Attempted characterisation of phenanthrene-4,5-quinone and electrochemical synthesis of violanthrone-16,17-quinone. How does the stability of bay quinones correlate with structural and electronic parameters?

Article abstract of DOI:10.1039/d0ra06519f

In bay quinones, two carbonyl moieties are forced into close proximity by their spatial arrangement, resulting in an interesting axially chiral and nonplanar structure. Two representatives of this little-explored class of compounds were investigated experimentally in this work. Electrochemical oxidation of 4,5-dihydroxyphenanthrene failed to provide evidence for the reversible formation of phenanthrene-4,5-quinone. Even at temperatures as low as T = 229 K, cyclic voltammograms did not show any evidence for reversibility, indicating that phenanthrene-4,5-quinone likely is a reactive intermediate even at low temperatures. Electrochemical oxidation of the larger homologue 16,17-dihydroxyviolanthrone, on the other hand, was reversible, and the quinone could be characterised by spectroelectrochemical means. The results of quantum chemical calculations confirm the experimental findings and indicate that a bay dicarbonyl moiety, also found in a number of angucycline antibiotics, does not necessarily have to confer extreme reactivity. However, in a series of phenanthrene quinones with an equal number (zero) of Clar sextets and a varying number of bay carbonyl groups (zero to two), there was a clear correlation between the triplet energy, taken as a measure of biradical character, and the number of bay carbonyl moieties, with the lowest triplet energy predicted for phenanthrene-4,5-quinone (two bay carbonyl moieties).

Full text of DOI:10.1039/d0ra06519f

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Attempted characterisation of phenanthrene-4,5-
quinone and electrochemical synthesis of
violanthrone-16,17-quinone. How does the stability
of bay quinones correlate with structural and
electronic parameters?†
Cite this: RSC Adv., 2020, 10, 38004
*
Dylan Wilkinson, Giacomo Cioncoloni, Mark D. Symes
and Gotz Bucher
¨
In bay quinones, two carbonyl moieties are forced into close proximity by their spatial arrangement, resulting in an
interesting axially chiral and nonplanar structure. Two representatives of this little-explored class of compounds were
investigated experimentally in this work. Electrochemical oxidation of 4,5-dihydroxyphenanthrene failed to provide
evidence for the reversible formation of phenanthrene-4,5-quinone. Even at temperatures as low as T ¼ 229 K,
cyclic voltammograms did not show any evidence for reversibility, indicating that phenanthrene-4,5-quinone
likely is a reactive intermediate even at low temperatures. Electrochemical oxidation of the larger homologue
16,17-dihydroxyviolanthrone, on the other hand, was reversible, and the quinone could be characterised by
spectroelectrochemical means. The results of quantum chemical calculations confirm the experimental findings
and indicate that a bay dicarbonyl moiety, also found in a number of angucycline antibiotics, does not necessarily
have to confer extreme reactivity. However, in a series of phenanthrene quinones with an equal number (zero) of
Clar sextets and a varying number of bay carbonyl groups (zero to two), there was a clear correlation between
the triplet energy, taken as a measure of biradical character, and the number of bay carbonyl moieties, with the
lowest triplet energy predicted for phenanthrene-4,5-quinone (two bay carbonyl moieties).
Received 27th July 2020
Accepted 24th September 2020
DOI: 10.1039/d0ra06519f
rsc.li/rsc-advances
Bay quinones are a little explored sub-class of quinones. The
close spatial interaction of the two oxygen atoms forces the
Introduction
system out of planarity, resulting in reduced p-overlap and
strain. The synthesis of the simplest bay quinone,
phenanthrene-4,5-quinone 4, had been attempted several
times, but without success. Oxidation of 4,5-dihydroxyphenan-
threne 3 with a range of oxidants, such as PbO2 or Fremy's salt,
merely yielded either a derivative of phenanthrene-1,4-quinone
or intractable tars (Scheme 1).5 Adding methyl groups to the
dihydroxyphenanthrene did not improve matters.6 Somewhat
more successful was the attempted synthesis of a t-butylated
derivative 6, where oxidation of the dihydroxyphenanthrene
precursor 5 instead yielded the rearranged product 7. The latter
could then be de-t-butylated to yield the natural product mor-
phenol 8. During the oxidation of 5, the authors noted the
appearance of a green intermediate, but could not characterise
this intermediate by NMR spectroscopy.7
Quinones are a class of mildly oxidising compounds, characterized
by the presence of a six-membered ring or a sequence of annelated
six-membered rings, with two oxygen atoms bound as carbonyl
groups. The simplest quinones are p-benzoquinone 1 and o-ben-
zoquinone 2, where the p-quinoid system in 1 is far more stable
than the o-quinoid system in 2. The reactivity of quinones, e.g., in
their practical use as dehydrogenating agents, is explained in terms
of biradicaloid resonance structures such as 1b or 2b. Over the
years, many quinones have been synthesized, and this mature eld
of research has been reviewed authoritatively.1–4
The bay quinone or -dicarbonyl substructure is not limited to
reactive species such as phenanthrene-4,5-dione. Among
angucycline8 antibiotics, there are several examples of
compounds showing two carbonyl groups in a bay arrangement,
such as tetrangomycin9 or oviedomycin.10 A bay quinone
subunit has also been reported to be formed upon photooxi-
dation of alkynyl-perylenes.11 The current work intends to shed
light on the reactivity of bay quinones, attempting the
School of Chemistry, University of Glasgow, University Avenue, Glasgow G12 8QQ, UK.
E-mail: goetz.bucher@glasgow.ac.uk
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06519f
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characterisation of parent phenanthrene-4,5-quinone 4 by
spectroelectrochemical measurements at low temperature. We
will also investigate the larger system 16,17-dihydroxyviolan-
throne 9/violanthrone-16,17-quinone 10 via cyclic voltammetry
and spectroelectrochemical measurements. Bay quinone 10 is
an intermediate in the industrial synthesis of vat dye 9,12–14 and
therefore its electrochemistry is of more than academic interest
(Scheme 2).
Results and discussion
Electrochemistry
The cyclic voltammogram of 16,17-dihydroxyviolanthrone 9 in
dry dimethylformamide (containing 100 mM TBAPF6) is shown
in Fig. 1 (black trace). There are two fairly reversible reduction
waves (peak separation ꢀ70 mV) at ꢁ1.4 V and ꢁ1.6 V (vs.
ferrocenium/ferrocene) possibly corresponding to the reduction
of the two carbonyl moieties. There also is a quasi-reversible
oxidation event whose mid-point lies near ꢁ0.4 V, but whose
oxidative and reductive processes are rather widely separated
(by about 550 mV).
In addition to the experimental work, we also present results of
DFT calculations on a series of bay quinones, shedding light on
structure–reactivity relationships in this class of compounds.
Scheme 1 Published attempts at synthesising phenanthrene-4,5-quinone 4 and derivatives.
Scheme 2 Oxidation of 16,17-dihydroxyviolanthrone 9.
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was passed (see Fig. S3b, ESI†) and the original green colour was
restored. Hence the electrochemically quasi-reversible wave
centred at around ꢁ0.4 V appears to be a two-electron process
whereby reduction of the oxidised species restores the original
molecule. This supposition is supported by UV-Vis analysis of
the original, oxidised and re-reduced forms (as shown in Fig. 2
and S4†), which conrms that all the salient features of the
fresh 16,17-dihydroxyviolanthrone are restored upon re-
reduction.
In contrast to the fairly well-dened behaviour of 16,17-
dihydroxyviolanthrone, the simpler analogue 4,5-dihydrox-
yphenanthrene 3 does not show any similar reversible electro-
chemistry over the potential range shown in Fig. 1 (red trace),
even at a low temperature of T ¼ ꢁ44 ꢂC. Indeed, on this basis,
there is no evidence even for the formation of phenanthrene-
4,5-quinone 4, let alone its re-reduction to 4,5-dihydrox-
yphenanthrene 3.
Fig. 1 Cyclic voltammetry in dimethylformamide/0.1 M TBAPF6 at
room temperature and a scan-rate of 100 mV sꢁ1. See Experimental
section for further experimental details. Black trace: 3 mM 16,17-
dihydroxyviolanthrone, red trace: 15 mM 4,5-dihydroxyphenanthrene.
A scan-rate dependency study on this quasi-reversible wave
over the range 10–1000 mV sꢁ1 suggests that the broad reduc-
tion peak may in fact contain two waves whose relative ratio
changes with scan rate (Fig. S1, see ESI†). This is borne out by
a cyclic voltammogram at low temperature (ꢁ44 ꢂC), which
clearly shows that this reductive feature has two peaks (see
Fig. S2, ESI†). However, even at scan rates as high as 1000 mV
sꢁ1, this wave remains only electrochemically quasi-reversible at
room temperature, with very little reduction in the peak-to-peak
separation. Hence it seems that the electron transfer kinetics
for this process are slow, with some evidence for rapid non-
electrochemical re-organisation process(es) occurring aer the
oxidation event.
Bulk electrolysis was conducted on the oxidation event
occurring at ꢁ0.15 V by applying a potential of 0 V (vs.
ferrocenium/ferrocene) as shown in Fig. S3a (ESI†). This
showed that the oxidative wave was a two-electron process, with
the bulk electrolysis consuming around 90% of the charge ex-
pected on the basis of an oxidation by 2 electrons per molecule.
This oxidation was accompanied by a colour change from dark
green to dark yellow (see Fig. 2). Upon reduction of this oxidised
sample at ꢁ1 V, charge consistent with a two-electron reduction
Results of calculations
In order to understand the very high reactivity and short-lived
nature of phenanthrene-4,5-quinone 4, we used density func-
tional theory (M06-2X/cc-pVTZ) to explore its potential energy
hypersurface. The results are shown in Fig. 3.
The results shown in Fig. 3 indicate that the intramolecular
cyclization of 4, yielding ketone 12 (which would then tauto-
merise to 8) via oxirane 11, would suffer from signicant
barriers. The calculated triplet free energy of 4, on the other
hand, is extremely small with DGT ¼ 2.1 kcal molꢁ1. This value
indicates that the quinone should have signicant biradical-
type reactivity. Thus, our experimental failure in detecting 4
via electrochemical means, even at low temperatures, does not
come as surprise. If the intermolecular reactivity of
phenanthrene-4,5-quinone is impeded by steric protection, as
in 6, however, the sequence of cyclization reactions shown in
Scheme 1 should be feasible, in agreement with the ndings by
Hewgill and Stewart (see Scheme 1).7 A calculation of the UV-Vis
spectrum of
4 (TD-B3LYP/6-311++G(d,p)//M06-2X/cc-pVTZ)
gives a longest-wavelength absorption maximum of l ¼
696 nm, corresponding to a deep green colour (for the full
spectrum, see Fig. S5, ESI†). Hence, the temporary observation
of a green colour during the oxidation of 5 can reasonably be
attributed to the formation of 6, while their failure to detect the
species by NMR spectroscopy probably is due to the lowest
triplet excited state of 6 being so low in energy that it is ther-
mally populated, which would signicantly broaden its NMR
bands.7 As a nal note, the peroxide 13, which is a valence
tautomer of 4, is predicted to be a minimum structure, albeit
higher in energy than 4. Unlike the severely twisted singlet
ground and rst triplet excited state of quinone 4, the rst
triplet excited state of 13 is calculated to be planar (point group
C2v). The barrier for interconversion of 4 and 13 on the singlet
spin manifold is signicant. On the triplet spin manifold, we
were unable to locate a transition state, presumably because the
symmetry-breaking reaction 313 / 34 has to go via a valley-
ridge inection point.
Fig. 2 Comparison between the electronic spectra of fresh 16,17-
dihydroxyviolanthrone (black) and the same sample after electro-
chemical 2-electron oxidation to violanthrone-16,17-quinone (red).
The samples were diluted to approximately 20 mM in order to collect
these spectra.
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Fig. 3 Free energies (in kcal molꢁ1) of relevant stationary points on the potential energy hypersurface of 4 (M06-2X/cc-pVTZ). All stationary
points are on the singlet spin manifold, unless indicated otherwise.
Calculations (M06-2X/cc-pVTZ) on the potential energy singlet excited state being higher in free energy by DGS
hypersurface of violanthrone-16,17-quinone 10 gave the results 6.5 kcal molꢁ1, in spite of being of closed shell nature.
¼
shown in Fig. 4.
We calculated (TD-B3LYP/6-31+G(d)/M06-2X/cc-pVDZ) the
The results shown in Fig. 4 show that a cyclization sequence UV-Vis spectra of diol 9 and bay quinone 10. The results, shown
10 / 14 should not be viable in this system, as it is both in Fig. 5, show that the UV-Vis spectrum of precursor 9 is
endothermic and inhibited by a very large barrier. Unlike reasonably well reproduced by the DFT calculation. The UV-Vis
quinone 4, 10 does not have an unusually small triplet free spectrum of bay quinone 10 is calculated to show absorption in
energy, as a value of DGT ¼ 35.5 kcal molꢁ1 is rather typical of the range 300–500 nm (where 9 is calculated to have only a weak
large annelated aromatic hydrocarbons. Like in the system 4/13, band), which is fully consistent with the experimental spectrum
a valence tautomerism exists with peroxide 15. Remarkably, this show in Fig. 2, the only difference being that the ne structure
peroxide is predicted to have a triplet ground state, with the rst of the 300–500 nm band system of 10 is not resolved in the
experimental spectrum.
Fig. 4 Free energies (in kcal molꢁ1) of relevant stationary points on the potential energy hypersurface of 10 (M06-2X/cc-pVTZ). All stationary
points are on the singlet spin manifold, unless indicated otherwise.
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all compounds, the triplet energy, as a measure of diradical
character, was calculated (Fig. 6).
Some angucycline antibiotics, like oviedomycin 24 or tet-
rangomycin 25 show a bay quinone (24) or bay dicarbonyl (25)
moiety. In order to assess whether this particular arrangement
might inuence the properties of 24 and 25, we performed DFT
calculations (M06-2X/cc-pVTZ) on 24 and 25, along with
hydroxyanthraquinone 26 as a model compound, in their
singlet ground and lowest triplet excited states. The results are
shown in Fig. 7. Table 1 summarises the results of all
calculations.
Taken together, the results presented in Fig. 3, 4, 6 and 7
indicate that aromatic stabilisation likely is the crucial factor in
determining the stability and biradical character of the
quinones and cyclic peroxides investigated. Triplet energies and
therefore biradical character correlate with the number of Clar
sextets available.15 E.g., in the case of 4/10, with zero Clar sextets
for 4, and two for 10, the free energy of the peroxide tautomer
(which, with its unfavourable O–O bond would normally be very
high in energy), is only slightly above the free energy of the
Fig. 5 Black: calculated UV-Vis spectrum (TD-B3LYP/6-31+G(d)//
M06-2X/cc-pVDZ) of diol 9. Red: calculated UV-Vis spectrum (TD-
B3LYP/6-31+G(d)//M06-2X/cc-pVDZ) of bay quinone 10.
Taken together, the results presented in Fig. 3 and 4 show quinone. A similar observation is made for 19/22. In both cases,
that a bay quinone moiety, such as in 4 or 10, may result in the quinone (4 or 19) also has a very low triplet free energy,
a biradicaloid system with a very small triplet energy, but it does indicating a large degree of biradical character. If, on the other
not necessarily have to. What about bay dicarbonyl moieties in hand, the quinone has an unambiguous Clar sextet, as in 18, the
general? Which electronic factors would favour a pronounced triplet energy calculated is signicant. In this case, the peroxide
biradical character in bay quinones? In order to assess these tautomer 21 (which does not have any Clar sextets) is calculated
questions, we performed further DFT calculations on to be a minimum structure only on the triplet potential energy
phenanthrene-2,7-quinone 16, phenanthrene-2,5-quinone 17, hypersurface. A geometry optimisation on the singlet PES
phenanthrene-1,4,5,8-diquinone 18, phenanthrene-4,5,9,10- resulted in optimisation of quinone 18.16 Considering quinone
diquinone 19, and the reduced violanthrone derivative 20. For 20/peroxide 23, it is the peroxide tautomer 23 that contains the
18–20, the isomeric peroxides 21–23 were also investigated. For larger number of Clar sextets (20 : 2; 23 : 4). Consistent with
Fig. 6 Free energies (M06-2X/cc-pVTZ, in kcal molꢁ1) of lowest singlet and triplet states of 16–23. The reference points are the lowest singlet
states of the quinone tautomers 16–20. n.a.m.: not a minimum.
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Fig. 7 Triplet free energies (M06-2X/cc-pVTZ) of 24, 25, and 26.
this, the energy difference between the quinone tautomer and energies, yet there is some variation. A comparison of the triplet
the peroxide tautomer 23 here is calculated to be smaller than energies of 4, 16 and 17 reveals that they correlate with the
for all other systems calculated. Remarkably, quinone 20 is number of carbonyl groups in bay positions, the triplet energy
predicted to have lowest triplet and open-shell lowest singlet being the lowest when both carbonyl groups are in bay posi-
states that are energetically essentially degenerate (in terms of tions, as in 4.
U, the singlet state is slightly lower in energy, while the inclu-
sion of entropy leads to a triplet ground state in terms of G).
The discussion of the triplet free energies of the angucyclines
investigated is complicated by the fact that optimisation of the
Quinones 4, 16 and 17 all have the same number of Clar triplet states in each case resulted in convergence of the
sextets (zero). Correspondingly, they all have fairly low triplet geometry optimisation to a product in which one phenolic
Table 1 Selected parameters (energy, excitation energy, geometric) of a series of quinones or endoperoxides. Optimizations at M06-2X/cc-
pVTZa
˚
DU
lmax,calc
Triplet free energy
F(C(]O)–C–C–C(]O))
[ꢂ]
R(O–O) [A]
F(C(]O)–C–C–C(]O))
(triplet) [ꢂ]
[kcal molꢁ1
]
R(O–O) [A]
(triplet)
˚
System
(HOMO–LUMO) [eV]
[nm]
4
2.011
4.490
2.637
2.215
3.932
n.a.m.
2.591
3.206
2.357
2.863
2.057
1.675
2.579
3.371
3.622
3.597
695.7b
345.4b
572.6c
646.8c
452.2c
n.a.m.
613.2c
547.1c
653.4d
542.5b
685.7d
1151.6d
505.8d
447.1c
420.0c
411.7c
2.1
8.2
17.2
8.4
2.825
1.437
n.a.
19.8
10.3
n.a.
2.644
1.985
n.a.
23.7
0.0
n.a.
n.a.
15.8
0.0
32.1
5.9
26.8
29.2
0.0
25.0
10.5
32.4
22.2
n.a.
13
16
17
18
21
19
22
9
10
15
20
23
24
25
26
n.a.
n.a.
n.a.
65.7
n.a.
10.8
51.3
28.9
35.5
6.5
ꢁ0.8
29.6
46.2
42.7
42.6
2.726
n.a.m.
2.835
1.432
2.653
2.772
1.435
2.703
1.441
2.699
2.786
n.a.
22.7
n.a.m.
19.3
11.6
30.8
23.5
10.6
24.7
9.3
2.801
1.990
2.648
1.424
2.671
2.600
1.990
2.703
1.436
2.690
2.760
n.a.
23.2
22.2
n.a.
a
b
c
d
n.a.m.: not a minimum. n.a.: not available. TD-B3LYP/6-311++G(2df,p)//M06-2X/cc-pVTZ. TD-B3LYP/6-311++G(d,p)//M06-2X/cc-pVTZ. TD-
B3LYP/6-31+G(d)//M06-2X/cc-pVTZ.
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hydrogen had shied its position (as indicated by the letter “b”).
Nevertheless, the results suggest that a bay quinone or -dicar-
bonyl moiety does not modify the electronic properties of these
angucyclines in a way that would confer signicant biradical-
type reactivity.
Experimental part
16,17-Dihydroxyviolanthrone 9 was obtained from Sigma-
Aldrich and was used (with identical results) both as received
and aer column ltration over SiO2 using DMF as eluent. The
synthesis of 4,5-dihydroxyphenanthrene 3 (Scheme 3) initially
followed the route to 2,20-dimethoxy-biphenyl-6,60-dialdehyde-
dicyclohexylimine 29, as published by Breit and Reichert.17
Thus, 3-methoxybenzaldehyde 26 was converted to imine 27,
which was iodinated in 2-position by treatment with 2 eq. n-
BuLi followed by quenching of the organolithium compound
with iodine. Iodoarene 28 was then homocoupled under
Ullmann-conditions, employing copper(I) thiophene-2-carbox-
ylate18 as coupling reagent. Acid hydrolysis of biphenyldialdi-
mine 29 then yielded dialdehyde 30. In order to convert 30 into
4,5-dimethoxyphenanthrene 31, we followed the synthetic
protocol by Jung and Hagiwara, converting 30 into the bisto-
sylhydrazone 32, aer which a Bamford–Stevens reaction in
reuxing 1,4-dioxane yielded 31 in fairly low yield.19 Finally, 31
was demethylated using HBr/HOAc.
Conclusion
The presence of a bay dicarbonyl moiety or a bay quinone in
a molecule may, but does not have to, confer signicant bir-
adical character. As far as quinones are concerned, aromaticity
(e.g., as expressed by the number of Clar sextets present)
appears to be the single most dominant criterion determining
the triplet energy and thus biradical character. For isomeric
quinones with equal number of Clar sextets, the presence of one
or both carbonyl moieties in bay positions then additionally
reduces the triplet energy. Among the molecules investigated in
this study, this is most prominent for phenanthrene-4,5-
quinone, which is predicted to have a very small triplet free
energy of only DGtriplet ¼ 2.1 kcal molꢁ1. Bay quinones may have
peroxide valence tautomers. These cyclic peroxides normally are
energetically well above the bay quinone tautomers. However, if
the peroxides are more aromatic than the quinones, this energy
6,60-Dimethoxybiphenyl-2,20-dicarbaldehyde 30
difference is reduced. Under certain conditions (relatively large To a solution of 28 (ref. 17) (7.5 g, 21.8 mmol) in NMP (50 mL)
p-system, peroxide tautomer more aromatic than bay quinone was added copper(I)-thiophenecarboxylate (10.4 g, 54.5 mmol,
tautomer), the bay quinone may also have a triplet ground state. 2.5 eq.). The resulting suspension was stirred for 24 h at room
Finally, the presence of a bay quinone or bay dicarbonyl moiety temperature with exclusion of light. Addition of conc. aqueous
in some angucycline natural products is not expected to have NH3 (250 mL) resulted in a blue solution, which was extracted
signicant impact on the electronic properties of these quinoid with EtOAc (4 ꢃ 100 mL). The combined organic layers were
compounds.
washed with aqueous ammonia solution (3 ꢃ 50 mL, 12.5%),
Scheme 3 Synthesis of 4,5-dihydroxyphenanthrene 3.
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brine (2 ꢃ 50 mL) The organic phase was dried over MgSO4 and The solution was extracted with DCM (3 ꢃ 50 mL). The organic
the solvent was removed under vacuum yielding crude residue phase was dried over MgSO4 then the solvent was removed
of 29. The residue 29 was dissolved in DCM (50 mL) and HCl (35 under vacuum. The crude material was then puried using
mL, 6 M) was added. The mixture was stirred at room temper- column chromatography (SiO2, DCM) yielding a grey powder
ature for 2 h. The layers were separated and the aqueous phase (0.09 g, 0.44 mmol, 47% yield). 1H NMR (400 MHz, CDCl3):
was extracted with DCM (3 ꢃ 40 mL). The combined organic d 7.0–7.51 (m, 8H), 10.08 (broad s, 2H) 13C NMR (100 MHz,
layers were washed with HCl (2 ꢃ 20 mL, 1 M), sat. aqueous CDCl3): d 115.2 (2C), 118.5 (2C), 121.9 (2C), 127.3 (2C), 127.7
ꢂ
NaHCO3 (40 mL) and brine (40 mL) The organic phase was dried (2C), 135.2 (2C), 151.4 (2C); mp: 160–163 C.
over MgSO4 and the solvent was removed under vacuum. The
residue was puried by column chromatography (hexane/EtOAc
5 : 1 / 1 : 1) yielding a white solid 30 (0.8 g, 3.1 mmol, 28%
Electrochemistry and UV-Vis spectroscopy
All solvents for electrochemistry were obtained from Sigma Aldrich
and used as supplied. Tetrabutylammonium hexauorophosphate
(TBA-PF6) (99%) was supplied by Sigma Aldrich. Electrochemical
studies were performed in a single chamber cell in a three-
electrode conguration using a CH Instruments CHI700 series
potentiostat. The supporting electrolyte was 0.1 M TBA-PF6 in
dimethylformamide, unless otherwise noted. A Pt wire was used as
the counter electrode, along with an Ag/AgNO3 pseudo reference
electrode. Potentials are reported relative to the ferrocenium/
ferrocene couple, the position of which was judged by adding
ferrocene to the samples analyzed. Working electrodes were
washed with acetone and deionized water prior to use. Cyclic vol-
tammograms were collected at room temperature under an
atmosphere of Ar at a scan rate of 100 mV sꢁ1 (unless otherwise
stated). A glassy carbon button electrode (area ¼ 0.071 cm2, CH
Instruments) was used as the working electrode for cyclic vol-
tammetry. Measurements were conducted without stirring and
with iR compensation enabled. Bulk electrolyses were carried out
in 0.1 M TBA-PF6 in dimethylformamide using an Ag/AgNO3
pseudo reference electrode, a Pt wire counter electrode and a large
area carbon felt working electrode. Solutionsꢂwere stirred during
bulk electrolysis. Room temperature was 20 C in these studies.
1
yield). H NMR (400 MHz, CDCl3): d 3.76 (s, 6H), 7.25 (dd, J ¼
0.9,8.2, 2H), 7.57 (t, J ¼ 8.0, 2H), 7.70 (dd, J ¼ 1.0,7.9, 2H), 9.69
(s, 2H), [impurity d 7.28, chloroform]; 13C NMR (100 MHz,
CDCl3): d 56.0 (2C), 115.9 (2C), 119.8 (2C), 125.5 (2C), 129.8 (2C),
ꢂ
135.9 (2C), 157.2 (2C), 191.7 (2C); mp: 110 C.
6,60-Dimethoxybiphenyl-2,20-dicarbaldehyde-bis-
tosylhydrazone 32 (ref. 19)
Dialdehyde 30 (0.51 g, 1.9 mmol) was dissolved in methanol (1
mL) and added dropwise to a rapidly stirred suspension of p-
toluenesulfonyl hydrazide (0.88 g, 4.75 mmol, 2.5 eq. excess) in
methanol (3 mL). A mildly exothermic reaction ensued and the
hydrazide dissolved. Aer 10 min the bis-tosylhydrazone began
to precipitate. Aer 25 min the reaction was cooled to 0 ꢂC and
the product removed by ltration and washed with methanol (3
mL). The product was dried under vacuum to remove any
residual methanol. The crude product 32 was recrystallized
from methanol yielding 0.67 g (1.11 mmol), 58% yield. 1H NMR
(400 MHz, (CD3)2SO): d 2.37 (s, 6H), 2.51 (m, 2H), 3.52 (s, 6H),
7.09 (d, J ¼ 5.8 Hz, 4H), 7.10 (d, J ¼ 5.8 Hz, 4H), 7.29 (s, 2H),
7.37–7.41 (m, 4H), 7.67 (d, J ¼ 8.3 Hz, 2H), 13C NMR (100 MHz,
(CD3)2SO)): d 21.5, 56.0, 112.8, 117.1, 123.9, 127.6 (2C), 129.7,
130.2 (2C), 134.0, 136.6, 143.9, 145.5, 157.3; mp: 225 ꢂC (sharp).
ꢂ
Temperatures of ꢁ44 C were achieved by immersing the cell in
a dry-ice/acetonitrile bath. UV-Vis spectra were recorded on
a JASCO V-670 spectrophotometer using 1 cm path length cuvettes.
4,5-Dimethoxyphenanthrene 31
Computational methods
Sodium hydride (0.07 g, 2.70 mmol, 2.4 eq) was added to 32
(0.67 g 1.11 mmol) in anhydrous 1,4-dioxane (10 mL). The
mixture was stirred at 2 h under argon then heated to 120 C
All calculations were performed employing the Gaussian 09
suite of programmes.20 All stationary points were characterised
as minima or transition states by performing a vibrational
analysis. DFT calculations were performed employing the M06-
2X hybrid functional,21 employing cc-pVDZ or cc-pVTZ basis
sets,22 or using the B3LYP method.23 UV-Vis spectra were
calculated at the TD-B3LYP24 level of theory with a 6-31+G(d), 6-
311++G(d,p) or 6-311++G(2df,p)25 basis set.
ꢂ
and set to reux for 12 h. The reaction mixture was cooled to
room temperature then ethanol (10 mL) and water (200 mL)
were added. The solution was extracted with DCM (5 ꢃ 50 mL).
The organic phase was washed with water (2 ꢃ 50 mL) then
dried over MgSO4. The solvent was removed under vacuum
1
yielding a white solid (0.05 g, 0.20 mmol, 18% yield). H NMR
(400 MHz, CDCl3): d 3.96 (s, 6H), 7.03 (dd, J ¼ 1.0, 8.0 Hz, 2H),
7.34 (dd, J ¼ 1.1, 8.0 Hz, 2H), 7.43 (t, J ¼ 7.8, 2H), 7.49 (s, 2H) 13
C
Conflicts of interest
NMR (400 MHz, CDCl3): d 55.8 (2C), 108.3 (2C), 119.2 (2C), 119.7
(2C), 126.6 (2C), 126.8 (2C), 134.6 (2C), 157.8 (2C).
There are no conicts to declare.
4,5-Dihydroxyphenanthrene 3
Acknowledgements
31 (0.22 g, 0.93 mmol) was added to a HBr solution (5 mL of
ꢂ
33 wt% in acetic acid) and set to reux at 120 C. The reaction MDS thanks the Royal Society for a University Research
was monitored using TLC and aer 2 h the reaction was Fellowship (UF150104). This work was supported by the EPSRC
complete. The reaction was cooled to room temperature then (EP/K031732/1). The data underpinning this work can be ob-
quenched with saturated Na2CO3 (50 mL), neutralising the acid. tained by contacting GB.
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Deoxygenation with Trimethylsilyl Iodide, Eur. J. Org.
Chem., 2009, 5452, DOI: 10.1002/ejoc.200900666.
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