Rhodium-catalyzed C-H alkynylation of arenes at room temperature

Article abstract of DOI:10.1002/anie.201309198

The rhodium(III)-catalyzed ortho C-H alkynylation of non-electronically activated arenes is disclosed. This process features a straightforward and highly effective protocol for the synthesis of functionalized alkynes and represents the first example of merging a hypervalent iodine reagent with rhodium(III) catalysis. Notably, this reaction proceeds at room temperature, tolerates a variety of functional groups, and more importantly, exhibits high selectivity for monoalkynylation. Hot rhod: A rhodium-catalyzed, electronically reversed Sonogashira reaction between unbiased arenes and the hypervalent iodine reagent 1 proceeds through C-H activation. This reaction displays excellent functional-group tolerance and high efficiency, and thus opens a new synthetic pathway to access functionalized alkynes. Cp=C₅Me₅, DCE=1,2-dichloroethane, Piv=pivaloyl, TIPS=triisopropylsilyl.

Full text of DOI:10.1002/anie.201309198

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Angewandte
Communications
DOI: 10.1002/anie.201309198
Cross-coupling
À
Rhodium-Catalyzed C H Alkynylation of Arenes at Room
Temperature**
Chao Feng and Teck-Peng Loh*
À
Abstract: The rhodium(III)-catalyzed ortho C H alkynyla-
tion of non-electronically activated arenes is disclosed. This
process features a straightforward and highly effective protocol
for the synthesis of functionalized alkynes and represents the
first example of merging a hypervalent iodine reagent with
rhodium(III) catalysis. Notably, this reaction proceeds at room
temperature, tolerates a variety of functional groups, and more
importantly, exhibits high selectivity for monoalkynylation.
phene and anilines), catalyzed by gold or palladium, have
been reported by Waser and co-workers.[7]
While considerable progress has been achieved in the
À
arena of C H alkynylation, the reported methods commonly
rely on the use of electronically activated arenes, with
electron-neutral or the more-challenging electron-deficient
arenes being largely underdeveloped. However, Chatani and
co-workers described the alkynylation of an aromatic acid
and 2-phenylpyridine derivatives by using palladium or
ruthenium as a catalyst and alkynyl bromide as an alkynyla-
tion reagent.[8,9] However, the need for the reactions to be
carried out at high temperature, the use of impractical
directing groups, and more importantly, the lack of selectivity
for mono and bis-alkynylation encouraged us to search for
a highly selective alkynylation of arenes. Specifically, we were
interested in discovering a method for alkynylation of
unactivated substrates under much milder reaction condi-
tions, which would be applicable in the late-stage function-
alization of complex molecules. With our continuing interest
in the rhodium catalysis,[10] we report herein the first example
of rhodium-catalyzed electronically reversed Sonogashira
coupling of electron-poor arenes by taking advantage of the
hypervalent iodine reagent 1 as the alkyne source without the
need for undesirable metallic oxidants (Scheme 1).[11] Our
A
lkynes rank among the most important functional groups
in synthetic chemistry because of their versatility to be
transformed into various useful molecules and also their
ubiquity in naturally occurring compounds, advanced materi-
als, and pharmaceutics.[1] The most popular and reliable
method for the introduction of alkynyl functionality is the
Sonogashira coupling reaction by virtue of its operational
simplicity and wide spectrum of functionality tolerance.[2]
À
With the surge in C H functionalization, much effort has
been focused on the discovery of new synthetic methods
which could obviate the use of halogenated starting materials
and, more importantly, allow the ease of transformation for
cases where specific aryl halides are not easily accessible.[3]
Compared with the substantial advancement attained in the
[4]
À
À
area of C H alkenylation, the analogous C H alkynylation
is largely underexplored, mainly because of the susceptibility
of terminal alkynes to homocoupling under the commonly
employed oxidative reaction conditions.[5] In this context, the
electronically reversed Sonogashira coupling reaction
between non-prefunctionalized arenes and an electrophilic
alkyne species is an attractive approach, and several elegant
works, using different transition metals and electrophilic
alkynes, have already been reported in the past few years.[6]
Elegant examples using hypervalent alkynyl iodine as the
À
electrophilic alkyne reagent for C H alkynylation of elec-
tron-rich aromatic substrates (indole, pyrrole, furan, thio-
À
Scheme 1. Rhodium-catalyzed C H alkynylation of electronically
unbiased arenes. Cp*=C5Me5, DCE=1,2-dichloroethane, Piv=piva-
loyl, TIPS=triisopropylsilyl.
[*] Prof. Dr. T.-P. Loh
Department of Chemistry
University of Science and Technology of China
Hefei, Anhui 230026 (P. R. China)
C. Feng, Prof. Dr. T.-P. Loh
protocol also allows selective access to monoalkynylation,
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University
Singapore 637371 (Singapore)
À
which represents a major issue in directed C H functional-
izations, with no bis-alkynylation products observed in all
cases examined.
E-mail: teckpeng@ntu.edu.sg
With 4-methyl-N-(pivaloyloxy)benzamide (2a) and 1 as
model substrates, the reaction conditions were investigated
(Table 1). We were rather pleased to find that with
[{RhCp*Cl2}2] as the catalyst, NaOAc as an additive, and
methanol as the solvent, the reaction worked smoothly at
room temperature to produce the desired alkynylation
[**] We gratefully acknowledge the USTC Research Fund, the Nanyang
Technological University, and the Singapore Ministry of Education
Academic Research Fund (ETRP 1002 111, MOE2010-T2-2-067,
MOE 2011-T2-1-013) for the funding of this research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309198.
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Table 1: Optimization of reaction conditions.[a]
Entry
Additive
Yield [%][b]
1
2
3
4
5
6
7
8
9
NaOAc
CsOAc
CsOPiv
NaOAc
PivOH
–
NaOAc
NaOAc
NaOAc
89
89
88
92[c]
[d]
–
[d]
–
9[e,g]
8[f,g]
n.r.[h]
[a] Unless otherwise noted, the reactions were carried out at room
temperature using 2a (0.1 mmol), 1 (0.11 mmol), additive
(0.025 mmol), catalyst (0.002 mmol) in solvent (0.5 mL) for 12 h.
[b] Yields of isolated products. [c] NaOAc (0.1 mmol). [d] No desired
product. [e] Bromotriisopropylsilylethyne was used instead of 1.
[f] Iodotriisopropylsilylethyne was used instead of 1. [g] Yield was
1
determined by H NMR spectroscopy using mesitylene as an internal
standard. [h] No rhodium catalyst added. n.r. =no reaction.
product 3a in 62% yield (see the Supporting Information).
Screening of the solvent proved DCE to be the optimal
choice, as it delivered 3a in 89% yield (Table 1, entry 1).
When CsOAc and CsOPiv were employed in place of NaOAc,
comparable chemical yields were obtained (Table 1, entries 2
and 3). On the contrary, PivOH, which is known to be
À
involved in the key step of C H bond cleavage in many
À
documented C H functionalization reactions, did not show
any positive effect on this reaction (Table 1, entry 5).[12] This
result indicated that although the acetate or pivalate anion
À
may promote the envisioned C H bond cleavage, the base-
promoted association of amide substrates with a rhodium
catalyst proved to be the prerequisite. By increasing the
stoichiometry of NaOAc, the yield of 3a was further
improved to 92% (Table 1, entry 4). Furthermore, NaOAc
was found to be essential for the success of this reaction and
no desired product could be detected with its absence
(Table 1, entry 6). It should be noted that the use of other
commonly employed alkynylation reagents such as bromo- or
iodotriisopropylethylene gave rise to the desired product in
less than 10% yield (Table 1, entries 7 and 8). Finally, the
control experiment clearly demonstrated the indispensability
of the rhodium catalyst, without which, no reaction occurred
(Table 1, entry 9).
Having obtained the optimal reaction conditions, the
generality of the amide coupling partner was investigated, and
the results are summarized in Scheme 2. In general, this
reaction showed extremely high reaction efficiency with
diversified functionalities ranging from electron-withdrawing
to electron-releasing ones, thus affording the desired alkyny-
lation products in high to excellent yield. Specifically,
electron-donating groups such as alkyl, methoxy, trimethyl-
silyl, and even oxidative labile methylsulfanyl were all well
tolerated. When the ortho-methyl-containing substrate 2c
was employed, the desired product 3c was isolated in 67%
Scheme 2. Reaction scope of amide derivatives. Unless otherwise
noted, the reactions were carried out at room temperature using 2
(0.1 mmol), 1 (0.11 mmol), NaOAc (0.1 mmol), and [{RhCp*Cl2}2]
(0.002 mmol) in DCE (0.5 mL) for 12 h. Yield of isolated products are
given. [a] Reaction time 6 h.
yield, and is attributed to the distorted coplanar geometry
arising from the steric repulsion which weakens the inter-
action between the rhodium catalyst and proximate aromatic
À
C H bond. When substrates holding meta substituents were
employed, the reaction tended to occur at a sterically more
accessible site to avoid the steric congestion (3d, 3e, 3k, 3t).
Interestingly, when the meta-methoxy-substituted amide 2h
was submitted to the optimal reaction conditions, the two
regioisomeric products 3h and 3h’ were produced in 54 and
33% yield, respectively, thus highlighting the coordination
effect of the methoxy substituent (3d versus 3h/3h’) in
stabilizing the aryl rhodium intermediate. Furthermore,
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biphenyl and naphthalyl substrates (2j,k) also worked nicely
to afford the desired product in excellent yield. Substrates
containing electron-withdrawing substituents such as NO2,
CF3, Ac, and CN all participated in this reaction. Notably,
heterocycles such as pyridine, furan, and indole were also
effective substrates and compatible with these reaction
conditions. The compound 2p is much more intriguing
considering that the pyridine moiety exhibits a strong chela-
tion effect to the rhodium catalyst. What also needs to be
mentioned is that when 2p and 2q were subjected to the
optimized reaction conditions, the corresponding alkynyla-
tion reactions tended to occur at the site of the more acidic
À
Scheme 4. C H alkynylation of an estrone derivative.
À
proton. This phenomenon suggested that such C H alkyny-
lation may proceed through a concerted metallation–depro-
tonation pathway. As was expected, the halogens Cl, Br, and
even I, were all nicely tolerated under these reaction
conditions, thus providing the opportunity for further manip-
ulation through traditional cross-coupling reactions.[13] In
addition, the alkyne-containing substrate 2u successfully
engaged in such transformations for the synthesis of dialkynyl
benzene derivatives, but the product 3u was isolated in
moderate yield by using a shorter the reaction time.[14] It is
À
worth mentioning that this C H alkynylation reaction was
scalable and a comparable yield was obtained when the
reaction was conducted on a gram scale.[15]
To evaluate the generality and limitations of the hyper-
valent alkynyl iodine reagents, tert-butyl- and phenyl-based
alkynylation reagents were tested using 2s as the model
substrate (Scheme 3). It was pleasing to observe that when
Scheme 5. Mechanistic studies.
reaction between 2n and 2a afforded the desired products 3n
and 3a in 75 and 25% yield, respectively. These experiments
clearly demonstrate that electronically-deficient arenes are
À
Scheme 3. C H alkynylation using tert-butyl and phenyl ethynyl benzio-
doxolones.
À
more favored in the C H activation step of this reaction, thus
ruling out the electrophilic aromatic substitution pathway. To
À
determine whether the C H activation is involved as the rate-
determining step in the catalytic cycle, an intermolecular KIE
experiment was conducted with 2b and [D5]-2b, and it
produced a KIE value of 9.0 (refer to the Supporting
Information). Furthermore, the individual initial reaction
rate of 2b or [D5]-2b with 1 was measured and the KIE value
was obtained as 6.8.[17] When the 13C-labeled reagent 1 was
employed, the reaction of 2b afforded product [13C]-3b, and
thud precluded the involvement of a silyl migration step in the
catalytic cycle.
tert-butyl-substituted ethynyl benziodoxone was employed,
the reaction proceeded smoothly to afford the desired
product 4s in 92% yield. However, when phenyl-substituted
reagent was used, the reaction turned out to be rather messy
and none of the desired 5s could be obtained.
To further showcase the applicability of this protocol to
the synthesis of natural products or related complex mole-
cules, the estrone-derived substrate 2v was synthesized and
subjected to the optimized reaction conditions (Scheme 4).
The reaction proceeded smoothly to afford the desired
alkynylation product 3v in 83% yield.
À
Based on the precedent of rhodium-catalyzed C H
activation involving regioselective insertion of unsymmetrical
alkynes, the reaction mechanism was tentatively proposed as
shown in Scheme 6.[18] The reaction starts with base-assisted
ligand exchange of the rhodium catalyst and 2b to generate
À
To further determine the electronic bias of the C H
activation step, competition experiments using 2n/2g and 2n/
2a were examined (Scheme 5). When an equimolar mixture
of 2n, 2g, and 1 were subjected to the optimized reaction
conditions, the desired products 3n and 3g were formed in 80
and 20% yield, respectively.[16] Similarly, the competition
À
the intermediate A, which undergoes reversible C H bond
activation through a concerted metalation-deprotonation
(CMD) pathway to afford the rhodacycle B.[19] The following
regioselective carborhodation of 1 occurs to produce C, which
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Received: October 22, 2013
Revised: December 19, 2013
Published online: February 7, 2014
À
Keywords: alkynes · C H activation · cross-coupling · iodine ·
rhodium
.
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À
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Experimental Section
An oven-dried 5 mL round-bottomed flask was charged with the
hypervalent alkynyl iodine regent 1 (47 mg, 0.11 mmol), 2a (23.5 mg,
0.1 mmol), [{RhCp*Cl2}2] (1.2 mg, 0.002 mmol), and NaOAc (8.2 mg,
0.1 mmol) in sequence, with subsequent addition of anhydrous DCE
(0.5 mL) by syringe. After stirring at room temperature for 12 h,
saturated sodium bicabonate was added and the resulting mixture was
extracted with dichloromethane (2 ꢀ 20 mL). Removal of the solvent
in vacuo and purification of the residue by silica gel column
chromatography afforded the desired product 3a (38.3 mg, yield:
92%).
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