the
intensity. By contrast, successive additions of H2O to the
system gradually decreased the intensity of the band at
350 nm which eventually disappeared.
Since the H/D exchange occurs rapidly, concentration of 3
in MeOD must remain extremely low, thus precluding the
detection by spectrophotometric investigation. Much for the
same reasons, it was impossible to obtain direct evidence of
the intermediate formation of species 4. Therefore, in
order to give support to our hypothesis, we compared the
reactivity of 1 towards iodine in the absence and in the
presence of Sc(OTf)3; in fact, it is known that arenes
undergo easy iodination with I2 in the presence of ef®cient
metalating salts such as tri¯uoroacetates of TlIII, HgII, and
AgI.14 In particular, the mechanism of benzene iodination in
the presence of thallium(III)tri¯uoroacetate has been
investigated in detail.15 The reaction proceeds through the
initial thalliation of the arene ring, followed by the electro-
philic substitution of thallium by iodine; the so formed I2 is
oxidized to I2 by TlIII which is recovered as TlI. Thus, one
molar equivalent of iodine produces two equivalents of
monoiodobenzene. Instead, one molar equivalent of iodine
produces one molar equivalent of monoiodobenzene, if AgI
or HgII tri¯uoroacetates are used, since they are not capable
to oxidize the I2 species.
Altogether, in our opinion, the above results strongly
support a mechanism of the type outlined in Scheme 2.
According to this, an electrophilic scandium species, of
the type [Sc(OTf)2(solv)]1 (solvˆCD3OD), would form
the metalated arenium ion 3 which can then restore the
aromatic sextet either reverting to the starting compound 1
or leading to the arylscandium species 4.
The solvated Sc(OTf)12 cation is thus supposed to be the
active species in the same way as Tl(OTf)12 and
Tl(O2CCF3)12 have been recognized to be the active electron
acceptors in the thalliation of arenes promoted by Tl(OTf)3
and Tl(O2CCF3)3 respectively.12 The analogy of the
catalytic behaviour of these ions can be ascribed to the
analogous closed shells con®gurations of ScIII and TlIII. It
is worthy to underline that Tl(OTf)3 is a more effective
thalliating agent than Tl(O2CCF3)3 due to a greater ioniza-
tion of the former salt and/or to a greater electrophilicity of
Tl(OTf)12 compared to Tl(O2CCF3)21; moreover, having ScIII
We examined the iodination of 1 in the presence of
Sc(OTf)3, both by H NMR spectroscopy and by conven-
1
1
tional methods (see experimental section). The H NMR
spectra showed that the reaction at room temperature
reached the equilibrium after about 15 min, giving the
monoiodinated product in 15% yield, while in the absence
Scheme 3. Proposed mechanism for the Sc(OTf)3 catalyzed iodination of 1,3,5-trimethoxybenzene (1).

C. Bisi Castellani et al. / Tetrahedron 56 (2000) 8161±8166
8165
of the catalyst, the same product was obtained in 5% yield.
These results were qualitatively con®rmed monitoring the
reaction by TLC.
ately disappeared after the initial additions, while yellow
PbI2 readily precipitated making it dif®cult to appreciate
the end-point. This roughly occurred after the addition of
0.25 molar equivalents of I2. A parallel experiment was
carried out in the absence of Sc(OTf)3 (Solution B). Under
these conditions, iodine was consumed very slowly since the
very beginning of the experiment, the reacting solution
assuming a dark orange color after few additions. Both reac-
tion mixtures were then warmed to 608C for 20 min. While
solution A remained unaltered, solution B turned to dark
brown and then to blue-green. 2-Iodo-1,3,5-trimethoxyben-
zene was isolated from solution A, following a procedure
similar to that reported in the literature.16,19 The reaction
mixture was cooled, ®ltered, and added with an aqueous
solutions of NaHCO3 and Na2S2O3. MeCN was removed
by evaporation under reduced pressure and the aqueous
phase was extracted with CHCl3. The organic solution
was dried over MgSO4 and evaporated to dryness under
reduced pressure. The crude product was crystallized from
CH2Cl2 to give crystals of 2-iodo-1,3,5-trimethoxybenzene,
mp 122±1238C, identical with an authentic sample.
The iodination of 1 was then carried out in MeCN on a
preparative scale in the presence of Pb(NO3)2 as scavenger
1
of I2. A preliminary test done by H NMR spectroscopy
ascertained the inactivity of Pb(NO3)2 towards iodination.
The reaction appeared to be clean (TLC) and crystals of the
monoiodinated product 5 were eventually isolated. These
experiments, which parallel the reported iodination of
arenes in the presence of AgI, HgII or TlIII species, support
the hypothesis that a labile scandiated derivative of the type
4 is also involved in the iodination of 1 (Scheme 3).
The instability of the scandiated species 4 can be easily
understood considering that ScIII, contrary to TlIII, HgII,
and AgI lacks the d-electrons necessary for the d±p back
donation.
Conclusions
This paper describes
a new kind of electrophilic
Acknowledgements
aromatic substitution reaction catalyzed by Sc(OTf)3.
The proposed mechanism involves an arylscandium
species as a likely intermediate. The activation of chemi-
cally inert C±H bonds through a transient metalation might
be involved in other organic reactions catalyzed by
scandium tri¯ate.
Financial support by Italian C.N.R. is gratefully
acknowledged.
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