Acid-Catalyzed Versus Thermally Induced C1-C1′ Bond Cleavage in 1,1′-Bi-2-naphthol: An Experimental and Theoretical Study

Article abstract of DOI:10.1021/acs.joc.9b00915

Experiments show that 1,1′-bi-2-naphthol (BINOL) undergoes facile C1-C1′ bond cleavage under action of triflic acid at temperatures above 0 °C to give mainly 2-naphthol along with oligomeric material. CASSCF and MRMP//CASSCF computations have demonstrated unambiguously that this unusual mode of scission of the biaryl bond can occur in the C1,C1′-diprotonated form of BINOL via a mechanism involving homolytic cleavage prompted by the intramolecular electrostatic repulsion. These findings also provide insights into the mechanism of a comparatively easy thermal cleavage of BINOL, implying the intermediacy of its neutral diketo form.

Full text of DOI:10.1021/acs.joc.9b00915

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Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
Acid-Catalyzed Versus Thermally Induced C1−C1′ Bond Cleavage in
1,1′-Bi-2-naphthol: An Experimental and Theoretical Study
Alexander M. Genaev,† Lyudmila N. Shchegoleva,† George E. Salnikov,†,‡,§ Andrey V. Shernyukov,†,‡
Leonid A. Shundrin,†,‡ Inna K. Shundrina,†,‡ Zhongwei Zhu,‡ and Konstantin Yu. Koltunov*,‡,∥
†N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Pr. Akademika Lavrentieva 9, Novosibirsk 630090, Russia
‡Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia
§International Tomography Center, Institutskaya 3a, Novosibirsk 630090, Russia
∥Boreskov Institute of Catalysis, Pr. Akademika Lavrentieva 5, Novosibirsk 630090, Russia
S
* Supporting Information
ABSTRACT: Experiments show that 1,1′-bi-2-naphthol (BINOL) undergoes facile C1−C1′ bond cleavage under action of
triflic acid at temperatures above 0 °C to give mainly 2-naphthol along with oligomeric material. CASSCF and MRMP//
CASSCF computations have demonstrated unambiguously that this unusual mode of scission of the biaryl bond can occur in
the C1,C1′-diprotonated form of BINOL via a mechanism involving homolytic cleavage prompted by the intramolecular
electrostatic repulsion. These findings also provide insights into the mechanism of a comparatively easy thermal cleavage of
BINOL, implying the intermediacy of its neutral diketo form.
with the oligomeric material formed from the other half of the
substance.9
Notably, the intriguing inclination of 1 to acidic cleavage is
apparently opposed to a typical mode of behavior of biaryls in
strong acid media, favoring more likely to an additional Ar−Ar′
bond formation (Scholl reaction) or intramolecular rearrange-
ments.10−16
Moreover, 1 can also be converted into 2-naphthol as a
result of pyrolysis, as communicated by Dianin as early as
1874.17 Although thermally stable (but not enantiostable) up
to 300 °C,18−20 1 certainly undergoes partial conversion into
2-naphthol already at 400 °C.21 Again, such behavior of 1
seems to be rather surprising, given the fact that nonstrained
biaryl bonds are normally the most resistant to breakage
carbon−carbon single bonds,22−26 while simple pyrolysis of
biaryls (including binaphthyls) results in the formation of
polycyclic aromatics.27,28
INTRODUCTION
■
1,1′-Bi-2-naphthol (BINOL) (1) is widely known and
recognized as a versatile chiral scaffold to obtain numerous
asymmetric reagents, catalysts, and materials.1−6 In addition,
BINOL derivatives have attained great importance as peptide
and DNA alkylating and cross-linking agents.7,8 Despite such
impressive practical use, some fundamental properties of this
unique compound have so far remained unstudied.
When examining protonation behavior of 1 as a mechanistic
aspect of its atropisomerization, we have found, among other
things, that low-temperature reactions of 1 with superacids led
to generation of C-mono- and C,C-diprotonated forms 2−4
(Figure 1).9 However, when dissolved in triflic acid at room
temperature, 1 underwent surprisingly facile C1−C1′ bond
cleavage to yield a near equimolar amount of 2-naphthol along
Here, we report a mechanistic exploration of the unusual
C1−C1′ bond cleavage in 1. A comparative study on the
thermal cleavage of 1 and its dimethyl ether 5 is also presented.
RESULTS AND DISCUSSION
■
Initially, the behavior of 1 in triflic acid, diluted with CHCl3
(molar ratio of 1:30:300, respectively), has been studied
Figure 1. Long-lived mono- and dicationic species derived from 1 in
HSO3F−SbF5−SO2ClF−CD2Cl2, HSO3F−SO2ClF−CD2Cl2 at −95
°C, and in CF3SO3H−CD2Cl2 at −40 °C.
Received: April 3, 2019
Published: May 14, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.9b00915
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The Journal of Organic Chemistry
Article
a
(Scheme 1). After 1 h stirring at 0 °C, the reaction was
quenched with ice, and the mixture was extracted with diethyl
Scheme 2. Reaction of 1 with Triflic Acid at −5 °C
a
Scheme 1. Reactions of Biaryls 1, 5, and 7 with Triflic Acid
a
The weight percentage of ions is given based on 1H NMR
spectroscopy data. The balance is unidentified products, which are
most likely oligomers.
bond in 3 is weakened by electrostatic repulsion, which can
lead to the formation of the radical cation/radical cation pair
(11/11) (Scheme 3). The radical cation 11 or its
a
The percentage of products is given based on uncorrected GC−MS
data (Supporting Information).
Scheme 3. Proposed Mechanism for the Acid-Induced
Cleavage of 1
ether to give a mixture composed mainly of the starting
compound 1 and 2-naphthol in ∼1:1 molar ratio (>85% in
sum, 1H NMR data). The major side products were an
isomeric binaphthol, traces of dinaphtho[2,1-b:1′,2′-d]furan
(6), and an unidentified product of molecular weight 342 [gas
chromatography−mass spectrometry (GC−MS) data, see the
Supporting Information].29 The lack of material balance for
the extracted material appeared close to the amount of 2-
naphthol produced, obviously due to partial oligomerization of
deprotonated form, 2-naphthoxy radical 12, can abstract the
hydrogen atom from another molecule (compound 1 or an
oligomer) to afford 2-naphthol and a free radical (cf. refs 32
and 33). Recombination of the latter will give the final
oligomeric material. A similar case is obviously true of the
cleavage of compound 5.
A free radical formation upon dissolving 1 in triflic acid has
been confirmed by EPR experiments. A singlet EPR signal (g =
2.0029) without the hyperfine structure was observed in the
range of temperatures −60 < T < 20 °C (Figure 2). The radical
concentration (determined with Bruker’s SpinCount quantifi-
cation package) was found to be about 3% of the starting
amount of 1. In view of the known pKa value for radical cation
1
the substance. Indeed, the H NMR spectrum of the ether-
insoluble residue (dark brown solid) in deuterated acetone
showed only a set of broadened signals at δH 6.5−8.5 ppm.
In like manner, diether 5 has been shown to convert into 2-
methoxynaphthalene and trace amounts of two isomeric
dimethoxybinaphthalens. In contrast, biaryl ether 730 turned
out to be unreactive under similar reaction conditions (Scheme
1).
Next, we have estimated the free energy barrier, ΔG⧧ = 19
kcal mol−1, for the cleavage of 1 in TfOH−CD2Cl2 at −5 °C,
1
based on H NMR kinetics experiment (see the Supporting
Information). Worthy of note is the substantial concurrent
generation of isomeric 1,6′- and 6,6′-bi-2-naphthols (in their
diprotonated forms 8 and 9) at this temperature (Scheme 2).
Notably, the structures of dications 8 and 9 are determined
unambiguously by 2D NMR (Supporting Information).
It should be noted also that 1 does not undergo C1−C1′
bond cleavage in aqueous mineral acids at a temperature of up
to 100 °C, when the formation of cation 2 and its tautomeric
keto form 10 has proved to be possible.9,31 Hence, no direct
contribution of monocations 2 and 10 to the cleavage reaction
can be concluded.
In contrast, dication 3, existing only in superacids,9 seems to
be a more feasible intermediate responsible for the triflic acid
mediated cleavage of 1. One can surmise that the C1−C1′
Figure 2. EPR spectra of 0.005 M solution of 1 in TfOH at −60
(blue) and 20 °C (red).
B
DOI: 10.1021/acs.joc.9b00915
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Article
11 (−5.5),34 radical 12 should undergo exhaustive protonation
in TfOH (Ho = −14). Therefore, one may assign the observed
EPR signal to 11. Notably, when the temperature was
increased to 80 °C, two additional EPR signals (g = 2.0142
and 2.0080) were observed (Supporting Information). Taking
into account that these radicals are formed only after
substantial heating, they cannot be considered as primary
intermediates of cleavage of 1. One of these additional radicals
is characterized by well-resolved isotropic hyperfine splitting
typical for aromatic protons in radical species. However, this
signal cannot be related to radical 12.35 As expected, no EPR
signal was observed for a solution of 2-naphthol in TfOH in a
“blank” experiment.
In order to estimate the energy barrier for the C1−C1′ bond
cleavage in 3 and thus support or discard the mechanism
suggested, the stationary energies for the transition state 3# and
dication 3 have to be found. It must be taken into account,
however, a potentially significant singlet biradical character of
3#. Therefore, single-configuration methods of calculation,
such as HF, density functional theory (DFT), or MP2 would
be inapplicable here. Indeed, the NFOD value for the DFT/
Perdew−Burke−Ernzerhof (PBE)/Λ1 calculated 3# is 1.88
(see the corresponding FOD plot in the Supporting
Information), thus falling under point (c) of Grimme’s rules
of thumb: “significant and delocalized ρFOD: use multireference
methods”.36,37
Figure 4. The active MOs and their Mulliken populations of a (left)
and b (right) symmetries at the CASSCF(2,2)/cc-pVDZ level of
calculations: transition state 3b# (top) and dication 3b (bottom).
level of theory but still remains notably higher compared to
that calculated by the MP2 method, which is commonly used
for closed shell molecules (Table 1). Therefore, biradical
nature of dication 3b (in the ground state) appears to be
somewhat overestimated at the CASSCF(6,6)- and CASSCF-
(2,2)/cc-pVDZ levels of calculations.
As can be seen in Table 1 (entries 1−3), the computed
energy barrier (ΔE) of the C1−C1′ bond cleavage in 3b
ranges from 25.3 to 28.0 kcal mol−1 and does not depend
significantly on the used basis set, active space dimension, or
upon the employing of the polar solvent correction. The found
values of ΔE exceed somewhat the experimental value of ΔG⧧
= 19 kcal mol−1 (see above). However, accounting for the
dynamic electron correlation by MRMP method (multi-
reference Møller−Plesset perturbation theory)39 leads to a
decrease in the value of ΔE, which becomes even below the
experimental estimation (Table 1, entries 4−5).
Also note that dication 3 persists mainly in three most stable
stereoisomeric forms, such as 3a−c (Figure 3).9 Among them,
structure 3b is preferable for the purpose of calculations owing
to its C2 symmetry, and it will be used solely in the following
computations.
Certainly, the calculations performed do not guarantee
thermochemical accuracy of the ΔE values, and their
refinement by means of thermodynamic correction or another
technique is, in principle, possible. A more accurate estimation
of the energy barrier ΔE requires geometry optimization both
of the key structures, 3b and 3b#, on the basis of dynamic
electron correlation. Nevertheless, such complex permanent
computations are not essential for the particular purpose of the
study because the calculated ΔE values fit well, in general, with
the proposed reaction mechanism.
The suggested mechanism is also in line with the appearance
of dications 8 and 9 on dissolving 1 in TfOH (Scheme 2).
According to DFT/B3LYP/cc-pVDZ estimations, the unpaired
electron density in 11 is localized mainly on carbon atoms 1
and 6 (Figure 5). Therefore, besides the reverse building of 1,
recombination of 11 should lead to the formation of the
protonated forms 8 and 9 of 1,6′- and 6,6′-bi-2-naphthols. The
additional formation of 6,8′- and especially 1,8′-, 8,8′-bi-2-
naphthols due to a residual spin density at the C8 atom seems
to be restricted by the unfavorable peri interactions.40 From an
energetic viewpoint, the intermediate generation of dications 8
and 9 is also plausible as these species are 15.0 and 28.4 kcal
mol−1 more stable compared to the parent dication 3c (DFT/
PBE/Λ1).
Figure 3. DFT/PBE/Λ1-optimized minimum-energy geometries of
dication 3. The symbols in parentheses indicate the symmetry group
and relative DFT/PBE/Λ1 energy (kcal mol−1).9
The CASSCF geometry optimization of 3b# and 3b was
performed using the CASSCF method. In its simplest
implementation, such as CASSCF(2,2), this method is quite
applicable in the case of the biradical state. The results
obtained are illustrated by Figure 4, where the active space
both of 3b# and 3b is composed of two molecular orbitals
(MOs) of a and b symmetry, populated by two electrons
(boundary MOs in the reference closed-shell configuration).
The active MO Mulliken populations are 1.666 (a), 0.336 (b)
for 3b#, and 1.980 (a), 0.020 (b) for 3b. This shows that the
transition state 3b# has indeed considerable biradical nature,
while a notable occupation of antibonding MO b in dication
3b is in line with the increased C1−C1′ bond length (1.64 Å)
in this species.
In a more extended CASSCF(6,6) computation of 3b, two
electron configurations ensure the main contribution to the
electron structure (with the same MOs as shown in Figure 4).
The C1−C1′ bond length in 3b is slightly decreased at this
It should be noted that the possibility of single electron
transfer reactions related to dicationic superelectrophiles has
previously been discussed in the literature.41 However, to the
best of our knowledge, only one study had in fact confirmed
C
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Table 1. Calculation Results for the Stationary Structures
a
no.
1
calculation method
CASSCF(2,2)
E (a.u.)
ΔE, kcal mol−1
C1−C1′ bond length, Å
3b#
3b
−915.915171
−915.956377
−916.147227
−916.189339
−915.942852
−915.987425
−918.838592
−918.859866
−918.843335
−918.868980
−918.868943
25.3
2.796
1.639
3.230
1.603
2.699
1.595
b
2
3
4
5
6
CASSCF(2,2) TfOH
CASSCF(6,6)
3b#
3b
26.4
28.0
13.3
16.1
3b#
3b
MRMP//CASSCF(2,2)
MRMP//CASSCF(6,6)
MP2/cc-pVDZ
3b#
3b
3b#
3b
3b
1.553
a
b
The cc-pVDZ basis set was used in all the calculations. The PCM parameters for triflic acid are taken from ref 38.
In contrast to 1, the analogous enol−keto tautomerization is
impossible for diether 5. Hence, it could be assumed that 5
would be thermally more stable than 1 (cf. ref 31). The
differential scanning calorimetry (DSC) and thermogravimet-
ric (TG) experiments showed that this is indeed true but not
dramatically so. By results of synchronous TG/DSC analysis,
the maximal rate of decomposition of 1 and 5 is observed at
425 and 436 °C, respectively (Supporting Information).
Besides, both the compounds have been shown to have
comparable enthalpies of decomposition (−23.62 and −23.72
kcal mol−1, respectively), which can be related to a similar
thermal process for 1 and 5.
Figure 5. Unpaired electron MO (DFT/B3LYP/cc-pVDZ) in radical
cation 11. The recombination sites responsible for the formation of
dications 8 and 9 are labeled by green arrows.
When 1 and 5 were heated simultaneously in a sealed tube
under Ar at 400 °C, dinaphthofuran 6 was formed as the major
product in both cases (Scheme 5). This may explain the
these types of events.42 Thus, cation radicals generated via
reaction of ferrocene with some oxazole, thiazole, and pyridine-
based superelectrophiles in triflic acid underwent coupling to
afford dicationic dimerization products. Obviously, such a
result is reverse to our findings, probably due to a more
efficient positive charge delocalization in the dications
obtained.42
Scheme 5. Thermolysis Reactions of 1 and 5
In addressing the thermally induced cleavage of 1, one can
propose that this reaction has a similar mechanistic origin as
the acid-catalyzed cleavage with respect to the preliminary
Csp3−Csp3 bond formation. Certainly, nonprotonated 1 can
be doubly tautomerized into a neutral diketo form 13 followed
by the C1−C1′ bond breaking to give a pair of radicals 12 and
thereby 2-napththol (Scheme 4). This reaction, therefore, can
be regarded as the reverse process of the standard oxidation
synthesis of 1 from 2-naphthol.1
similarity in the DSC data for 1 and 5. However, thermolysis of
diether 5 did not give even traces of 2-methoxynaphthalene,
while 1 underwent the concurrent cleavage to give a notable
amount of 2-naphthol (cf. ref 21). Consequently, the data
obtained are generally in line with the suggested mechanism
for the cleavage of 1 presented in Scheme 4.
Scheme 4. Proposed Mechanism for the Thermal Cleavage
of 1
CONCLUSIONS
■
In summary, the surprisingly facile C1−C1′ bond cleavage in
the parent BINOL 1 and its dimethyl ether 5 under action of
triflic acid, leading to the formation of 2-naphthol and 2-
methoxynaphthalene, respectively, is described. The homolytic
C1−C1′ bond dissociation in C1,C1′-diprotonated forms of 1
and 5 has proved to be the best rationale for the observed
reactions, which cannot proceed under moderate acidic
conditions. The strong acid-mediated cleavage of BINOLs
represents thereby a result of a special case of super-
electrophilic41 activation. Alternatively, the neutral diketo
form 13, derived from 1 at elevated temperature, provides
the comparatively easy thermal cleavage of 1 to give again 2-
According to DFT calculations, tautomer 13 is considerably
less stable than 1 (by 29.6 kcal mol−1).9 Nevertheless, the
Eyring equation predicts a much higher energy barrier, about
50 kcal mol−1, for the thermal cleavage of 1, being it possible
only at 400 °C. Therefore, the intermediacy of 13 is quite
permissible in that case.
D
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naphthol but along with dinaphthofuran 6. Overall, an
intermediate C1 sp3−C1′ sp3 bond formation is the key step
in both the acid-catalyzed and thermally induced cleavages of
1.
Experimental (NMR, EPR, TGA/DSC, and GC−MS)
and computational data (PDF)
Raw data of 1D and 2D NMR experiments (ZIP)
AUTHOR INFORMATION
Corresponding Author
*E-mail: koltunov@catalysis.ru.
ORCID
■
EXPERIMENTAL SECTION
■
Racemic BINOL (1) and CF3SO3H were purchased from chemical
suppliers and used as received. Diethers 5 and 7 were prepared
according to refs 31 and 30, respectively.
George E. Salnikov: 0000-0002-5868-9563
Konstantin Yu. Koltunov: 0000-0002-9466-1592
Notes
NMR spectra were recorded using Bruker AV-600 and Bruker AV-
400 spectrometers. The chemical shifts were measured relative to the
residual proton and carbon signals of CD2Cl2 (δH 5.33 ppm, δC 53.6
ppm), CDCl3 (δH 7.24 ppm, δC 76.9 ppm), and acetone-d6 (δH 2.07
ppm, δC 29.2 ppm). The chemical structures and the complete NMR
signals assignment for all the relevant species were unambiguously
determined by 2D NMR (HSQC, HMBC, COSY, NOESY, see the
Supporting Information). GC−MS data were obtained on an Agilent
6890N/5973N EI/PCI instrument using a HP-5MS column. TG
analysis (TGA) and DSC measurements were performed on a
NETZSCH STA 409 instrument. EPR spectra were recorded using a
Bruker ELEXSYS-II E500/540 spectrometer.
The CASSCF calculations were carried out with the GAMESS-US
package using the MRMP method for finding the dynamic correlation
energy.39 The MP2 and B3LYP calculations were performed with the
same program as well. The cc-pVDZ basis set was used in all these
calculations. DFT/PBE calculations were performed using the
PRIRODA program43,44 with the Λ145 basis set (similar to the cc-
pVDZ basis) as a gas-phase model.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.M.G., L.N.S., and G.E.S. gratefully acknowledge the support
of the RFBR under grant no. 17-03-00564. We also
acknowledge the Multi-Access Chemical Research Center SB
RAS for spectral and analytical measurements and Cluster of
the Information Computation Center, Novosibirsk State
University (http://www.nusc.ru/) for computing resources.
REFERENCES
■
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̌
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1
C1-monoprotonated form of 2-naphthol: H NMR (600.3 MHz,
CF3SO3H−CD2Cl2, −29 °C): δ 4.84 (s, 2H), 7.25 (d, J = 9.1 Hz,
1H), 7.82 (t, J = 7.7 Hz, 1H), 7.85 (d, J = 7.8 Hz, 1H), 8.03 (t, J = 7.7
Hz, 1H), 8.10 (d, J = 7.8 Hz, 1H), 9.11 (d, J = 9.1 Hz, 1H). 13C{1H}
NMR (151 MHz, CF3SO3H−CD2Cl2, −29 °C): δ 40.7, 119.2, 129.7,
130.0, 130.5, 135.9, 138.5, 140.4, 174.9, 207.9.46,47
Ion 8: 1H NMR (600.3 MHz, CF3SO3H−CD2Cl2, −29 °C): δ 4.91
(s, 2H), 5.84 (s, 1H), 7.31 (d, J = 9.2 Hz, 1H), 7.34 (d, 1H), 7.56 (d,
J = 8.0 Hz, 1H), 7.75 (d, J = 7.8 Hz, 1H), 7.87 (t, J = 7.5 Hz, 1H),
7.91 (d, J = 8.2 Hz, 1H), 7.98 (s, 1H), 8.00 (t, J = 7.7 Hz, 1H), 8.22
(d, J = 8.0 Hz, 1H), 9.05 (d, J = 9.4 Hz, 1H), 9.26 (d, J = 9.1 Hz,
1H).48 13C{1H} NMR (151 MHz, CF3SO3H−CD2Cl2, −29 °C): δ
40.8, 55.2, 118.3, 120.6, 129.6, 130.9, 131.0, 131.3, 131.3, 136.5,
136.7, 137.1, 138.4, 139.8, 140.9, 144.3, 173.0, 176.2, 207.7, 209.7.
Ion 9: 1H NMR (600.3 MHz, CF3SO3H−CD2Cl2, −29 °C): δ 4.95
(s, 4H), 7.36 (d, 2H), 8.02 (d, 2H), 8.37 (d, J = 8.0 Hz, 2H), 8.44 (s,
2H), 9.23 (d, J = 9.3 Hz, 2H).48 13C{1H} NMR (151 MHz,
CF3SO3H−CD2Cl2, −29 °C): δ 40.8, 120.3, 130.6, 130.7, 133.8,
136.6, 140.0, 141.0, 174.2, 208.9.
Dinaphtho[2,1-b:1′,2′-d]furan (6) (the data were taken from the
spectrum of the mixture of compounds 1, 6 and 2-naphthol,
Supporting Information): 1H NMR (600.3 MHz, acetone-d6): δ
7.64 (ddd, J = 8.1, 6.8, 1.1 Hz, 2H), 7.82 (ddd, J = 8.5, 6.8, 1.4 Hz,
2H), 7.95 (d, J = 8.8 Hz, 2H), 8.12 (dt, J = 8.8, 0.6 Hz, 2H), 8.19
(ddt, J = 8.1, 1.4, 0.6 Hz, 2H), 9.20 (dm, J = 8.5 Hz, 2H). 13C{1H}
NMR (151 MHz, CF3SO3H−CD2Cl2, −29 °C): δ 112.9, 119.2,
124.9, 125.6, 126.8, 128.7, 129.0, 129.9, 131.7, 154.6.49
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ASSOCIATED CONTENT
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* Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.9b00915.
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DOI: 10.1021/acs.joc.9b00915
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DOI: 10.1021/acs.joc.9b00915
J. Org. Chem. XXXX, XXX, XXX−XXX