Formylation of activated arenes by phenyl formate: Implications for the mechanism of the Fries rearrangement of aryl formates

Article abstract of DOI:10.1002/poc.1356

We present an NMR and DFT investigation of the reaction of phenyl formate with 3-methoxyphenol and 3,5-dimethoxyphenol with excess BCl₃. The products obtained (3-methoxy- and 3,5-dimethoxy-salicylaldehyde, respectively) are the same as those resulting from the Fries rearrangement of 3-methoxy- and 3,5-dimethoxy-phenyl formate. These results represent a novel regioselective synthetic route to aromatic aldehydes, using phenyl formate as a source of formylating agent. They also unambiguously prove that the Fries rearrangement of aryl formates (that we recently investigated in J. Org. Chem. 71, 9331-9340, 2006) is intermolecular: the intermediate formyl chloride is released in situ and, in turn, it formylates the intermediate dichloroborate ester of 3-methoxy- and 3,5-dimethoxy-phenol in a second independent step. The -BCl₂ moiety bound to the aryl oxygen of the substituted phenol interacts with the formyl chloride strongly favouring the ortho substitution. Copyright

Full text of DOI:10.1002/poc.1356

Special Issue
Received: 4 October 2007,
Revised: 7 February 2008,
Accepted: 8 February 2008,
Published online in Wiley InterScience: 23 May 2008
(
www.interscience.wiley.com) DOI 10.1002/poc.1356
Formylation of activated arenes by phenyl
formate: implications for the mechanism of
the Fries rearrangement of aryl formates
a
b
c
Alessandro Bagno , Willi Kantlehner and Giacomo Saielli *
We present an NMR and DFT investigation of the reaction of phenyl formate with 3-methoxyphenol and
3
3,5-dimethoxyphenol with excess BCl . The products obtained (3-methoxy- and 3,5-dimethoxy-salicylaldehyde,
respectively) are the same as those resulting from the Fries rearrangement of 3-methoxy- and 3,5-dimethoxy-phenyl
formate. These results represent a novel regioselective synthetic route to aromatic aldehydes, using phenyl formate
as a source of formylating agent. They also unambiguously prove that the Fries rearrangement of aryl formates
(
that we recently investigated in J. Org. Chem. 71, 9331–9340, 2006) is intermolecular: the intermediate formyl
chloride is released in situ and, in turn, it formylates the intermediate dichloroborate ester of 3-methoxy- and
,5-dimethoxy-phenol in a second independent step. The —BCl moiety bound to the aryl oxygen of the substituted
3
2
phenol interacts with the formyl chloride strongly favouring the ortho substitution. Copyright ß 2008 John Wiley &
Sons, Ltd.
1
1
Keywords: Fries rearrangement; B NMR; reaction mechanisms; DFT calculations; formylation
Therefore the reaction is intermolecular. In the second step, a
INTRODUCTION
Friedel-Crafts acylation takes place; nevertheless, only the ortho
product is obtained since the Ar—OBCl2 moiety of the
intermediate interacts with the formylating agent (formyl
chloride) driving the reaction through the ortho path. MP2
calculations (on DFT-optimized geometries) agreed with this
view: in fact, at the level of theory used in our previous work, the
para Wheland intermediate was not found on the potential
The Fries rearrangement transforms an aryl ester into a
[
1,2]
hydroxycarbonyl compound, typically a ketone.
Excess of a
Lewis acid is needed; whether the para or ortho isomer is
obtained it depends strongly on the experimental conditions like
temperature, substrate and, most important, the Lewis acid. In
[
3,4]
the last decade new catalysts, such as heteropoly acids
and
[
5]
[19]
metal triflates, as well as new reaction methodologies, such as
microwave irradiation and ionic liquids as solvents, have been
successfully applied to the Fries rearrangement.
energy surface.
[
6]
[7]
Despite the clear-cut evidence collected in our previous work,
regarding the formation of formyl chloride as formylating agent,
the intermolecular nature of the reaction remained open to
doubt, for example, concerning the extent of separation of the
two intermediates in the solvent cage.
The proposed mechanism amounts to a formylation of
activated arenes (the intermediate obtained from the substrate
in the first step) using an aryl formate (the substrate itself) as a
source of formyl chloride. It is then conceivable a ‘‘cross’’ Fries
rearrangement where the aryl formate providing the formylating
agent is different from the substituted phenol to be formylated.
Despite the importance of the reaction in organic synthesis,
the details of the mechanism are still unclear. Intramolecular
mechanisms have been proposed, particularly in those cases
[
8]
where the ortho isomer is the main or the only product, as well
as intermolecular ones, particularly where the para isomer is the
[
9–13]
dominant product.
Recently, the Fries rearrangement has been extended to aryl
formates, thus providing a convenient route to the synthesis of
[
14]
hydroxybenzaldehydes, as shown in Scheme 1.
BCl and BBr , under appropriate experimental conditions,
3
3
[
14]
were found to be good catalysts for this reaction. The choice of
the Lewis acid appears particularly important since earlier
attempts to perform the Fries rearragement of aryl formates
* Correspondence to: G. Saielli, Istituto per la Tecnologia delle Membrane del
CNR, Sezione di Padova, Via Marzolo, 1-35131 Padova, Italy.
E-mail: giacomo.saielli@unipd.it
[
15–17]
using BF
We have been involved in a detailed mechanistic study of the
Fries rearrangement of aryl formates promoted by BCl , using
NMR spectroscopy ( H, H, C, and B) and DFT calcu-
3 3
, AlCl , HF, or polyphosphoric acid failed.
a A. Bagno
Dipartimento di Scienze Chimiche, Universit a` degli Studi di Padova, Via
Marzolo, 1-35131 Padova, Italy
3
1
2
13
11
[
18,19]
lations.
The general picture that emerged was the following:
b W. Kantlehner
the reaction, after the rapid complexation of the substrate at the
carbonyl oxygen (shown in Scheme 2), proceeds with two main
steps: in the first step, the aryl oxygen–carbon bond is cleaved
and formyl chloride is released in situ, together with the
formation of a dichloroborate ester of the corresponding phenol.
Institut f u¨ r Organische Chemie der Universit a¨ t Stuttgart, Pfaffenwaldring 55,
D-70569 Stuttgart, Germany
c G. Saielli
Istituto per la Tecnologia delle Membrane del CNR, Sezione di Padova, Via
Marzolo, 1-35131 Padova, Italy
J. Phys. Org. Chem. 2008, 21 682–687
Copyright ß 2008 John Wiley & Sons, Ltd.

FORMYLATION OF ARENES BY PHENYL FORMATE
soon as BCl is added and eventually it becomes dark red as the
3
substrate has completely reacted.
NMR spectroscopy
Scheme 1. Fries rearrangement of aryl formates
1
H measurements have been run at 400 MHz with a 5-mm
multinuclear BBI probehead and at 300 MHz with a 5-mm
1
1
multinuclear BBO probehead. B NMR measurements have been
run with a 5-mm multinuclear BBO probehead at 96 MHz.
The outcome of such an experiment would be twofold: first, it
would provide further evidence of the proposed reaction
mechanism of the Fries rearrangement of aryl formates, definitely
supporting the intermolecular, regioselective, path; second, it
would represent a novel synthetic route to aromatic aldehydes
using a proper aryl formate as a source of the formyl group.
DFT calculations
Dealing with boron compounds requires an appropriate
20]
description of dative bonds. The MPW1K functional
[
has
proved to be effective, in conjunction with 6-311þG** basis set,
for boron compounds. This level of theory has been used in this
work for geometry optimization, frequency analysis, and for the
calculation of the Gibbs free energy corrections and ZPE
EXPERIMENTAL
Preparation of samples
corrections at 298 K. The electronic energy has been calculated
at the MP2/6-311þG** level of theory, using the structures
The reaction mixtures were prepared by mixing the substituted
phenol and phenyl formate directly in the NMR tube with a
concentration of ca. 0.06 M each (ca. 0.6 ml), and then adding ca.
optimized at the DFT level, and the final Gibbs free energy has
been calculated adding the thermal corrections calculated at the
DFT level, following the protocol of Reference [19]. Calculations
0.1 ml of a 1 M solution of BCl
3
in heptane, that is, with a nominal
[21]
were run using the Gaussian 03 software package.
0
.15 M concentration of BCl . This 2.5-fold excess is necessary in
3
order to allow for the consumption of one equivalent of BCl by
3
reaction with the phenol, thus leaving another equivalent to form
the complex with phenyl formate and still leaving some excess
RESULTS AND DISCUSSION
free BCl
3
.
In order to get more insight on the reaction mechanism of the
Fries rearrangement of 3-methoxyphenyl formate we have
investigated the behaviour of a mixture of phenyl formate and
The reaction mixture was kept in ice during the first few
minutes before inserting the tube into the probehead, except for
the time needed to mix the reactants using a vortex and to insert
the tube into the spinner and wiping it. Usually the tube was
allowed to equilibrate for a few minutes before starting the
acquisition. The solution turns to a pale orange/yellow colour as
3
3-methoxyphenol with excess BCl , shown in Scheme 3.
Phenyl formate, with excess BCl , does not rearrange to the
3
Fries product: neither the para- nor the ortho-hydroxy-aldehyde
[
14]
are observed.
Nonetheless, according to our proposed
Scheme 2. The main steps of the Fries rearrangement of aryl formates as proposed in Reference [19]. The occurrence of the 2nd step depends on the
substituent R
Scheme 3. Reaction of phenyl formate and 3-methoxyphenol with excess BCl
3
J. Phys. Org. Chem. 2008, 21 682–687
Copyright ß 2008 John Wiley & Sons, Ltd.
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A. BAGNO, W. KANTLEHNER AND G. SAIELLI
1
3 3
Figure 1. Final H NMR spectrum (300 MHz, 283 K, CDCl ) of the 1:1 mixture (0.06M) of phenyl formate and 3-methoxyphenol with excess (0.15 M) of BCl
mechanism, the first step of Scheme 2 takes place after the
pre-equilibrium, producing the intermediate phenol dichlorobo-
occurrence. In fact, during the course of the reaction, only three
aldehyde signals (phenyl formate at 8.98 ppm, formyl chloride at
9.70 ppm and final product at 8.70 ppm) and two methoxy signals
(3-methoxyphenol at 3.83 ppm and final product at 4.02 ppm) are
observed. More important, the aromatic region between 6.80 and
7.00 ppm, where most aromatic signals of the complex of
[
19]
rate ester with the release of formyl chloride.
The latter, in
absence of a sufficiently reactive substrate to formylate,
slowly decomposes into CO and HCl. However, in the presence
of 3-methoxyphenyl dichloroborate, which is the intermediate
in the 1st step of Scheme 2 for the rearrangement of
[
19]
3-methoxyphenyl formate with BCl3 lie,
resonances. It is likely that the quantitative reaction of
3-methoxyphenol with BCl (note that the phenol OH resonance
shows no new
3-methoxyphenyl formate, we should obtain 3-methoxysali-
cylaldehyde exactly as from the Fries rearrangement of
3
3-methoxyphenyl formate itself.
at 4.83 ppm disappears after the addition of BCl ) prevents any
3
The initial spectrum of the equimolar mixture of phenyl
nucleophilic attack of 3-methoxyphenol to phenyl formate.
In the proposed mechanism, an essential role is played by the
formate and 3-methoxyphenol with excess BCl
3
is just the
superposition of the H NMR spectrum of the complex of phenyl
formate with BCl at the carbonyl oxygen (e.g., we observe the
formyl resonance at 9.23 ppm ) and the dichloroborate ester of
-methoxyphenol.
In Fig. 1, we show the final H NMR spectrum of the same
reaction mixture. The formation of the Fries product
3-methoxysalicylaldehyde) is clearly demonstrated by the
aldehyde resonance at 8.73 ppm with its typical broad, doublet-
1
Ar—OBCl intermediate, where the tri-coordinated boron can still
2
3
act as Lewis acid, in coordinating formyl chloride and driving the
[
19]
[15]
reaction through the ortho pathway.
The transition state for
[
19]
3
the formylation that we identified in Reference , regarding
3-methoxyphenyl formate, is shown in Fig. 2.
1
(
1
0
11
like signal, due to long-range coupling with B and B, as
obtained from the direct Fries rearrangement of 3-metho-
[
19]
xyphenyl formate.
1
1
Also, the time evolution of the B NMR spectrum of the above
reaction mixture closely resembles the one observed for
[
19]
11
3
-methoxyphenyl formate with excess BCl3.
The B signal
of the final product is observed at 8.3 ppm.
Therefore, the mixture of phenyl formate and
3
-methoxyphenol with an excess of BCl yields the same product
3
as the Fries rearrangement of 3-methoxyphenyl formate under
the same experimental conditions.
One might reasonably wonder whether or not a transester-
ification occurs, in such acidic conditions, first producing
3-methoxyphenyl formate which then would undergo a Fries
rearrangement as observed in Reference [19]. In such a case, the
fact that the mixture of phenyl formate and 3-methoxyphenol
yields the same product as 3-methoxyphenyl formate would not
add any new information. However, we can safely discard this
Figure 2. Structure of the transition state for the formylation of
3
-methoxyphenyl dichloroborate (MPW1K/6-311þG ). The B–O distance
**
(oxygen of formyl chloride) is 1.553 A˚ , while the C–C distance of the C–C
˚
bond being formed is 2.192 A. C blue; H grey; O red; Cl green; B purple
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J. Phys. Org. Chem. 2008, 21 682–687

FORMYLATION OF ARENES BY PHENYL FORMATE
1
Figure 3. Final H NMR spectra of the reaction mixtures (0.06 M in substrates) obtained from treatment with excess BCl
3
(0.08 M in the first two cases and
3 3
0.15 M in the last case) of: (top) phenyl formate, 400 MHz, 298 K, CDCl ; (middle) 3,5-dimethoxyphenyl formate, 300 MHz, 301 K, CDCl ; 1:1 mixture of
phenyl formate and 3,5-dimethoxyphenol, 300 MHz, 301 K, CDCl
3
1
1
We note that the B—O bond is formed ahead of the C—C
bond. Therefore, the interaction of the oxygen of HCOCl with the
Lewis acid moiety is essential to lower the reaction barrier for the
formylation. Accordingly, we have investigated the reaction of a
B data can be fitted by first-order kinetics in the product
(Fig. 4). However, only the reaction of 3-methoxyphenol
with phenyl formate is sufficiently slow to allow for a reliable
collection of data points, leading to an effective rate constant
ꢁ
5
ꢁ1
1
:1 mixture of phenyl formate and anisole with excess BCl : in
k ¼ 9.4 ꢀ 10
s . The reaction of 3,5-dimethoxyphenol is
3
agreement with the proposed mechanism, no anisaldehyde was
significantly faster even at a lower temperature: by the time
the NMR tube has been inserted into the probe and equilibrated
about 2/3 of the product have already been formed, so that the
derived rate constant is only crudely estimated. These circum-
stances somewhat limit the mechanistic proposals that can be
drawn. At this stage, we can only make the following
observations. (a) A low concentration of HCOCl is maintained
1
observed in the H NMR spectrum (as inferred from the lack of
1
1
resonances above 8 ppm), while the final B NMR spectrum
showed only the signal of free BCl
dichloroborate (31.8 ppm).
3
(46.5 ppm) and phenyl
These results represent an unambiguous proof that the Fries
rearrangement of aryl formates proceeds via an intermolecular,
albeit highly regioselective, pathway. Phenyl formate, in the
presence of an excess of boron trichloride, behaves as a source of
formyl chloride which, in turn, formylates 3-methoxyphenyl
dichloroborate present in the mixture. This observation may be
exploited to devise a synthetic route to aromatic aldehydes using
phenyl formate as a source of formylating agent.
However, the substrate to be formylated needs to be activated
for electrophilic substitution: phenyl dichloroborate does not
react, in contrast to the analogous 3-methoxyphenyl dichlor-
oborate. Similarly, toluene was not formylated to tolualdehyde
[
19]
under the same experimental conditions. It is then interesting
to study the reaction of a substrate more reactive towards
formylation than 3-methoxyphenyl formate, like 3,5-dimethoxy-
phenyl formate.
1
In Fig. 3 we show the final H NMR spectra of solutions of (top)
phenyl formate, (middle) 3,5-dimethoxyphenyl formate and
(
bottom) a 1:1 mixture of phenyl formate and 3,5-dimethox-
yphenol, all with an excess of BCl . The last spectrum is clearly the
superposition of the first two, indicating that, also in this case, the
reaction of a mixture of phenyl formate and 3,5-dimethoxy-
3
Figure 4. Time evolution of the normalized integrated intensity, I, of the
11
8.3 ppm signal of the salicylaldehyde in the B spectrum for the solution
of 1:1 mixture of phenyl formate (0.06 M) with: (open circles)
3
phenol with excess of BCl leads to 3,5-dimethoxysalicylaldehyde
3
2
-methoxyphenol at 283 K, (open squares) 3,5-dimethoxyphenol at
68 K, and excess of BCl (0.15 M). Dashed lines represent the best fit
exactly as obtained from the Fries rearrangement of 3,5-
dimethoxyphenyl formate under the same experimental
conditions.
3
to the function I(t) ¼ 1ꢁexp[ꢁk (tꢁt )] (however, this may not imply
0
first-order kinetics; see text)
J. Phys. Org. Chem. 2008, 21 682–687
Copyright ß 2008 John Wiley & Sons, Ltd.
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A. BAGNO, W. KANTLEHNER AND G. SAIELLI
Figure 5. Gibbs free energy profile (with thermal corrections at 298 K) for the Fries rearrangement of (~) phenyl formate; (*) 3-methoxyphenyl
formate; (&) 3,5-dimethoxyphenyl formate (MP2/6-311þG**//MPW1K/6-311þG**)
[
19]
during the reaction; (b) the reactivity of 3,5-dimethoxyphenol
is higher than that of 3-methoxyphenol. These two results
provide conflicting evidence, suggesting electrophile formation
or electrophilic attack to be the rate-determining step,
respectively. This contradiction can be reconciled assuming that
both steps have comparable rates, which may lead to a reaction
profile hardly distinguishable from first-order kinetics.
CONCLUSIONS
We have presented experimental and computational results
concerning the reaction of phenyl formate with activated arenes
with excess BCl
3
. The present study fully confirms the
intermolecular mechanism of the Fries rearrangement of aryl
formates proposed in our previous work.
[
19]
Summarizing, the
A systematic investigation of the kinetics, by varying the
relative concentrations of substrates and Lewis acid, and the
temperature, is needed to elucidate the details of the mechanism.
QM calculations of the reaction profile are in agreement with
the experimental findings. Starting from the various structures
Fries rearrangement promoted by BCl is understood to be a
3
typical Friedel-Crafts acylation where the acylating agent is
HCOCl generated in situ by cleavage of a formate ester. HCOCl is a
weak acylating agent, also because it decomposes rapidly.
However, despite the intermolecular pathway, high ortho
regioselectivity is achieved because HCOCl remains coordinated
(
reactants, intermediates, transition states and products) inves-
tigated in Reference [19] concerning 3-methoxyphenyl formate,
we have recalculated the reaction profile for the reaction of
phenyl formate and 3,5-dimethoxyphenyl formate focussing
on the two transition states identified therein. The first one
concerns the migration of the Lewis acid moiety from the
carbonyl oxygen to the aryl oxygen (after nucleophilic attack of
2
to the —OBCl moiety present in the intermediate phenol ester,
thus driving the formylation towards the closest nucleophilic
ortho carbon.
On the other hand, the reaction investigated here may
represent a synthetically useful procedure for formylation of
activated arenes, using phenyl formate as a source of formyl
chloride, thereby avoiding the use of CO and HCl under high
[
19]
a chloride anion to the carbonyl ); the second is a standard
intermolecular Friedel-Crafts acylation. A summary of the
reaction profile is shown in Fig. 5.
[
22]
pressure as in the well-known Gattermann-Koch method.
z
The activation barrier for the first step (DG of about 18 kcal/
mol) of the reaction is essentially unaffected by the substitution
of the aromatic ring. In fact, we recall that all the formates
Acknowledgements
We thank Dr Ralf Kress (University of Stuttgart) for the synthesis of
phenyl formate and Prof. G. Scorrano (Universit a` di Padova) for
useful comments. We also acknowledge insightful suggestions by
a reviewer.
3
investigated, in the presence of an excess of BCl , undergo
ArO—CHO bond cleavage with the release of formyl chloride. In
contrast, the activation barrier for the second step, the Friedel-
Crafts acylation is, as one might expect, indeed influenced by
substitution. In particular, QM calculations predict that the
activation barrier for the formylation of phenyl dichloroborate is
z
rather high, DG being more than 30 kcal/mol, while it is reduced
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 682–687

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Copyright ß 2008 John Wiley & Sons, Ltd.
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