A [2+2+2]-Cyclotrimerization Approach to Selectively Substituted Fluorenes and Fluorenols, and Their Conversion to 9,9′-Spirobifluorenes

Article abstract of DOI:10.1002/chem.201502370

Synthesis of selectively substituted fluorenes and fluorenols was achieved by using catalytic [2+2+2]cyclotrimerization. Various starting diynes were reacted with different alkynes in the presence of a catalytic amount of Wilkinson's catalyst (RhCl(PPh₃)₃) providing the compounds possessing the fluorene scaffold in good isolated yields. A set of four regioselectively substituted fluorenols was converted to the corresponding 9,9′-spirobifluorenes and their spectral characteristics were measured. A select group! Selectively substituted fluorenes and fluorenols were synthesized by RhCl(PPh₃)₃ catalyzed [2+2+2]-cyclotrimerization of the diynes with alkynes. Fluorenols were subsequently transformed into a set of four regioselectively substituted 9,9′-spirobifluorenes.

Full text of DOI:10.1002/chem.201502370

DOI: 10.1002/chem.201502370
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&
Spiro Compounds |Hot Paper|
A [2+2+2]-Cyclotrimerization Approach to Selectively Substituted
Fluorenes and Fluorenols, and Their Conversion to 9,9’-
Spirobifluorenes
[a]
[a]
[b]
[b]
[a]
ˇ
ˇ
Reinhard P. Kaiser, Filip Hessler, Jirí Mosinger, Ivana Císarovµ, and Martin Kotora*
Dedicated to Professor Tamotsu Takahashi on the occasion of his 60th birthday
Abstract: Synthesis of selectively substituted fluorenes and
fluorenols was achieved by using catalytic [2+2+2]cyclotri-
merization. Various starting diynes were reacted with differ-
ent alkynes in the presence of a catalytic amount of Wilkin-
son’s catalyst (RhCl(PPh3)3) providing the compounds pos-
sessing the fluorene scaffold in good isolated yields. A set of
four regioselectively substituted fluorenols was converted to
the corresponding 9,9’-spirobifluorenes and their spectral
characteristics were measured.
Introduction
development of OLEDs (organic light emitting diodes) and es-
pecially recently as blue-light-emitting OLEDs. Specifically, the
most promising candidates are 9,9’-spirobifluorenes that are
prepared from the parent fluorenes or fluorenones.[8] Worth
mentioning in this respect are recent pioneering experiments
indicating that tuning of their physico-optical properties can
be achieved by selective functionalization of each aromatic
ring of the 9,9’-spirobifluorene fluorene scaffold.[9,10]
The transition-metal-catalyzed or mediated [2+2+2] cycloaddi-
tion of alkynes is a useful and atom-economical method for
the construction of the benzene ring.[1] It has also been applied
for synthesis of compounds with extended aromatic systems,
for example, polyphenylenes,[2] helicenes,[3] phenanthrenes and
other condensed polyaromatics,[4] and linear acenes.[5]
Among the class of aromatic compounds can be found
a number of compounds possessing privileged structures that
are important substances for application in various fields of
chemistry and, recently, also in material science based on or-
ganic molecules. One such group of aromatic compounds in-
cludes substances possessing a fluorene scaffold.[6] Due to its
structural features, that is, a five-membered ring sandwiched in
between two benzene rings, it combines properties typical for
cyclopentadienes and benzenes. For example, properties of
such fluorine–metal complexes can be finely tuned by steric
and electronic effects of substituents attached to the fluorene
scaffold.[7]
As far as synthesis of substituted fluorenes and fluorenones
is concerned (they can be easily interconverted into each other
by using simple redox techniques), a number of processes
have been developed. The historical one leading to alkylated
fluorenes was based on Lewis acid-catalyzed alkylation of pris-
tine fluorene, which gave rise to numerous regioisomers.[11]
A
range of polyethylated fluorenes was also prepared by Friedel–
Crafts reactions.[12] 1,2,3,4-Tetrasubstituted fluorenes could be
accessed through the TiCl4 induced rearrangement of bis-
(indenyl)zirconacyclopentadienes.[13] Another approach to tet-
rasubstituted fluorenes relies on a Diels–Alder reaction of cy-
clobutadienes with phenylpropynoates (Dewar benzene path-
way), followed by rearrangement and Friedel–Crafts acyla-
tion.[14] A sequential cycloaddition reactions of phenylated
cyclopentadienones provided phenylated fluorenes.[15] Biphenyl
amides underwent intramolecular Friedel–Crafts acylation in
moderate to high yields.[16] Synthesis of monosubstituted fluo-
renes was based on cascade allenylation, electrocyclization,
and intramolecular Friedel–Crafts reactions.[17] ortho-Metallation
of biphenyl derivatives was exploited as well.[18]
Another application of fluorenes and their congeners has
emerged within the last 20 years. They constitute a distinct
family of compounds used as highly attractive candidates for
[a] R. P. Kaiser, Dr. F. Hessler, Prof. Dr. M. Kotora
Department of Organic Chemistry
Faculty of Science, Charles University in Prague
Albertov 6, 128 43 Praha 2 (Czech Republic)
E-mail: kotora@natur.cuni.cz
In addition to the above mentioned list of methods,
a number of transition metal catalyzed processes have been
developed as well. These include a) Pd-, Rh-, or Ir-catalyzed de-
hydrogenative cyclization of benzophenones[19] or diphenyl-
methanes.[20] b) Pd-catalyzed intramolecular Heck reaction,[21]
c) carbonylation followed by intramolecular acylation,[22] d) Pd-
catalyzed cross-coupling reactions followed by addition to the
ˇ
[b] Prof. Dr. J. Mosinger, Dr. I. Císarovµ
Department of Inorganic Chemistry
Faculty of Science, Charles University in Prague
Albertov 6, 128 43 Praha 2 (Czech Republic)
Supporting information for this article, including all experimental proce-
dures, compound characterization data, spectroscopic measurements, and
1H and 13C NMR spectra, is available on the WWW under http://dx.doi.org/
10.1002/chem.201502370.
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carbonyl group,[23] or Friedel–Crafts reaction,[24] e) Pd-catalyzed
arylboration of the carbonyl group followed by Heck reac-
tion,[25] f) Pd- or Rh-catalyzed cyclization of biphenyl deriva-
tives,[26] g) Pd-catalyzed cross-coupling/CÀH activation process-
es suitable for preparation of methylated fluorenes,[27] h) elec-
trophilic bromination followed by Pd-catalyzed Suzuki coupling
used for preparation of phenylated fluorenes,[28] i) Au-catalyzed
retro-Buchner reaction of aryl substituted cycloheptatrienes,[29]
and j) Fe-catalyzed cross-dehydrogenative coupling of ortho-
formyl biphenyls.[30]
Table 1. Cyclotrimerization of 3a and 3b with alkynes 5 to fluorenes 1.
However, to the best of our knowledge, the use of catalytic
[2+2+2]cyclotrimerization for the synthesis of fluorenes has
not been reported yet. This is surprising because it would
allow synthesis of selectively substituted regioisomers under
mild and neutral reaction conditions.
We envisioned that regioselectively substituted fluorenes
1 or fluorenols 2 could be accessed by using cyclotrimerization
of suitably substituted diynes 3 or diynols 4 with alkynes 5
(Scheme 1). In this respect it is worth noting that several struc-
turally related diynes have been prepared but they have not
been subjected to cyclotrimerization reactions.
Results and Discussion
The respective diynes 1 and 2 (Figure 1) were prepared by
using standard synthetic tools. The 1a and 1b were prepared
by Negishi coupling of the corresponding alkynylzinc halides
with 2-iodobenzyl bromide in the presence of Pd(DPE-
Phos)Cl2.[31] Syntheses of the symmetrically or unsymmetrically
substituted diynes 2 started from 2-ethynylbenzaldehyde or 2-
iodobenzadehyde, followed by Sonogashira coupling and addi-
tions of corresponding alkynyl acetylides to the carbonyl
group (for details, see the Supporting Information).
[a] A) RhCl(PPh3)3 (10 mol%), toluene, 908C, 16 h; B) CpCo(CO)2
(10 mol%), THF, MW, 1808C, 1 h; C) Cp*RuCl(cod) (10 mol%), toluene,
908C, 16 h; D) Ni(cod)2/2PPh3 (10 mol%), THF, 208C, 3 h. [b] Isolated
yields.
Our initial endeavors started with screening of cyclotrimeri-
zations of 3a with various alkynes 5 (Table 1) At the outset we
reacted 3a with 2-butyne 5a in the presence of Wilkinson’s
catalyst (RhCl(PPh3)3, 10 mol%) in toluene. No reaction was ob-
served at 208C, but heating of the reaction mixture at 908C
provided the desired fluorene 1aa in 34% isolated yield. The
volatility of 2-butyne 5a could account for a lower yield of
1aa. Carrying out the reaction with 3-hexyne 5b under the
same reaction conditions gave 1ab in an isolated yield of
83%. We then decided to test other frequently used catalysts,
such as CpCo(CO)2, Cp*RuCl(cod), and Ni(cod)2/2PPh3 (cod=
1,5-cyclooctadiene). However, only the first and third of these
catalysts gave the desired product 1ab, albeit in very low
yields of 7 and 17%, whereas the use of the second catalyst
did not promote any reaction and the starting material re-
mained intact. The reaction was also tested in dichlorometh-
ane, but once again it did not proceed. Next, cyclotrimerization
with 4-octyne 5c was carried out and the corresponding fluo-
rene 1ac was isolated in 23% yield. The lower yield of 1ac
was probably due to the bigger steric hindrance exerted by
nPr groups in comparison with the Et one. Gratifyingly, the re-
Scheme 1. Retrosynthetic analysis of fluorene formation.
Figure 1. The prepared diynes 3 and 4.
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action with 2-butyn-1,4-diol 5d proceeded with high conver-
sion, furnishing 1ad in a high yield of 93%. Then our attention
turned to diyne 3b and similar results were observed. The re-
actions with alkynes 5a–d furnished the corresponding fluo-
renes 1ba–1bd in 23, 66, 18, and 69% isolated yields, respec-
tively.
Table 2. Cyclotrimerizations of 4 with various alkynes 5 to fluorenols 2.
In addition to the above-mentioned reactions, cyclotrimeri-
zations of 3a with terminal alkynes such as phenylethyne 5e
and ferrocenylethyne 5 f were attempted. The former gave rise
to a 1:1 mixture of regioisomers 1ae and 1ae’ (85% yield),
whereas the latter produced a 4:1 mixture of regioisomers 1af
and 1af’ (75% yield). The structure of 1aa, 1ab (see the Sup-
porting Information), and 1af (Figure 2) were unequivocally
Figure 2. PLATON drawing of 1af showing displacement ellipsoids on the
30% probability level.
zations of the protected diyne TBS-4a, bearing phenyl groups
with 2-butyne 5a, 3-hexyne 5b, 4-octyne 5c, and 2-butyn-1,4-
diol 5d, were carried out, and the corresponding fluorenols
TBS-2a–d were obtained in 19, 71, 56, and 82% isolated
yields, respectively. In order to avoid the use of the protective
group and thus save a few synthetic steps, cyclotrimerizations
of the free diyne 4a were also carried out with alkynes 5a–d
confirmed by X-ray structure analyses (see Supporting Informa-
tion for details). Although speculative, the difference in the re-
gioisomer ratio could probably be attributed to a different
steric hindrance exerted by the ferrocene moiety in ferroceny-
lethyne 5 f, affecting preferential insertion into one of the RhÀ under the same reaction conditions. Gratifyingly, in all cases
C bonds in the intermediate rhodacyclopentadiene. Interest-
ingly, attempts to bring about cyclotrimerization of 3a or 3b
with diphenylethyne or its substituted congeners such as
bis(pCF3C6H4)ethyne or bis(pCH3C6H4)ethyne in the presence of
a catalytic amount of RhCl(PPh3)3, Cp*RuCl(cod), CpCo(CO)2, or
Ni(cod)2/2PPh3, were not met with success. Only homocyclotri-
merizations of 1a or 1b were observed. The preferential
homocyclotrimerization was probably caused by steric effects
in the alkynes used.
the corresponding fluorenols 2aa–2ad were obtained in good
isolated yields of 25, 76, 26, and 86%, respectively. The reac-
tion of 4a with 3-hexyne 5b was also carried out in the pres-
ence of 5, 2, and 1 mol% of RhCl(PPh3)3 to check the effect of
catalyst loading on the reaction yield. 2ab was obtained in
a good isolated yield of 65% in the presence of 5 mol% of the
catalyst. However, negligible yields (5 and 2%) were obtained
in the presence of 2 and 1 mol% of the catalyst. These results
clearly indicate that rather high catalyst loading (10 mol%) is
required for the successful course of the reaction. Reactions of
4a with 3-hexyne 2b were also run in the presence of catalytic
amounts of CpCo(CO)2, Cp*RuCl(cod), and Ni(cod)2/2PPh3.
In the next phase, we decided to screen the scope of the cy-
clotrimerization reaction with fluorenols 4 in the presence of
10 mol% of Wilkinson’s catalyst (Table 2). Initially, cyclotrimeri-
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However, only the use of the first and the third catalysts gave
the desired product 2ab, albeit in low yields of 11 and 19%.
The use of Cp*RuCl(cod) in toluene or dichloromethane did
not provide the desired product. The results obtained with 4a
were similar to those obtained with TBS-4a, clearly demon-
strating that protection of the hydroxyl group is not essential
for the successful course of the reaction. Cyclotrimerization of
diyne 4b, possessing propyl groups with 3-hexyne 5b and 2-
butyn-1,4-diol 5d, furnished fluorenols 2bb and 2bd in 38 and
77% isolated yield, respectively The use of Ni(cod)2/2PPh3 pro-
vided 2bb in 11 % yield.
Our attention then turned to unsymmetrically substituted
diynes 4c and 4d. Both of them were cyclotrimerized with 3-
hexyne 5b and furnished the corresponding tetrasubstituted
fluorenols 4cb and 4db in 62 and 48% isolated yields, respec-
tively. Finally, reactions of 4a and 4b with ferrocenylethyne 5 f
were carried out. The former provided a 3:1 mixture of re-
gioisomers 2af and 2af’ in 80% yield, and the latter gave rise
to an inseparable 2.5:1 mixture of regioisomers 2bf and 2bf’
in 67% yield. Attempts to carry out cyclotrimerization of 4a
with diphenylethyne in the presence of RhCl(PPh3)3 or
Ni(cod)2/2PPh3 were again not met with success. The reaction
failed to produce even traces of the desired products.
Their structure was unequivocally confirmed by single-crystal
X-ray structure analysis of 7a (see the Supporting Information)
and 7d (Figure 3).
With 9,9’-spirobifluorenes 7a–d on hand, their optophysical
properties were studied. All four compounds exhibit similar ab-
sorption spectra; moreover, in the range 260–320 nm with four
absorption bands (Figure 4 and the Supporting Information, SI-
Table 2) similar to those observed for 9,9’-spirobifluorene (SBF)
and 4-phenyl-spirobifluorene (4-Ph-SBF) analogs in the same
solvent (257, 275, 296, and 308 nm).[9] The absorption band at
312 nm can be attributed to redshifted p–p* transition of the
fluorenyl unit of SBF (308 nm).[9] Despite similar absorption
spectra, fluorescence spectra of the four compounds show dif-
ferent emission profiles depending on the substitution on the
SBF core (Figure 5). The first type of emission spectra recorded
for 7b and 7c presents well-resolved emission spectra, practi-
cally identical for both compounds, with maxima at 315 and
328 nm. The same profile of spectra was reported for SBF
(lmax =311 and 323 nm) and for 2-substituted SBF derivatives,
Having a set of variously substituted fluorenols on hand, we
decided to convert them into 9,9’-spirobifluorenes (Table 3).
The regioselectively substituted fluorenols 2ab–2 db were oxi-
dized with PCC in dichloromethane into the corresponding flu-
orenones 6. This transformation proceeded uneventfully and
the desired fluorenones 6a–d were obtained in excellent iso-
lated yields (89–94%). Finally, fluorenols were converted into
the corresponding 9,9’-spirobifluorenes 7 according to the pre-
viously reported two-step method[32] comprising addition of
ortho-lithiobiphenyl to fluorenols followed by acidic treatment.
Thus, a set of four regioselectively substituted 9,9’-spirobifluor-
enes 7a–d was prepared in good isolated yields (66–84%).
Table 3. Synthesis of four 9,9’-spirobifluorenes 7.
Figure 3. PLATON drawing of 7d showing displacement ellipsoids on the
30% probability level.
[a] DCC, CH2Cl2, 208C. [b] 1: 2-LiC6H4-Ph, THF, -78 to 208C, 12 h; 2: 12 m
HCl, AcOH.
Figure 4. Absorption spectra of 10À6 m solutions of 7a (black), 7b (green) 7c
(red), and 7d (blue) in cyclohexane.
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Acknowledgements
This work was supported by Czech Science Foundation (grant
No. 13-15915S). R.P.K. would like to gratefully acknowledge the
support from the Cusanuswerk e.V.. The authors also would
like to thank Lach-Ner s.r.o. for the generous gift of chemicals
as a part of the award given to M.K.
Keywords: aromatic compounds · catalysis · cycloaddition ·
rhodium · spiro compounds
Figure 5. Normalized and corrected emission spectra of 10À6 m solutions of
7a (black), 7b (green), 7c (red), 7d (blue) in cyclohexane. (lex =250 nm).
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midine (lmax =338 and 355 nm) at the C2 position of the SBF
core. The second, structureless type of emission spectra with
one broad emission band, was observed for 7a (lmax =333 nm)
and 7d (lmax =330 nm). The same character of spectra was re-
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phenylphosphine oxide-SBF (lmax =346 nm),[33] 4-benzothio-
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369 nm).[35]
=
The emission quantum yields (Fs) of the derivatives were
determined in cyclohexane by standard procedures with DPA
as reference (see the Supporting Information, SI-Table 2 and SI-
Figures 4–7). All samples exhibit a high quantum yield of emis-
sion; in particular, 7c derivative appears to be very efficiently
fluorescent with Fs =0.87. Therefore, it is likely that substitu-
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Published online on August 6, 2015
Chem. Eur. J. 2015, 21, 13577 – 13582
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