On the decarboxylation of 2-methyl-1-tetralone-2-carboxylic acid-oxidation of the enol intermediate by triplet oxygen

Article abstract of DOI:10.1039/c3nj00457k

The formation of 2-methyl-1-tetralone from the metal-free and base-free decarboxylation of 2-methyl-1-tetralone-2-carboxylic acid involves 2-methyl-3,4-dihydro-1-naphthol as an intermediate. The reaction of this enol with atmospheric oxygen leads to 2-hyd

Full text of DOI:10.1039/c3nj00457k

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On the decarboxylation of 2-methyl-1-tetralone-2-
carboxylic acid – oxidation of the enol intermediate
by triplet oxygen†
Cite this: NewJ.Chem., 2013,
37, 2245
Received (in Montpellier, France)
1st May 2013,
Abdelkhalek Riahi,a Jacques Muzart,*a Manabu Abe*bc and Norbert Hoffmann*a
Accepted 13th May 2013
DOI: 10.1039/c3nj00457k
www.rsc.org/njc
The formation of 2-methyl-1-tetralone from the metal-free and attributed the evolution of the UV spectrum with time to an
base-free decarboxylation of 2-methyl-1-tetralone-2-carboxylic oscillation reaction rather than to the sequence shown in
acid involves 2-methyl-3,4-dihydro-1-naphthol as an intermediate. Scheme 1.4 According to them, ‘‘no reliable indication of the
The reaction of this enol with atmospheric oxygen leads to formation of an enolic intermediate could be obtained by spectro-
2-hydroperoxy-2-methyl-1-tetralone. The oxidation mechanism is scopic methods’’.4
confirmed by quantum chemical calculations at the (U)B3LYP/
6-31G(d) level of theory.
Optically active 2 can also be attained from the Pd-catalyzed
hydrogenolysis of 2-methyl-1-tetralone-2-carboxylic acid benzyl
ester (4) in the presence of chiral aminoalcohols.5 Repeating
In 1997, we reported that the decarboxylation of 2-methyl-1- this hydrogenolysis, Baiker et al. have, besides 2, isolated small
tetralone-2-carboxylic acid 1 in the presence of a chiral amino amounts of 2-hydroxy-2-methyl-1-tetralone (5) and 2-methyl-1-
alcohol affords enantioenriched 2-methyl-1-tetralone 2.1 Moni- tetralol (6) (Scheme 2).4 They assumed that the Pd-catalyzed
1
toring, using UV and H NMR spectroscopies, the transforma- debenzylation produces the ketoacid 1,3,4 and suggested the
tion of 1 in the absence of any additive led us, in 2004, to assign formation of 5 from the reaction of 2 with traces of oxygen, and
2-methyl-3,4-dihydro-1-naphthol (3) as an intermediate leading that of 6 from the reduction of 2.4
to 2 (Scheme 1).2
These two reports of Baiker prompted us to revisit the
In 2008, Baiker et al. reinvestigated the additive-free decarboxy- 1H NMR monitoring of the decarboxylation of 1 and to inves-
lation of 1.3,4 They were able to reproduce our UV experiments tigate its reactivity under an oxygen atmosphere.
but not those concerning the NMR analyses.4 They, however,
We have to point out that we attributed the structure of the
intermediate, which leads to 2 from 1, from the comparison of
its spectroscopic characteristics with those of 2-methylinden-3-
ol obtained from the Norrish type II photoreaction of 2-methyl-
2-isobutyl-1-indanone.6 This photo-enol was identified from its
1H NMR (methyl group as a singlet) and UV (lmax: 265–270 nm)7
characteristics, and its quantitative tautomerisation into
Scheme 1 Decarboxylation of 2-methyl-1-tetralone-2-carboxylic acid 1 invol-
ving the formation of enol 3.
a
´
´
CNRS-Universite de Reims Champagne-Ardenne, Institut de Chimie Moleculaire de
Reims, UMR 7312, UFR des Sciences Exactes et Naturelles, BP 1039,
51687 Reims Cedex 2, France. E-mail: norbert.hoffmann@univ-reims.fr,
jacques.muzart@univ-reims.fr; Fax: +33 3 26 91 31 66; Tel: +33 3 26 91 33 10
b Hiroshima University, Department of Chemistry, Graduate School of Science,
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan.
E-mail: mabe@hiroshima-u.ac.jp; Fax: +81-82-424-7432; Tel: +81-82-424-7432
c Institute for Molecular Science (IMS), Okazaki, Aichi 444-8787, Japan
† Electronic supplementary information (ESI) available: Experimental section
with spectroscopic data and quantum chemical calculation data. See DOI:
10.1039/c3nj00457k
Scheme 2 Palladium catalysed transformation of keto ester 4.
c
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2-methyl-1-indanone. Consequently, the structure 3 agreed with
the 1H NMR (a singlet at 1.37 ppm) and UV (lmax: 280 nm)
spectra obtained from the monitoring of the decomposition of
1. Moreover, both 1H NMR and UV monitoring showed that this
unstable compound evolved in the formation of 2.
After several attempts, we found that the problem of the
1H NMR reproducibility apparently resulted from the crystal-
lization procedure of the substrate sample. We suspect that
Scheme 4 Formation of the hydroperoxide 7 induced by decarboxylation of
2-methyl-1-tetralone-2-carboxylic acid 1.
10
traces of an undetermined compound can favor either the P(OEt)3 provided, in a few minutes, the a-ketol 5 (Scheme 4).
stability of the enol, or its tautomerisation into the ketone. Similar peroxides were reported in the silyl enol ethers with
Indeed, we obtained erratic NMR spectra when 1 was isolated singlet oxygen.11 In contrast, an acetonitrile solution of 2 under
according to the Baiker procedure, whereas reproducible the same experimental conditions did not afford 7. Thus, this
results, similar to those reported previously,2 were issued for oxidation concerned an intermediate obtained from 1 and not
samples of 1 isolated as described in the experimental part.† from 2.
Such samples are relatively stable at room temperature, the
The 1H NMR spectrum of 5 agrees with that provided in the
decarboxylation occurring slowly. A part of a new 1H NMR literature,4,12 and the mass spectrum corresponds to this
spectrum of a solution of 1 (1.5 mg) in CD3CN (0.8 mL) is structure. Surprisingly, the 1H NMR signals of the methyl group
depicted in Scheme 3, curve a, the singlet (d = 1.44 ppm) of 5 (d = 1.40 ppm) and 7 (d = 1.48 ppm) in CDCl3 disagree with
corresponding to the methyl substituent.1,3,4 After 24 h at rt our above proposal on the meaning of the singlet at 1.32 ppm
1
and then 1 h at 45 1C, this sample led to the NMR spectrum in the spectra in Scheme 3. Actually, the H NMR spectra of 5
depicted in Scheme 3, curve b, which shows two new signals: a and 7 depend on the solvent, the above signal of 5 and 7 in
singlet at 1.37 ppm and a doublet at 1.20 ppm (J = 6.7 Hz), the CD3CN being at 1.32 ppm and 1.37 ppm, respectively. Conse-
doublet corresponding to the methyl of 2,3,4,8 whereas the quently, traces of 5 were obtained from 1 under the conditions
singlet is attributed to the methyl of 3.1,6 An additional reaction leading to the spectra depicted in Scheme 3. Given these
time of 96 h at rt followed by 10 min at 45 1C led to results, we examined the stability of 7: at rt in MeCN, its slow
the disappearance of the signal at 1.44 ppm, indicating the transformation into 5 has been observed. Thus, low amounts of
complete consumption of 1 (Scheme 3, curve c). This was 7 could be formed when monitoring the decarboxylation of 1,
accompanied by the increase in the signals at 1.37 and 1.20 ppm, their presence would not be detected from the spectra b and c
the singlet disappearing at the profit of the doublet when the in Scheme 3, because the signal of the corresponding Me group
reaction time was prolonged to 14 days at rt (Scheme 3, curve d). would be very small and hidden under that of 3.
These results agree with the reaction cascade shown in
Scheme 1, and also with the usually accepted cyclic, six-center are known. Basic conditions with oxygen and trialkyl phos-
mechanism of the decarboxylation of b-ketoacids.9 phites provide 5 from 2.12,13 A radical reaction was suspected by
Procedures for the synthesis of compounds such as 5 and 7
The NMR spectra of the decarboxylation of 1 show a weak Sajiki’s team for the synthesis of 2-hydroxy-2-methyl-1-indanone
singlet at 1.32 ppm (Scheme 3, curves b–d), which could from 2-methyl-1-indanone using oxygen, Pd/C as the catalyst
correspond to the methyl of an oxygenated compound, i.e. 5 and triethylamine in ethanol at room temperature.14 Under
or 2-hydroperoxy-2-methyl-1-tetralone 7. This led us to examine these conditions, the formation of 2-hydroperoxy-2-methyl-1-
the reaction of 1 in MeCN under an oxygen atmosphere. indanone15 and its reduction with the Pd/C–NEt3 system is
Interestingly, 7 was isolated in 78% yield, its reduction with conceivable.16 The recent procedure of Ritter uses the dinuclear
PdII complex Pd2(hpp)2, to catalyze the a-hydroxylation of 2
with oxygen, leading to 5 in 91% yield.17 The corresponding
mechanism remains unknown; it was speculated ‘‘that enol
formation is important for hydroxylation’’.17 From experiments
and meticulous analysis, it appeared that this reaction has no
radical character and that 2-hydroperoxy-2-methyl-1-tetralone is
not an intermediate.17 These literature reports do not give
elements allowing explanation of the formation of an oxidation
compound from 1 and not from 2 under both base-free and
palladium-free conditions. This leads us to assume that 7 is
produced from 3. Actually, the formation of a-hydroperoxy
ketones from the reaction of enols with atmospheric oxygen
in the absence of any additive has already been reported.18 As
compound 3 possesses a styrene moiety, it should also be
mentioned that triplet oxygen may be added to styrene which
plays a role in the polymerization under RAFT conditions of
this monomer.19
Scheme 3 NMR spectra of the decarboxylation process of 2-methyl-1-tetra-
lone-2-carboxylic acid 1.
c
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Letter
Doubts about the oxidation of enols with atmospheric structure, suggesting that the hydrogen transfer process is a
oxygen may originate from a lack of mechanistic understanding barrierless process in S-DR. Actually, a non-hydrogen-bonded
although the transformation was observed in the past18 and it structure of the singlet diradical S-DR0 (S2 = 0.86, DGrel
=
has also been reported in recent literature.20 In neutral and 29.9 kcal molꢀ1) was found as an equilibrated structure at the
aprotic media and in contrast to enolates,21 electron transfer BS-UB3LYP/6-31G(d) level of theory. The singlet–triplet energy
from the enols such as 3 or the corresponding enolalkylethers gap in DR0 was, thus, nearly negligible.
to triplet oxygen (3O2) is hardly possible.22 Furthermore, the
The intersystem crossing (ISC) process from T-DR to S-DR is
generation of singlet oxygen (1O2) is not possible under the key for the formation of the hydroperoxide 7. For such a
dark reaction conditions in the absence of a sensitizer diradical with the short distance of the two radical sites, the
(Scheme 4). Via calculations, we checked whether the direct rate of the ISC process is directly related to the magnitude of
addition of triplet oxygen to 3 may occur.
spin–orbit coupling (SOC). Strong SOC for two-center inter-
Quantum chemical calculations for the oxidation reaction actions is proposed to be favored by the following factors: (1) if
were performed at the (U)B3LYP/6-31G(d) level of theory23 with the angle of the axes of the two radical orbitals is close to 901;
the Gaussian 09 suite of program (Scheme 5).24 The transition (2) for ‘‘ionic’’ admixture in the singlet state wave function; and
state TS1 (RQH), DGrel = 27.4 kcal molꢀ1, with an imaginary (3) for special proximity of the two radical orbitals.27 However,
frequency (ni) of 381 cmꢀ1 and S2 = 2.03 was found in the the process is spin-forbidden, thus, the lifetime of the triplet
addition reaction of the enol 3 (RQH) with 3O2 to give the diradicals is in general microsecond time scale at room tem-
triplet diradical T-DR (S2 = 2.03), DGrel = 26.7 kcal molꢀ1
.
perature. For diradicals including oxy radicals such as T-DR, in
The intramolecular hydrogen-bonding stabilization interactions which one-center spin-flip on oxygen is possible (Scheme 5),
were found in TS1 and T-DR. Thus, the atom distance between Minaev and coworkers found that the one-center spin–orbit
the peroxy oxygen and hydrogen of OH was calculated to be interaction effectively proceeds from the triplet state to the
198 pm and 201 pm as shown in the structures TS1 and T-DR, singlet state.28 Thus, the ISC process from T-DR to S-DR would
respectively. The O–H–O angles in TS1 and T-DR were found to be a fast process to give the hydroperoxide 7. All the computa-
be 1381 and 1371, respectively. The calculated atom-distances tional results clearly indicate that the hydroperoxide formation
3
and angles suggest that the hydrogen-bonding interactions are is possible in the reaction of 3 with O2. The fast ISC process
categorized into the ‘‘moderate hydrogen bonds’’.25 Indeed, the and the spontaneous hydrogen transfer in S-DR are the key for
non-hydrogen bonded structures TS10 (S2 = 2.04) and T-DR0 the oxidation reaction.
(S2 = 2.03) were found to be less stable than the hydrogen-bonded
ones TS1 and T-DR by 2.9 and 3.3 kcal molꢀ1, respectively.
A small solvent effect was computed at the same level of
theory in the reaction profile of the oxidation of 3 (Scheme 5).
The optimization of the singlet state of S-DR from the Thus, the energy barrier of the reaction of 3O2 with 3 was
hydrogen-bonded structure of T-DR at the broken-symmetry calculated to be 27.0 kcal molꢀ1 in acetonitrile, for which
(BS)26-UB3LYP/6-31G(d) level of theory finally gave the hydro- Polarized Continuum Model (PCM)29 was used for estimating
peroxide 7 as an equilibrated structure. Thus, the hydrogen- the solvent effect. This value is nearly the same as 27.4 kcal
bonded singlet diradical S-DR was not found as an equilibrated molꢀ1 obtained in the calculation without the solvent.
The additive-free decarboxylation of 2-methyl-1-tetralone-2-
carboxylic acid in acetonitrile involves the formation of 3,4-
dihydro-2-methylnaphthalen-1-ol as an intermediate leading to
2-methyl-1-tetralone. In the presence of oxygen, the decarboxy-
lation of this ketoacid under base-free and metal-free condi-
tions leads to 2-hydroperoxy-2-methyl-1-tetralone through,
most likely, the oxidation of the enol intermediate. Calculations
revealed that triplet oxygen may be directly added to 3,4-
dihydro-2-methylnaphthalen-1-ol. A previous electron transfer
from the enol to triplet oxygen followed by charge combination
is not necessary. Under the reaction conditions such a redox
process would be endothermic.
Experimental section
2-Methyl-1-tetralone-2-carboxylic acid (1)
A
solution of 2-carboethoxy-2-methyl-1-tetralone (710 mg,
3.05 mmol) in H2O–MeOH (1 : 3, 6 mL) was added to a solution
of KOH (550 mg, 9.80 mmol) in H2O–MeOH (1 : 3, 14 mL). The
mixture was stirred at room temperature under air for 24 h.
After addition of CH2Cl2 (30 mL), the recovered aqueous
phase was acidified using 2 M HCl, then extracted with CH2Cl2
Scheme 5 Quantum chemical calculations upon the addition of triplet oxygen
(3O2) to enol 3 forming 2-methyltetralone.
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to white crystals of 1 (209 mg, 40% yield); its 1H NMR char-
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was stored in the freezer.
´ˇ ´ ˇ ´
´ ˇ
´
12 M. Paˇzick´y, V. Semak, B. Gaspar, A. Bılesova, M. Salisova
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250 apparatus employing tetramethylsilane as internal refer-
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the text.
2-Hydroperoxy-2-methyl-1-tetralone (7)30
A solution of 1 (30 mg, 0.15 mmol) in MeCN (15 mL) was stirred
at room temperature under oxygen (balloon) for 129 h. Filtration
of the mixture over silica gel (petroleum ether/ethyl acetate:
70/30) afforded 2-hydroperoxy-2-methyl-1-tetralone (22 mg,
78% yield).
2-Hydroxy-2-methyl-1-tetralone (5)
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for 5 min. Purification of the mixture over silica gel (petroleum
ether/ethyl acetate: 70/30) afforded 2-hydroxy-2-methyl-1-tetralone
(16 mg, 60% yield).
´
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