Structurally Diverse π-Extended Conjugated Polycarbo- and Heterocycles through Pd-Catalyzed Autotandem Cascades

Article abstract of DOI:10.1002/chem.201503080

The Pd-catalyzed reaction between 2,2′-dibromobiphenyls and related systems with tosylhydrazones gives rise to new π-extended conjugated polycarbo- and heterocycles through an autotandem process involving a cross-coupling reaction followed by an intramolecular Heck cyclization. The reaction shows wide scope regarding both coupling partners. Cyclic and acyclic tosylhydrazones can participate in the process. Additionally, a variety of aromatic and heteroaromatic dibromoderivatives have been employed, leading to an array of diverse scaffolds featuring a fluorene or acridine central nucleus, and containing binaphthyl, thiophene, benzothiophene and indole moieties. The application to appropriate tetrabrominated systems led to greater structural complexity through two consecutive autotandem cascades. The photophysical properties of selected compounds were studied through their absorption and emission spectra. Fluorescence molecules featuring very high quantum yields were identified, showing the potential of this methodology in the development of molecules with interesting optoelectronic properties.

Full text of DOI:10.1002/chem.201503080

DOI: 10.1002/chem.201503080
Full Paper
&
Fluorenes |Hot Paper|
Structurally Diverse p-Extended Conjugated Polycarbo- and
Heterocycles through Pd-Catalyzed Autotandem Cascades
Raquel Barroso,[a] María-Paz Cabal,[a] Rosana Badía-LaiÇo,[b] and Carlos ValdØs*[a]
Abstract: The Pd-catalyzed reaction between 2,2’-dibromo-
biphenyls and related systems with tosylhydrazones gives
rise to new p-extended conjugated polycarbo- and hetero-
cycles through an autotandem process involving a cross-
coupling reaction followed by an intramolecular Heck cycli-
zation. The reaction shows wide scope regarding both cou-
pling partners. Cyclic and acyclic tosylhydrazones can partici-
pate in the process. Additionally, a variety of aromatic and
heteroaromatic dibromoderivatives have been employed,
leading to an array of diverse scaffolds featuring a fluorene
or acridine central nucleus, and containing binaphthyl, thio-
phene, benzothiophene and indole moieties. The application
to appropriate tetrabrominated systems led to greater struc-
tural complexity through two consecutive autotandem cas-
cades. The photophysical properties of selected compounds
were studied through their absorption and emission spectra.
Fluorescence molecules featuring very high quantum yields
were identified, showing the potential of this methodology
in the development of molecules with interesting optoelec-
tronic properties.
Introduction
aryl halides as a very reliable method for the synthesis of sub-
stituted alkenes.[9a] In these reactions the tosylhydrazone acts
as a carbene precursor and behaves as the nucleophilic com-
ponent of the cross-coupling. From a mechanistic point of
view, the differencial steps of this transformation include the
formation of a palladium carbene complex and the migratory
insertion reaction. Since our initial contribution, and as a result
of the efforts of several research groups, the application of sul-
fonylhydrazones and diazo compounds in metal-catalyzed pro-
cesses involving metal carbenes and migratory insertion reac-
tions has undergone remarkable development.[10] More recent-
ly, we have incorporated the Pd-catalyzed cross-coupling with
tosylhydrazones in autotandem reactions, that is, processes in
which the same Pd-catalyst is able to promote more than one
independent bond-forming catalytic step.[11] Thus, cascade re-
actions that combine the CÀC bond-forming reaction of tosyl-
hydrazones with a CÀN bond-forming reaction have been de-
veloped to obtain quite complex heterocyclic systems from
simple starting materials (Scheme 1b).[12] Moreover, the concat-
enation of the tosylhydrazone cross-coupling followed a CÀH
functionalization on an azole has led to a straightforward syn-
thesis of pyrroloisoquinolines through an autotandem process
with the creation two CÀC bonds (Scheme 1c).[13]
Polycyclic aromatic molecules with extended p-conjugation
have attracted great interest over the last decades because of
their vast array of applications as advanced materials[1] and key
components of electronic devices. Organic synthesis have pro-
vided tools to incorporate a large diversity of p-conjugated
molecular frameworks[2] allowing for the tuning of the elec-
tronic and photophysical properties.[3] Among the different
structural subunits present in organic functional materials, the
fluorene system stands as an ubiquitous moiety in p-extended
conjugated systems with applications in organic photovolta-
ics[4] and electroluminescent materials.[5] Moreover, the fluorene
structure is also present in molecules with biological activity[6]
and in ligands for organometallic chemistry.[7] For these rea-
sons, the synthesis of fluorenes and structurally related aromat-
ic molecules has attracted a great deal of attention recently.[8]
In the last few years we have been devoted to the develop-
ment of new synthetic methodologies based on the employ-
ment of sulfonylhydrazones in metal-catalyzed and uncata-
lyzed reactions.[9] In particular, in 2007 we reported the Pd-cat-
alyzed cross-coupling reactions between tosylhydrazones and
[a] R. Barroso, Prof. M.-P. Cabal, Prof. C. ValdØs
Departamento de Química Orgµnica e Inorgµnica and
Instituto Universitario de Química Organometµlica
“Enrique Moles”, Universidad de Oviedo
c/Juliµn Clavería 8, Oviedo 33066 (Spain)
E-mail: acvg@uniovi.es
Pd-catalyzed autotandem processes have been particularly
attractive in the synthesis of heterocyclic molecules by com-
bining CÀC and CÀN cross-coupling reactions.[14–16] These ap-
proaches generally rely on the employment of complementary
and preorganized bifunctional coupling partners. For instance,
in the examples presented in Scheme 1, ortho-dihalobenzene
derivatives behave as bis-electrophiles, whereas the b-aminoto-
sylhydrazone (Scheme 1b) and the a-N-azoletosylhydrazone
(Scheme 1c) are the ambidentate nucleophilic partners, respec-
tively. Alternatively, two coupling partners, both featuring an
[b] Prof. R. Badía-LaiÇo
Departamento de Química Física y Analítica
Universidad de Oviedo
c/Juliµn Clavería 8, Oviedo 33066 (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503080.
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hydrazone of cyclohexanone 1a. We expected that, after the
initial cross-coupling reaction to give cyclohexenone 3a, a 5-
exo-trig intramolecular Heck-type cyclization[18] might occur,
giving rise to spirofluorene 4a (Scheme 2). We selected sym-
metrical coupling partners to avoid the formation of re-
gioisomers, hence simplifying analysis of the results.
Scheme 2. Initial CÀC/CÀC Pd-catalyzed auto-tandem reaction considered
Scheme 1. Examples of typical Pd-catalyzed autotandem reactions: a) with
complementary bifunctional coupling partners; b) CÀC/CÀN and c) CÀC/CÀC
cascades of ambidentate electrophiles with functionalized tosylhydrazones;
d) this work’s approach (new bonds are represented in bold).
and some optimization results
As starting point to develop suitable experimental condi-
tions, we selected the standard reaction conditions that we
had developed for the cross-coupling reactions with tosylhy-
drazones and aryl bromides, that is, [Pd2(dba)3] (dba=dibenza-
lacetone) as the Pd-source, Xphos as the ligand, LiOtBu as the
base in dioxane as the solvent. The initial experiments employ-
ing a 1:1 ratio of both coupling partners and 2 mol% of
[Pd2(dba)3] led to a mixture of the cross-coupling product 3a
and the desired spirocycle 4a. To drive the reaction to the ex-
clusive formation of the spirocyclic compound 4a, we in-
creased the catalyst loading as well as the 1a/2a ratio. Thus,
employing 4 mol% of [Pd2(dba)3] and tosylhydrazone 1a in
excess, the spirocyclic compound 4a derived from the auto-
tandem reaction was isolated in an excellent yield of 93%. The
electrophilic and a nucleophilic reacting point could be com-
bined for the construction of cyclic structures through the au-
totandem cascade (Scheme 1a).[15b]
Continuing with our interest in Pd-catalyzed autotandem
processes employing tosylhydrazones, we designed a concep-
tually different approach, in which the functionality formed in
the first Pd-catalyzed step would react in a subsequent Pd-cat-
alyzed CÀC bond-forming reaction. Considering that a new
double bond is formed in the Pd-catalyzed cross-couplings
with tosylhydrazones, we envisioned that it might then react
in an intramolecular Heck reaction, to provide cyclic systems
through a CÀC/CÀC autotandem sequence. This approach
turned out to be successful, and in a preliminary communica-
tion we reported the preparation of spirofluorene and spiro-
acridine derivatives employing this methodology.[17]
1
structure 4a could be assigned based on the 13C, H NMR, and
MS spectra. In particular, the presence of two dt at 5.35 (3J=
4
3
9.9 Hz, J=2.1 Hz), and 6.12 ppm (3J=9.9 Hz, J=3.8 Hz) in the
1H NMR spectrum was a clear indication of the presence of the
endocyclic double bond. Moreover, the spirocyclic structure
was later confirmed by X-ray crystallographic analysis in anoth-
er member of this family of spiro compounds (Figure 1). Inter-
estingly, during the autotandem reaction, the unusual forma-
tion of two CÀCAr bonds on the same carbon atom has taken
place.[6b,19,20]
Encouraged by our preliminary results on these tandem re-
actions as a new methodology for the synthesis of new p-ex-
tended materials, we decided to investigate the scope of the
transformation in more detail. Herein, we present a full ac-
count of our results that show that this chemistry can be ap-
plied to the generation of a diversity of polycarbo- and hetero-
cycles with extended p-conjugation with promising optoelec-
tronic properties. Moreover, some photophysical properties
and potential applications of selected p-extended systems are
also discussed. In this paper, we wish to report a detailed de-
scription of the progress of our work.
The formation of 4a can be explained by considering two
independent processes catalyzed by the same Pd species. As
represented in Scheme 3, a first catalytic cycle involves the for-
mation of 3a through oxidative addition of the Pd0 species to
one of the CÀBr bonds of 2a to form arylpalladium complex A,
generation of the Pd-carbene intermediate B, migratory inser-
tion to give the alkylpalladium complex C, and b-hydride elimi-
nation, that releases the intermediate alkene 3a. Then, alkene
3a will be incorporated to a second catalytic cycle that in-
volves an intramolecular Heck reaction, which ultimately leads
Results and Discussion
Our initial experiments were carried out employing 2,2’-dibro-
mobiphenyl 2a as the ambidentate electrophile and the tosyl-
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reactions involving the formation of seven-membered rings
(see later, Schemes 9 and 10).
With this initial result in hand, and taking into consideration
the mechanistic proposal, we set out to study the scope of
this reaction. We considered the preparation of spirofluorene
derivatives, a type of structure that is currently employed in
the development of electroluminescent and optoelectronic
materials.[21] First, the scope of the reaction was examined re-
garding the structure of the tosylhydrazone. As presented in
Scheme 4 the synthesis of spirofluorenes 4 could be accom-
Figure 1. ORTEP representation of the molecular structure of 4i determined
by X-ray crystallographic analysis.[17]
Scheme 4. Synthesis of spirofluorenes 4 by reaction between cyclic N-tosyl-
hydrazones 1 and 2,2’-dibromobiphenyl 2a. Yields of the isolated product
obtained after column chromatography. [a] Reaction time 48 h. [b] Yield de-
1
termined by H NMR spectroscopy employing triphenylmethane as internal
reference. [c] Structure confirmed by X-ray diffraction.
plished by employing a variety of tosylhydrazones 1 derived
from cyclic ketones. The reaction proceeded successfully with
tosylhydrazones of 4-substituted cyclohexanones and cyclo-
heptanone to give spirofluorenes 4a–c, and 4d, respectively,
but failed with cyclopentanone. Moreover, tetralone tosylhy-
drazone was also an appropriate substrate for the reaction
leading to the interesting spiro derivatives 4e and 4 f which
feature and additional aromatic ring. Similarly, the reaction
with the hydrazone of 4,4,-dimethylcyclohexenone led to the
symmetric spirofluorene 4g featuring two unconjugated
double bonds. Interestingly, the cascade reaction could also be
achieved with the tosylhydrazones of N-benzyl-4-piperidone
and 4-piranone, which led to the corresponding spiro deriva-
tives 4h and 4i featuring enamine and enol ether functionali-
ties, respectively. In particular, the structure of 4i was con-
firmed by X-ray crystallographic analysis (Figure 1).[17] Finally,
the reaction with the hydrazone of 2-methoxycyclohexanone
furnished spiro compound 4j in a completely regioselective
manner.[9d]
Scheme 3. Mechanism proposed for the Pd-catalyzed autotandem process.
to the formation of 4a after the second b-hydride elimination
from the alkylpalladium complex F. Importantly, a requirement
for tosylhydrazones to participate in this tandem reaction
would be to feature hydrogen atoms that could be eliminated
at both the a and b positions. Moreover, for the reaction to be
successful, the intramolecular cyclization must be favored
against the incorporation of a second molecule of tosylhydra-
zone from intermediate D. This is the case for 5-exo-trig cycli-
zations, and for 6-exo-trig Heck reactions, however, it failed for
We next turned our attention to the employment of tosylhy-
drazones derived from acyclic ketones. Indeed, under similar
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reaction conditions, the autotandem reaction proceeded suc-
cessfully for an array of acyclic systems, if the precursor ketone
features hydrogens at the vicinal a and b positions. Thus, the
reactions with hydrazones of ethyl ketones, such as 3-penta-
none and propiophenones led to 9-vinylsubstituted fluorenes
4k, 4l and 4m, respectively (Scheme 5). On the other hand,
the reactions with a hydrazone substituted with longer alkyl
group provided the alkene with the expected trans stereo-
chemistry 4n. Noteworthy, the reaction tolerates the presence
of the ester functionality as represented in 4o. Thus, this set of
examples shows the wide applicability of this methodology as
a general synthesis of functionalized 9,9-disubstituted fluo-
renes.
Scheme 6. Synthesis of symmetrically substituted spirofluorenes 5a–c by re-
action between cyclic N-tosylhydrazones 1 and 5,5’disubstituted-2,2’-dibro-
mobiphenyls 2b,c.
fluorenes feature appropriate functionalities to be further ela-
borated or incorporated into oligomeric structures.
The unsymmetrical 2,2’-dibromides 2d and 2e were pre-
pared from commercially available para-substituted anilines 6,
by aromatic bromination with N-bromosuccinimide (NBS) fol-
lowed by Sandmeyer reaction to yield the corresponding 2-
bromo-1-iodo-4-substituted benzenes 7 (Scheme 7).[24] Next,
the palladium-catalyzed Suzuki–Miyaura cross-coupling reac-
Scheme 5. Synthesis of 9,9-disubstituted fluorenes by reaction between acy-
clic N-tosylhydrazones 1 and 2,2’-dibromobiphenyl 2a.
Considering that substituted fluorenes are structural constit-
uents of molecules with interesting photophysical properties,
and also might be original scaffolds in medicinal chemistry, we
decided to explore the versatility and scope of the autotandem
reaction towards the synthesis of structurally diverse spirofluor-
ene derivatives by employing substituted dibrominated scaf-
folds. Thus, we selected a series of symmetrically substituted
[5,5’-diamino- (2b);[22] 5,5’-dichloro- (2c);[21] and unsymmetrical-
ly substituted [4-chloro- (2d) and 4-methyl-2,2’dibromobiphen-
yl (2e)[23]]2,2’-dibromobiphenyls.
Direct manipulation of commercial 2,2’-dibromobiphenyl led
to symmetrically substituted derivatives 2b and 2c. An auto-
tandem reaction on 2,2’-dibromo-5,5’-dichlorobiphenyl 2c led
to the desired substituted spirofluorenes 5b and 5c, albeit
with moderate to poor yields (Scheme 6). Importantly, al-
though chloroarenes can also react in cross-couplings with to-
sylhydrazones, the formation of the spirofluorene occurred se-
lectively. The reaction with 2,2’-dibromo-5,5’-diaminobiphenyl
2b provided also the spirocyclic compound 5a (Scheme 6).
This time the autotandem reaction takes place in the presence
of the free NH2 groups. Noteworthy, both classes of substituted
Scheme 7. Synthesis of unsymmetrically substituted spirofluorenes 5d–g by
reaction between N-tosylhydrazones 1 and 5,5’disubstituted-2,2’-dibromobi-
phenyls 2d, 2e, and 2 f.
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tion with 2-(2-bromophenyl)-1,3,2-dioxaborinane in the pres-
ence of K3PO4·7H2O in DMF produced 2,2’-dibromo-4-substitut-
ed-biphenyl 2d and 2e.[25] Then, the autotandem reactions
under typical conditions afforded, as desired, the substituted
spirofluorenes 5d,e (Scheme 7).
Additionally, benzyl bromination of 2e led to tribromide 2 f,
which was converted into the new dibromide 2g by cross-cou-
pling with the tosylhydrazone of p-methoxybenzaldehyde, em-
ploying a modification of the conditions reported by Wang.[26]
As expected, under the standard reaction conditions, the new
dibromide 2g provided the fluorenes 5 f and 5g, featuring ad-
ditional conjugation (Scheme 7). Interestingly, two sequential
Pd-catalyzed cross-couplings with tosylhydrazones are em-
ployed in the synthesis of these highly conjugated materials.
The process is also very efficient for the preparation of spiro-
dibenzofluorenes 9 from the corresponding commercially
available 2,2’-dibromobinaphthyl 8, with yields very similar to
those of the reactions with the biphenyl derivatives. Again,
both carbo- and heterocyclic tosylhydrazones were compatible
with this cascade reaction (Scheme 8). These new structures
Scheme 9. Synthesis of spiroacridines 12 and spirodihydroanthracenes 13 by
autotandem reactions. [a] Tosylhydrazone (4 equiv) was employed. [b] Reac-
tion time 48 h. [c] Tosylhydrazone (3 equiv) and LiOtBu (5 equiv) were em-
ployed. [d] N-Methylcarbazole derived from an intramolecular cyclization of
aniline 10 was isolated as major compound.
tions, the diarylanilines showed some tendency to cyclize and
form carbazoles.[16b] Nevertheless, it was found that employing
a larger excess of the tosylhydrazone, the spiroacridines could
be synthesized in preparatively useful yields. Interestingly, the
autotandem reaction under these conditions proceeded suc-
cessfully with the NÀH-free anilines, leading to the spiroacri-
dines 12, valuable structures for the development of molecules
with optoelectronic properties, that could be further elaborat-
ed through the free NÀH.[28] However, the N-methylated ana-
logue 10b, showed a higher tendency to cyclize to the carba-
zole even in the presence of a large excess of tosylhydrazone,
and the spirocompound 12g could be isolated only in very
poor yield. Finally, the reactions with bis(2-bromophenyl)me-
thane 11 occurred uneventfully providing the spirodihydroan-
thracenes 13 with fairly good yields.
Scheme 8. Synthesis of spirodibenzofluorenes 9 by reaction between N-to-
sylhydrazones and 2,2’-dibromobinaphthyl 8.
On the other hand, the reaction with benzyl ether 14 failed
to provide the spirocyclic compound. In this case, we obtained
a mixture of compounds 15 and 16, derived from the mono-
and dialkenylation reaction, respectively, (Scheme 10). Clearly,
the formation of a seven-membered ring through a 7-exo-trig
cyclization is not a favored process.
are particularly appealing because of their very extended p-
conjugation.[27] The presence of the five-membered ring enfor-
ces a very rigid structure that enables conjugation of the two
naphthalene moieties. A discussion on fluorescence and poten-
tial applications of these molecules will be discussed later in
the paper.
To expand the applicability of the autotandem cascade, the
process was also examined employing other brominated scaf-
folds such as bis(2-bromophenyl)amines 10, and bis(2-bromo-
phenyl)methane 11 (Scheme 9). These reactions were expected
to afford the corresponding spiro compounds 12 and 13, fea-
turing a central six-membered ring through a 6-exo-trig cycliza-
tion. Although the initial experiments were carried out under
the same reaction conditions, these cascade processes turned
out to be more challenging. Indeed, under the standard condi-
Scheme 10. Reaction with dibromide 14, which did not afford the spirocyclic
structure.
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To explore the robustness and further applications of this
cascade transformation, more elaborated scaffolds were exam-
ined. Thus, we turned our attention to the incorporation of
heterocycles, considering that the new heterocyclic structures
might be of interest in medicinal chemistry as well as in the
development of molecules with optoelectronic properties. Our
initial effort was directed towards thiophene and benzothio-
phene derivatives that would lead to polycyclic structures,
which are widely present in functional organic materials.[29]
Thus, for our study we chose the 3,3’-dibromo-2,2’-bithiophene
17a[30] and the 3,3’-2,2’-dibenzodithiophene 17b.[31] Delightful-
ly, the autotandem cascade reactions with 17a proceeded as
expected to yield spirodithiophenes[32] 18 with moderate to
good yields (Scheme 11). It is worth noting that molecules 18
Scheme 12. Synthesis of spirocyclic derivatives of indeno[1,2-b]indoles 24
and benzo[b]indeno[2,1-d]thiophenes 25.
no[2,1-d]thiophene derivatives 25 (Scheme 12). The reactions
are not restricted to the preparation of the spirocyclic deriva-
tives; tosylhydrazones derived from acyclic ketones can be also
incorporated, leading to functionalized indenoindoles 24c and
indenobenzothiophene 25d and e, the latter usually with
higher overall yields.
Scheme 11. Synthesis of spirocyclic derivatives of cyclopentadithiophenes
18 and cyclopentadibenzothiophenes 19.
are unprecedented 4H-cyclopenta[2,1-b:3,4-b’]dithiophenes,
structures that are employed as monomers in conductive poly-
mers with applications in organic photovoltaics.[29] This reac-
tion represents a new entry into spiro- and functionalized de-
rivatives of these systems. Moreover, the reactions with the
bis(benzothiophene) 17b provided another class of unknown
spirocyclic structures 19 (Scheme 11) with doubtless interest in
the development of molecules with optoelectronic properties.
A preliminary study on the fluorescence properties of some
members of this family of compounds is discussed below.
Next, we considered using dibromide scaffolds combining
two different aromatic rings that might provide unsymmetrical
spirobenzofluorene analogues. For our study we chose 3-
bromo-2-(2-bromophenyl)-1-tosyl-1H-indole 20 and 3-bromo-2-
(2-bromophenyl)benzo[b]thiophene 21 (Scheme 12). The re-
quired dibrominated indole 20[33] was readily prepared by reac-
tion of N-tosyl-2-ethynylaniline 22 with 1-bromo-2-iodoben-
zene[34] under typical Sonogashira conditions, which afforded
the corresponding indole derivative 23, followed by bromina-
tion with NBS (Scheme 12). The benzothiophene analogue 21
was prepared following a known procedure by selective Suzuki
cross-coupling of 2,3-dibromothiophene with o-bromophenyl-
boronic acid.[35]
These results show clearly the versatility of this approach in
the synthesis of heterocycle-containing p-extended polycyclic
systems. By combining cross-coupling reactions to access the
required dibromides with the cascade cyclization, a wide varie-
ty of structurally diverse spirocyclic or disubstituted analogues
of fluorenes could be efficiently prepared.
Finally, continuing with our idea of employing this autotan-
dem reaction in the synthesis of new fluorene-based structures
with extended p-conjugation, we selected tetrabrominated
scaffolds 26, 27, and 28 (Figure 2), expecting that in each case,
two independent autotandem reactions might occur, giving
rise to new flat polyaromatic structures.
The cascade reactions under typical reaction conditions led
to the expected indeno[1,2-b]indoles 24[36] and benzo[b]inde-
Figure 2. Tetrabrominated scaffolds selected for double autotandem reac-
tions.
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The double autotandem processes were studied, employing
the corresponding excess of tosylhydrazone. The treatment of
2,2’,6,6’-tetrabromobiphenyl 26[37] with cyclohexanone tosylhy-
drazone failed to give the product derived from a double cas-
cade reaction. Instead, a mixture of compounds was obtained,
from which the intermediate 29 (that incorporates two cyclo-
hexene subunits, but in which only one cascade reaction has
taken place) could be isolated, although with very poor yield
(Scheme 13). Clearly, after the formation of the first spirofluor-
Scheme 14. Synthesis of 6,12-dihydroindeno[1,2-b]fluorenes 30 through
a double autotandem cascade.
Scheme 13. Pd-Catalyzed reaction of tosylhydrazone 1a with tetrabromide
26.
ene, a distortion in the bond angles occurs as a result of the
formation of the five-membered ring, leaving carbon atoms
a and b far apart, and a second 5-exo-trig cyclization would be
disfavored. Nevertheless, the formation of 29, although in very
poor yield, indicates that the formation of spirofluorenes sub-
stituted at 4’ and 5’ positions is feasible. We plan to investigate
on those intriguing class of structures in the future.
We next turned our attention to tetrabromide 27, a scaffold
that had been previously employed in the synthesis of func-
tional organic materials with heterocyclic ladderane struc-
tures.[23,38] Unlike the reactions with 26, the autotandem pro-
cesses with tetrabromide 27 proceeded successfully, giving rise
to 6,12-dihydroindeno[1,2-b]fluorenes 30. Noteworthy, these
molecules feature an all-carbon ladderane structure, which has
been employed as building block for blue emitters,[39] and mol-
ecules with other organic electronics applications.[40] The tosyl-
hydrazones of 4-pyranone and propiophenone were employed
this time as representatives of cyclic and acyclic tosylhydra-
zones, respectively (Scheme 14).
The synthesis of tetrabromide 28 was accomplished from tri-
bromide 2 f (Scheme 15). On one side, compound 2g was
transformed into the tosylhydrazone 32 through aldehyde 31.
Next, the Pd-catalyzed cross-coupling reaction between 2g
and 32 occurs chemoselectively through the benzylic position
of the tribromide, leading to tetrabromide 28 as a single dia-
stereoisomer. Then, the cascade reactions with an excess of
the appropriate tosylhydrazone led to the new p-extended
structures 33, which include the fluorene and vinylphenylene
fragments, very common moieties in functional organic materi-
als (Scheme 15). Again, both cyclic and acyclic tosylhydrazones
can be employed in the coupling reaction, leading to structur-
ally diverse polyaromatic structures (Scheme 15). It must be
noted that compounds 30 and 33 are expected to be formed
as a mixture of two diastereoisomers, which have shown iden-
Scheme 15. Synthesis of spirofluorene-vinylphenylene hybrids 33 through
a double autotandem cascade.
Notably, the polycyclic compounds obtained through the
autotandem reactions discussed along this paper feature an
additional double bond that comes from the final b-hydride
elimination in the catalytic cycle. The presence of this function-
ality might be useful to accomplish further modifications, such
as oxidations, ring openings, or eventually to attach these frag-
ments to other molecules through addition reactions or cross-
metathesis. So far, we have concentrated in a quite simple
transformation on the derivatives obtained from the 4-oxa-
none tosylhydrazone, which feature a dihydropyran moiety.
Treatment of the binaphthyl derivative 9e with an alcohol in
1
tical H and 13C NMR spectra in all cases.
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acidic media leads to the formation of the corresponding
acetal 34, emulating the reactivity of dihydropyran, a very
commonly used protective group for alcohols.[41] As an exam-
ple, a terminal triple bond, that could be later elongated
through click chemistry was attached to the fluorescence spi-
robinaphthyl to yield the functionalized derivative 34b
(Scheme 16). This very simple reaction might enable the modi-
fication of these structures to tune their physical properties,
such as solubility or aggregation in solid phase, or to attach
the fluorophore structures to polymers, solid supports or mon-
olayers, as well as open the door to the employment of these
molecules as fluorescence tags that can be introduced and
cleaved under the same conditions as the THP protective
group.
From our point of view, the data obtained from spirodiben-
zofluorenes 9 and 34 are quite appealing. Their absorption
spectra in CH2Cl2 highly dilute solution feature p!p* transi-
tions at about lmax =353 and 370 nm, showing a large batho-
chromic shift when compared with the absorption spectra re-
ported for 1,1’-binaphthyl (lmax =280 and 295 nm).[42] Interest-
ingly, whereas in typical 2,2’-binaphthyls there is no conjuga-
tion between the two napthyl rings because of the steric re-
pulsions between the substituents at positions 8 and 8’, the
geometrical distortion created by the formation of the five-
membered ring enforces a rigid structure in which both naph-
thyl rings are arranged in a situation closer to planarity, there-
fore promoting effective conjugation between the rings. In
fact, DFT molecular modeling studies predict a structure in
which both naphthalene rings are slightly bent (torsion angle
of 218) to avoid the steric interaction between the hydrogens
at the internal positions (Figure 3). Nevertheless, despite the
distortion from planarity, conjugation between both naphtha-
lene fragments occurs: the graphical representations of HOMO
and LUMO orbitals clearly show the extension of the p-conju-
gation along both aromatic structures.[43] Interestingly, these
compounds turned out to be highly fluorescent, showing an
emission band at about 403 nm with also high quantum
yields.
Scheme 16. Attachment of alcohols to fluorescence spirobinaphthyl 9 taking
advantage of the enol ether functionality.
Regarding the thiophene-containing spiroderivatives, the
simplest 4H-cyclopenta[2,1-b:3,4-b’]dithiophene 18 showed an
absorption band at lmax =318 nm, but no significant emission
bands. The non-symmetric compound 25a presented an emis-
sion band at 357 nm, although with a modest quantum yield.
However, the benzocondensed analogues 19a and 19c fea-
tured significant emission bands at 407 and 383 nm, respec-
tively. These results indicate the potential interest of these pre-
viously unknown structures in the design of new electrolumi-
nescent materials by modifying the aromatic backbones at-
tached at the spirocyclic central core.
Photophysical properties
Most of the molecules described along this work feature un-
precedented structures with extended p-conjugation. To evalu-
ate the potential interest of these classes of molecules in the
development of functional organic materials and luminescent
devices, we carried out a study of the photophysical properties
of some selected compounds by means of their UV/Vis absorp-
tion and fluorescence spectra. Thus, thiophene- and benzothio-
phene-containing spirocompounds (18, 19, and 25), spiro di-
benzofluorenes (9 and 34) and bisfluorenes (33) were studied.
A summary of the results is presented in Table 1.
Finally, the UV/Vis spectra of bisfluorenes 33 show an ab-
sorption band at lmax =367 nm. The importance of the pres-
ence of the fluorene structure to enforce the extended p-con-
jugation is shown by comparing
these data with the spectra of
the tetrabromide precursor 28,
which features absorption at
Table 1. Absorption and emission spectral data for selected compounds.[e]
labs
[nm][a]
(eÆs[b])”104
lem
[nm][a]
F[d] Æs[b]
lmax =320 nm (Figure 4). More-
[LmolÀ1 cmÀ1
]
[%]
over, compounds 33 are strongly
9e
250
250
250
318 (315)
316 (305 sh, 315)
315
355 (350)
334, 351 (333 347)
326
366
366
366
4.1Æ0.1
385 sh, 403
385 sh, 401
385 sh, 402
399 (372)
357
356
407 (404)
383, 403 (377 398)
379
399–422
60Æ10
fluorescence in CH2Cl2 solution,
34a
34b
18b
25a[c]
25c[c]
19a[c]
19c[c]
28
2.15Æ0.1
2.15Æ0.06
2.8Æ0.1
110 Æ20
with sharp emission bands at
60Æ10
399 and 423 nm, and very high
1.0Æ0.1 (0.3)
quantum yields (Figure 4 and
1.7Æ0.1 (0.97Æ0.01)
1.0Æ0.2
16Æ2 (11.8Æ0.4)
17Æ2
Table 1). The type of substitution
1.2Æ0.1 (0.28Æ0.01)
0.1 (0.77Æ0.3)
2.3Æ0.2
24Æ3 (49Æ3)
at C9 on the fluorene moieties
18Æ3 (25Æ1)
has no significant influence on
the position of the absorption
and emission bands, neverthe-
less, the determined quantum
yields differ slightly (Table 1) but
consistently maintaining very
high values.
33a
33b
33c
3.18Æ0.08
68Æ9
0.98Æ0.06
399–423
399–422
98Æ6
1.0Æ0.1
100Æ10
[a] Measured in CH2Cl2 (1”10À5 m) at RT. [b] Standard deviation. [c] Measurements carried out in hexanes are in-
dicated in brackets. [d] Fluorescence quantum yields relative to quinine sulfate 0.1 N in H2SO4 (F=0.55 at
360 nm). [e] More detail is provided in the Supporting Information.
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Figure 3. Above: Three-dimensional optimized structure[44] (b3Lyp/6-
31+G**) for the spirobinaphthyl parent structure. A dihedral angle of 21.68
between the two naphthyl moieties facilitates the conjugation over the
whole aromatic surface. Below: Representation of the HOMO and LUMO mo-
lecular orbitals that show the delocalization over the whole aromatic sur-
face.
Figure 5. Three-dimensional optimized structure (b3Lyp/6-31+G**) for the
spirofluorene-vinylphenylene hybrid 33b. A 2 nm-long planar structure is
predicted. Representation of the HOMO and LUMO molecular orbitals that
show the delocalization over the whole p-surface.
already discovered new structures with promising properties
as blue-light emitters.[45] Moreover, the modular nature of our
methodology might allow for the combination of different aro-
matic structures with suitable substitutions and different hy-
drazones, offering the opportunity of tuning the physical prop-
erties as well as the absorption and emission properties of the
molecules.
Conclusion
We have shown that the autotandem Pd-catalyzed reaction be-
tween tosylhydrazones and 2,2’-dibromobiphenyls and related
systems represents a very versatile method for the synthesis of
p-extended conjugated molecules. The method allows for the
introduction of a variety of structures, substituents, and func-
tional groups around the basic fluorene structure, enabling the
tuning of the electronic properties of the materials. As a proof-
of-concept, we have already developed a very straightforward
method for the preparation of highly fluorescence blue-emit-
ting molecules through a synthetic route that involves two
consecutive Pd-catalyzed reactions with tosylhydrazones.
Figure 4. Fluorescence spectra for compounds 28 and 33a–c 1”10À5 m sol-
utions in CH2Cl2). a: excitation spectra; solid lines: emission spectra. The
inserted picture corresponds to solutions of 33a and 33b in CH2Cl2 upon
UV irradiation at 365 nm.
DFT-based molecular modeling studies on 33b shows a total-
ly planar structure for these fluorene-vinylphenylene hybrids,
which enables the efficient conjugation of the all-carbon-p-ex-
tended system responsible for the blue emission properties
observed (Figure 5). Noteworthy, the nearly 2 nm long planar
structure was synthesized from simple materials through two
consecutive cross-couplings based on tosylhydrazones
(Scheme 15). Interestingly, these structures resulted in very effi-
cient blue-emitting molecules in solution (Figure 4).
Experimental Section
General procedure for the Pd-catalyzed auto-tandem reac-
tion between tosylhydrazones 1 and dibromides
These results clearly show the versatility of our methodology
for the generation of structurally diverse small molecules with
extended p-conjugation and interesting photophysical proper-
ties. Although we have not attempted so far to optimize the
conjugated structures into any particular application, we have
Synthesis of polycyclic aromatic molecules with extended p-conju-
gation: A carousel reaction tube was charged under nitrogen at-
mosphere with the corresponding N-tosylhydrazone (2 equiv), biar-
yldibromide compound (0.2 mmol, 1 equiv), tris(dibenzylideneace-
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Acknowledgements
Financial support of this work by Ministerio de Economía y
Competitividad of Spain: Grant CTQ2013-41336-P. A FICYT
(Principado de Asturias, Spain) predoctoral fellowship to R.B. is
gratefully acknowledged.
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Keywords: tandem catalysis · cross-coupling · fluorenes ·
fluorescence · palladium
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Received: August 5, 2015
Published online on September 25, 2015
Chem. Eur. J. 2015, 21, 16463 – 16473
www.chemeurj.org
16473
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