Rhodium-catalyzed intramolecular hydroarylation of 1-halo-1-alkynes: Regioselective synthesis of semihydrogenated aromatic heterocycles

Article abstract of DOI:10.1002/chem.201303008

The regioselective intramolecular hydroarylation of (3-halo-2-propynyl) anilines, (3-halo-2-propynyl) aryl ethers, or (4-halo-3-butynyl) aryl ethers was efficiently catalyzed by Rh₂(OCOCF₃)₄ to give semihydrogenated aromatic heterocycles, such as 4-halo-1,2-dihydroquinolines, 4-halo-3-chromenes, or 4-(halomethylene)chromans, in good to excellent yields. Some synthetic applications taking advantage of the halo-substituents of the products are also illustrated. Ring a rhodium! Regioselective intramolecular hydroarylation of haloacetylenes efficiently catalyzed by [Rh ₂(tfa)₄] (see scheme; tfa=OCOCF₃) gave semihydrogenated aromatic heterocycles in good to excellent yields. Copyright

Full text of DOI:10.1002/chem.201303008

DOI: 10.1002/chem.201303008
Full Paper
&
Organic Synthesis
Rhodium-Catalyzed Intramolecular Hydroarylation of 1-Halo-1-
alkynes: Regioselective Synthesis of Semihydrogenated Aromatic
Heterocycles
Hirohiko Murase, Kousuke Senda, Masato Senoo, Takeshi Hata, and Hirokazu Urabe*[a]
Abstract: The regioselective intramolecular hydroarylation of
(3-halo-2-propynyl)anilines, (3-halo-2-propynyl) aryl ethers, or
(4-halo-3-butynyl) aryl ethers was efficiently catalyzed by
Rh2(OCOCF3)4 to give semihydrogenated aromatic heterocy-
cles, such as 4-halo-1,2-dihydroquinolines, 4-halo-3-chro-
menes, or 4-(halomethylene)chromans, in good to excellent
yields. Some synthetic applications taking advantage of the
halo-substituents of the products are also illustrated.
Intramolecular Friedel–Crafts reaction and its variants are tradi-
tional methods for the preparation of cyclic compounds via
electrophilic substitution to various aromatic compounds.[1] Al-
though a wide variety of electrophiles and their equivalents
could be used in these reactions, there still exists some limita-
tion on the electrophiles, especially incipient ones such as ace-
tylenes.[2] Recent rapid development in transition-metal cata-
lysts, which are more susceptible to interact with an acetylenic
bond than traditional Lewis or Brønsted acids, has made it
more reactive toward the aromatic substitution. For this
reason, this type of reaction is often called transition-metal-cat-
alyzed hydroarylation of acetylenes, irrespective of its actual
mechanism, and has attracted much attention recently.[3]
During the course of our studies on the synthetic utility of
haloacetylenes,[4] we experienced that the cyclization of (halo-
propargyl)aniline 1 to 2 with a typical Lewis acid, BF3·OEt2,
proved unsuccessful, probably due to the moderately electron-
deficient nature of haloacetylenes not permitting the effective
interaction with such an activator (Scheme 1, method a). How-
ever, to the contrary, we were pleased to find that a Rh catalyst
([Rh2(tfa)4], tfa=CF3CO2)[5] is quite promising for the same cycli-
zation from 1 or 3,[6] nicely giving desired products 2 or 4 in
good yields, without being accompanied by isomeric product
5 (method b). The carbon–carbon bond formation exclusively
took place at the acetylenic carbon atom a to halogen, yield-
ing the six-membered dihydroquinoline derivatives 2 or 4, the
structure of which was verified by the derivatization to known
Scheme 1. Rh-catalyzed hydroarylation of chloroacetylene.
4-chloroquinoline 6. On the other hand, 4-chloroquinoline 6
was not detected as a byproduct in the crude reaction mixture
derived from method b, which shows that the aromatization of
the product during this cyclization was negligible. A plausible
mechanism is shown in Scheme 2, in which the more cationic
acetylenic carbon atom a to halogen appears to be preferen-
[a] H. Murase, K. Senda, M. Senoo, Prof. Dr. T. Hata, Prof. Dr. H. Urabe
Department of Biomolecular Engineering
Graduate School of Bioscience and Biotechnology
Tokyo Institute of Technology
4259-B-59 Nagatsuta-cho, Midori-ku, Yokohama
Kanagawa 226-8501 (Japan)
Fax: (+81)(0)45-924-5849
E-mail: hurabe@bio.titech.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303008.
Scheme 2. A proposed mechanism.
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tially attacked by the aromatic ring to exhibit the observed re-
gioselection.
Table 1. Preparation of various 4-chloro-1,2-dihydroquinolines.[a]
Equations (1) and (2) show some scope of the substrates.
The above reaction is valid for the conversion of both chloro-
and bromoacetylenes 3 and 7 to desired products 4 and 9, but
not for an iodoacetylene 8, due perhaps to the increasing in-
stability of the acetylenic carbon–iodine bond [Eq. (1)]. The
protection of the aniline nitrogen atom is critical: while the sul-
fonyl derivatization is satisfactory as already shown in
Scheme 1, N-acyl protection found in 11 and 12 proved unsuit-
able, giving virtually no desired products 13 or 14 [Eq. (2)]
(Boc=tBuOCOÀ).
Entry
Substrate
Product
Isolated
yield [%]
1
91
74
97
91
96
2
3
4
5
6
83[b]
Partially hydrogenated aromatic heterocycles, such as those
shown above, are frequently found in naturally occurring prod-
ucts and artificial pharmaceuticals and are also useful inter-
mediates in organic synthesis.[7] The hydrogenation of the cor-
responding aromatic compounds is one of the most straight-
forward methods for their preparation, but the regioselective
transfer of controlled amounts of hydrogen (or its surrogates)
to the aromatic ring is not necessarily a trivial process. In addi-
tion, the halogenated structures prepared herein should be
quite useful for further functionalization and carbon–carbon
bond formation. Thus, we further investigated the generality
of this transformation. Table 1 summarizes other results of the
dihydroquinoline synthesis. Various p-toluenesulfonylanilines
participated in the reaction. A TBS-protected hydroxy group in
17 remained unattacked (Table 1, entry 4), and a sterically con-
gested aniline 18 and a-naphthylamine derivative 19 cyclized
as well to give 23 and 24 (entries 5 and 6). As already shown
in Equation (1), the corresponding bromoacetylenes could be
used as an alternative starting material to give similar prod-
ucts, which are summarized in Table 2. However, bromoacetyl-
enes appear less reactive than chloroacetylenes, as the former
requires prolonged reaction periods and more catalyst loading
to achieve good product yields. The fact that the carbon atom
a to bromide is less cationic than that a to chloride should ac-
count for the above observation (cf. Scheme 2). Nonetheless,
from various bromoacetylenes 25–29, the desired heterocyclic
products 30–34 were obtained in good yields. The TBS ether
[a] Ts=p-TolSO2À, TBS=tBuMe2SiÀ. [b] The reaction was performed for
7 h and with 10 mol% of catalyst.
moiety in 27 again survived the reaction conditions. The ani-
lines in Table 2 were protected with a benzenesulfonyl group
rather than the Ts that was used in Table 1, showing variation
in sulfonyl protecting groups.
4-Halo-1,2-dihydroquinolines prepared above should be ver-
satile synthetic intermediates for the preparation of substituted
heterocyclic compounds.[8] Scheme 3 exemplifies some applica-
tions.[8q,r] From a representative product 9, different types of
side chains including an sp3, sp2, or sp carbon terminus were
successfully introduced to its dihydroquinoline skeleton, illus-
trating the diverse synthesis from a common starting material.
During these transformations, an undesired side reaction, aro-
matization to the corresponding quinolines like the one men-
tioned in Scheme 1, was not observed at all. It should be em-
phasized that the extension of a functional group or a carbon
side chain from the halogen substituents may make it possible
to prepare the products that could not be obtained by the
direct hydroarylation of the corresponding alkynes[6d–g] owing
to the lack of either regioselectivity or functional-group com-
patibility (for example, the hydroxy group in 36).
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After having established the preparation of dihydroquino-
lines via the intramolecular hydroarylation of haloacetylenes
and their synthetic application, we next turned our attention
to the preparation of an oxygen heterocycle from a similar
halopropargyl aryl ether 39 (Scheme 4). Under analogous con-
Table 2. Preparation of various 4-bromo-1,2-dihydroquinolines.
Entry Substrate
[Rh2(tfa)4]
[mol%]
t
[h]
Product
Isolated
yield [%]
1
3
5
15.5
86
84
73
70
80
61
2
3
4
5
6
4
10
5
3
Scheme 4. Rh-catalyzed cyclization of chloropropargyl aryl ethers.
4
ditions as those described above, the desired cyclization grati-
fyingly proceeded to give exclusively chromene 40 containing
an endo-cyclic double bond in good yield.[6,9] The isomeric five-
membered product 41 was not detected, and the structure of
40 was unambiguously determined by its hydrogenation to
known 42 (yield not optimized). It should be noted that the
amount of the Rh catalyst could be reduced to 0.5 mol% of
the substrate whilst keeping a satisfactory yield.
5
4
10
7
The advantage of the Rh catalyst is again illustrated in Equa-
tions (3) and (4). Although the substrates 43 and 45 have an
electron-rich aromatic ring, traditional activators, such as
CF3CO2H or BF3·OEt2 proved not effective for their cyclization.
On the other hand, the Rh catalyst did effect the desired reac-
tion, giving chromenes 44 and 46, which is consistent with the
outcome of Scheme 1. Equations (3) and (4) also show a favora-
ble aspect of this Rh-catalyzed reaction that it tolerates the
presence of a nitrogen functional group in the substrates.[10]
However, to complete the reaction in an acceptable period,
a greater amount of the Rh catalyst (10 mol%) is necessary
than that for a non-functionalized substrate (0.5 mol% in
Scheme 4), which suggests that the catalyst is at least partially
deactivated by coordination with the nitrogen functional
group.
To show the generality of this functional-group compatibility,
several protected amino-substituted 4-chlorochromenes ob-
tained by this method are summarized in Table 3. In these re-
Scheme 3. Synthetic application of 4-bromo-1,2-dihydroquinoline.
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Table 4. Preparation of various 4-bromochromenes.
Entry
Substrate
Product
Isolated
yield [%]
1
2
3
70
87
84
Table 3. Preparation of various 4-chlorochromenes.
Entry
Substrate
Product
Isolated
yield [%]
4
5
88
95
1
2
3
X=COPr
Boc
Ts
(43)
(47)
(48)
(44)
(54)
(55)
67
95
96
4
5
X=COPr
Boc
(45)
(49)
(46)
(56)
62
94[a]
6
7
X=COPr
Ts
(50)
(51)
(57)
(58)
62
97[a]
8
80
69
9
[a] The reaction was performed for 2 h and with 3 mol% of catalyst.
actions, acyl-, Boc-, and Ts-protected aniline derivatives 43, 47,
and 48 can be used equally well (Table 3, entries 1–3) and, to
our satisfaction, broad substitution patterns on the aromatic
ring are also acceptable (entries 4–9).
Scheme 5. Hydroarylation of chlorobutynyl aryl ether.
Like the synthesis of dihydroquinolines, this cyclization is
not limited to chloroacetylenes, but is also applicable to
bromoacetylenes to give 4-bromochromenes, which are listed
in Table 4. In these transformations, the amount of the Rh cata-
lyst could again be reduced to as low as 1 mol% of the sub-
strates.
(Scheme 5). Considering the tendency discussed above that
the carbon–carbon bond formation always takes place at the
acetylenic carbon atom a to the halogen, we, at first glance,
expected that the reaction of Scheme 5 might give preferen-
tially seven-membered product 74. However, what was actually
observed was the ring closure at the acetylenic carbon atom
b to the halogen, selectively giving a chromene derivative 72
containing an exo-cyclic double bond with Z configuration.
It is interesting to compare the above cyclizations with that
starting from homologous halobutynyl aryl ether 71
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The olefin geometry of 72 was unambiguously determined by
a H NMR NOESY study, and another isomeric product 73 and
Table 5. Preparation of various 4-(halomethylene)chromans.
1
aforementioned 74 were not detected even in a crude reaction
mixture. A conventional Lewis acid, BF3·OEt2, again proved use-
less for this type of substrate. Further structural confirmation
of 72 was performed on an alternative product 75 (vide infra),
which was led to methylchroman 76 upon hydrogenolysis
Entry Substrate
[Rh2(tfa)4] Product
[mol%]
Isolated Z/E
yield
[%]
1
2
3
5
3
3
70[a]
97:3
97:3
98:2
79
81
Scheme 6. A proposed mechanism.
4
5
3
83
95:5
(Scheme 5). The reaction course is proposed in Scheme 6,
which is consistent with the formation of Z-olefinic products.
In addition, the ease of formation of a six-membered ring rela-
tive to a seven-membered ring should be a prevailing driving
force in this case. As a whole, the direction of the cyclization
appears to be controlled under the balance between the elec-
tronic (ring closure at the acetylenic carbon atom a or b to hal-
ogen) and structural (five-, six-, or seven-membered ring for-
mation) factors.
10
54[b]
>99:1
6
7
5
3
53[c]
>99:1
As Z-(chloromethylene)chroman 72 should be also a useful
building block, we investigated the cyclization of various halo-
butynyl aryl ethers and summarized the results in Table 5. In
each reaction, more catalyst loading was required than for the
cyclization of halopropargyl substrates shown in Scheme 4 and
Table 4. The olefin geometry was fixed to be Z with high selec-
tivities in most cases. An additional olefinic bond in the start-
ing material 80 survived the reaction conditions. The double
cyclization of 81 took place to give tricyclic products 86 with
the defined olefin geometry, although an aromatic regioisomer
88 was also formed in a small amount. A bromo-substituted
butynyl ether 82 could be used as well to give sterically con-
gested vinyl bromide 87.
91[d]
95:5
[a] The reaction was performed for 24 h. [b] The reaction was performed at
908C for 15 h. [c] The isomeric product 88 (see below) was also isolated in
17% yield. [d] The reaction period was 0.5 h and this product is an insepara-
ble 95:5 mixture of 87 and 89 (see below).
In conclusion, the regioselective intramolecular hydroaryla-
tion of haloacetylenes was efficiently achieved with [Rh2(tfa)4],
which was found to be superior to traditional Brønsted or
Lewis acids, to give 4-halo-1,2-dihydroquinolines, 4-halo-3-
chromenes, or 4-(halomethylene)chromans in good yields.
with the aid of ethyl acetate. The combined filtrates were concen-
trated in vacuo to give a crude oil, H NMR spectroscopic analysis
of which showed the presence of a single olefinic isomer. The
crude product was chromatographed on silica gel (hexane/ethyl
acetate) to afford the title compound (0.905 g, 86%) as a pale-
yellow solid.
1
Experimental Section
N-(Benzenesulfonyl)-4-bromo-1,2-dihydroquinoline (9): A mixture
of
N-(3-bromo-2-propynyl)-N-phenylbenzenesulfonamide
(7)
Acknowledgements
(1.05 g, 3.00 mmol) and rhodium trifluoroacetate dimer ([Rh2(tfa)4],
58.9 mg, 0.0895 mmol) in toluene (30 mL) was stirred in an oil bath
maintained at 808C for 15.5 h. After being cooled to room temper-
ature, the mixture was filtered through a short pad of silica gel
This work was supported by a Grant-in-Aid for Challenging Ex-
ploratory Research (22655014) from JSPS, Japan.
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Keywords: chromene · dihydroquinolines · heterocycles ·
hydroarylation · rhodium
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Received: July 31, 2013
Published online on December 2, 2013
Chem. Eur. J. 2014, 20, 317 – 322
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