Photo racemization and polymerization of (R)-1,1′-Bi(2-naphthol)

Article abstract of DOI:10.3390/molecules21111541

(R)-1,1′-Bi(2-naphthol) ((R)-BINOL) in an acetonitrile solution lost optical activity upon irradiation with an Hg-Xe lamp. HPLC resolution of the product indicated that (R)-BINOL was racemized upon irradiation, and SEC analysis suggested that a polymeric product was formed in the course of racemization. It is proposed that polymerization of BINOL can occur before it is racemized and that a unit in a polymer derived from BINOL may lose its optical activity afterwards due to in-chain racemization and/or reduction. The polymeric products seem to consist not only of BINOL residues but also of residues derived from acetonitrile as well as those derived through reduction of BINOL.

Full text of DOI:10.3390/molecules21111541

molecules
Communication
Photo Racemization and Polymerization
0
of (R)-1,1 -Bi(2-naphthol)
Zhaoming Zhang , Yue Wang 1 and Tamaki Nakano 1,2,
1
*
1
Macromolecular Science Research Division, Institute for Catalysis and Graduate School of Chemical Sciences
and Engineering, Hokkaido University, N21 W10, Kita-ku, Sapporo 001-0021, Japan;
zhang@polymer.cat.hokudai.ac.jp (Z.Z.); yue.wang@cat.hokudai.ac.jp (Y.W.)
Integrated Research Consortium on Chemical Sciences (IRCCS), Institute for Catalysis, Hokkaido University,
N21 W10, Kita-ku, Sapporo 001-0021, Japan
2
*
Correspondence: tamaki.nakano@cat.hokudai.ac.jp; Tel.: +81-11-706-9155
Academic Editor: Yoshio Okamoto
Received: 11 October 2016; Accepted: 11 November 2016; Published: 16 November 2016
Abstract: (R)-1,1’-Bi(2-naphthol) ((R)-BINOL) in an acetonitrile solution lost optical activity upon
irradiation with an Hg–Xe lamp. HPLC resolution of the product indicated that (R)-BINOL was
racemized upon irradiation, and SEC analysis suggested that a polymeric product was formed in the
course of racemization. It is proposed that polymerization of BINOL can occur before it is racemized
and that a unit in a polymer derived from BINOL may lose its optical activity afterwards due to in-chain
racemization and/or reduction. The polymeric products seem to consist not only of BINOL residues
but also of residues derived from acetonitrile as well as those derived through reduction of BINOL.
Keywords: photo-polymerization; racemization; isomerization; excited states; reduction;
0
1
,1 -bi(2-naphthol)
1
. Introduction
Interactions between substances and light play important roles in life. One of the most important
aspects is photosynthesis by plants and other organisms. In photosynthesis, sugars and molecular
oxygen are produced from carbon dioxide and water where photon energy drives the reaction.
Moreover, in artificial chemical synthesis, light promotes various reactions that are not driven by
thermal energy [
products can be obtained using circularly polarized light (CPL) [12
a preferred-handed helical conformation is induced for a polyfluorene derivative in the solid state
when the polymer is irradiated with CPL [20 22]. The mechanism of this chirality induction involves
1
–
11]. Further, in reactions where chiral compounds are produced, non-racemic
–
19]. We recently reported that
–
a twisted-coplanar transition (TCT) of an aromatic–aromatic junction in the polymer where one of the
enantiomeric, right- and left-handed twists is preferentially excited into the coplanar conformation.
TCT through photo excitation was first predicted for biphenyl through theoretical calculations [23].
In addition, CPL-assisted chirality induction has been reported for polymers consisting of fluorene
and azobenzene units [24,25].
In this work, in order to examine the applicability and limitations of the photo-induced TCT for
0
a molecule with a higher rotational barrier, we irradiated optically active (R)-1,1 -bi(2-naphthol)
(BINOL) with non-polarized light and investigated the structures of the products (Scheme 1).
BINOL has been used as an effective chiral ligand and as a building block for chiral functional
molecules and polymers; its stability has been considered to be sufficient under normal conditions
for organic synthesis [26–28]. However, the present study indicated that BINOL is readily racemized
and polymerized at the same time upon photo irradiation. Such stereo chemical and chemical
transformations of BINOL are unprecedented.
Molecules 2016, 21, 1541; doi:10.3390/molecules21111541
www.mdpi.com/journal/molecules

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Scheme 1. Twisted-coplanar transition (TCT) of BINOL.
2
. Results and Discussion
BINOL was irradiated in a solution of acetonitrile under N atmosphere with light using a 500-W
2
Hg–Xe lamp without any modulation or polarization. General information about experiments is found
in Appendix A. A drastic change was observed through irradiation on circular dichroism (CD) spectra
in acetonitrile solution (Figure 1a); CD intensity largely decreased through irradiation, and the signal
almost disappeared within ca. 15 min of irradiation (Figure 1b,c). In addition, along with the changes
in CD spectra, a decrease in UV intensity (hypochromism) was observed. These results are suggestive
of stereo chemical and chemical transformation of BINOL through photoexcitation. Hypochromism
has been observed in photo chirality induction to poly(9,9-di-n-octylflurene-2,7-diyl) using CPL, and it
was ascribed to a change in dihedral angle in the main chain [20
,21]; however, the hypochromism
observed in this work appears to have a connection with polymer formation as discussed later.
(
a)
(
b)
(c)
Figure 1. Changes in chirality of (R)-BINOL in CH CN solution upon irradiation ([BINOL]
o
=
3
−
4
1
.0
×
10 M, cell path 1 mm, room temperature): (a) CD and UV spectra taken upon irradiation for
13 min at 1-min interval; (
b
) gCD-vs.-relative irradiation energy plot at 223 nm; (
c) gCD-vs.-relative
irradiation energy plot at 318 nm.
In order to confirm the reason of the mutarotation, chromatographic resolution of the irradiated
BINOL samples was performed (Figure 2). For resolution, a Daicel Chiralpak IA column based
on amylose derivative was employed. Under the conditions in this work where the size of the

Molecules 2016, 21, 1541
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column was 25 cm
×
0.46 cm (i.d.), the flow rate was 0.5 mL/min, and the eluent was a mixture
of hexane and dichloromethane (50/50, v/v), the racemic BINOL was completely resolved with
chromatographic factors, k = 2.35 and = 1.61 (Figure 2a). The sample before irradiation indicated
α
1
only one signal which is ascribed to the (R)-isomer at an elution time of 14.9 min (Figure 2b) while
the irradiated samples indicated the signals of the (S)-isomer at an elution time of 10.6 min and the
relative intensity of this signal increased with irradiation time (Figure 2c–e). These observations
unambiguously indicate that the decrease in CD intensity under irradiation is ascribed to racemization
of BINOL. Photo racemization diaryl compounds having axial chirality including BINOL [29
–
31],
a BINOL derivative [32], 1,1 -binaphthyl [33], a biphenyl derivative [34], and a 1,1 -biphenanthrene
derivative [35 36] have been reported. In addition, photo-induced decomposition of one enantiomer
0
0
,
BINOL in the presence of a protein is known [37].
S
R
a. Racemic
b. Before irradiation
(ee > 99%)
c. Irradiation for 1 h
ee = 80%)
(
d. Irradiation for 2 h
ee = 60%)
(
e. Irradiation for 3 h
(ee = 44%)
0
5
10
15
20
25
Elution time (min)
Figure 2. HPLC chromatograms of samples obtained from (R)-BINOL in a CH CN solution upon
3
−
3
irradiation ([(R)-BINOL]
standard samples: ( ) standard racemic BINOL; (
for 1 h, 2 h, and 3 h, respectively.
o
= 1.0
×
10 M, cell path = 5 mm, solution volume = ca. 1.75 mL) and
a
b
) standard (R)-BINOL; ( ) samples after irradiation
c–e
The change in chirality due to irradiation was quantitatively analyzed by evaluating chirality in
terms of Kuhn’s anisotropy factor, gCD, [38] which is defined as follows:
gCD = 2(ε − ε )/(ε + ε )
L
R
L
R
where
ε and ε are molar absorptivities of the system toward left- and right-handed circularly
L R
polarized light. Figure 1b,c indicate the plots of gCD and enantiomeric excess (e.e.) against relative
irradiation energy sum (relative absorbed dose) by the system at 223 nm and 318 nm. Since a decrease
in UV intensity is observed in Figure 1a, “relative irradiation energy sum” (relative absorbed dose) [22
was chosen rather than irradiation time for the quantity on the horizontal axis in Figure 1b,c.
It corresponds to the sum of irradiation time intervals ( ti) used to change a CD spectrum to the
]
∆
directly following CD spectrum multiplied by UV absorbance (Abs ) corresponding to the former CD
i
spectrum according to the following equation:

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n
Relative irradiation energy sum =
∆t × Abs .
∑
i
i
i=0
Because gCD is proportional to the enantiomeric excess (e.e.) of a compound, gCD at each data point
in Figure 1b,c can be correlated to the e.e. of BINOL on the basis of gCD of optically pure (R)-BINOL
before irradiation. Under the present experimental conditions where light intensity is constant,
in other words, the number of photons per time hitting the system is constant throughout the reaction,
the rate of decrease in gCD and e.e. would depend only on and is proportional to the concentration
of un-racemized (R)-BINOL (a first-order relation), which would lead to a concave upward curve
with negative slopes increasing, with an increase in relative energy absorbed by the system. However,
the relation’s gCD and e.e. of BINOL with relative photon energy absorbed by the system at both
223 nm and 318 nm seem not to fit to a first-order relation. This would mean that the irradiated
solution undergoes not only racemization but also another chemical transformation which affects the
efficiency of photo racemization.
About this aspect, information was obtained by size-exclusion chromatography (SEC) analyses of
the products. Figure 3 shows SEC profiles of the samples before and after irradiation. The signal of
BINOL appeared at around 21.2 min before irradiation, and additional signals emerged in the range of
17.6–22.6 min whose relative intensity increased with irradiation time. This observation clearly means
that polymerization of BINOL takes place under irradiation. Although M of the product relative
n
to standard polystyrenes is only less than a few hundred, the highest molar mass of the product
corresponding to the rising point of the polymer peak at 17.6 min is ca. 2000. Thus, it is established
that BINOL undergoes polymerization as well as racemization upon irradiation. The deviation of
e.e.-vs.-relative energy plots in Figure 1 from a shape expected for a first-order reaction may be caused
by the polymerization.
Figure 3. Size-exclusion chromatography (SEC) chromatograms of samples obtained from (R)-BINOL
−
3
in a CH CN solution upon irradiation ([(R)-BINOL]
volume = ca. 1.75 mL) and a standard (R)-BINOL sample: (
o
= 1.0
×
10 M, cell path = 5 mm, solution
) samples after
3
a
) standard (R)-BINOL; (b–f
irradiation for 1 h, 2 h, 3 h, 4 h, and 5 h, respectively.

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The aforementioned decrease in UV spectral intensity of the BINOL solution in Figure 1a may
also be ascribed to polymer formation. Although the structure of the polymerization product is not
1
yet clear, the product seems to be almost optically inactive. Figures 4 and 5 show the H-NMR and
IR spectra, respectively, of the photo polymerization product along with those of (R)-BINOL. The broad
aromatic signals in the NMR spectrum of the photo reaction product support that the product is
a polymer (Figure 4). Further, it is noteworthy that rather intense signals were observed in the range
of 0.8~5.33 ppm in addition to the aromatic signals. These results suggest that the polymeric product
does not consist purely of BINOL units, but it may have a partially reduced structure and may have
residues arising not only from BINOL but also from acetonitrile, the solvent. This aspect was further
assessed by IR spectra (Figure 5). In the spectrum of the polymer, the signals were much broader
than those of BINOL, which is consistent with the formation of a polymer. In addition, signals due
−
1
to aromatic C–H out-of-plane bending vibration including the intense ones at 815 and 747 cm are
much weaker in the polymer spectrum which supports the partial loss of aromaticity due to reduction.
−
1
Further, the polymer spectrum indicated rather intense signals at around 1700 cm which would not be
observed in a polymer consisting only of BINOL units and may indicate the presence of C=N or C=O
functions, suggesting that acetonitrile might have a role in the photo polymerization or that C=O species
that have been proposed to form upon irradiation [30,31] might be incorporated into the polymer chain.
−
1
It is also noteworthy that, in the IR spectra, the signals in the range of 3000–4000 cm were largely
broadened through irradiation. This might entail the formation of intra-molecular hydrogen-bonding
between –OH groups of neighboring units in the polymeric product with various strengths and forms.
a. (R)-BINOL
Me Si
4
CHCl3
-
OH
Aromatic
H O
2
1
0
8
6
4
2
0
THF
b. Polymer
Me Si
4
THF
Aromatic
1
0
8
6
4
2
0
(
ppm)
Figure 4. 1H-NMR spectra: (
R)-BINOL in a CH CN solution upon irradiation for 5 h ([(R)-BINOL]
a) (R)-BINOL taken in CDCl ; (b) the polymeric product obtained from
3
−
3
(
o
= 1.0
×
10 M, cell path = 5 mm)
3
taken in THF-d [400 MHz, room temperature, Me Si].
8
4

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Figure 5. IR spectra: (
a) (R)-BINOL; (b) the polymeric product obtained from (R)-BINOL in a CH CN
3
−
3
solution upon irradiation for 5 h ([(R)-BINOL]o = 1.0 × 10 M, cell path = 5 mm) [KBr pellet].
Elemental analysis of a polymer prepared in a repeated experiment under conditions similar
to those for the HPLC and SEC experiments with an irradiation time of 4 h further supported the
conclusions obtained from the NMR and IR spectra. While the ideal ratio of elements for a polymer
comprising purely of BINOL units may be close to the value for BINOL (C, 83.90%; H, 4.93%),
the experimental results on the polymer was C, 58.95%; H, 4.73%; N, 3.54%. These results support
that the product has N atoms arising from CH CN and that BINOL was partially reduced on the
3
incorporation into the polymer chain. Although the exact structure of the polymer is not yet known
and the reaction mechanism leading to the polymer is not yet clear at this point, studies are under way
to shed light on these aspects.
Emission spectra of BINOL and the polymeric product obtained by irradiation are shown
in Figure 6. While (R)-BINOL indicated a rather sharp emission peak at around 355 nm, the polymeric
product showed much broader emission signals. Emission efficiencies of BINOL and the polymer
were 0.04 and 0.03, respectively, as determined using 9,10-diphenylanthracene (0.90 in cyclohexane) as
standard [39]. The value for BINOL well matches the one in the literature [40]. The broader signals
of the polymer may mean that the polymer has an extended
π-conjugation system derived from the
BINOL backbone. Changes in emission spectra of BINOL and related compounds due to their photo
degradation have been reported [31
have never been proposed.
,41,42]; however, polymer formation during the photo reactions
In the context of polymer formation, a question may be which occurs faster, racemization or
polymerization. Although exact kinetic comparisons between the two have not been conducted in
this work, Figure 1b,c indicate that the decrease in gCD is slower than expected from a first-order
reaction. In a first-order reaction, the drop in gCD with an increase in relative irradiation energy should
be sharper. This may mean that polymerization of BINOL can occur before it is racemized and that
a unit in a polymer derived from BINOL loses its chirality afterwards due to in-chain racemization
and/or reduction.

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Figure 6. Emission spectra: (
a) (R)-BINOL in a CH CN solution; (b) the polymeric product obtained
3
−
6
from (R)-BINOL in a CH CN solution upon irradiation for 4.5 min ([(R)-BINOL]
o
= 1.0
×
10 M,
3
cell path = 10 mm) [λex 300 nm]. Intensity was normalized to a constant absorbance at 300 nm.
In addition, it was found that the photo racemization and polymerization was faster at a lower
−
3
−4
concentration. The reactions at 1.0
×
10 M (Figures 2–5) and at 1.0
×
10 M (Figure 1) were
completed roughly within 5 h and 15 min, respectively, using the same irradiation set-up. This might
mean that inter-molecular interactions have a role in the reaction mechanism.
Further, whether TCT upon irradiation is feasible from the view of rotation barrier of BINOL
is an important point in discussing the reaction mechanism in this study. Rotation barriers of
BINOL have been assessed by thermal racemization experiments and theoretical calculations;
the experimental barriers in naphthalene and diphenyl ether were 155.5 kJ/mol and 158 kJ/mol,
respectively, and theoretically predicted barriers were in the range of 158.3–249.4 kJ/mol and varied
depending on the transition-state conformation [43]. In the absorbance spectrum of BINOL (Figure 1a),
the lowest-energy absorbance edge is at around 350 nm, which corresponds to 342 kJ/mol. This means
that even the lowest-energy photon absorbed by BINOL has a high enough energy to induce TCT.
3
. Conclusions
R)-BINOL was found to racemize and polymerize upon photo irradiation. The racemization may
(
occur through TCT of BINOL through photo excitation. Although TCT has been reported for biphenyl
through theoretical calculations [23] and has been experimentally established for aromatic–aromatic
junctions in polymers leading to a helical structure upon irradiation using CPL [20–22,44], reports
upon photo racemization of axially chiral compounds are rare. The finding of co-occurring photo
racemization and polymerization of optically active BINOL is unprecedented, while there have been
only a few related publications about the photo-induced racemization of diaryl compounds [29–36].
The observations in this work are in sharp contrast to the general recognition that BINOL is a rather
robust chiral backbone to be used for catalysis and for the construction of chiral materials. Studies
are under way to clarify the polymer structure and the mechanism of the photo racemization and
polymerization as well as to find a way to make an optically active polymer from racemic BINOL with
the aid of light.
Acknowledgments: Tamaki Nakano acknowledges the Mitsubishi Foundation for partial financial support for
the research and the cost to publish in open access. Zhaoming Zhang thanks the China Scholarship Council (CSC)
for a scholarship to study in Japan.
Author Contributions: Tamaki Nakano conceived and designed the experiments; Yue Wang and Zhaoming Zhang
performed the experiments; Tamaki Nakano, Yue Wang and Zhaoming Zhang analyzed the data; Tamaki Nakano
wrote the paper.

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Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the
decision to publish the results.
Appendix A
(
R)- and racemic BINOL (Wako Pure Chemical Industries, Ltd., Japan) and acetonitrile
(Kanto Chemical Co., Inc., Japan) were used as purchased. Irradiation was conducted using
an Ushio Optical Modulex SXUID500MAMQQ 500-W Hg–Xe lamp under a N atmosphere at ambient
2
◦
13
temperature (ca. 23 C). NMR spectra were recorded on a JEOL ESC400 spectrometer (400 MHz
1
for H, 100 MHz for C). Size-exclusion chromatography (SEC) measurements were carried out using
a chromatographic system consisting of a JASCO DG-980-50 degasser, a HITACHI L-7100 pump,
a HITACHI L-7420 UV-Vis detector, and a HITACHI L-7490 RI detector, equipped with TOSOH
TSKgel G3000HHR and G6000HHR columns (30
flow rate: 1.0 mL/min). HPLC resolution was conducted using a chromatographic system consisting of
a JASCO DG-980-50 degasser, a JASCO PU-980 pump, and a JASCO UV-2070plus UV detector (230 nm)
×
0.72 (i.d.) cm) connected in series (eluent: THF,
equipped with a Daicel Chiral Pak IA column (25 cm
×
0.46 cm (i.d.)) with a mixture of hexane and
dichloromethane (50/50, v/v) as eluent. Circular dichroism (CD) spectra were taken with a JASCO-820
spectrometer. UV-Vis spectra were measured on JASCO V-550 and V-570 spectrophotometers. FT-IR
spectra were recorded on a JASCO FT/IR-6100 spectrometer. Emission spectra were taken using
a JASCO FP-8500 fluorescence spectrophotometer.
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