Transition-metal-free intramolecular carbene aromatic substitution/Büchner reaction: Synthesis of fluorenes and [6,5,7]benzo-fused rings

Article abstract of DOI:10.1002/anie.201409982

Intramolecular aromatic substitution and Büchner reaction have been established as powerful methods for the construction of polycyclic compounds. These reactions are traditionally catalyzed by Rh^II catalysts with a-diazocarbonyl compounds as the substrates. Herein a transition-metal-free intramolecular aromatic substitution/Büchner reaction is presented. These reactions use readily available N-tosylhydrazones as the diazo compound precursors and show wide substrate scope.

Full text of DOI:10.1002/anie.201409982

Angewandte
Chemie
DOI: 10.1002/anie.201409982
Carbenes
Transition-Metal-Free Intramolecular Carbene Aromatic Substitution/
Bꢀchner Reaction: Synthesis of Fluorenes and [6,5,7]Benzo-fused
Rings**
Zhenxing Liu, Haocheng Tan, Long Wang, Tianren Fu, Ying Xia, Yan Zhang, and Jianbo Wang*
Dedicated to Professor Li-Xin Dai on the occasion of his 90th birthday
Abstract: Intramolecular aromatic substitution and Bꢀchner
reaction have been established as powerful methods for the
construction of polycyclic compounds. These reactions are
traditionally catalyzed by RhII catalysts with a-diazocarbonyl
compounds as the substrates. Herein a transition-metal-free
intramolecular aromatic substitution/Bꢀchner reaction is pre-
sented. These reactions use readily available N-tosylhydra-
zones as the diazo compound precursors and show wide
substrate scope.
aromatic substitution[2] and Bꢀchner ring expansion[3]
(Scheme 1). These reactions provide a unique approach for
the construction of ring systems. The Bꢀchner reaction has
been known to proceed through norcaradiene intermediate I.
Since the early 1990s, RhII-catalyzed intramolecular Bꢀchner
reaction has been extensively studied by the groups of
McKervey, Doyle, Moody, Maguire, and others.[4] This
unique approach toward seven-membered carbocycles has
been utilized in natural product synthesis.[5]
When the reaction leads to a highly strained five–six
bicyclic system, the reaction usually follows a different path
such as electrophilic aromatic substitution.[2] This type of
reaction also finds applications in the synthesis of benzo-fused
bicyclic systems. However, these reactions need transition
metal catalysts or acid catalysts to achieve high reactivity and
selectivity. Besides, a-diazocarbonyl compounds are used as
the substrate, imposing some limitations on the substrate
scope.
T
he reaction of diazo compounds with arenes has been
known for long time. In general, the diazo compounds
stabilized by electron-withdrawing substituents, namely a-
diazocarbonyl compounds, have been employed as the
substrates.[1] Due to the lack of selectivity of free carbenes
generated through photolysis or thermolysis, these reactions
are usually catalyzed by transition metals or acids. Classical
reactions of a-diazocarbonyl compounds with arenes include
On the other hand, nonstabilized diazo compounds can be
generated in situ from N-tosylhydrazones through the treat-
ment with base (Bamford–Stevens reaction).[6] Recently, N-
tosylhydrazones have been extensively explored as diazo
compound precursors in transition-metal-catalyzed coupling
reactions[7] and other related reactions.[8] However, to the best
of our knowledge, the nonstabilized diazo compounds gen-
erated in situ from N-tosylhydrazones have only been spor-
adically studied for intramolecular aromatic substitution and
Bꢀchner reactions.[9] As a continuation of our own interest in
this area, herein we report transition-metal-free reactions of
the N-tosylhydrazones as shown in Scheme 2. Depending on
the substrates, the reaction either undergoes intramolecular
aromatic substitution or intramolecular Bꢀchner reaction.
Both reactions are highly efficient, thus constituting useful
methods for the construction of aromatic polycyclic ring
systems.
Scheme 1. RhII-catalyzed intramolecular reaction of aromatic a-diazo-
carbonyl compounds.
[*] Z. Liu, H. Tan, L. Wang, T. Fu, Y. Xia, Dr. Y. Zhang, Prof. Dr. J. Wang
Beijing National Laboratory of Molecular Sciences (BNLMS) and
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education
College of Chemistry, Peking University
Beijing 100871 (China)
E-mail: wangjb@pku.edu.cn
At the outset of this investigation, we employed N-
tosylhydrazone 1a (R = H) as the substrate with Rh2(OAc)4
Prof. Dr. J. Wang
The State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences
Shanghai 200032 (China)
[**] The project was supported by the National Basic Research Program
of China (973 Program, number 2015CB856600), the NSFC (grant
numbers 21272010 and 21332002).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409982.
Scheme 2. Transition-metal-free reaction of aromatic diazo compounds
generated in situ from N-tosylhydrazones.
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Table 1: Optimization of the reaction conditions.[a]
Entry
Substrate
Cat. (mol%)
Base
T [8C]
Yield [%][b]
1
2
3
4
5
6
7
8
1a
1b
1b
1b
1b
1b
1b
1b
Rh2(OAc)4 (5)
Rh2(OAc)4 (5)
LiOtBu
LiOtBu
LiOtBu
Cs2CO3
NaOMe
NaH
110
110
110
110
110
110
90
10
96
91
86
45
99
50
18
–
–
–
–
–
–
NaH
NaH
70
[a] All reactions were carried out with 1a or 1b (0.40 mmol), base
(0.48 mmol) in 4.0 mL toluene for 4 h. [b] Yields of isolated product.
as the catalyst (Table 1). The reaction afforded the intra-
À
molecular aromatic C H insertion product fluorene 2a (R =
H) in only 10% yield, with dimerization being the major side
reaction (Table 1, entry 1). Switching the substrate to 1b (R =
Me) dramatically changed the reaction, affording the corre-
sponding fluorene 2b (R = Me) in 96% yield (Table 1,
entry 2). Surprisingly, we found that the reaction could
afford the product 2b in 91% yield in the absence of
Rh2(OAc)4 catalyst (Table 1, entry 3). Different bases were
then examined and NaH was found to be the best one,
affording fluorene 2b in almost quantitative yield (Table 1,
entry 6). The reaction was found to be significantly affected
by the reaction temperature. The yields diminished dramat-
ically at low temperature (Table 1, entries 7 and 8).
Scheme 3. Transition-metal-free synthesis of fluorene derivatives. All
reactions were carried out with 1a–u (0.40 mmol), NaH (0.48 mmol)
in toluene (4.0 mL) at 1108C for 4–15 h. All yields refer to isolated
products. [a] Reaction time was 11 h. [b] Reaction time was 4 h.
[c] Reaction time was 15 h. [d] Reaction time was 6 h.
The results shown in Table 1 demonstrate an efficient
synthesis of fluorene. Fluorene and its derivatives have found
applications in various fields, especially in material scien-
ces.[10] Traditional methods for the synthesis of the fluorene
structure call for multistep reactions.[11] Recently, a transition-
gave 99% yield (2h). Next, we examined the effect of the
substituent on the other aryl ring. When one of the insertion
positions was blocked (1i), the product 2i was isolated in 91%
yield. For 1j, there were two different insertion positions, and
the products were found to be 1:1 mixtures of the isomers 2j.
When the two insertion positions were symmetrical, it was
found that the yields were not significantly affected by the
substituents. Both electron-rich (1k–m) and electron-poor
substituents (1n–o) afforded the corresponding products 2k–
m and 2n–o in good to excellent yields. Moreover, the
reaction with substrates bearing a heteroaromatic ring (1s–u)
also proceeded smoothly to give 2r–t, albeit with diminished
yields in the case of 2t.
À
metal-catalyzed C H activation strategy has been developed
for the synthesis of fluorenes.[12] In contrast to the previous
methods, the fluorene synthesis shown herein has some
unique merits. First, the starting materials could be readily
accessed through Suzuki–Miyaura coupling reaction and
subsequent condensation with TsNHNH2. Second, the tran-
sition metal catalyst is not involved in the reaction, thus
avoiding the problem of heavy metal contamination. Thus, we
proceeded to explore the substrate scope of the reaction
(Scheme 3).
The scope of the reaction was first tested by varying
a series of R groups in the substrates. When the R group is H,
Me, Ph, (1a, b, e), the yields are good to excellent. For the
substrates in which R is Et or iPr (1c, d), the yields are slightly
diminished due to the 1,2-H shift side reaction. In the case of
1 f (R = CF3), the diminished yield is presumably attributed to
the low reactivity of the corresponding carbene.
Next, the effect of substituents on the aromatic rings was
investigated. The p-Cl substituent on the phenyl ring bearing
a tosylhydrazone moiety (1g) showed a detrimental effect on
the reaction giving 2g. The naphthyl-substituted substrate
It has been well documented that the chemoselectivity of
the RhII-catalyzed intramolecular reaction of a-diazocarbonyl
compounds is drastically affected by the substrate structure.
We have thus anticipated that by extending the linker
between two aromatic rings, the intramolecular Bꢀchner
reaction may become the dominant reaction path. This was
indeed shown to be the case. When N-tosylhydrazone 3a was
submitted to the same reaction conditions, 4b-methyl-4b,10-
dihydrobenzo[a]azulene 5a was isolated in 82% yield. The
intramolecular aromatic substitution product 4 was not
detected [Eq. (1)].
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for the S-bridged substrate the reaction failed to afford the
expected product 5u.
Since the conversion of ketone to N-tosylhydrazone is
highly efficient, we next performed the experiments in a one-
pot reaction directly starting from the ketones (Scheme 5). To
our delight, the reaction with 2-biphenyl methyl ketones
worked smoothly under the one-pot conditions (2b, 2k, 2h).
The reaction with methylene and O-bridged methyl ketones
also proceeded well.
Since this highly efficient intramolecular Bꢀchner reac-
tion can serve as a straightforward method for the construc-
tion of the 4b,10-dihydrobenzo[a]azulene structure, we then
proceeded to study the substrate scope. As shown in
Scheme 4, the reaction with methylene-bridged substrates
(3a–e), which could also be easily prepared by Suzuki–
Miyaura cross-coupling, proceeded smoothly to afford the
corresponding products 5a–e. The structure of 5e was
confirmed by X-ray crystallography.[13]
Next, aromatic N-tosylhydrazones bearing a heteroatom
bridge were investigated. N-Me-bridged substrates afforded
the dihydrocyclohepta[b]indole derivatives (5 f–h) in moder-
ate yields. Similarly, a series of O-bridged substrates were
examined in the reaction. The reaction proceeded smoothly
and most of the yields were moderate to good (5i–l, 5n–q, 5s).
In some cases the yields were excellent (5m, r, t). However,
Scheme 5. One-pot reactions of methyl ketones. All reactions were
performed with methyl ketones (0.40 mmol), TsNHNH2 (0.44 mmol)
in toluene (1.0 mL) at 1108C for 15 h, then NaH (0.48 mmol) and
toluene (3.0 mL) were added to the reaction mixture, and stirring was
continued at 1108C for 15 h. All yields refer to isolated products after
two steps.
To gain some insights into the reaction, we have compared
the corresponding intramolecular reaction of a-diazoester 6
under the metal-free and RhII-catalyzed conditions. As shown
in Equation (2), the metal-free reaction afforded the intra-
molecular aromatic substitution product 7 in 51% yield
(conditions A). However, the Rh2(OAc)4-catalyzed reaction
afforded the same product in 95% yield (conditions B).[12f] In
contrast, both transition-metal-free and Rh2(OAc)4-catalzyed
reaction of N-tosylhydrazone 1b afforded the corresponding
intramolecular aromatic substitution product 2b in similarly
high yields (Table 1, entries 2 and 3). The results indicate the
effect of the a-substituent on the reactivity of the carbene
intermediate.
Moreover, the results in Scheme 3 illustrate that the
reaction time is dependent on the substituents. To more
precisely determine the rate influences by the substituents,
intermolecular competition experiments were carried out as
Scheme 4. Transition-metal-free synthesis of [6,5,7]benzo-fused ring
compounds. All reactions were performed with 3a–u (0.40 mmol),
NaH (0.48 mmol) in 4.0 mL toluene at 1108C for 15 h. All the yields
refer to isolated products.
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mechanism.
In summary, we have developed a simple and efficient
method to access fluorenes and [6,5,7]benzo-fused rings
through transition-metal-free intramolecular aromatic sub-
stitution/Bꢀchner reaction, using N-tosylhydrazones as non-
stabilized diazo compound precursors. A literature survey
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availability of the aromatic N-tosylhydrazone substrates and
the high efficiency of the transition-metal-free reaction, this
method may turn into a general methodology for preparing
these types of compounds.
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Received: October 11, 2014
Revised: November 20, 2014
Published online: && &&, &&&&
Keywords: aromatic substitution · Bꢀchner reaction · carbenes ·
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Communications
Carbenes
Z. Liu, H. Tan, L. Wang, T. Fu, Y. Xia,
Y. Zhang, J. Wang*
&&& — &&&
Transition-Metal-Free Intramolecular
Carbene Aromatic Substitution/Bꢀchner
Reaction: Synthesis of Fluorenes and
[6,5,7]Benzo-fused Rings
Transition-metal-free: The synthesis of
fluorenes and [6,5,7]benzo-fused rings is
achieved through a transition-metal-free
protocol of intramolecular aromatic sub-
stitution and Bꢀchner reaction. The new
synthetic method uses readily available
aromatic N-tosylhydrazones as the diazo
compound precursors and shows wide
substrate scope.
6
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