Explorations of the solubilizing effectiveness of CH3OCH2CH2O substituents in the photocyclizations of some 1,2-diarylethylenes to [n]phenacenes

Article abstract of DOI:10.1016/j.tetlet.2015.02.124

We have previously published many photocyclizations of 1,2-diarylethylenes to produce zig-zag aromatic ring systems with n = 5, 7, and 11 fused benzene rings. We invented the name [n]phenacenes for these compounds. To increase their solubilities we attach

Full text of DOI:10.1016/j.tetlet.2015.02.124

Tetrahedron Letters 56 (2015) 3342–3345
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Explorations of the solubilizing effectiveness of CH OCH CH O
3
2
2
substituents in the photocyclizations of some 1,2-diarylethylenes
to [n]phenacenes
⇑
Alyssa A. Bohen, Kimberly C. Mullane , Joseph M. Bohen, Clelia W. Mallory , Frank B. Mallory
Department of Chemistry, Bryn Mawr College, Bryn Mawr, PA 19010-2899, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
We have previously published many photocyclizations of 1,2-diarylethylenes to produce zig-zag
aromatic ring systems with n = 5, 7, and 11 fused benzene rings. We invented the name [n]phenacenes
for these compounds. To increase their solubilities we attached either various alkyl or phenyl
Received 1 December 2014
Revised 22 February 2015
Accepted 24 February 2015
Available online 28 February 2015
substituents. We present here our recent explorations of the effectiveness of CH
as solubilizing groups in three examples of diarylethylene photocyclizations that produced one
5]phenacene and two [7]phenacene systems.
3 2 2
OCH CH O substituents
[
Keywords:
Ó 2015 Elsevier Ltd. All rights reserved.
Photocyclizations
Diarylethylenes
Phenacenes
Solubilizing groups
In a series of earlier publications1a–d we have reported the
photocyclizations of a number of 1,2-diarylethylenes to produce
this way one can more easily adjust the solubility of the molecule
to make it either more soluble or less soluble.
[n]phenacenes having n = 5, 7, or 11 benzene rings fused in an
We report here our preliminary investigations to explore the
extended zig-zag pattern. In these previous studies we attached
several alkyl or phenyl substituents along the aromatic ring
skeletons of the diarylethylenes in order to provide adequate
solubilization for both the diarylethylenes and their [n]phenacene
photoproducts. These solubilizing substituents were very useful
for the recrystallizations as well as for the spectral analyses of
these compounds.
In our choice of effective solubilizing groups there had to be a
compromise that allowed sufficient solubility to permit further
functionalization and elongation as well as ease of purification
through recrystallization. Some earlier attempts using long-chain
tertiary alkyl substituents gave oils throughout the synthetic
schemes, and the resulting difficulties in purification contributed
to failures in the subsequent chemistry attempted. Unfortunately
the ability of the side chains to adequately solubilize large
3 2 2
solubilizing ability of pairs of CH OCH CH O substituent groups
(which we have abbreviated in Schemes 1–4 as RO groups) in the
photochemical syntheses of the three [n]phenacene compounds
1, 2, and 3 shown in Scheme 1. The syntheses of these three
compounds are presented in Schemes 2–4.
Schemes 2–4 indicate that modifications of the chemistry used
for the syntheses of the polyether-substituted phenacenes were
necessary as compared with the syntheses of the alkyl- and
1
phenyl-substituted phenacenes. One complication arose with
the free-radical bromination reactions using N-bromosuccinimide
in carbon tetrachloride because the competing electrophilic
aromatic brominations on the phenacene rings were sometimes
significant competing reactions. Conditions were optimized for
free-radical chemistry using an external light source and a
free-radical initiator, and this succeeded in eliminating (or at least
minimizing) the substitution pathways for some reactions (e.g.,
7 ? 8 and 18 ? 19). In other cases, especially in the [5]phenacene
and [7]phenacene systems, these competing substitution reactions
persisted which is likely a result of the fact that polyether substi-
tution on an aromatic ring results in a more electron-rich system
that will be more susceptible to ring bromination. However, there
was good improvement in several of these cases when the free-
[n]phenacenes is not known until well into the synthetic scheme.
However, an advantage of employing polyether groups rather than
2
alkyl side chains is that they can be exchanged with either longer
or shorter ether groups as needed along the synthetic pathway. In
⇑
Corresponding author. Tel.: +1 610 526 5105; fax: +1 610 526 5086.
E-mail address: fmallory@brynmawr.edu (F.B. Mallory).
radical bromination was carried out using
as solvent.
a,a,a-trifluorotoluene
3
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-
6
323, United States.
http://dx.doi.org/10.1016/j.tetlet.2015.02.124
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
0

A. A. Bohen et al. / Tetrahedron Letters 56 (2015) 3342–3345
3343
Scheme 1.
Scheme 3. Synthesis of [7]phenacene 2.
Scheme 2. Synthesis of [5]phenacene 1.
Further evidence of more reactive aromatic rings was observed
in compounds 7 and 18 in our unsuccessful attempts to transform
a bromo substituent on an aromatic ring bearing a polyether group
to an aldehyde using the standard butyllithium and DMF proce-
dure. In the syntheses described in Schemes 2 and 3 it was neces-
sary to use a two-step procedure where the aromatic bromo
compound was reacted first with cuprous cyanide and the result-
ing nitrile was then converted to the aldehyde by treatment with
DIBAL (e.g., 7 ? 9 ? 10 in Scheme 2, and 18 ? 20 ? 22 in
Scheme 3).
Scheme 4. Synthesis of [7]phenacene 3.
This work was begun with the assumption that the polyether
substituents would be more effective at solubilizing the phenace-
nes and their precursors than had been found for the alkyl- and

3
344
A. A. Bohen et al. / Tetrahedron Letters 56 (2015) 3342–3345
Table 1
Solubility measurements in benzene at room temperature
Compound
Sample amt
Benzene amt
mol/L
mg
mmol
g
mL
0.939
0.954
0.107
0.106
4.492
0.00711
d
d
2
2
2
2
6
6
7
7
1
1
2
2
3
3
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
0.0067
0.0067
0.0074
0.0074
0.0077
0.0077
0.0064
0.0064
0.0059
0.0059
0.8229
0.8366
0.0935
0.0926
3.9375
3.9785
1
0.0070
1
d
0.069
0.0701
d
0.0017
0.0017
4.539
44.9570
44.6863
17.0489
17.1450
51.2915
50.9826
19.4511
19.5608
0.00012
0.00013
0.00030
0.00030
Figure 1. Optimized geometry of compound 3, sideview.
phenyl-substituted compounds. A comparison of the solubilities of
A similar effect might be occurring with the polyether groups.
Unfortunately, we have not been successful in growing an X-ray
quality crystal of any polyether-substituted phenacene, but we
have carried out a calculation for compound 3 using Gaussian 09
1d
tert-butyl and phenyl substitution was reported earlier. As listed
in Table 1, in the two [5]phenacenes 26 and 27 shown above, the
phenyl substituents in compound 26 were less effective
solubilizing groups than were the tert-butyl substituents in
compound 27. This had not been anticipated and our intuition
failed us once again for the case of the polyether-substituted
4
Revision D.01 with the B3LYP hybrid DFT method and the
⁄
5
6-31G basis set. This calculation confirms our suggestion that
the linear nature of the polyether chains should not interfere with
the close and effective packing of the phenacene rings in the crystal
lattice as do the tert-butyl groups. This is illustrated dramatically in
the sideview representation of the compound 3 molecule shown
above in Figure 1 that emerges from the calculation.
[
5]phenacene 1 which we found to be about 4 times less soluble
in benzene than its tert-butyl counterpart! (The comparison is
not exact because the tert-butyl- and phenyl-substituted
compounds 26 and 27 each have three solubilizing groups
attached to the [5]phenacene backbone whereas the polyether
derivative 1 has only two solubilizing groups.)
Two direct solubility comparisons were possible with the poly-
ether-substituted [5]phenacene and [7]phenacenes as shown in
Table 1. First, in comparing the mol/L solubilities of the two
A logical extension of the current work could be to use either
longer polyether groups as solubilizing substituents or more of
them to achieve the appropriate solubility for the synthesis of
longer [n]phenacene derivatives.
[
7]phenacenes 2 and 3, another solubility pattern emerges.
Acknowledgements
Compound 3, the symmetrically substituted [7]phenacene, is
slightly more than twice as soluble in benzene at room tem-
perature than the unsymmetrically substituted [7]phenacene 2.
Second, both the [5]phenacene 1 and the [7]phenacene 2 have
unsymmetric substitution patterns and two polyether solubilizing
groups. Solubility measurements on 1 and 2 showed that the addi-
tion of the two extra aromatic rings in the [7]phenacene 2
decreased its solubility in benzene by more than a factor of ten
as compared with that of the [5]phenacene 1. This behavior may
help explain why subsequent chemistry that was attempted to
elongate compound 1 to produce an [11]phenacene was unsuc-
cessful as low solubility became a major factor.
Acknowledgement is made to the donors of the Petroleum
Research Fund, administered by the American Chemical Society,
for partial support of this research through Grant 41685-AC1,
and to Bryn Mawr College for other research support. The authors
also acknowledge the generous support of the National Science
Foundation for
a
Major Research Instrumentation Award
(
CHE-09589960) to Bryn Mawr College that enabled the purchase
of the NMR spectrometer used in these studies.
Supplementary data
It is believed that crystal packing of the phenacene molecules in
the solid state contributes to their overall solubility. The less-
soluble phenyl-substituted phenacenes were found by X-ray
crystallography to pack much more closely in the crystal lattice
Supplementary data (experimental procedures, characteriza-
tions and spectroscopic data) associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.
2015.02.124.
1
d
than did their more soluble tert-butyl-substituted counterparts.

A. A. Bohen et al. / Tetrahedron Letters 56 (2015) 3342–3345
3345
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