Photochemical generation of 9-Fluorenyl radicals

Article abstract of DOI:10.1002/poc.3920

A series of 9H-fluorenols and 9H, 9′H-bifluorenyls were irradiated in less polar solvents giving photoproducts derived from their corresponding 9H-fluorenyl radicals. These transient species were directly observed by laser flash photolysis and their UV/visible spectra compared with those of their corresponding cations. Theoretical calculations (density functional theory [DFT]) of these intermediates indicate their destabilizing nature in similar fashion to the antiaromatic character of the corresponding cations.

Full text of DOI:10.1002/poc.3920

Received: 27 August 2018
DOI: 10.1002/poc.3920
Revised: 29 October 2018
Accepted: 30 October 2018
R E S E A R C H A R T I C L E
Photochemical generation of 9‐Fluorenyl radicals
2
2
1,3
1,4
1
Ian R. Duffy | William J. Leigh | Hanan Afifi | Abdelaziz Ebead | René Fournier |
1
Edward Lee‐Ruff
1
Department of Chemistry, York
Abstract
University, Toronto, Canada
2
A series of 9H‐fluorenols and 9H, 9′H‐bifluorenyls were irradiated in less polar
solvents giving photoproducts derived from their corresponding 9H‐fluorenyl
radicals. These transient species were directly observed by laser flash photolysis
and their UV/visible spectra compared with those of their corresponding cat-
ions. Theoretical calculations (density functional theory [DFT]) of these inter-
mediates indicate their destabilizing nature in similar fashion to the
antiaromatic character of the corresponding cations.
Department of Chemistry, McMaster
University, Hamilton, Canada
3
Faculty of Science, Beni‐Suef University,
Beni‐Suef, Egypt
4
Faculty of Science, Arish University,
Arish, Egypt
Correspondence
Edward Lee‐Ruff, Department of
Chemistry, York University, Toronto,
Ontario, Canada, M3J 1P3.
Email: leeruff@yorku.ca
KEYWORDS
fluorenols, bifluorenyls, fluorenyl radicals, laser flash photolysis, fluorenyl cations, computational
studies, aromatic stabilization, resonance stabilization energy
1
| INTRODUCTION
Our interests in carbocations incorporated into
destabilizing electronic environments such as bonding to
Ever since the first characterization of a persistent carbon‐
electron‐deficient moieties or as part of an antiaromatic
[1]
[7]
centered radical in 1900, the importance of these ubiq-
cyclic configuration, such as cyclopentadienyl,
[2]
[2]
[8]
uitous species in atmospheric chemistry, combustion,
prompted the current studies of their radical analogues.
[3]
[4]
polymerization, and biology has been well recog-
nized. The inherent uncharged nature of these species
renders them susceptible to bimolecular reactions such
as hydrogen atom abstraction and additions to multiple
bonds and C‐C bond formation such as dimerization.
Their open‐shell electronic configuration also make these
prone to reactions with ground state (triplet) molecular
oxygen with formation of peroxides and subsequent
autoxidation accounting for their transient nature under
aerobic conditions. In spite of their transitory nature,
these species have been utilized in organic synthesis
The 9‐fluorenyl cation can be formally regarded as an
antiaromatic carbocation possessing the 12π cyclic
electronic configuration. The 9‐substituted carbocations
can be generated by photolysis of their alcohol precursors
in highly solvating polar medium. For example,
irradiation of 9H‐fluorenols in polar solvating solvents
such as 1,1,1,3,3,3‐hexafluoroisopropanol (HFIP) or
1,1,1‐trifluoroethanol (TFE) results in formation of the cor-
responding carbocation intermediates. By contrast, pho-
tolysis of these substrates in less polar solvents gives
products which are formally derived from their radical
intermediates (Scheme 1). One of the principal products
obtained is the corresponding 9H, 9′H‐bifluorenyl formally
derived from coupling of the radical intermediate. We
observed that these dimers are themselves photolabile
and reversibly form the corresponding 9‐fluorenyl radicals
under the irradiation conditions. To our knowledge, no
[
7]
[
5]
through their selective C‐C bond forming reactions.
The electron‐deficient nature of these open shell interme-
diates permits facile additions to alkenes and alkynes
5c
resulting in C‐C bond formation. Furthermore, substitu-
ent effects on the relative stabilizations of these neutral
[
6]
species parallel those for their carbocation analogues.
J Phys Org Chem. 2018;e3920.
wileyonlinelibrary.com/journal/poc
© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/poc.3920

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DUFFY ET AL.
2
.3 | Irradiation of 9‐cyano‐9,9′bifluorenyl
(3e)
A solution of 9,9′‐bifluorenyl 3e (50 mg, 0.14 mmol) in
methanol (20 mL) was irradiated for 15 minutes. Evapo-
ration of the solvent under vacuum followed by
chromatographic purification (7% ethyl acetate‐hexane)
yielded 6 mg (13%) of 9H‐fluorene 2a, 3 mg (6%) of
fluoren‐9‐one,
bifluorenyl (3d), 8 mg (17.4%) of 9,9′‐bifluorenyl 3a, and
mg (8%) of unreacted starting material 3e. Characteriza-
3
mg (5.7%) of 9,9′‐dicyano‐9,9′‐
4
tion of the photoproducts was based on comparison with
authentic samples.
SCHEME 1 Irradiation of Cyanobifluorenyls 3d and 3e
systematic study of the photochemistry of 9,9′‐bifluorenyls
as a source of these radicals has been reported.
2.4 | Irradiation of 9‐cyanofluorene (2d)
A solution of 9‐cyanofluorene 2d (35 mg, 0.183 mmol) in
methanol (25 mL) was irradiated for 1 hour. Evaporation
of the solvent under vacuum followed by chromato-
graphic purification (7% ethyl acetate‐hexane) yielded
4 mg (13.3%) of 9H‐fluorene 2a, 3 mg (9.1%) of fluoren‐
2
2
| EXPERIMENTAL
.1 | Apparatus and reagents
9‐one, 4 mg (13.3%) of 9,9′‐bifluorenyl 3a, 2 mg (5.5%)
All reactions were carried out in dried glassware. Proton
of 9‐methoxyfluorene, and 13 mg (37.1%) of recovered 2d.
spectra were obtained on Bruker ARX‐400, ARX‐300, or
ARX‐600 spectrometers. Solutions in 99.8% CDCl with
3
1
% tetramethylsilane (TMS) as internal standard were
used. Photolyses were performed using a Hanovia
50 W medium pressure mercury arc lamp in a quartz
2
.5 | Laser flash photolysis
4
Excitation wavelength for these experiments is 248 nm. A
KrF excimer laser (248 nm, ~25 ns, 94‐102 mJ/pulse) was
used and deoxygenated solutions of methanol or hexanes
as solvent were analyzed. Transient spectra were
immersion well. All solutions were purged with argon
prior to irradiation. Melting points of all compounds
were determined on a Reichert melting point apparatus
and are uncorrected. 9‐Trifluoromethylfluorenol, 9,9′‐
bifluorenyl,
[
9]
[13]
recorded using a flow system.
[
10]
[8]
9,9′‐bistrifluoromethyl‐9,9′‐bifluorenyl,
[11]
9
,9′‐dicyano‐9,9′bifluorenyl,
and
9‐cyano‐9,9′
[12]
bifluorenyl
were prepared by literature procedures.
2.6 | Computational studies
Geometries were optimized, and energies were calcu-
lated, using density functional theory (DFT) with the
B3LYP hybrid exchange‐correlation functional and a
“cc‐pVTZ” basis set (correlation‐consistent triple zeta
plus polarization). B3LYP is a good general purpose func-
tional, and by far the most popular for calculations of
equilibrium geometries and energies of molecules of
2
9
.2 | Irradiation of
,9′‐dicyano‐9,9′bifluorenyl (3d)
A
solution containing 40 mg (0.105 mmol) of
dicyanobifluorenyl 3d in 80 mL of methanol was irradi-
ated for 2 hours. Removal of the solvent under vacuum
followed by chromatographic purification (7% ethyl
acetate‐hexane) yielded 4 mg (11.5%) of 9H‐fluorene 2a,
[
14]
main group elements.
The cc‐pVTZ is a standard dif-
fuse basis set that gives medium to high accuracy on
[
15]
valence excitations.
Vibrational frequencies were cal-
3
9
4
mg (7.9%) of fluoren‐9‐one,
‐cyanofluorene 2d, 3 mg (7.3%) of 9‐methoxyfluorene,
mg (11.5%) of 9,9′‐bifluorenyl 3a, and 5 mg (12.5%) of
8
mg (20%) of
culated for each minimum energy structure to verify that
the structures found were indeed equilibrium geometries
and to get zero‐point energies (ZPEs). The bond dissocia-
tion energies, BDEs, were calculated by taking differences
of free energies, which include the ZPE and thermal cor-
rections calculated at room temperature. Excitation
unreacted 3d. The characterization of each product was
based on comparison of physical and spectral data with
those of authentic samples.

DUFFY ET AL.
3 of 6
energies were calculated in vacuum, and in methanol
using the polarizable continuum model (PCM).
derived from 9‐cyanofluorenyl radical leading to the
reduced 9‐cyanofluorene(2d)or coupling tothebifluorenyl
[16]
3d as is evident from the photolysis of the unsymmetrical
compound 3e (Scheme 1). The latter substrate appears to
dissociate into both the 9‐cyanofluorenyl and fluorenyl
radicals as evident from formation of both bifluorenyls 3d
and 3a. The intermediacy of both radicals is shown from
the transient spectra obtained from flash photolysis of
3
3
| RESULTS AND DISCUSSION
.1 | Photoproducts from 9‐Fluorenols
and 9,9′‐Bifluorenyls in methanol
3
e (vide infra). Furthermore, 9‐cyanofluorene was found
[
8]
We reported recently on the photochemistry of a series
of 9H‐fluorenols 1a‐c giving the reduced fluorenes 2a‐c
and 9,9′‐bifluorenyls 3a‐c as principal products. Evidence
for the intermediacy of 9‐fluorenyl radicals was the trap-
ping of the transient with TEMPO in the case of the
to be photolabile, and independent experiments have
shown that it is transformed to fluorene and bifluorenyl
under the irradiation conditions. Small amounts of fluore-
noneand9‐methoxyfluorene werealsoobservedandattrib-
uted to incomplete deoxygenation purging, and solvent
nucleophilic quenching of the carbocation formed in
competition.
[8]
parent 9H‐fluorenol (1a). The 9,9′‐bifluorenyls (3a‐c)
undergo photoexcitation to give the reduced fluorenes
in hydrogen‐donating solvents including methanol, indic-
ative of homolysis of the benzylic C‐C bond. In one
instance, the photolysis of 3c led to isomerization to the
bifluorenyl 4c in alcohols and other polar solvents. In
order to probe the mechanism of this unprecedented
transformation, these studies were extended to the
electron‐withdrawing cyano substituted bifluorenyl 3d.
Furthermore, the presence and nature of the fluorenyl
radical intermediates was probed by laser flash photolysis
of 9‐fluorenols 1 and 9,9′‐bifluorenyls 3. The relative
stabilities of these intermediates were assessed using the-
oretical calculations and compared with their correspond-
ing antiaromatic carbocations.
3
.3 | Time resolved spectroscopy
Laser flash excitation at 248 nm of fluorenols 1a and 1c,
and bifluorenyls 3a, 3c, 3d, and 3e generated transients
which exhibit UV/visible spectra characterized by bands
in the 500‐540 nm wavelength region (Figure 1). These
transients decay by second order kinetics with rate coef-
ficients 2 k/ελmax~0.3‐2.1 × 10 cm s in deoxygenated
solutions. The lifetimes decrease with increasing
oxygen atmosphere indicative of radical species. The
similarity of the transient spectra generated from
fluorenols 1a and 1c with the corresponding bifluorenyls
7
‐1
3a and 3c, respectively, suggests that these transients are
associated with the fluorenyl radicals derived from C‐O
and C‐C bond homolysis, respectively. The radicals
exhibit peaks that are similar to their corresponding
[
7]
carbocations except that the latter peaks are slightly
red‐shifted (15 nm for R = H, 20 nm for R = CF3).
The peaks observed below 400 nm decay more slowly
and are likely associated with the triplet state of the
substrates.
The transient spectrum obtained from irradiation of
3e shows two species in the low energy region as evi-
dent from the overlapping peaks at 500‐540 nm with
the slower decaying transient at 540 nm corresponding
to the 9‐cyanofluorenyl radical. This value is consistent
[17]
with that reported for the 9‐cyanofluorenyl radical.
The bathochromic shift seen for the 9‐substituted
fluorenyl radicals bearing electron‐withdrawing groups,
3.2 | Photoproduct distribution from
CF and CN, is corroborated by theoretical calculations
3
continuous irradiation of cyano substituted
bifluorenyls
(Table 1) even though the absolute values are in
discrepancy due to nonaccountability of solvation effects
in the theoretical model. The bathochromic shifts are
associated with the greater spin density delocalization
for the radicals and charge delocalization for the
Continuous irradiation of methanolic solutions of
bifluorenyls 3d and 3e gives rise to products formally

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DUFFY ET AL.
FIGURE 1 LFP data (248 nm excitation in argon purged methanol solutions)
TABLE 1 Theoretical and experimental low energy transition
3
.4 | Kinetic and thermodynamic
absorptions for Fluorenyl radicals
stabilities of 9‐Fluorenyls
λ
simulated in
λ
exp. obs.
λ
simulated in
R
vacuum (nm)
(nm)
MeOH (nm)
As can be seen from the second order rate constants for
H
450
430
485
490
500
‐
451
425
495
515
the transient decay profiles, the parent 9‐fluorenyl
decays faster (1.6‐2.1 × 10 cm s ) than the 9‐
7
‐1
CH
3
substituted CN and CF analogues by a factor of 4 to 5
3
CF
3
530
540
(
Figure 1), assuming that the extinction coefficients for
CN
these compounds are the same. This observation is con-
[
17]
sistent with the data reported by Scaiano
for the rel-
ative reactivity of 9‐fluorenyl and 9‐cyano‐9‐fluorenyl
radicals toward oxygen quenching, which is rationalized
in terms of electronic factors associated with spin delo-
calization of the unpaired electron into CN. The case
carbocations with electron‐withdrawing substituents.
Theoretical calculations to include methanol solvation
effects also show bathochromic shifts closer to the exper-
imental values for the fluorenyl radicals bearing
for the lower reactivity of 9‐CF substituted fluorenyl
3
electron‐withdrawing groups, CF and CN.
compared with parent fluorenyl radical may be due to
3

DUFFY ET AL.
5 of 6
SCHEME 2 Gibbs free energy
difference (ΔG) of 9‐substituted Fluorenyls
TABLE 2 Free energy difference of Fluorenyl C‐H bond dissoci-
ation and for the reaction in Scheme 2 (kcal Mol )
4
| CONCLUSIONS
‐1
In summary, photoexcitation of 9,9′‐bifluorenyls gener-
ates the corresponding fluorenyl radicals in methanol
solutions as evident from the laser flash photolysis of
both bifluorenyls and the corresponding fluorenols.
These transients show peaks in the region λ 500‐540 nm
and decay by second order kinetics. Their spectral
features are similar to the corresponding carbocations,
with the latter showing red shifts of 15‐20 nm. These rad-
icals can undergo hydrogen abstraction from solvent to
produce the reduced 9‐substituted fluorenes, or dimerize
to the bifluorenyls as was evident from the photolysis of
monosubstituted bifluorenyl 3e, which results in both
fluorenyl and 9‐cyanofluorenyl radicals. Computed reso-
nance stabilization energies indicate an inherent destabi-
lization of fluorenyl radicals except for the 9‐CF3
analogue, which is rationalized in terms of delocalization
of spin density into the planar benzene rings. This
also accounts for the formation of the unusual dimeriza-
tion to the unsymmetrical bifluorenyl 4c. Whereas
the fluorenyl radicals are destabilized (except for
9‐trifluoromethylfluorenyl) relative to their nonaromatic
analogues, their kinetic stabilities increase with substitu-
tion at C‐9 due to stereoelectronic factors.
9
H‐Fluorene
9,10‐Dihydroanthracene
(ΔG)
R
(ΔG)
(ΔGrxn)
H
76.4
72.1
77.4
67.0
71.2
72.0
78.2
63.7
‐5.2
‐0.1
CH
3
CF
3
+1.2
‐3.3
CN
steric factors. The relative thermodynamic stabilities of
‐substituted fluorenyl radicals may be computed by
comparison of the C‐H bond dissociation energies of
the C(9)‐H bond of fluorene with that of the benzylic
C‐H bond of the nonaromatic dihydroanthracene
according to Scheme 2. The computed values are tabu-
lated in Table 2.
As can be seen, the fluorenyls are all destabilized
relative to the nonaromatic model except for 9‐CF . Of
note is the nonplanarity of the comparative model system
dihydroanthracene) relative to fluorenyl, which dimin-
9
3
(
ishes the extent of electron delocalization into the
benzene ring of the former. The relative free energy dif-
ference values point to an inherent destabilization of
fluorenyls. In regard to the 9‐CF substituted fluorenyl,
3
ORCID
the strong electron‐withdrawing substituent may induce
greater spin delocalization into the conjugated benzene
rings, which can be better accommodated in the more
planar fluorene ring than the model system. This observa-
tion can also account for the unusual formation of 4c
Edward Lee‐Ruff https://orcid.org/0000-0003-3631-1081
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