Dimerization of 9-phenylethynylfluorene to di-indeno-naphthacene and dispiro-[fluorene-dihydronaphthacene-fluorene]: An X-ray crystallographic and NMR study

Article abstract of DOI:10.1021/ol049967o

The attempted Diels-Alder reaction between 9-phenylethynylfluorene and tetracyclone yields instead three products resulting from the dimerization of the isomeric allene. The major product is 8,16-diphenyl-diindeno[1,2,3-de: 1′,2′,3′-mn]naphthacene, in which each terminal ring is derived from a fluorenyl unit; aerial oxidation then yields a peroxide. A dihydronaphthacene bearing fluorenyl moieties spiro-bonded at the C(5) and C(11) positions was also identified. The structures of the naphthacenes were elucidated by X-ray crystallography, and a mechanistic rationale is offered.

Full text of DOI:10.1021/ol049967o

ORGANIC
LETTERS
2004
Vol. 6, No. 5
787-790
Dimerization of 9-Phenylethynylfluorene
to Di-indeno-naphthacene and
Dispiro-[fluorene-dihydronaphthacene-
fluorene]: An X-ray Crystallographic
and NMR Study
,†,‡
Laura E. Harrington,† James F. Britten,† and Michael J. McGlinchey*
Department of Chemistry, McMaster UniVersity, 1280 Main St. W., Hamilton,
Ontario, Canada L8S 4M1, and Department of Chemistry, UniVersity College Dublin,
Belfield, Dublin 4, Ireland
michael.mcglinchey@ucd.ie
Received January 5, 2004
ABSTRACT
The attempted Diels−Alder reaction between 9-phenylethynylfluorene and tetracyclone yields instead three products resulting from the dimerization
of the isomeric allene. The major product is 8,16-diphenyl-diindeno[1,2,3-de:1′,2′,3′-mn]naphthacene, in which each terminal ring is derived
from a fluorenyl unit; aerial oxidation then yields a peroxide. A dihydronaphthacene bearing fluorenyl moieties spiro-bonded at the C(5) and
C(11) positions was also identified. The structures of the naphthacenes were elucidated by X-ray crystallography, and a mechanistic rationale
is offered.
In continuation of our studies on the dynamic behavior of
sterically hindered organic or organometallic systems of the
general formula C6Ar5X, where Ar is phenyl or â-naphthyl
and X is phenyl, â-naphthyl, or ferrocenyl,1 we wished to
prepare 9-fluorenylpentaphenylbenzene, 1. The synthesis
required the Diels-Alder addition of 9-phenylethynylfluo-
rene, 2,2 to tetraphenylcyclopentadienone (tetracyclone), prior
to thermolysis and cheletropic elimination of carbon mon-
oxide (Scheme 1). However, after chromatographic separa-
Scheme 1. Attempted Route to Fluorenylpentaphenylbenzene,
1
† McMaster University.
‡ University College Dublin.
(1) (a) Mailvaganam, B.; Sayer, B. G.; McGlinchey, M. J. J. Organomet.
Chem. 1990, 395, 177. (b) Gupta, H. K.; Brydges, S.; McGlinchey, M. J.
Organometallics 1999, 18, 115. (c) Brydges, S.; Harrington, L. E.;
McGlinchey, M. J. Coord. Chem. ReV. 2002, 232-234, 75. (d) Harrington,
L. E.; Britten, J. F.; McGlinchey, M. J. Can. J. Chem. 2003, 81, 1180.
(2) (a) Dem’yanov, P. I.; Stykow, I. M.; Krut’ko, D. P.; Petrosyan, V.
S.; Reutov, O. A. Metallorg. Khim. 1988, 1, 1039. (b) Dem’yanov, P. I.;
Stykow, I. M.; Krut’ko, D. P.; Vener, M. V.; Petrosyan, V. S. J. Organomet.
Chem. 1992, 438, 265. (c) Lee, M. S.; Jackson, J. E. Res. Chem. Intermed.
1994, 20, 223. (d) Harrington, L. E.; Britten, J. F.; McGlinchey, M. J.
Tetrahedron Lett. 2003, 44, 8057.
tion to remove unreacted starting materials, three products
were isolated and characterized spectroscopically and by
10.1021/ol049967o CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/03/2004

Figure 1. Views of the X-ray crystal structure of dispiro-[fluorene-dihydronaphthacene-fluorene], 4 (30% thermal ellipsoids).
X-ray crystallography, none of which was the desired Diels-
Alder adduct. We here describe these three products and offer
a mechanistic rationale.
structures of 4 and 5 are shown in Figures 1 and 2,
respectively, and reveal in each case a substituted naph-
thacene framework.
The attempted synthesis of 1 required the use of the
relatively harsh conditions necessary to drive Diels-Alder
reactions. However, rather than yielding the substituted
benzene derivative, 1, the synthesis led instead to a complex
mixture of products that proved to be difficult to separate
and identify. The most abundant fraction (45%) was a blue
material, 3, that gave a yellow solution in dichloromethane
when exposed to air and light. The identity of this product
was not immediately evident, though the fragments present
in the molecule could be deduced from the NMR spectrum.
The second major fraction (13%) was a yellow solid,
which did not appear to arise from decomposition of the blue
product. However, this compound readily formed large
crystals that were suitable for an X-ray crystallographic
investigation. The X-ray crystal structure revealed the
presence of two molecules that had fortuitously cocrystallized
together, allowing the identification of both products. The
Molecule 4, C42H26, is a dihydronaphthacene in which
fluorenyl groups are bonded in a spiro fashion at the C(5)
and C(11) positions. Unlike naphthacene (tetracene), pen-
tacene, and hexacene,3 the linear tetracylic skeleton in 4 is
not planar in the solid state but instead folds about the central
bond so as to make a bend angle of 22° and exhibits a slight
twist of the central rings. Moreover, the two spiro-bonded
fluorenyl substituents, which are aligned essentially orthogo-
nally to their attached six-membered rings, are bent slightly
in the opposite direction from the dihydronaphthacene
framework, thus producing a flattened saddle-type (C2)
structure.
This deformation contrasts with that observed in rubrene
(5,6,11,12-tetraphenylnaphthacene) bearing four (C5Ph5)Ru
cations, which instead twists about the central bond such
that the terminal rings are oriented at 67° to each other.4
However, in solution, the spectroscopic data for 4 are
Figure 2. X-ray crystal structure of the di-indeno-naphthacene peroxide 5 (30% thermal ellipsoids) and a plot of the cocrystal, illustrating
the relative arrangement of the molecules.
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Org. Lett., Vol. 6, No. 5, 2004

Scheme 2. Proposed Mechanism for the Formation of Products 3-5
consistent with a centrosymmetric molecule of C2h symmetry.
The fluorenyl substituents, and also the terminal phenyl rings,
for a 1,2,3-trisubstituted benzene, and also a phenyl ring.
These data, together with the 13C NMR and mass spectra,
are entirely in accord with the assignment of 3 as C42H24,
the C2h-symmetric naphthacene precursor to the peroxide 5,
as illustrated in Scheme 2. Indeed, when a solution of 3 in
dichloromethane was allowed to stand in air and light, the
blue color was gradually discharged as oxidation to 5
occurred. A pure sample of 5 was recovered from the
decomposition of 3 and identified by high-resolution mass
spectrometry and NMR spectroscopy. Compound 5 is not a
product of the original reaction but results from decomposi-
tion of 3 in solution.
The observed products are evidently not Diels-Alder
adducts but rather are derived from the dimerization of
9-phenylethynylfluorene, 2, presumably via its isomeric
allene, 6, with which it is known to be in equilibrium.5 The
mechanisms of dimerization of allenes have a long and
controversial history,6 but it is now generally understood that
initial formation of a bis-alkylidene-cyclobutane is the norm.7
Thus, when the allene 6 was heated in refluxing heptane,
two products were formed:5 a yellow material (84% yield)
each exhibit the expected doublet-triplet-triplet-doublet
1
patterns in the H NMR regime.
In 5, C42H24O2, each terminal ring of the naphthacene
skeleton is clearly derived from a fluorenyl moiety, and
phenyl substituents are located at C(8) and C(16). Moreover,
the C(7b) and C(16) positions are bridged by a peroxy
linkage (O-O 1.527(8) Å), thus inducing a folding and
twisting of the naphthacene framework such that the dihedral
angle between the terminal rings is now 116°. In addition,
the phenyl substituent attached at C(8) is stacked parallel to
the six-membered ring of the adjacent fluorenyl group. We
are unaware of any previous X-ray structures of peroxy-
bridged naphthacenes.
In light of these X-ray characterizations, the identity of
1
the blue compound, 3, was readily established from its H
NMR spectrum, which exhibited a COSY-connected doublet-
triplet-triplet-doublet pattern characteristic of an ortho-
disubstituted benzene, a doublet-triplet-doublet sequence
(3) (a) Campbell, R. B.; Robertson, J. M.; Trotter, J. Acta Cryst. 1962,
15, 289. (b) Bulgarovskaya, I. V.; Zavodnik, V. E.; Vozzhennikov, V. M.
Acta Crystallogr. 1987, C43, 764. (c) Holmes, D.; Kumaraswamy, S.;
Matzger, A. J.; Vollhardt, K. P. C. Chem. Eur. J. 1999, 5, 3399. (d)
Takahashi, T.; Kitamura, M.; Shen, B.; Nakajima, K. J. Am. Chem. Soc.
2000, 122, 12876.
(5) Kuhn, R.; Rewicki, D. Chem. Ber. 1965, 98, 2611.
(6) (a) Moureu, C.; Dufraisse, C. C. R. Acad. Sci. Paris 1926, 182, 1440,
1584. (b) Dufraisse, C.; Etienne, A.; Jolly, J. C. R. Acad. Sci. Paris 1950,
231, 5. (c) Dufraisse, C.; Mathieu, J.; Valls, J. J. C. R. Acad. Sci. Paris
1958, 246, 661. (d) Landor, P. D.; Landor, S. R. J. Chem. Soc. 1963, 2707.
(7) Fischer H. In The Chemistry of Alkenes; Patai, S., Ed.; Wiley: New
York, 1964; Vol. 1, pp 1064-1067.
(4) Fagan, P. J.; Ward, M. D.; Caspar, J. V.; Calabrese, J. C.; Krusic, P.
J. J. Am. Chem. Soc. 1988, 110, 2981.
Org. Lett., Vol. 6, No. 5, 2004
789

that was subsequently characterized by X-ray crystallography
as the head-to-tail [2 + 2] dimer, 7,8 and a red dimer (8%)
whose identity remains to be unraveled.
minus the phenyl substituents) is reported to exhibit elec-
troluminescence.12 Unfortunately, however, its synthesis,
color, and other physical properties are not described in the
patent. Finally, in light of the present observations, it would
be interesting to speculate on the structure of the yet
uncharacterized allene dimer derived from 9-(ferrocenyl-
ethynyl)fluorene.13 The organometallic group may stabilize
a radical intermediate and so facilitate formation of a
naphthacene incorporating ferrocenyl moieties, which may
result in interesting redox properties.
With some prescience, Dreissig, Luger, and Rewicki8 noted
that the bond linking the tetrahedral carbons in the four-
membered ring of 7 is very long (1.606 Å) and should
therefore cleave readily to yield a 1,4-diradical. Important
studies by Capdeveille and Rigaudy,9 and also by Christl
and co-workers,10 demonstrated the intermediacy of bi-allyl
diradicals that can be delocalized onto the ortho positions
of neighboring aromatic rings, thus allowing the formation
of six-membered rings. In Scheme 2, we show how these
ideas can account for the generation of the observed products
3-5.
To conclude, under Diels-Alder conditions, the allene
isomer of 9-phenylethynylfluorene undergoes dimerization
reactions to yield 8,16-diphenyl-diindeno[1,2,3-de:1′,2′,3′-
mn]naphthacene (3) and dispiro-[fluorene-dihydronaphthacene-
fluorene] (4). Compound 3 oxidizes in the presence of light
and air to give the corresponding peroxide, 5. X-ray crystal
structures of 4 and 5 have been determined, and a mecha-
nistic rationale for their formation has been presented.
Cleavage of the initially formed [2 + 2] dimer, 7, leads
to the diradicals 8 and 9 whereby one can envisage coupling
either between an ortho-phenyl site and the C(9) position of
a neighboring fluorenyl ring (to give 10) or between an ortho-
fluorenyl position and the benzylic site (leading to 11).
Subsequent disrotatory electrocyclization of the 6π system
generates the required molecular frameworks, and aerial
oxidation yields the observed products 3 and 4. It has long
been known that linear polyaromatics such as naphthacene
or pentacene readily undergo addition of dioxygen,11 and so,
with hindsight, formation and isolation of the peroxide 5 is
not unexpected.
Acknowledgment. Financial support from the Natural
Sciences and Engineering Research Council of Canada
(NSERC) is gratefully acknowledged. L.E.H. thanks NSERC
for a Graduate Scholarship.
Supporting Information Available: Experimental pro-
cedures and characterization for compounds 3-5, as well
as X-ray crystallographic collection and refinement details
and structural data, with fully labeled thermal ellipsoid plots.
This material is available free of charge via the Internet at
http://pubs.acs.org.
We are aware of only one compound closely related to
those described here. In the Japanese patent literature, di-
indeno[1,2,3-de:1′,2′,3′-mn]naphthacene (i.e., molecule 3
(8) Dreissig, W.; Luger, P.; Rewicki, D. Acta Crystallogr. 1974, B30,
2037.
(9) (a) Rigaudy, J.; Capdevielle, P. Tetrahedron 1977, 33, 767. (b)
Capdevielle, P.; Rigaudy, J. Tetrahedron 1979, 35, 2093.
(10) Christl, M.; Rudolph, M.; Peters, E.-M.; Peters, K.; von Schnering,
H. G. Angew. Chem., Int. Ed. Engl. 1995, 34, 2730.
(11) Fieser, L. F.; Fieser, M. Organic Chemistry, 3rd ed.; Reinhold: New
York, 1956; pp 774-775.
OL049967O
(12) Ikeda, S.; Hosokawa, C.; Arakane, T. Japanese Patent 2001-102173;
Chem. Abstr. 2001, 134, P 287628e.
(13) Buchmeiser, M.; Schottenberger, H. Organometallics 1993, 12, 2472.
790
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