The Gomberg-type dimerization of bifluorenylidene radicals: An X-ray crystallographic investigation

Article abstract of DOI:10.1016/j.tetlet.2003.09.050

The reaction of phenylethynyllithium with 9-bromofluorene yields 9,9′-bifluorenylidene, 8, 9,9′-bifluorenyl, 9, and a molecule 10, of formula C₅₂H₃₂, in which coupling has occurred between a 9,9′-bifluorenyl fragment and a 9,9′-bifluorenylidene moiety such that a C(9) position of the former is attached to a C(3) site of the latter. This parallels the unsymmetrical radical coupling of triphenylmethyl radicals, which leads to the Gomberg dimer, rather than hexaphenylethane. The structure of 10 has been elucidated by X-ray crystallography, and a mechanistic rationale is offered.

Full text of DOI:10.1016/j.tetlet.2003.09.050

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 8057–8060
The Gomberg-type dimerization of bifluorenylidene radicals: an
ꢀ
X-ray crystallographic investigation
a
a
a,b,
Laura E. Harrington, James F. Britten and Michael J. McGlinchey
*
a
Department of Chemistry, McMaster University, 1280 Main St. W., Hamilton, ON L8S 1L5, Canada
b
Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
Received 20 August 2003; revised 5 September 2003; accepted 8 September 2003
Abstract—The reaction of phenylethynyllithium with 9-bromofluorene yields 9,9%-bifluorenylidene, 8, 9,9%-bifluorenyl, 9, and a
molecule 10, of formula C H , in which coupling has occurred between a 9,9%-bifluorenyl fragment and a 9,9%-bifluorenylidene
52
32
moiety such that a C(9) position of the former is attached to a C(3) site of the latter. This parallels the unsymmetrical radical
coupling of triphenylmethyl radicals, which leads to the Gomberg dimer, rather than hexaphenylethane. The structure of 10 has
been elucidated by X-ray crystallography, and a mechanistic rationale is offered.
©
2003 Elsevier Ltd. All rights reserved.
1
. Introduction
nucleophilic attack on 9-bromofluorene using
phenylethynyl-lithium was considered, and the reaction
was carried out at −78°C in an attempt to minimize
carbene formation. However, the reaction yielded
numerous products, and we now report the structures
and possible mechanisms of formation of the major
fractions.
We have recently reported studies on the dynamic
behavior of sterically hindered systems containing five-,
six- or seven-membered rings containing such bulky
substituents as phenyl, b-naphthyl, chloro or ferrocenyl
1
moieties (1–4). In continuation of these investigations,
we have targeted 9-fluorenylpentaphenylbenzene, 5, as
a ligand with the potential to exhibit restricted or
correlated rotation. Although fluorenyl-substituted ben-
2
. Results and discussion
2
zenes have been reported, there has not been a system-
atic study of the influence of these bulky groups on
their neighboring substituents in a fully substituted
system. The synthesis of 5 required the unsymmetrical
alkyne 9-phenylethynylfluorene, 6, which would then be
used in a Diels–Alder addition to the appropriate
cyclopentadienone, prior to thermolysis and cheletropic
elimination of carbon monoxide, as exemplified in
Scheme 1.
When 9-bromofluorene was treated with phenylethynyl-
lithium at −78°C, allowed to warm slowly to room
temperature and hydrolyzed, several products were
obtained after chromatographic separation. Two of
these were readily identified both spectroscopically and
by X-ray crystallography as the known molecules 9,9%-
4
5
bifluorenylidene, 8, and 9,9%-bifluorenyl, 9 (combined
yield 16%). The mass spectrum of the third product, 10
The original preparation of 9-phenylethynylfluorene, 6,
involved nucleophilic displacement of bromide from
9
-bromofluorene using
a
phenylethynyl Grignard3
reagent; however, the reported yield was only 10%.
Nevertheless, the possibility of a single-step synthesis by
Keywords: X-ray crystallography; fluorenyl radicals; Gomberg dimer-
ization.
ꢀ
Supplementary data associated with this article can be found at
doi:10.1016/j.tetlet.2003.09.050
Corresponding author. Tel.: 353-1-716-2880; fax: 353-1-716-2127;
e-mail: michael.mcglinchey@ucd.ie
Scheme 1. Diels–Alder synthesis of sterically crowded ben-
zene derivatives.
*
0
040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.09.050

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058
L. E. Harrington et al. / Tetrahedron Letters 44 (2003) 8057–8060
Figure 1. X-Ray crystal structure of (a) the tetramer 10 (50% thermal ellipsoids), and (b) comparison with the combination of
fragments 8 and 9.
13
(
12% yield), revealed a molecular weight of 656, corre-
radical 11, dimerization to 12, and finally aerial oxi-
dation to give the final product. The second, and
perhaps more attractive option, would proceed via ini-
tial coupling of the diradical forms of 8; subsequent
migration of a single hydrogen then yields 10 directly,
as illustrated in Scheme 2. However, a feature common
to both mechanisms is the coupling between a radical
located at a 9-fluorenyl position with a radical site on
the phenyl periphery of the molecule. This parallels the
classic ‘Gomberg-type’ dimerization of triphenylmethyl
radicals, which, for steric reasons, do not yield the
sponding to the formula C H , and indicating the
52
32
formation of a dimer of 8, that is, a tetramer of
fluorenylidene. Since a variety of such tetramers could
be readily envisaged, the product was characterized by
X-ray crystallography and the resulting structure
appears as Figure 1(a).
The tetramer 10 is comprised of a 9,9%-bifluorenyl frag-
ment linked to a 9,9%-bifluorenylidene moiety such that
a C(9) position of the former is attached to a C(3) site
of the latter (CꢀC 1.545(4) A). The steric crowding
,
between the two fluorenylidene fragments causes a
twisting such that the interplanar angle is 35°; this
compares favorably with the dihedral angles reported in
other 9,9%-bifluorenylidenes which lie in the range 31–
6
3
4°. Moreover, the structure of the 9,9%-bifluorenyl unit
in 10 closely resembles that found in the parent system
such that the two fluorenyls adopt a gauche conforma-
7
tion. Figure 1(b) illustrates the remarkable similarity
between 10 and its analogous molecular components 8
and 9.
It seems evident that the products 8, 9 and 10 arise
from a fluorenylidene moiety generated by dehydro-
bromination of 9-bromofluorene. It has been claimed
that treatment of 9-bromofluorene with KOH in
DMSO at room temperature yields 9,9%-bifluorenyli-
8
dene, 8, ‘almost quantitatively’, but Bethel reported
the formation of small quantities of ‘an amorphous
9
brown solid’ of unknown identity. Fluorenylidene car-
1
0
bene is well known to adopt a triplet ground state,
(
1
1
although the singlet state is also accessible) and so
dimerization and radical abstraction to yield 8 and 9
are not unexpected processes; however, the fluorenyli-
dene tetramer, 10, has not been previously reported.
Furthermore, other investigations involving such
fluorenyl-derived radicals or carbenes have not reported
1
2
the formation of tetrameric products.
One can envisage two possible routes to 10: the first
would involve generation of the triplet diradical form
of 8, abstraction of one hydrogen to form the known
Scheme 2. Proposed mechanism for the formation of tetra-
mer, 10.

L. E. Harrington et al. / Tetrahedron Letters 44 (2003) 8057–8060
8059
1
4
expected hexaphenylethane. One might speculate that
the triplet diradical state of bifluorenylidene is relatively
long-lived because of the sterically mandated non-pla-
nar orientation of the two C H moieties, thereby
PhCꢁC plane (91.2°), though it is bent away by 129.7°
from the linear alkyne. All other bond lengths and
angles are within acceptable ranges.
1
3
8
hindering radical coupling, as in Tomioka’s now classic
triplet carbenes.
1
5
3
. Conclusion
Treatment of 9-bromofluorene with an alkynyl-lithium
reagent unexpectedly yields the tetramer, 10, in which a
Other minor products were obtained after chromato-
graphic separation. Their mass spectra were inconclu-
sive, they provided very complicated NMR spectra,
even over a range of high and low temperatures and,
unfortunately, they did not yield crystals. Conse-
quently, their structures remain to be unraveled.
9
,9%-bifluorenyl fragment and a 9,9%-bifluorenylidene are
linked by a Gomberg-type radical coupling, as deter-
mined by X-ray crystallography.
Supporting information available: Experimental proce-
dures and characterization for compounds 6 and 10, as
well as X-ray crystallographic collection and refinement
details and structural data. Crystal data for 6 and 10
has been deposited with the Cambridge Crystallo-
graphic Data Center (CCDC c 217750, 217751).
In light of this unexpected result, an alternative route to
9
-phenylethynylfluorene, 6, was undertaken. We chose
16
to repeat a more recent synthesis that begins with
addition of phenylethynyllithium to fluorenone; subse-
quent treatment of the 9-phenylethynylfluoren-9-ol with
1
7
BF and triethylsilane furnishes the desired hydrocar-
3
bon in respectable yield (Scheme 3). Since 6 is known to
Acknowledgements
undergo ready rearrangement to the corresponding
3
allene, 7, its structure has now been unambiguously
confirmed by X-ray crystallography, as shown in Figure
Financial support from the Natural Sciences and Engi-
neering Research Council of Canada (NSERC) is grate-
fully acknowledged. L.E.H. thanks NSERC for a
Graduate Scholarship. We also thank our colleague Dr.
J. F. Valliant for helpful discussions.
2.
The CꢁC bond length is 1.195(2) A and the fluorenyl
,
substituent is almost exactly perpendicular to the
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