Photo-induced β-elimination of 9-fluorenylmethanol leading to dibenzofulvene

Article abstract of DOI:10.1039/c7cc03297h

An effective photo-induced β-elimination of an alcohol leading to a vinyl compound is introduced for the first time. 9-Fluorenylmethanol was irradiated in a solution using a Xe lamp and was efficiently converted to dibenzofulvene (DBF) (9-methylenefluorene) in the absence of base which is necessary in the corresponding ground-state reaction.

Full text of DOI:10.1039/c7cc03297h

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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Photo-induced β-Elimination of 9-Fluorenylmethanol Leading to
Dibenzofulvene†
Hassan Nageh,a Zhao Liming,a Akira Nakayama,a Jun-ya Hasegawa,a, b Yue Wang,a and
.3Received 00th January 20xx,
Accepted 00th January 20xx
Tamaki Nakanoa, b,
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Effective photo-induced β-elimination of an alcohol leading to a such as high charge mobility and remarkable hypochromism as
vinyl compound is introduced for the first time. 9- well as charge delocalization over the stacked fluorene units.
Fluorenylmethanol was irradiated in a solution with an Xe lamp DBF monomer has been synthesized in the ground state from
and was efficiently converted to dibenzofulvene (DBF) (9- FM in the presence of a base such as t-BuOK where
methylenefluorene) in the absence of base which is necessary in dehydration is considered to occur through an E1cb
the corresponding ground-state reaction.
mechanism.11 The present study discloses that DBF is obtained
in the absence of base on photo irradiation and proposes that
Photochemistry is an important discipline in organic synthesis
because light promotes various reactions which are not driven
by heat.1 Typical elemental reactions induced by light include
E-Z isomerization, bond scission leading to radical or carbene,
ring closure and cycloaddition. Although various products have
been obtained through these photochemical reactions, clean,
the reaction occurs through
mechanism.
β-elimination through an E2
photo-induced dehydration (β-elimination) of alcohol leading
to olefin in the absence of side reactions is unprecedented.
Because olefins can be used as source materials for functional
vinyl polymers, their synthesis through alcohol dehydration is
important as well as the other methods including Aldol
condensation,2 Wittig reaction,3 Tebbe-reagent-based
olefination,4 Peterson olefination,5 Julia-Lythgoe olefination,6
McMurry reaction (carbonyl coupling),7 Shapiro and related
reactions,8 and hydrogenation of acetylenes.9 In the ground
Scheme 1. Synthesis of dibenzofulvene (DBF) through
photo-induced -elimination of 9-fluorenylmethanol (FM)
β
Figure 1 shows the UV spectral change of a hexane solution
of FM on irradiation (panel A) along with the UV spectrum of
pristine DBF (panel B). On irradiation, the signals due to DBF
appeared and they became more intense with an increase in
irradiation time at the cost of the signals due to FM. The
presence of clear isosbestic points at 225 nm, 259 nm, and 304
nm indicates that the reaction system contains only FM and
DBF and that no side reaction occurred through irradiation.
The production of DBF in hexane on irradiation was further
confirmed by IR spectrum (ESI, Figure S1). Additionally,
formation of poly(DBF) was not confirmed by size-exclusion
chromatography (SEC) (ESI, Figure S3), confirming that the
reaction is clean.
state, dehydration of alcohol can be achieved by
through contact with a base or an acid.
β-elimination
Herein we report the first, efficient photo-induced
β-
elimination of alcohol to olefin through an E2 mechanism
which converts 9-fluorenylmethanol (FM) to dibenzofulvene
(DBF) (Scheme 1). DBF is an important olefin that leads
through vinyl polymerization to poly(DBF) having a unique π-
stacked conformation.10 On the basis of the conformation, the
polymer exhibits characteristic photo electronic properties
In order to obtain information on the photochemical
dehydration of FM, kinetic analyses were conducted on
reactions carried out under various conditions, i.e., at different
concentrations in hexane, in the presence and absence of a
radical scavenger and in different solvents (hexane,
acetonitrile, methanol, and H2O) (Figure 2 and Table 1). The
amount of DBF generated by the reaction was quantified on
aInstitute for Catalysis (ICAT) and Graduate School of Chemical Sciences and
Engineering, Hokkaido University, N 21, W 10, Kita-ku, Sapporo 001-0021, Japan.
Email:tamaki.nakano@cat.hokudai.ac.jp; Fax: +81-11-7069156; Tel: +81-11-
7069155.
bIntegrated Research Consortium on Chemical Sciences (IRCCS), Institute for
Catalysis, Hokkaido University, N21W10, Kita-ku, Sapporo 001-0021, Japan.
†Electronic Supplementary Information (ESI) available: Experimental details,
spectral data, SEC profiles, and tabulated data. See DOI: 10.1039/x0xx00000x
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the basis of the intensity of the UV signal at 256 nm photo-induced
Journal Name
β
-elimination of FM is an almost unimolecular
corresponding to DBF (ESI, Figure S7).
reaction.
Figure 2 indicates the first-order plots of the reactions in
different solvents. All the reactions at low conversions (<5.0%)
exhibited almost linear relations according the following
equation:
ln([FM]o/[FM]t) = kt
where [FM]o and [FM]t are the concentrations of FM before
starting irradiation and at a given duration of irradiation (t),
respectively, k is the first-order rate constant, and t is the
duration of irradiation. The data points in all the reaction plots
in this work were well approximated by linear lines, and first-
order rate constant (k) values were determined in the range of
conversion of 0-5.0 % (Table 1).
Table 1. First-order rate constants for DBF production from FM
upon irradiation in hexane under various conditions at 23oCa
εc
Φd
[FM]
(M)
kb
Run
Solvent
(sec-1)
Figure 1. UV spectra of a hexane solution of FM observed on
irradiation with a 300-W Xe lamp (A) ([FM]o = 1.6 x 10-5 M], cell
path = 1 cm) and UV spectrum of pristine DBF in hexane (B)
([DBF] = 2.0 x 10-5 M, cell path = 1 cm).
1
2e
3
Hexane
Hexane
MeOH
CH3CN
H2O
3.9 x 10-5
3.9 x 10-5
4 x 10-5
8.6 x 10-5
8.5 x 10-5
7 x 10-5
3.2 x 10-5
~0f
1.89
0.12
33.62
37.50
80.37
0.29
0.35
0.33
4
3.9 x 10-5
5
3.9 x 10-5
aIrradiation with a 300-W Xe lamp. bDetermined at a
conversion of 5.0 % or lower by UV spectroscopy. cDielectric
constants data cited from ref. 12. Deteremined using 9,10-
d
diphenylanthracene as a standard compound (F = 0.90, ref. 13).
eIn the presence of 2,2,6,6-Tetramethyl-4-piperidinol (5.1 x 10-
4 M). fNo products were detected.
Solvent remarkably affected the reaction rate; the rate
constant was greater in the following order of solvent, i.e., in
hexane, in MeOH, in CH3CN, and in H2O (Figure 2 and Table 1).
The order of the solvents seems to be in accordance with the
order of their dielectric constants (ε). The solvent polarity
Figure 2. First-order rate plots for DBF production from FM on
irradiation in hexane ([FM]o = 3.9 x 10-5 M) (a), acetonitrile
([FM]o = 3.9 x 10-5 M) (b), methanol ([FM]o = 4 x 10-5 M) (c),
and H2O ([FM]o = 3.9 x 10-5 M) (d).
effect is opposite to that expected from the deprotonation-
based mechanism in the ground state;11 a polar reaction
involving deprotonation would be faster in a solvent with a
higher dielectric constant. The reaction therefore may not go
through an E1cb mechanism involving a raw carbanion at the
9-position of the fluorene backbone. The reaction in exited
states may proceed rather through an E2 mechanism where
bond scission and formation concertedly occur.
First, dependence of initial reaction rate on concentration of
FM was examined in hexane at [FM]o = 7.5 x 10-5 M, 5.0 x 10-5
M, 3.9 x 10-5 M, 1.5 x 10-5 M and 1.0 x 10-5 M (ESI, Figure S4)
for 600-sec duration of reaction (conversion < 5.0%); the
reaction was found to be almost first order with respect to
[FM]o where the slope was 1.06. This result suggests that the
In regard of these experiments in different solvents, it was
confirmed that absorbance of FM had a linear relation with
2 | J. Name., 2012, 00, 1-3
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concentration in the range of concentration discussed in this contained in air did not significantly affect the reaction (ESI,
work (ESI, Figures S5 and S6). This result strongly suggests that Figure S9). These results suggest that the reaction system
FM is molecularly dispersed without aggregate formation in all does not involve free-radical species.
solvents including H2O.
In order to shed light on the reaction mechanism, density
In addition, as far as the organic solvents are concerned, the function theory (DFT) calculations were conducted on the
rate constant was higher in a solvent in which the fluorescent transformation of FM (Figure 3). Some of the transition states
efficiency (Φ
) of FM was lower. This result may be in line with were obtained using the GRRM package.14 The black, purple,
an assumption that the reaction would occur through a triplet and red paths correspond to those in the S0, S1, and T1 states,
excited state similarly to many other photo chemical reactions. respectively. On photo excitation from the S0 state to the S1
A lower fluorescence quantum yield may be caused in part by state followed by structural relaxation, the conformation of
a higher efficiency of intersystem crossing leading to a higher FM slightly changes mainly through rotation around the single
chance of generating a triple-state active species (quenching bond connection the 9-poisiton carbon and the –CH2- group
by the reaction) though efficiency non-radiative deactivation (S1-min). FM then proceeds to the key transition state in the
to the ground state directly from singlet excited states may triplet state (T1-TS1) involving homolysis of the CH2-O bond
also contribute.
leading to an OH radical and an 9-fluorenylmethyl radical. The
Additionally, molar absorptivity of FM was rather smaller in OH radical appears to interact with the hydrogen at the 1-
actetonitrile and in H2O than in hexane and in MeOH (ESI, position of the fluorene backbone possibly through weak
Table S1) (hypchromicity) which may be suggestive of
π
-
hydrogen-bonding (T1-IM1, T1-IM2) before it interacts with the
stacking interactions between FM molecules in the ground hydrogen at the 9-position in the second transition state (T1-
state. However, the emission spectral shape of FM with the TS2) which leads to abstraction of the hydrogen leading to DBF
emission maximum wavelength of 304 nm was unchanged (T1-IM3). The methylene unit in DBF is perpendicular to the
without any new signals based on excimer formation on fluorene backbone in the triplet state (T1-IM3) and becomes
changing solvent (ESI, Figure S8). These results suggest that
π
stacking interactions in the ground state, if any, may be too energy intersystem crossing point (MEISCP).
-
coplanar when it is deactivated to the S0 state via minimum
weak to promote excimer interactions in excited states which
could affect the reaction rate.
The β-elimination is thus expected to occur through an E2
mechanism where radical species have a role. Although the
Further, the presence of 2,2,6,6-tetramethyl-4-pieridinol, a reaction in the presence of a radical scavenger did not affect
radical scavenger had little effect on the reaction in hexane the rate constant (run 2 in Table 1), life time of the proposed
(runs 1 and 2 in Table 1). In addition, the presence of oxygen OH and 9-fluorenylmethyl radicals may be so short in the
117.5
Ea = 14.0
S1
109.5
104.6
104.7
103.4
Ea =3.4
86.8
T1
84.9
83.4
81.9
T1
72.2
72.2
72.0
54.7
71.4
70.9
55.2
h
ν
Ea = 0.5
11.7
Product
5.1
10.8
Reactant
7.6
S0
S0
0.0
S0-min
T1-twisted
S1-min
T1-min
T1-TS1
T1-IM1
S0 min
T1-IM2
T1-TS2
T1-IM3
MEISCP
Photo
excitation
H-abstraction
by · OH
Intersystem
crossing
Homolysis
leading to ·OH
Rearrangement
of ·OH
Transition from S1 to T1
Figure 3. Proposed mechanism of the photo-induced β-elimination of 9-fluorenylmethanol leading to dibenzofulevene. Numbers
and Ea detnote relative potential energies and activation energy in kcal/mol, respectively. MEISCP stands for minimum energy
intersystem crossing point.
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3
4
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concerted E2 reaction that the two species cannot collide with
the scavenger or oxygen. The nature of the proposed radical
species may be different from that of general free-radical
species.
5
6
In connection to the proposed mechanism, the polar solvent
effects discussed earlier may be interpreted in terms of
mobility of the reactive species. The OH radical may be
interacted with MeOH or CH3CN or H2O through hydrogen-
bonding and/or dipole-dipole interactions, and therefore have
lower mobility in the polar solvents than in hexane leading to
less efficiency in the rearrangements (T1-IM1, T1-IM2) depicted
in Figure 3.
7
8
9
10 (a) T. Nakano, K. Takewaki, T. Yade and Y. Okamoto, J. Am.
Chem. Soc., 2001, 123, 9182-9183. (b) T. Nakano and T. Yade, J.
Am. Chem. Soc., 2003, 125, 15474-15484. (c) T. Nakano, O.
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Fukuda, T. Yamaguchi and S. Okumura, Macromolecules, 2005,
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Okamoto, Macromolecules, 2003, 36, 1433-1435. (f) T. Nakano,
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The mechanism predicted by the DFT calculations is in a
sharp contrast to the facts that fluorene derivatives15,16 and
benzylalcohol derivatives17 have been expected to form ion or
radical on the carbon atom at the benzylic position where the
active species can be stabilized by conjugation with the
aromatic system. Homolysis of the CH2-O bond of FM is hence
unexpected. As DFT methods have been used to successfully
interpret radical reactions,18,19 we believe that the predicted
radial mechanism can be trusted.
11 (a) R. M. O'Ferrall and S. J. Slae, Chem. Soc. D, 1969, 486-487.
(b) R. M. O'Ferrall and S. J. Slae, Chem. Soc. B, 1970, 260-268. (c)
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In summary, we disclosed the first photo-induced
β-
elimination reaction through an E2 mechanism leading to a
useful olefin compound. The observed effects of solvents and
additives and the results of theoretical calculations suggested
that the reaction occur through an E2 mechanism where
radical species may have a role. Though the scope of the
photo-induced
β-elimination reaction is still limited to the
conversion of 9-fluorenylmethanol so far, further studies
aiming to expand the scope of the substrate are currently
ongoing.
This work was supported in part by the MEXT program of
Integrated Research Consortium on Chemical Sciences. TN
acknowledges the Mitsubishi Foundation for partial financial
support. HN is indebted for the MEXT scholarship. A part of the
study was supported by JSPS KAKENHI Grant Number JP
15H05805 and by Research Center for Computational Science,
Okazaki, Japan.
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H. Uno, Y. Naruta and K. Maruyama, Tetrahedron, 1984, 40,
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