A versatile rhodium(I) catalyst system for the addition of heteroarenes to both alkenes and alkynes by a C-H bond activation

Article abstract of DOI:10.1002/anie.201200120

Rhod to Addition: A highly efficient and convenient rhodium catalyst system was developed for the title transformation. A base co-catalyst was found to facilitate the key arene C-H bond-activation step and substrate scope was very broad, including both electron-deficient pyridine N-oxides, and electron-rich azoles. The catalytic system was effective for the hydroheteroarylation of both alkenes and alkynes and gave excellent regio- and stereoselectivity.

Full text of DOI:10.1002/anie.201200120

Angewandte
Chemie
DOI: 10.1002/anie.201200120
Hydroheteroarylation
AVersatile Rhodium(I) Catalyst System for the Addition of
À
Heteroarenes to both Alkenes and Alkynes by a C H Bond
Activation**
Jaeyune Ryu, Seung Hwan Cho,* and Sukbok Chang*
The transmetalation of organometallic reagents that are
is crucial to make this functionalization method more reliable.
In this respect, the introduction of directing groups is highly
effective in facilitating selective activation at the ortho
position through the in situ formation of chelating metallic
intermediates.[3,4]
À
employed in reactions with unsaturated C C bonds has
become a well-established method since the development of
highly efficient and selective catalytic systems. Indeed, a wide
range of organometallic reagents, including boron, silicon,
zinc, and titanium species, were introduced for rhodium-
catalyzed arylation reactions (Scheme 1a).[1] On the other
hand, the direct addition of (hetero)arenes across alkenes and
À
The hydroheteroarylation of multiple C C bonds to
directly afford functionalized heteroarenes is an important
reaction of high synthetic utility. Representatively, Ellman,
Bergman, and co-workers reported the alkylation of hetero-
cycles with alkenes using a catalytic system composed of a RhI
complex bearing a monodentate phosphine, and an acid.[2c,d]
The reaction was found to proceed via substrate-based Rh/N-
heterocyclic carbene intermediates, which were generated by
À
alkynes through metal-mediated C H bond activation has
À
an isomerization-initiated C H bond activation. Hiyama and
co-workers developed a Ni/Lewis acid binary catalyst system
for the reaction of heteroarenes with alkenes and alkynes.[5]
Although other catalytic systems and reaction conditions
have also been developed for the hydroarylation of double
and triple bonds,[6] there is still room for improvement,
especially with respect to selectivity, reaction conditions, and
substrate scope. We present herein the first example of
a rhodium catalyst system that enables the facile addition of
heteroarenes to both alkenes and alkynes (Scheme 1b).
Furthermore, the presence of a base was important and its
role is discussed in the context of mechanism.
Scheme 1.
In our continuing efforts to develop metal-catalyzed
reactions,[7] we found that certain well-defined rhodium-
emerged as an attractive alternative to the conventional
approach because it offers a more straightforward and
economical route to alkyl or alkenyl (hetero)arenes.[2] In
based catalytic systems displayed excellent selectivity and
[8]
À
reactivity in the C H bond activation of heteroarenes. In
this context, we envisioned the use of a highly selective ortho
activation of heterocycles for the hydroarylation of unsatu-
rated compounds.
À
fact, the direct C H bond functionalization method does not
require preactivation of (hetero)arenes, thus avoiding the
generation of stoichiometric amounts of byproducts. How-
Initially, we investigated various combinations of rhodium
species, ligands, and bases for the reaction of 4-phenyl-
pyridine N-oxide (1a, 2.5 equivalents) with tert-butyl acrylate
(2a).[9] The desired hydroarylation reaction was observed,
albeit in low yield (Table 1, entry 1), when the dimeric species
[{Rh(cod)Cl}2] (2 mol%) was used, in the presence of PCy3
ligand (5 mol%) and CsOAc (25 mol%). When the bidentate
ligand L2 was used, the yield of the monoaddition product 3a
was dramatically increased, although a trace amount of
bishydroarylation compound 4a was observed (entry 2). No
conversion was observed without base and its nature was
crucial to the reaction efficiency (entries 2–5). The absence of
ligand led to low yield (entry 6) and the highest selectivity and
yield was observed with 1,2-bis-(diphenylphosphino)ethane
(L4, entry 8), although the use of other ligands, including N-
heterocyclic carbene, gave poor yields (entries 7 and 9).
À
ever, the ability to selectively activate a single C H bond in
À
the presence of electronically or sterically similar C H bonds
[*] J. Ryu, Dr. S. H. Cho, Prof. Dr. S. Chang
Department of Chemistry and Molecular-Level Interface Research
Center
Korea Advanced Institute of Science and Technology (KAIST)
Daejeon 305–701 (Korea)
E-mail: shchom@kaist.ac.kr
sbchang@kaist.ac.kr
[**] This research was supported by the Korea Research Foundation
Grant (KRF-2008-C00024), MIRC (NRF-2011–0001322), a T. J. Park
Postdoctoral Fellowship (S.H.C.), and a Global Ph.D. Fellowship
(NRF-2011-0008452, J.R.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200120.
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Table 1: Optimization of reaction conditions.[a]
À
sistent with the postulate that ortho C H bond cleavage of
substrate 1a proceeds through a base-assisted proton abstrac-
tion with the help of metal coordination.[12] In fact, several
research groups,[13] including our own,[8a–b] validated this
mechanistic proposal through experimental and theoretical
studies.
A competition experiment between pyridine N-oxide and
its deuterated analogue revealed a significant kinetic isotope
Entry
Rh catalyst
Ligand
Base
Yield [%][b]
3a
4a
À
effect (KIE), kH/kD = 3.21, thus suggesting that the C H bond
cleavage step is rate-limiting [Eq. (1)]. In addition when the
1
2
3
4
5
6
7
8
[{Rh(cod)Cl}2]
[{Rh(cod)Cl}2]
[{Rh(cod)Cl}2]
[{Rh(cod)Cl}2]
[{Rh(cod)Cl}2]
[{Rh(cod)Cl}2]
[{Rh(cod)Cl}2]
[{Rh(cod)Cl}2]
[{Rh(cod)Cl}2]
Rh(acac)3
L1
L2
L2
L2
L2
none
L3
L4
L5
L4
L4
CsOAc
CsOAc
none
11
90 (84)
<1
29
24
5
–
4
tBuONa
K2CO3
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
–
–
–
–
3
–
–
–
7
95 (90)
<1
<1
<1
9
10
11
cross-over experiment of 4-phenylpyridine N-oxide and
[D5]pyridine N-oxide was carried out under the optimal
conditions a similar ratio of intermolecular deuterium scram-
bling was observed in each isolated product [Eq. (2)], thus
implying that the reaction does not proceed through inner-
sphere 1,4-conjugate addition.
[Rh(cod)2(BF4)]
[a] Reaction conditions: 1a (0.625 mmol), 2a (0.25 mmol), Rh catalyst
(2 mol%), ligand (5 mol%) and base (25 mol%) in toluene (0.5 mL).
[b] Yield as determined by NMR spectroscopy (yield of isolated product
in parentheses). acac = acetoacetate , cod = 1,5-cyclooctadiene.
Alternative rhodium precursors to [{Rh(cod)Cl}2] afforded
poorer results (entries 10 and 11).
On the basis of the above observations, a mechanistic
proposal, involving a generalized heterocyclic structure, is
presented in Scheme 2. We assume that the [{Rh(cod)Cl}2]
precursor and cesium acetate undergo ligand exchange to give
I,[14] which is presumed sufficiently active to facilitate a key
base-promoted proton abstraction and metalation step to
generate the Rh/heteroaryl species II.[15] Subsequent olefin
insertion into the rhodium–aryl bond would result in the
To gain mechanistic insight, the role of the base was
investigated by means of a H/D exchange experiment.[10]
A
significant level of deuterium incorporation (61%) was
observed at the ortho position of 4-phenylpyridine N-oxide
(1a) when the reaction mixture was treated with D2O
following its subjection to the optimized conditions in the
absence of olefin (Figure 1A).[11] In contrast, negligible
exchange was observed when the base was excluded (Fig-
ure 1B) and in the case where the rhodium precursor and
ligand were excluded (Figure 1C). This observation is con-
À
desired C C bond, thus leading to a Rh/alkyl intermediate III
or its enolate species (not shown). Although protonolysis of
Figure 1. Rhodium/base-mediated H/D exchange experiments. w/o =
without.
Scheme 2. A plausible mechanism for the present reaction.
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III may occur to deliver the desired product VI, the result of
the aforementioned cross-over experiment led us to presume
that the final protonation step occurs predominantly from
isomeric intermediate V, which can be generated from III
through a b-hydride elimination and re-insertion process.[16]
In fact, an alkenyl derivative IV was isolated as a byproduct
(5%) when 2-methylpyridine N-oxide was treated with an
increased amount of tert-butyl acrylate (2a, 3.0 equiva-
lents).[4d,17]
In the present hydroarylation protocol double addition of
the alkene substrate could readily be enforced simply by
reversing the stoichiometric ratio between the N-oxide and
alkene substrate. Therefore, the use of an excess of alkene
together with a prolonged reaction time gave dialkylated
products in excellent yield [Eq. (3)].
We next investigated the substrate scope of both the N-
oxide and the alkene substrate (Scheme 3). Variation of the
electronics of the pyridine N-oxide through substitution at the
4-position did not have a significant effect on reaction
efficiency (3b–3d). Reactions with ortho-substituted pyridine
N-oxides took place smoothly (3e and 3 f). Other types of N-
oxides, such as those derived from quinoline, pyrazine, and
pyridazine, were also viable in the hydroarylation of tert-butyl
acrylate (3g–3j). Notably, the reaction was highly regiose-
lective and occurred only at the position ortho to the N-oxide
group, even in the presence of additional heterocyclic nitro-
gen atoms (3h–3j). Other O-alkylated conjugated esters also
participated in the reaction with similar yields (3k and 3l).
Notably, although the hydroarylation of conjugated amides
(3m and 3n) and ketones (3o and 3p) was highly efficient,
that of acrylonitrile was not (3q). Internal electron-rich
alkenes were unreactive under the reaction conditions.[18]
Deoxygenation of the initially produced N-oxide products
was successfully performed in a one-pot sequence by treat-
ment of the crude reaction mixture with PCl3, thus delivering
the corresponding pyridine derivatives in high yields (3m and
3n).[10]
After the successful application of electron-deficient N-
oxides, we were interested in whether our catalytic system
could also be effective when electron-rich heterocycles were
used. We were pleased to observe that various types of azoles,
including benzimidazole, benzoxazole, benzthiazole, and
thiazole, readily reacted with tert-butyl acrylate under the
slightly altered reaction conditions where an excess of the
olefin (2 equivalents) relative to the heteroarene was
employed (Scheme 4). The desired alkylated heteroarenes
were obtained in excellent yields in all cases examined. The
isolation of olefinated byproduct 6 f from the reaction of
caffeine reinforces our proposed mechanism, specifically the
involvement of a b-hydride elimination process (Scheme 2;
between III and V via IV).
As part of these studies, we preliminarly investigated the
viability of an addition reaction across triple bonds using the
same catalytic system. To our delight, we found that the
hydroheteroarylation of disubstituted alkynes was very facile,
Scheme 3. Hydroarylation of alkenes with N-oxides. Reaction condi-
tions: 1 (0.625 mmol), 2 (0.25 mmol), Rh catalyst (2 mol%), L4
(5 mol%), and base (25 mol%) in toluene (0.5 mL). [a] For 24 h in 1,4-
dioxane (0.5m). [b] 1/2=1:4. [c] 1/2=1:2.5. [d] Yield over two steps
after deoxygenation using PCl3 in toluene at 258C. morph = morpho-
line.
Scheme 4. Hydroarylation of alkenes with azoles. Reaction conditions:
5 (0.25 mmol), 2a (0.5 mmol), Rh catalyst (2 mol%), ligand (5 mol%),
and base (25 mol%) in toluene (0.5 mL).[a] 3.0 Equiv. of alkene was
used for 24 h. [b] A byproduct (6 f) was also obtained from this
reaction.
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thus affording alkenylated heteroarenes with excellent regio-
and stereoselectivity (Scheme 5).
Experimental Section
General procedure: A mixture of [{Rh(cod)Cl}2] (2.5 mg, 2mol%),
1,2-bis(diphenylphosphino)ethane (DPPE, 5.0 mg, 5 mol%), CsOAc
(12 mg, 25 mol%) and 4-phenylpyridine N-oxide (107 mg,
0.63 mmol) was weighed into a 1 mL screw-capped vial equipped
with a spinvane triangular-shaped Teflon stirbar. Toluene (0.5 mL)
was added followed by tert-butyl acrylate (37 mL, 0.25 mmol). The vial
was sealed with a Teflon-lined cap and the reaction mixture was
stirred at 120oC for 12 h in an oil bath. The reaction mixture was
cooled to room temperature, filtered through a plug of Celite, and
then washed with EtOAc (30 mL). The filtrate was concentrated, and
evaporated to dryness under high vacuum. The residue was purified
by silica gel column chromatography (CH2Cl2/acetone 10:1) to give 2-
(3-tert-butoxy-3-oxopropyl)-4-phenylpyridine N-oxide in 90% yield.
Scheme 5. Hydroarylation of alkynes with N-heterocycles. Reaction
conditions: heteroarene (0.25 mmol), 7 (0.5 mmol), Rh catalyst
(2 mol%), ligand (5 mol%), and base (25 mol%). Yield of isolated
product and E/Z ratio, as determined by 1H NMR spectroscopy, in
parentheses.
Received: January 6, 2012
Published online: March 1, 2012
Keywords: alkenes · alkynes · homogeneous catalysis ·
.
hydroheteroarylation · pyridine N-oxides
À
The C C bond formation of the addition reaction of
caffeine to the unsymmetrical triple bond of 1-phenyl-1-
propyne occurs exclusively at the methyl-substituted acety-
lenic carbon center presumably because of steric reasons, thus
affording 8a in high yield. The newly generated double bond
was E configured, as determined by NOE analysis, thus
suggesting that the addition step proceeds in a syn manner.
The structure of 8a was unambiguously confirmed by X-ray
crystallographic analysis.[10] Analogous results were obtained
with symmetric alkynes such as diphenylacetylene and 4-
octyne, the addition of which was completely stereoselective
(products 8b and 8c, respectively). Notably, the newly
generated double bond of alkenylated heteroarenes did not
isomerize under the present reaction conditions as was
reported to occur in the system developed by Hiyama and
co-workers.[5d] The alkyne hydroarylation procedure was also
readily applied to electron-deficient N-oxides. For example,
the reaction of 1-phenyl-1-propyne with pyrazine N-oxide
proceeded with complete regioselectivity, although the ste-
reoselectivity was slightly decreased with the E isomer 8d
being still favored. Additionally, we observed that the Rh-
catalyzed hydroarylation of alkenes (tert-butyl acrylate)
proceeded faster than that of alkynes (4-octyne) under
identical reaction conditions (see the Supporting Information
for details).
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À
arene C H bond-activation step. Substrate scope was very
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[10] See the Supporting Information for details.
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