Hexabenzo[4.4.4]propellane: A helical molecular platform for the construction of electroactive materials

Article abstract of DOI:10.1021/ol7015466

Helical hexabenzo[4.4.4]propellane (a relative of hexaphenylethane) and its derivatives are synthesized and their structures are established by X-ray crystallography. Isolation and X-ray crystallographic characterization of a robust trication-radical salt

Full text of DOI:10.1021/ol7015466

ORGANIC
LETTERS
2007
Vol. 9, No. 21
4091-4094
Hexabenzo[4.4.4]propellane: A Helical
Molecular Platform for the Construction
of Electroactive Materials
Paromita Debroy, Sergey V. Lindeman, and Rajendra Rathore*
Department of Chemistry, Marquette UniVersity, P.O. Box 1881,
Milwaukee, Wisconsin 53201
rajendra.rathore@marquette.edu
Received June 30, 2007
ABSTRACT
Helical hexabenzo[4.4.4]propellane (a relative of hexaphenylethane) and its derivatives are synthesized and their structures are established by
X-ray crystallography. Isolation and X-ray crystallographic characterization of a robust trication-radical salt of hexamethoxypropellane derivative
confirms that its framework is stable toward oxidative (aliphatic) C−C bond cleavage. It is also demonstrated that propellane can be easily
brominated at the 4,4 -positions of the biphenyl linkages for its usage as a molecular platform for the preparation of electroactive materials.
′
Hexabenzo[4.4.4]propellane1 is a relative of hexaphenyl-
ethane (HPE) in which the phenyls of HPE from the opposite
carbons are linked at the ortho positions to form biphenyl
linkages (see structure A, Figure 1). Interestingly, unlike
central C-C bond,2 hexabenzo[4.4.4]propellane (referred to
hereafter as propellane) is shown to be completely inert
toward the reductive (aliphatic) C-C bond cleavage despite
its close structural resemblance to the parent HPE.1
Density functional theory (DFT) calculations at B3LYP/
6-31G* level showed that the molecular geometry of
propellane (D3h) features three biphenyl moieties lying
orthogonal to each other in a helical arrangement at an
aliphatic C-C bond (i.e., Figure 1).3
It was conjectured that propellane may serve as a next-
generation molecular platform containing three orthogonally
(1) Wittig, G.; Schoch, W. J. Liebigs Ann. Chem. 1971, 749, 38. Also
see: Kimura, M.; Kuwano, S.; Shimada, K.; Kamata, R.; Yasuda, S.;
Sawaki, Y.; Fujikawa, H.; Noda, K.; Taga, Y.; Takagi, K. Bull. Chem. Soc.
Jpn. 2006, 79, 1793.
(2) (a) Vreven, T.; Morokuma, K. J. Phys. Chem. A 2002, 106, 6167.
(b) Yannoni, N.; Kahr, B.; Mislow, K. J. Am. Chem. Soc. 1988, 110, 6670
and references therein.
Figure 1. Orthogonal helical arrangement of the biphenyl moieties
in parent propellane.
(3) Note that propellane possesses geometric similarity to the metal-
tris-bipyridyls in which three bipyridyl moieties lie almost orthogonally
around the octahedral metal center.
hexaphenylethane (HPE) which is yet to be isolated due to
its significantly lengthened and consequently weakened
10.1021/ol7015466 CCC: $37.00
© 2007 American Chemical Society
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juxtaposed and readily functionalizable biphenyl linkages
(Figure 2) for the preparation of electroactive materials for
Scheme 1. Preparation of Propellane and Its Derivatives
Figure 2. Comparison of (helical) propellane and spiro-bifluorene
platforms for designing electroactive materials.
the applications in the emerging areas of molecular electron-
ics and nanotechnology.4 In this context, it is important to
note that spiro-bifluorene is extensively utilized as a platform
for the preparation of molecular wires that lie orthogonal to
each other at an insulating juncture,5 i.e., Figure 2.
4,4′-positions by reaction with tert-butyl chloride in dichloro-
Before undertaking the exploration of propellane and its
derivatives for the preparation of electroactive materials, one
needs to establish (i) that it is stable toward both reductive
as well as oxidative cleavage of the central C-C bond and
(ii) to demonstrate that it can be readily functionalized at
the 4,4′-positions, i.e., Figure 2.
methane using ferric chloride as a catalyst8 (Scheme 1).
An electron-rich propellane derivative containing methoxy
groups at the 4,4′-positions was also obtained using a similar
strategy as summarized in Scheme 1. Thus, 2,7-dimethoxy-
9-fluorenone9 was subjected to reductive coupling using Zn/
ZnCl2 in THF-H2O followed by an acid-catalyzed re-
arrangement of the resulting pinacol to the corresponding
pinacolone 7 in nearly quantitative yield (Scheme 1). A
reaction of 7 with 4,4′-dimethoxy-2-biphenyllithium10 in
anhydrous ether followed by acid-catalyzed rearrangement/
cyclization of the corresponding carbinol 8 afforded propel-
lane 9 in 53% isolated yield.
Accordingly, herein we present, for the first time, the
structures of propellane and its derivatives by X-ray crystal-
lography and establish that they are stable toward oxidative
degradation by the isolation and X-ray crystallographic
characterization of a stable tri-cation-radical salt of an
electron-rich propellane. Furthermore, we will show that the
parent propellane can be easily brominated at the 4,4′-
positions of all three biphenyls for attachment of various
electroactive groups using standard palladium-catalyzed
coupling reactions.
The molecular structures of 3, 5, 6, and 9 were established
with the aid of 1H/13C NMR spectroscopy (see the Supporting
Information) and were further confirmed by X-ray crystal-
lography as follows.
Thus, parent propellane 3 (or structure A) was obtained
in ∼60% overall yield by a reaction of 2-biphenyllithium
with readily available pinacolone 1 (derived from fluo-
renone)6 followed by an acid-catalyzed Wagner-Meerwein
rearrangement/cyclization of the resulting carbinol 2 (Scheme
1) using a slightly modified procedure of Wittig.1 A similar
reaction with easily available 4,4′-di-tert-butyl-2-biphenyl-
lithium7 similarly afforded di-tert-butylated derivative 5 in
75% yield. It is noteworthy that 5 can be easily and
quantitatively de-tert-butylated to produce 3 by a reaction
of 5 in benzene with AlCl3-CH3NO2 as a catalyst7 (Scheme
1). Moreover, both 3 and 5 can be fully tert-butylated at the
The determination of precise molecular structures of
various propellanes by X-ray crystallography (see Figure 3
for a representative structure) established that the central
(aliphatic) C-C bond is only slightly elongated [3, 1.563(2)
Å; 5, 1.572(17) Å; 6, 1.560(3) Å; and 9, 1.564(3) Å] as
compared to the standard value of 1.530 Å for a C(sp3)-
C(sp3) bond.11 It is also noted that the central C-C bonds
in various propellanes are relatively shorter than the average
value of 1.588 Å for the C-C bonds between two tertiary
carbons.11 The observation of relatively normal C-C bond
length in various propellanes as compared to the calculated
C-C bond length of 1.72 Å for hexaphenylethane2 is easily
reconciled by the fact that phenyl groups (due to the linkage
(4) (a) Introduction to Molecular Electronics; Petty, M. C., Bryce, M.
R., Bloor, D., Eds.; Oxford University Press: New York, 1995. (b) Maiya,
B. G.; Ramasarma, T. Curr. Sci. 2001, 80, 1523.
(5) (a) Tour, J. M. Chem. ReV. 1996, 96, 537. (b) Wu, R.; Schumm, J.
S.; Pearson, D. L.; Tour, J. M. J. Org. Chem. 1996, 61, 6906. (c) Wu, Y.;
Li, J.; Fu, Y.; Bo, Z. Org. Lett. 2004, 6, 3485 and references therein.
(6) Hoang, M.; Gadosy, T.; Ghazi, H.; Hou, D.-F.; Hopkinson, A. C.;
Johnston, L. J.; Lee-Ruff, E. J. Org. Chem. 1998, 63, 7168.
(8) Rathore, R.; Burns, C. L. J. Org. Chem. 2003, 68, 4071.
(9) Epperson, J. R.; Bruce, M. A.; Catt, J. D.; Deskus, J. A.; Hodges, D.
B.; Karageorge, G. N.; Keavy, D. J.; Mahle, C. D.; Mattson, R. J.; Ortiz,
A. A.; Parker, M. F.; Takaki, K. S.; Watson, B. T.; Yevich, J. P. Bioorg.
Med. Chem. 2004, 12, 4601.
(10) Sparling, J.; Fung, D.; Safe, S. Biomed. Mass Spectrom. 1980, 7,
13.
(7) Tashiro, M.; Yamato, T. J. Org. Chem. 1979, 44, 3037.
(11) Cambridge Structural Database (CSD).
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Org. Lett., Vol. 9, No. 21, 2007

Figure 4. (A) Cyclic and square-wave voltammograms of 2 ×
10-3 M 9 in CH2Cl2 (0.1 M n-Bu4NPF6) at a scan rate 200 mVs-1
.
(B) Comparison of the UV-vis absorption spectra of cation radicals
of 9 (green) and dimethoxybiphenyl 11 (pink), obtained using 10+•
(blue) as an oxidant in CH2Cl2 at 22 °C.
Figure 3. ORTEP diagram of [9‚CH2Cl2] and its triple helical
packing arrangement.
respectively.13 Similarly, tert-butylated derivatives 5 and 6
also showed reversible waves for the first oxidation at a
potential of 1.60 and 1.55 V vs SCE, while the later
oxidations occurring at relatively higher potentials with
irreversible waves (see Figures S1 and S2 in the Supporting
Information). The reversibility of electrochemical oxidations
of 5, 6, and 9 indicates that the central C-C bond in these
molecules do not undergo oxidative cleavage at least on the
electrochemical time-scale.14
at ortho-carbons) lie almost in (non-hindered) staggered
arrangement along the central C-C bond, i.e., the average
dihedral angles æ(Ph-C-C-Ph) are close to the ideal 60°
value. (For a complete listing of important bond lengths,
angles, and dihedral angles of various X-ray structures
reported herein, see Table S1 in the Supporting Information).
Despite the inherent helicity (chirality) of the propellane
framework and a very high barrier of racemization, the
synthesis described in Scheme 1 produces only the racemic
mixtures of various propellanes (i.e., 3, 5, 6, and 9).
Expectedly, the crystallizations of the racemic 3, 5, and 6
yield the crystals containing closely packed stacks of
molecules of alternating chirality. Interestingly, however,
hexamethoxypropellane 9 forms triple helical columns by
stacking molecules of one enantiomer onto each other (see
Figure 3). The molecules of 9 in the helix are linked together
via an alternating dichloromethane molecule by C-H‚‚‚O
contacts with the distance and angle of 2.63 Å and 139°,
respectively (see Figure 3). Note that “helixes” of alternating
chirality (formed from the enantiomers of 9) pack adjacent
to each other in the crystal with multiple contacts. As such,
this observation suggests that propellane framework may
represent an interesting building block for devising supra-
molecular helical arrays for interesting nonlinear optoelec-
tronic properties and electrical conductivity.12
The electrochemical reversibility and relatively low oxida-
tion potential of 9 prompted us to carry out its oxidation to
the corresponding cation radical using a stable naphthalene
cation radical (10•+)15 as an aromatic one-electron oxidant
(Eox ) 1.34 V vs SCE).
Thus, Figure 4B shows the spectrum of 9•+ obtained by a
reaction of a 6.1 × 10-5 M solution of 10•+ (λmax ) 672,
616, 503, and 396 nm; ꢀ672 ) 9300 M-1 cm-1)15 with 1 equiv
of 9 in dichloromethane at 22 °C, i.e., eq 1.
The characteristic absorption spectrum of green-colored
9
•+ with a structured absorption band (λmax ) 430, 861 nm)
was expectedly similar to that of the 4,4′-dimethoxybiphenyl
(11) cation radical16 (λmax ) 428, 812 nm), i.e. Figure 4B.
(For complete details for the generation of cation radicals
of 9 and 11, see Supporting Information.)
With the various propellane derivatives at hand, we next
evaluated their electron-donor strength and the initial indica-
tion of the oxidative stability by cyclic voltammetry.
Thus, each propellane was subjected to electro-chemical
oxidation at a platinum electrode as a 2 mM solution in
dichloromethane containing 0.1 M n-Bu4NPF6 as the sup-
porting electrolyte. Figure 4A shows the cyclic voltammo-
grams of 9 with three reversible 1-electron oxidations
occurring at the potentials of 1.17, 1.37, and 1.51 V vs SCE
for the formation of mono-, di-, and trication radical,
(13) There is little or no possibility of “through-space” interaction
between the cationic biphenyl moieties in propellanes due to their orthogonal
juxtapostion with respect to each other. Thus, splitting of the oxidation waves
in 9 most likely arises due to the Coulombic repulsion amongst the
orthogonally juxtaposed cationic biphenyls.
(14) The parent propellane (3) showed only an irreversible oxidation wave
at ∼1.8 V. Such an observation does not necessarily imply the instability
of the resulting cation radical towards C-C bond cleavage but most likely
indicates its high electrophilic reactivity similar to that observed for the
unsubstituted biphenyl cation radical.
(12) Compare: Rajca, A.; Safronov, A.; Rajca, S.; Shoemaker, R. Angew.
Chem., Int. Ed. Engl. 1997, 36, 488.
(15) Rathore, R.; Le Mague’res, P.; Lindeman, S. V.; Kochi, J. K. Angew.
Chem., Int. Ed. 2000, 39, 809.
Org. Lett., Vol. 9, No. 21, 2007
4093

It is noteworthy that the green-colored 9•+, obtained in eq
1, is highly persistent at ambient temperatures and did not
show any decomposition during a 12 h period at 22 °C as
confirmed by UV-vis spectroscopy. It is to be further noted
that the reduction of 9•+ with zinc dust regenerated the neutral
9 quantitatively, as confirmed by 1H/13C NMR spectroscopy.
The high stability of the 9•+ lends support to the fact that
propellane derivatives are stable toward oxidative C-C bond
cleavage, and thus, we next attempted the isolation of
crystalline 9•+ as follows.
we next explored if they can be functionalized at the 4,4′-
postions of three orthogonal biphenyl moieties (see Figure
2) for the preparation of electro-active materials as follows.
Thus, parent propellane 3 was subjected to a bromination
reaction in dichloromethane in the presence of powdered iron
as a catalyst for 4 h to afford 12 in quantitative yield with
selective bromination at 4,4′-positions as established by X-ray
crystallography (see Scheme 2). Moreover, hexabromopro-
Scheme 2. Preparation of Electroactive Propellanes
Treatment of a solution of 9 with triethyloxonium hexa-
-
chloroantimonate (Et3O+SbCl6 )san efficient 1e- oxidant
for the isolation of crystalline cation-radical salts17 - in
dichloromethane at ∼0 °C immediately resulted in a dark-
green solution which was spectrally identical to that obtained
above as in eq 1. The green-colored solution was carefully
layered with toluene and stored in a refrigerator at -10 °C
for 2 days. The crystallographic analysis of the dark-colored
crystals thus obtained, indicated that the crystals consisted
-
of a tri-cation-radical salt [93•+3SbCl6 ], most likely formed
-
by crystallization induced disproportionation [9•+SbCl6 ].18
-
Figure 5 shows that the crystal structure of [93•+3SbCl6 ]
consists of columns of tricationic 93•+ with alternating
pellane 12 underwent efficient Suzuki coupling reactions with
2,5-dimethoxytolylboronic acid and 9,9-dihexyl-2-fluorene-
boronic acid, under standard conditions, to afford arylated
derivatives 13 and 14 in excellent yields (see Scheme 2).
In summary, we have synthesized and established struc-
tures of propellane and its derivatives by X-ray crystal-
lography. The helical propellane framework is stable toward
both reductive and oxidative C-C bond cleavage as shown
by isolation and X-ray crystallographic characterization of
a stable trication-radical salt. Moreover, parent propellane 3
can be easily transformed to the hexabromopropellane 12
which in turn was utilized for the preparation of electroactive
derivatives 13 and 14 (Scheme 2). These findings demon-
strate that the propellane framework can serve as a platform
for the preparation of futuristic electroactive materials for
practical applications.
Figure 5. Packing arrangement of racemic tricationic 9 with
multiple O‚‚‚Ar contacts.
chirality as opposed to neutral 9 (vide supra) along one axis
surrounded by 4 equiv of toluene molecules (two of them
are rotationally disordered within molecular plane) and
-
separated by layers containing 3 equiv of SbCl6 (two in
general positions and two halves on 2-fold axes) and several
heavily disordered solvent molecules. The columns of 93•+
are built on a 6-fold O‚‚‚Ar contacts (as close as 3.05 Å)
which may correspond to attractive interactions between the
dipoles of the methoxy groups with delocalized positive
charge on the aromatic moieties. Unfortunately, the low
experimental precision severely limits discussion of the bond-
length changes in 93•+, but the averaged bond lengths are in
a good agreement with the expected quinoidal distortion of
dimethoxybiphenyl moieties (see Table S1 in the Supporting
Information).
It is also noted that reduction of 93•+ using Zn dust or
borane-dimethyl sulfide complex19 regenerated the neutral
9 in quantitative yield and thereby further attesting to the
stability of the central C-C bond in propellanes toward
oxidative cleavages.
Acknowledgment. We thank the National Science Foun-
dation (CAREER Award) for financial support.
Supporting Information Available: Synthetic details, 1H/
13C NMR data, and X-ray structural data of 3, 5, 6, 9, and
12. This material is available free of charge via the Internet
at http://pubs.acs.org.
OL7015466
(16) Singular absence of any additional absorption bands (in near IR
region) in hexamethoxypropellane cation radical when compared to 4,4′-
dimethoxybiphenyl cation radical suggests the lack of electronic coupling
amongst the orthogonally juxtaposed biaryl moieties in propellanes.13
(17) Rathore, R.; Kumar, A. S.; Lindeman, S. V.; Kochi, J. K. J. Org.
Chem. 1998, 63, 5847.
(18) Compare: Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J.
K. J. Am. Chem. Soc. 1998, 120, 6931.
(19) Rathore, R.; Burns, C. L.; Guzei, I. A. J. Org. Chem. 2004, 69,
1524.
Having established that propellanes are stable toward both
reductive1 and oxidative cleavage of the central C-C bond,
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Org. Lett., Vol. 9, No. 21, 2007