C-H insertion processes on stabilized indolyl and ortho-aminophenyl fischer carbene complexes: Synthesis of azepino[3,2,1-hi] indole, benzazepine and indole derivatives

Article abstract of DOI:10.1002/chem.200801132

A study was conducted to demonstrate a new sequential reaction of alkoxy indolyl and ortho-aminophenyl alkynyl Fischer carbene complexes that lead to azepino(3,2,1-hi)indoles and benzazepines. It was demonstrated that these transformations involve an init

Full text of DOI:10.1002/chem.200801132

DOI: 10.1002/chem.200801132
À
C H Insertion Processes on Stabilized Indolyl and ortho-Aminophenyl
Fischer Carbene Complexes: Synthesis of Azepino
Benzazepine and Indole Derivatives
A
JosØ Barluenga,* Martín FaÇanµs-Mastral, and Fernando Aznar[a]
Fischer carbene complexes have become very useful ma-
terials in organic synthesis. Since their discovery in 1964,[1]
their chemistry has been extensively studied, and at the
present, there are a large number of useful transformations
involving Fischer carbene complexes that find application in
synthetic organic chemistry,[2] mostly oriented to the prepa-
ration of carbo- and heterocycles.[3] In fact, a particularly in-
teresting feature of the chemistry of Fischer carbene com-
plexes is their ability to provide complex polycyclic struc-
carbonyl(benzylidenemethoxy)chromium complex into the
À
b-C H bond of (E)- and (Z)-crotonitrile which involves a b-
elimination and a reductive elimination reaction.[8] However,
À
as far as we know, only one example of a direct C H inser-
tion process, in which a typical methoxycarbene chromium
complex is involved, has been described thus far.[9]
As a part of our work based on the study of cycloaddi-
tion-promoted cascade processes of alkynyl Fischer carbene
complexes,[10] we have recently turned our attention to the
development of cascade reactions of indolylalkynyl carbene
complexes[11] which are interesting starting materials for the
synthesis of complex polycyclic systems containing the
indole core, a prominent structural unit frequently found in
numerous natural products and pharmaceutically active
compounds.[12]
tures from simple starting materials in cascade processes
[4]
À
that usually involve the formation of several C C bonds.
On the other hand, a very versatile methodology for the
À
À
formation of C C bonds is the C H carbene insertion.
These type of processes are commonly observed in reactions
involving metal carbenoids.[5] However, they are rare for sta-
bilized Fischer carbene complexes. The more electrophilic
character of the carbene carbon atom of the former, which
requires the presence of at least one electron-withdrawing
group,[5e] could be the reason for this difference behavior in
In the present paper, we report a new sequential reaction
of alkoxy indolyl and ortho-aminophenyl alkynyl Fischer
carbene complexes that lead to azepino[3,2,1-hi]indoles and
A
benzazepines. These unusual transformations involve an ini-
À
À
À
C H insertion reaction. In fact few examples of C H inser-
tion are known for stabilized Fischer carbene complexes.
We have previously described the formation of oxaborolane
tial cycloaddition step followed by an intramolecular C H
insertion process. Based on this transformation we have also
À
extended the C H insertion reaction to simpler ortho-ami-
nophenyl Fischer carbene complexes, in a process that leads
to the formation of indoles.
or oxazaborolidine derivatives from boroxy Fischer carbene
complexes via an intramolecular C H insertion process.
Takeda reported that the reaction of acetoxyalkenylcarbene
complexes with electron-rich butadienes afforded bicyclo-
heptane derivatives, along with other compounds, derived
[6]
À
First, we focused on indolylalkynylcarbene complex 1a,
which bears an allyl group at the nitrogen atom (Scheme 1).
In order to break the linear arrangement of the triple bond
enabling a subsequent intramolecular reaction we chose, as
a first approach, the well-known [2+2]-cycloaddition reac-
tion.[10c–d,11a,13] Thus, when we treated carbene complex 1a
with 2,3-dihydrofuran in THF at room temperature and the
reaction mixture was subsequently warmed at 908C in a
sealed tube, we obtained a separable mixture of compounds
2a and 3 in 57% combined yield and in a 4.2:1 ratio. Both
products were obtained as unique diastereoisomers
(Scheme 1). Structural assignments of these new compounds
were based on a series of NMR studies. Additionally, the
relative configuration of the new stereogenic centres of both
compounds was determined by NOESY experiments.
À
from an intramolecular C H insertion product of the initial-
ly formed Diels–Alder adduct.[7] Wienand and Reissig re-
ported the formal insertion of the carbene ligand of penta-
[a]Prof. J. Barluenga, Dr. M. FaÇanµs-Mastral, Prof. F. Aznar
Instituto Universitario de Química Organometµlica “Enrique Moles”
Unidad Asociada al C.S.I.C., Universidad de Oviedo, Juliµn Clavería,
8
33006 Oviedo (Spain)
Fax : (+34)985-103-450
E-mail: barluenga@uniovi.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801132.
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COMMUNICATION
Table 1. Synthesis of azepino[3,2,1-hi]indoles 2 and benzazepines 6 by a
A
À
sequential [2+2]cycloaddition/C H insertion reaction of Fischer car-
bene complexes 1 and 5.
Entry 1/ R1
R2
Product Yield
[%][a]
dr 2:2’/
6:6’
5
1
2
1a CH=CH2
1b (E)-CH=
CHMe
–
–
2a
2b
46[b]
53
1:0
1:0
3
4
5
6
7
8
1c Ph
1d OEt
–
–
–
–
2c
2d
–
–
6a
6b
49
1:0
1.3:1
50[c]
–
1e
H
1 f Et
5a Ph
5b 4-Me-C6H4
–
Ph
59[d]
62[d]
3.5:1
3.4:1
4-Me-
C6H4
H
Scheme 1. Synthesis of 2a and 3 from indolylalkynylcarbene complex 1a.
9
5c Ph
6c
54[d]
3.1:1
[a]Isolated yield based on starting alkynylcarbene complex. [b]The cy-
clopropanation product 3 was also obtained with an 11% yield. [c]The
product was obtained as a mixture of diastereoisomers which could not
be separated. [d]The product was obtained as a mixture of diastereoiso-
mers which could be separated by column chromatography.
The formation of both products can be explained consid-
ering first the formation of the intermediate carbene com-
plex 4a, generated by a formal [2+2]-cycloaddition reaction
between the alkynyl Fischer carbene 1a complex and the
2,3-dihydrofuran. Benzazocine derivative 3, which could be
initially considered as the expected product, would come
from the intramolecular cyclopropanation of the double
bond of the allylic moiety in carbene complex 4a. On the
other hand, the major product, the azepinoindole derivative
4). While compound 2c was formed with complete diaste-
reoselectivity, the azepinoindole 2d was generated as a 1.3:1
mixture of diastereoisomers. Conversely, when the reaction
was carried out with alkynyl carbene complexes 1e and 1 f,
which present a methyl and a propyl group respectively at
the nitrogen atom (Table 1, entries 5 and 6), the correspond-
ing azepinoindole derivatives could not be obtained. Taking
into account these results, it seems that the presence of an
additional activating group is necessary to give rise to the
À
2a could be explained by the intramolecular C H insertion
À
reaction of the carbene carbon atom into a C H bond in a
position to the nitrogen atom in 4a. As far as we know this
is the first example in which a seven-membered ring is
[14]
À
formed in a C H carbene insertion process.
À
In order to examine the scope of this new C H insertion
reaction, a set of experiments were carried out with differ-
À
C H insertion products.
This sequential [2+2]-cyclization/C H insertion reaction
À
ent N-substituted indolylalkynylcarbene complexes
1
(Scheme 2 and Table 1).
could also be accomplished with ortho-aminophenylalkynyl
Fischer carbene complexes 5[15] which can be considered
structural analogues of carbene complexes 1. So, when car-
bene complexes 5 was treated with 2,3-dihydrofuran under
the conditions described above, benzazepine derivatives 6
were obtained as a mixture of two diastereoisomers which
could be separated by column chromatography (Scheme 3
and Table 1, entries 7–9). The structure of these compounds
was determined by NMR analysis and the relative configu-
ration of the new stereogenic centres could be confirmed by
X-ray analysis of the major diastereoisomer of benzazepine
6a.[16]
Scheme 2. Synthesis of azepino[3,2,1-hi]indoles 2 through a sequential
G
À
[2+2]-cycloaddition/C H insertion reaction of indolylalkynylcarbene
complexes 1.
Thus, when the reaction was conducted with the N-crotyl
carbene complex 1b (Table 1, entry 2) and 2,3-dihydrofuran
under the same reaction conditions described above, the
azepinoindole derivative 2b was exclusively formed in 53%
yield and as a single diastereoisomer. This sequential [2+2]-
À
cyclization/C H insertion reaction could also be successfully
carried out with alkynyl carbene complexes 1c and 1d,
which bear a benzyl and a methoxymethyl group at the ni-
trogen atom, respectively, leading to azepinoindole deriva-
tives 2c and 2d in moderate yields (Table 1, entries 3 and
Scheme 3. Synthesis of benzazepine derivatives 6 from a sequential [2+
À
2]cycloaddition/C H insertion reaction of alkynylcarbene complexes 5.
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J. Barluenga et al.
The scope of this sequential process was evaluated by
changing the nature of the triggering cycloaddition reaction.
The cascade process can be promoted by a [4+2]cycloaddi-
tion[17] when cyclopentadiene is employed as diene compo-
nent. Thus, the reaction of alkynylcarbene complexes 1b,c
and 5a with cyclopentadiene led exclusively to the forma-
Due to the aromatization of the Diels–Alder cycloadduct
promoted by the elimination of one molecule of methanol,
the complete planar arrangement in 11 would probably
place the carbene carbon atom away from the methylene
À
group in a position to the nitrogen atom, disfavoring the C
H insertion reaction. This fact would on the contrary favor
the cyclopentannulation reaction.
À
tion of the C H insertion products 7 and 8 as a mixture of
two out of four possible diastereoisomers with moderate
yields (Scheme 4).
Taking into account the chemical behavior shown by alky-
nylcarbene complexes 1 and 5, we decided to further investi-
À
gate the possibility of carrying out a C H insertion process
with the simpler methoxy ortho-aminophenyl Fischer car-
bene complexes 12 (Scheme 6). Thus, Fischer carbene com-
plexes 12 were synthesized from 2-bromoanilines following
standard procedures (see Supporting Information). Interest-
ingly, although complexes 12 could be efficiently synthe-
sized, a little amount of compound 13 was obtained during
the reaction work-up. Moreover, in a single experiment, we
observed that carbene complexes 12 was fully transformed
À
into indoles 13 through an intramolecular C H insertion re-
action at room temperature in 24 h.[19] The same transforma-
tion takes place in 30 min at 508C (Scheme 6).
Scheme 4. Synthesis of azepino[3,2,1-hi]indoles 7 and benzazepine deriva-
G
À
tive 8 by a sequential [4+2]-cycloaddition/C H insertion reaction of
Fischer carbene complexes 1 and 5.
However, when the reaction was carried out with 1-me-
thoxy-3-trimethylsililoxy-1,3-butadiene
(Danishefskyꢁs
diene) was used as the diene component the corresponding
À
C H insertion products were not obtained. In this case, the
reaction of carbene complexes 1a,b gave rise exclusively to
indenoindoles 10 as a result of a sequential [4+2]-cycloaddi-
tion/cyclopentannulation[18] process (Scheme 5).
Scheme 6. Transformation of o-aminophenyl carbene complexes 12 into
À
indoles 13 by an intramolecular C H insertion reaction.
This different behavior could be explained taking into ac-
count the geometry of the carbene complex intermediate 11.
The formation of 1,2-disubstituted indoles 13 could be ex-
plained through an intramolecular insertion reaction of the
À
carbene carbon into the C H bond of the benzylic carbon
of Fischer carbene complex 12. The resulting intermediate
À
14 of this C H insertion would experiment a spontaneous
aromatization through the loss of a molecule of methanol
affording indole 13. As it was observed for alkynylcarbene
À
complexes 1 and 5, the C H insertion only took place in the
benzylic positions and no product from an insertion in the
methyl group was obtained when the reaction was carried
out with N-methyl substituted complexes 12c and 12d.
In conclusion, we have described a new sequential [n+2]
À
cycloaddition/C H insertion reaction of stabilized indolyl
and ortho-aminophenyl alkynyl Fischer chromium carbene
complexes which leads to the formation of azepino[3,2,1-
G
hi]indoles and benzazepines, respectively. It is also remark-
Scheme 5. Formation of indenoindoles 10 through a sequential [4+2]cy-
cloaddition/cyclopentannulation reaction
able that for the best of our knowledge this is the first time
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À
C H Insertion in Fischer Carbene Complexes
COMMUNICATION
À
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carbene insertion process. On the other hand, azepino[3,2,1-
AHCTREUNG
hi]indole moiety have a high synthetic interest because it is
present in compounds with important and specific biological
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have also describe the transformation of ortho-aminophenyl
carbene complexes into 1,2-disubstituted indoles.
Experimental Section
General procedure for preparation of azepino[3,2,1-hi]indoles 2 or ben-
A
zazepine derivatives 6: 2,3-Dihydrofuran (2 mmol) was added to a solu-
tion of Fischer carbene complex 1 or 5 (0.5 mmol) in THF (10 mL), and
the mixture was stirred at room temperature under argon atmosphere
until the TLC analysis showed the disappearance of 1 or 5. Then the tube
was sealed and the solution was heated at 908C during two hours. The
mixture was diluted with hexane (30 mL) and was exposed to air and
light. Finally, the mixture was filtered through a pad of Celite, solvents
were removed under reduced pressure and the crude product was puri-
fied by column chromatography to afford 2 or 6.
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General procedure for preparation of 1,2-disubstituted indoles 13: A so-
lution of Fischer carbene complex 12 (0.5 mmol) in THF (10 mL) was
stirred under argon atmosphere at room temperature during 24 h or at
508C during 30 min. Then the mixture diluted with hexane (30 mL) and
was exposed to air and light. Finally, the mixture was filtered through a
pad of celite, solvents were removed under reduced pressure and the
crude indole 13 was purified by column chromatography.
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We gratefully acknowledge financial support from the DGICYT
(CTQ2004-08077-C02-01/BQU and CTQ2007–61048/BQU), MEC (FPU
grant to M. F.M.). We also thank Dr. Angel L. Suµrez for help in the X-
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À
C H
Keywords: benzazepines
insertion · heterocycles
·
carbenes
·
cascades
·
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Received: June 10, 2008
Published online: July 30, 2008
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