Sequential oxidative transformation of tetraarylethylenes to 9,10-diarylphenanthrenes and dibenzo[g, p]chrysenes using DDQ as an oxidant

Article abstract of DOI:10.1021/ol200069c

Tetraarylethylenes can be sequentially transformed into 9,10-diarylphenanthrenes and dibenzo[g,p]chrysenes using 1 and 2 equiv of DDQ, respectively, in CH₂Cl₂ containing methanesulfonic acid, in excellent yields. Efficient access to substituted dibenzochrysenes from tetraarylethylenes establishes the versatility of this procedure over the existing multistep syntheses of dibenzochrysenes. Moreover, the ready regeneration of DDQ from easily recovered reduced DDQ-H₂ continues to advance the use of DDQ/H⁺ for the oxidative C-C bond forming reactions.

Full text of DOI:10.1021/ol200069c

ORGANIC
LETTERS
2011
Vol. 13, No. 7
1634–1637
Sequential Oxidative Transformation of
Tetraarylethylenes to 9,10-Diaryl-
phenanthrenes and Dibenzo[g,p]-
chrysenes using DDQ as an Oxidant
Tushar S. Navale, Khushabu Thakur, and Rajendra Rathore*
Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, Wisconsin 53201,
United States
Raj.Rathore@marquette.edu
Received January 10, 2011
ABSTRACT
Tetraarylethylenes can be sequentially transformed into 9,10-diarylphenanthrenes and dibenzo[g,p]chrysenes using 1 and 2 equiv of DDQ,
respectively, in CH2Cl2 containing methanesulfonic acid, in excellent yields. Efficient access to substituted dibenzochrysenes from
tetraarylethylenes establishes the versatility of this procedure over the existing multistep syntheses of dibenzochrysenes. Moreover, the ready
regeneration ofDDQfromeasily recovered reducedDDQ-H2 continuestoadvancetheuse of DDQ/Hþ for the oxidative C-C bond forming reactions.
Efforts toward the design and synthesis of polycyclic aro-
matic hydrocarbons (PAHs) for use as functional materials
in electronic and optoelectronic devices continue to grow
with an unabated pace owing to their potential applications
in the ever evolving areas of molecular electronics and nano-
technology.1,2 Of these, dibenzochrysene, a twisted polycyclic
aromatic hydrocarbon, and its derivatives have been explored
by Swager and co-workers3 and others4 for the preparation of
sensors, nonlinear optical and liquid-crystalline materials, etc.
Unfortunately, however, the preparation of parent dibenzo-
chrysene (DBC)5 as well as its various substituted derivatives
requires multistep syntheses.3-5
In principle, various dibenzochrysenes should be accessi-
ble via a pair of oxidative C-C bond formations in readily
available symmetrical (via McMurry coupling)6 as well as
unsymmetrical7 tetraarylethylenes (via a recently developed
procedure from our laboratory) using oxidants similar to
those employed for the Scholl reaction.8 Unfortunately, such
oxidative transformations of tetraarylethylenes (TAE) to
dibenzochrysenes (DBC) are rarely employed9 (Scheme 1).
In this context, it is noteworthy that Olah and co-
workers10 have shown that treatment of tetraarylpinacols
with a superacid (CF3SO3H) affords 9,10-diarylphenan-
threnes (DAP) via a dicationic intermediate TAEþ2
(Scheme 1). Interestingly, it is also well-known11 that the
same dicationic intermediate (i.e., TAEþ2) can also be
(1) Heath, J. R. Annu. Rev. Mater. Res. 2009, 39, 1–23 and references
cited therein.
€
(2) Watson, M. D.; Fechtenkotter, A.; Mullen, K. Chem. Rev. 2001,
101, 1267.
(3) (a) Yamaguchi, S.; Swager, T. M. J. Am. Chem. Soc. 2001, 123,
12087. (b) Rose, A.; Tovar, J. D.; Yamaguchi, S.; Nesterov, E. E.; Zhu,
Z.; Swager, T. M. Philos. Trans. R. Soc. London, Ser. A 2007, 365, 1589
and references therein.
(4) (a) Li, C.-W.; Wang, C.-I.; Liao, H.-Y.; Chaudhuri, R.; Liu, R.-S.
J. Org. Chem. 2007, 72, 9203. (b) Toal, S. J.; Trogler, W. C. J. Mater.
Chem. 2006, 16, 2871. (c) Chaudhuri, R.; Hsu, M.-Y.; Li, C.-W.; Wang,
C.-I.; Chen, C.-J.; Lai, C. K.; Chen, L.-Y.; Liu, S.-H.; Wu, C.-C.; Liu,
R.-S. Org. Lett. 2008, 10, 3053.
(6) McMurry, J. E. Acc. Chem. Res. 1983, 16, 405.
(7) Banerjee, M.; Emond, S. J.; Lindeman, S. V.; Rathore, R. J. Org.
Chem. 2007, 72, 8054.
(8) (a) Scholl, R.; Mansfeld, J. Ber. Dtsch. Chem. Ges. 1910, 43, 1734.
(b) Kovacic, P.; Jones, M. B. Chem. Rev. 1987, 87, 357.
(9) Navale, T. S.; Zhai, L.; Lindeman, S. V.; Rathore, R. Chem.
Commun. 2009, 2857.
(10) Klumpp, D. A.; Baek, D. N.; Prakash, G. K. S.; Olah, G. A.
J. Org. Chem. 1997, 62, 6666.
(5) Buckles, R. E.; Serianz, A.; Naffziger, D. Proc. Iowa Acad. Sci.
1973, 80, 45.
(11) Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am.
Chem. Soc. 1998, 120, 6931.
r
10.1021/ol200069c
Published on Web 03/01/2011
2011 American Chemical Society

(10 mL) was reacted with 1 equiv of DDQ at ∼0 °C, under
an argon atmosphere, to afford a blue-violet solution14
which turned brown during the course of 30 min. The
resulting brown mixture was quenched by an addition of
saturated aqueous sodium bicarbonate solution (20 mL).
The dichloromethane layer was separatedand washedwith
a saturated brine solution (2 ꢀ 10 mL), dried over anhy-
drous magnesium sulfate, and evaporated to afford the
corresponding 9,10-diphenylphenanthrene (1b) in quanti-
tative yield (Scheme 3).
Scheme 1. Sequential Oxidative Transformation of TAE to
DAP and DBC
Scheme 3. Oxidative Transformation of 1a to 1b Using DDQ as
Oxidant
accessed from tetraarylethylenes via two successive 1-e-
oxidations (i.e., Scheme 1).
Accordingly, our initial attempts to transform parent tetra-
phenylethylene to either 9,10-diphenylphenanthrene or diben-
zochrysene using FeCl3 (an extensively utilized oxidant for the
oxidative C-C bond forming reactions)8 gave a complex
mixture of products.12 We recently demonstrated13 that the
DDQ/Hþ system, which oxidizes a variety of aromatic elec-
tron donors with oxidation potentials as high as ∼1.6 V vs
SCE to their cation radicals, can be used as an effective oxidant
for C-C bond forming reactions; e.g., see Scheme 2.
It is important to note that the reduced hydroquinone
(DDQ-H2) in Scheme 3 readily dissolves into the aqueous
sodium bicarbonate layer and can be recovered quantita-
tively by acidification (using aqueous hydrochloric acid,
10%) followed by extraction with diethyl ether. Further-
more, DDQ can be regenerated from DDQ-H2 by a
simple oxidation using either conc. nitric acid or N2O4.15
A series of TAE 1a-8a (with oxidation potentials vary-
ing from ∼1.1 to ∼1.5 V vs SCE) were converted to the
corresponding DAP 1b-8b in good to excellent yields
when subjected to the oxidative cyclodehydrogenation
reaction according to the reaction conditions depicted in
Scheme 3 (see Table 1). It is noteworthy that the reaction
time for the conversion of TAEs to DAPs increased from
∼30 min to 24 h with the increasing oxidation potentials of
TAEs (see Table 1). The most electron-poor tetrabromo
derivative 8a (Eox = 1.51 V), among the TAEs in Table 1,
underwent only partial conversion (56%) to 8b in 96 h.
The structures of various DAPs in Table 1 were estab-
lished by 1H/13C NMR spectroscopy and further confirmed
by X-ray crystallography (see Supporting Information).
The symmetrical tetraarylethylenes with both electron-
donating (2a) and electron-withdrawing substituents (8a)
also produced minor amounts of diarylmethylidene-fluorene
2c and 8c, respectively, together with the expected 2b and
8b as major products (Table 1).16a The unsymmetrical
tetraarylethylenes 3a, 5a, 6a, and 7a,16b on the other hand,
underwent regioselective conversion to the corresponding
Scheme 2. Oxidative Transformation of a o-Terphenyl to the
Corresponding Triphenylene Using DDQ as Oxidant
Herein, we now report that the same DDQ/Hþ system as
in Scheme 2 can also be employed for the sequential
transformations of a variety of symmetrical and unsym-
metrical tetraarylethylenes to corresponding 9,10-diaryl-
phenanthrenes (DAP) using 1 equiv of DDQ or to
dibenzochrysenes (DBC) using 2 equiv of DDQ in excel-
lent yields. The preliminary details of these findings in-
cluding the involvement of the tetraarylethylene cation
radicals as intermediates in a rapid conversion of TAE to
DAP, as well as a relatively slow transformation of DAP to
DBC, are discussed in the context of an electron transfer
mechanism as follows.
(14) The blue-violetþc•olor in reaction of 1a with DDQ/Hþ arises due
to the formation of 1a and 1þ2, as confirmed by comparison of the
UV-vis absorption spectrum ofþt2he reaction mixture with that of the
Thus, a 0.1 M solution of tetraphenylethylene (1a) in a
9:1 mixture of dichloromethane and methanesulfonic acid
authentic spectra11 of 1aþ• and 1
.
(15) (a) Brook, A. G. J. Chem. Soc. 1952, 5040. (b) Newman, M. S.;
Khanna, V. K. Org. Prep. Proc. Int 1985, 17, 422.
(12) An NMR analysis of the crude reaction mixture suggested that it
contained traces of parent dibenzochrysene 1d and 9,10-diphenylphena-
threne (1b) together with a multitude of products arising from the
electrophilic aromatic chlorination of 1a, 1b, and 1d.
(13) (a) Zhai, L.; Shukla, R.; Rathore, R. Org. Lett. 2009, 11, 3474.
(b) Zhai, L.; Shukla, R.; Wadumethrige, S. H.; Rathore, R. J. Org.
Chem. 2010, 75, 4748.
(16) (a) Oxidative cyclizations of symmetrical TAEs to correspond-
ing diarylphenanthrenes and diarylmethylidenefluorene have been pre-
viously observed; see: Ciminale, F.; Lopez, L.; Mele, G. Tetrahedron
1994, 50, 12685. (b) It is well known that the cation radicals and dications
of of TAEs are twisted around the ethylenic CdC bond by ∼30°-60°
and undergo ready isomerization under oxidative conditions; see ref 11.
Org. Lett., Vol. 13, No. 7, 2011
1635

Table 1. Oxidative Transformation of TAEs to DAPs Using
DDQ/Hþ Oxidant System in Dichloromethane at 0 °C
Figure 1. UV-vis absorption spectra of dicationic 10a (blue) and9a
(red) formed quantitatively from the reactions of the corresponding
TAEs with 1 equiv of DDQ in CH2Cl2/MeSO3H at 22 °C.
the fact that they form rather stable dications (i.e., 9aþ2
and 10aþ2) owing to the effective quinoidal delocalization
of a pair of cationic charges (Figure 1). Moreover, the high
stability of dicationic 9aþ2 and 10aþ2 was further demon-
strated by their isolation and structuralcharacterization by
X-ray crystallography.11
Surprisingly, the oxidative transformations of various
TAEs to DAPs using 1 equiv of DDQ, according to
Scheme 3, did not show any indication of the formation
of corresponding dibenzochrysenes. However, a reaction
of isolated 9,10-diphenylphenanthrene (1b) with DDQ/Hþ
for an extended period at 22 °C showed a partial conver-
sion of 1b to dibenzochrysene (1c) (Scheme 4).
Scheme 4. Oxidative Transformation of a 1a to 1b Using DDQ
as Oxidant
a The yields refer to both b and c products in cases of entries 2, 5, 7, and8.
b cis-1,2-bis(4-bromophenyl)-1,2-diphenylethylene also gave an identical
mixture of products (i.e. 7b and 7c). c Yield is based on conversion (56%)
and recovered 8a (44%). d See Figure 1.
As noted above, the reaction times for the oxidative
conversion of various TAEs to the corresponding DAPs
increased with increasing oxidation potentials of TAEs, and
therefore, the slow reactivity of 1b toward the DDQ/Hþ
oxidation system suggested that its oxidation potential must
be relatively high as compared to tetraphenylethylene. In-
deed, a cyclic voltammetric analysis of 1b showed that its
oxidation potential (Eox1 = 1.59 V vs SCE) was ∼230 mV
higher than the tetraphenylethylene (Eox1 = 1.36 V vs SCE)
and was comparable to tetrakis(4-bromophenyl)ethylene
(Eox1 = 1.51 V vs SCE) whose (partial) oxidative conversion
to the corresponding DAP required 96 h (see Table 1).
9,10-diarylphenanthrenes where the oxidative C-C bond
formation occurred between the relatively electron-rich
aryl rings (see Table 1).
Interestingly, the most electron-rich 1,2-dianisyl-1,
2-ditolylethylene11 (9a, Eox1 = 0.89 V) and tetraanisyl-
ethylene11 (10a, Eox1 = 0.79 V) produced intensely colored
solutions when treated with DDQ and methanesulfonic acid
according to the Scheme 3. The quenching of the resulting
reaction mixtures, after 24 h, using aqueous sodium bicarbo-
nate, afforded a quantitative recovery of the starting TAEs 9a
and 10a. Photospectrometric analyses in Figure 1 of the
highly colored reaction mixtures of 9a and 10a, prior to
aqueous workup, produced characteristic absorption spectra
which were readily assigned to the corresponding dications of
the TAEs (i.e., 9aþ2 and 10aþ2) by comparison with the
Encouraged by the partial oxidative conversion of 1b to
dibenzochrysene (1d) in Scheme 4, we subjected various
isolated DAPs to reaction with DDQ/Hþ, according to
Scheme 4, and found that they can be transformed to the
corresponding dibenzochrysenes with excellent to moderate
conversions depending on their redox potentials (see Table 2).
Table 2 shows that DAP 2b with the lowest oxidation
potential (i.e., Eox1 = 1.37 V) undergoes an efficient and
quantitative conversion to the corresponding tetramethyl-
dibenzochrysene 2d while electron-poor bromo-substituted
reported spectra of 9aþ2 and 10aþ2 11
.
As such, the singular lack of formation of DAPs from
tetraarylethylenes 9a and 10a is not surprising in light of
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Org. Lett., Vol. 13, No. 7, 2011

Table 2. Oxidative Transformation of DAPs to DBCs Using
DDQ/Hþ Oxidant System in Dichloromethane at 22 °C
Scheme 5. Sequential Oxidative Transformation of TAE 2a to
DAP 2c and DBC 2d Using 1 and 2 equiv of DDQ, Respectively
ing DBCs with 1 equiv of DDQ warrants further explana-
tion, especially in light of the fact that the conversion of the
TAE 2a to DAP 2b or DAP 2b to DBC 2d, under identical
conditions, occurs on a similar time scale (e.g., see
Scheme 5). As such, the explanation for this remarkable
selectivity may lie in the fact that both of the oxidative
transformations (i.e., TAEfDAP and DAPfDBC) pro-
ceed via an electron transfer mechanism.
As noted above the oxidation potentials of DAPs are
considerably higher than TAEs, and therefore in the presence
of TAE, the concentration of the DAP cation radical is
expected to be insignificant. For example, based on the
oxidation potential difference of 260 mV between 2a and
2b, one can easily estimate that, even in a 1:1 mixture of 2aþ•
and 2b, the concentration of 2bþ• will be negligible (see eq 1).11
Therefore the oxidative transformation of 2b to correspond-
ing dibenzochrysene 2d via a cation radical mechanism will
not occur while 2a is present in the reaction mixture.
1
a Conversion percentages were determined by H NMR spectroscopy
and yields refer to the isolated dibenzochrysenes based on recovered DAPs.
b The Eox1 and Eox2 values were determined by cyclic voltammetry in CH2Cl2
containing 0.1 M nBu4PF6 at a scan rate of 200 mV s-1 c An isomeric
.
mixture of 5b/5c was used.
DAPs 6b, 7b, and 8b with Eox1 values g1.64 V, expectedly,
do not react under these conditions.17
Synthesis of various substituted dibenzochrysenes in
Table 2 can also be accomplished directly from tetra-
arylethylene using 2 equiv of DDQ under otherwise identical
conditions listed in Scheme 4 (see Scheme 5). A one-pot
sequential oxidative transformation of 2a to 2d was also
carried out, and it showed that an ∼2:1 mixture of 2b and
diarylmethylidenefluorene 2c formed after the addition of
the first equivalent of DDQ was cleanly transformed into a
single dibenzochrysene 2d upon addition of a second
equivalent of DDQ (Scheme 5).18
Similarly, tetraarylethylene 3a, 4a, and 5a were con-
verted to the corresponding dibenzochrysenes 3d, 4d, and
5d, respectively, in yields comparable to those in Table 2,
using 2 equiv of DDQ in a one-pot procedure. Note that
bromo-substituted TAEs (i.e., 6a, 7a, and 8a) even with
excess DDQ (3-5 equiv) afforded only the DAP deriva-
tives (see Tables 1 and 2).
In summary, we have shown that substituted 9,10-diaryl-
phenanthrenes and dibenzochrysenes can be easily accessed
from readily available symmetrical and unsymmetrical
tetraarylethylenes by sequential oxidative transformation
using 1 and 2 equiv of DDQ, respectively, in dichloro-
methane in the presence of methanesulfonic acid. The
selective formation of 9,10-diarylphenanthrenes, without
contamination from dibenzochrysenes, with 1 equiv of
DDQ strongly suggests that these sequential oxidative
transformations proceed via paramagnetic cation radicals
as reactive intermediates. The detailed mechanisms of
these oxidative transformations of tetraarylethylenes to
9,10-diarylphenanthrenes and dibenzochrysenes are currently
under investigation and will be presented in due course.
The observation of the selective oxidative transformation
of various TAEs to DAPs without the contamination form-
Acknowledgment. We thank the National Science
Foundation (Chemsitry) for financial support.
(17) Note that DDQ (Ered = þ0.60 V vs SCE), in the presence of an
acid, oxidizes a variety of aromatic donors with oxidation potentials as
high as ∼1.6 V vs SCE to the corresponding cation radicals; however, the
rate of formation of these cation radicals decreases with the increasing
oxidation potential of donors; see: Rathore, R.; Kochi, J. K. Acta Chem.
Scand. 1998, 52, 114 and references cited therein.
Supporting Information Available. Experimental pro-
1
cedures, H/13C NMR data, X-ray structure data for
(18) The bifluorenylidene formed from 2a, after oxidative C-C bond
formation, can easily be isomerized to DBC 2d by an acid or electron-
transfer catalysis; see: Suzuki, K.; Yamaguchi, U. Bull. Chem. Soc. Jpn.
1962, 35, 735 and references cited therein.
representative DAPs and spectra of various compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Org. Lett., Vol. 13, No. 7, 2011
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