Mn-catalyzed aromatic C-H alkenylation with terminal alkynes

Article abstract of DOI:10.1021/ja311689k

The first manganese-catalyzed aromatic C-H alkenylation with terminal alkynes is described. The procedure features an operationally simple catalyst system containing commercially available MnBr(CO)₅ and dicyclohexylamine (Cy₂NH). The reaction occurs readily in a highly chemo-, regio-, and stereoselective manner delivering anti-Markovnikov E-configured olefins in high yields. Experimental study and DFT calculations reveal that (1) the reaction is initiated by a C-H activation step via the cooperation of manganese and base; (2) manganacycle and alkynylmanganese species are the key reaction intermediates; and (3) the ligand-to-ligand H-transfer and alkynyl-assisted C-H activation are the key steps rendering the reaction catalytic in manganese.

Full text of DOI:10.1021/ja311689k

Communication
pubs.acs.org/JACS
Mn-Catalyzed Aromatic C−H Alkenylation with Terminal Alkynes
Bingwei Zhou,† Hui Chen,*,‡ and Congyang Wang*,†
‡
†Beijing National Laboratory of Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, CAS Key
Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
S
* Supporting Information
Scheme 1. Mn-Promoted C−H Alkenylations with Alkynes
ABSTRACT: The first manganese-catalyzed aromatic C−
H alkenylation with terminal alkynes is described. The
procedure features an operationally simple catalyst system
containing commercially available MnBr(CO)5 and
dicyclohexylamine (Cy2NH). The reaction occurs readily
in a highly chemo-, regio-, and stereoselective manner
delivering anti-Markovnikov E-configured olefins in high
yields. Experimental study and DFT calculations reveal
that (1) the reaction is initiated by a C−H activation step
via the cooperation of manganese and base; (2) mangana-
cycle and alkynylmanganese species are the key reaction
intermediates; and (3) the ligand-to-ligand H-transfer and
alkynyl-assisted C−H activation are the key steps
rendering the reaction catalytic in manganese.
H activation by Mn due to MnR(CO)5 being indispensably
employed for such stoichiometric reactions,4,8 as it is generally
ecently, the direct functionalization of inert CH bonds
accepted that the R group of MnR(CO)5 serves as an
intramolecular H-acceptor in the C−H activation step (Mode
A); (iii) the transformation of seven-member manganacycles
into alkenylated products and, more importantly, regenerating
the catalytic species of manganese. With these concerns, we
herein uncover a novel catalyst system containing commercially
available MnBr(CO)5 and dicyclohexylamine (Cy2NH), which
enables efficient aromatic C−H activation via Mn/amine
cooperation (Mode B). Accordingly, we demonstrate the first
Mn-catalyzed aromatic C−H alkenylations with terminal alkynes
(Scheme 1b).10 Of note, terminal alkynes are often nontrivial
substrates in C−H alkenylation reactions due to their relatively
acidic terminal protons and ease of self-di- or trimerizations in
transition metal catalysis.11
We commenced our reaction with arylpyridine 1g and alkyne
2a as model substrates (Table 1). No products were detected in
toluene with mere MnBr(CO)5 as a catalyst (entry 1).
Surprisingly, the alkenylated product 3ga was formed in 24%
NMR yield when a catalytic amount of triethylamine (Et3N), a
weak base, was added into the reaction (entry 2). The screening
of solvents revealed Et2O as being optimal (entry 3).12 An array
of bases were then examined, and secondary amines bearing
proper steric bulkiness were noted to be beneficial for the
reaction (entries 4−6).12 Inspired by the N-heterocyclic carbene
(NHC) and pyridine ligands developed by Glorius13a and Yu13b
respectively, we tested a series of secondary amines of varied
carbocycle ring sizes ranging from A-1 to A-4.12 To our delight,
largely enhanced yields of 3ga were generally obtained with
dicyclohexylamine (A-2) being the best (entries 7−10). Further
R
ubiquitous in organic molecules has attracted intensive
attention since it holds great potential in reshaping traditional
organic synthesis.1 Second- and third-row transition metals, such
as Pd, Rh, and Ru, are major players in this arena and have
witnessed explosive success in catalytic CC and CX bond
formation reactions.2 In contrast, using the first-row transition
metals for such catalytic transformations has been considerably
less studied despite their high abundance in earth’s crust, low
cost, and promising reactivities.3 For manganese in particular,
although Bruce et al.4 reported seminal work on stoichiometric
ortho-manganation of benzylideneaniline as early as in 1971,
there are only a few examples of Mn-catalyzed CC bond
formation reactions via inert CH activation.5−7 Kuninobu and
Takai et al. elegantly demonstrated a Mn-catalyzed imidazole-
directed addition of aromatic CH bonds to the polar CO
bond of aldehydes.5 Note that the presence of silanes plays a key
role in making this reaction catalytic in Mn.
For the related addition to unpolarized C−C triple bonds of
alkynes, however, only stoichiometric versions have been
disclosed to date.8 The groups of Liebeskind, Nicholson, Suar
́
ez,
and others showed the formation of five-member manganacycles
with a stoichiometric MnBn(CO)5 reagent, which was prepared
from stepwise reactions employing Mn2(CO)10, Na/Hg
amalgam, and benzylchloride (Scheme 1a).9 The ensuing
insertion of alkynes into Mn−Caryl bonds gave seven-member
manganacycles upon heating, photoirradiation, or with the
reagent of trimethylamine N-oxide.8 Clearly, challenges still
remain for the successful development of practical Mn-catalyzed
C−H alkenylation reactions: (i) the avoidance of tedious
syntheses and stoichiometric use of MnR(CO)5 (R = Bn, Me,
Ph) reagents; (ii) the ensuing need to develop new modes of C−
Received: November 30, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja311689k | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
a
,b
Table 1. Screening of Reaction Parameters
Scheme 2. Scope of Alkynes
b
entry
cat. (10 mol %)
base (20 mol %)
solvent
yield (%)
c
1
MnBr(CO)5
MnBr(CO)5
MnBr(CO)5
MnBr(CO)5
MnBr(CO)5
MnBr(CO)5
MnBr(CO)5
MnBr(CO)5
MnBr(CO)5
MnBr(CO)5
Mn2(CO)10
ReBr(CO)5
Ru3(CO)12
Mn(acac)3
−
toluene
toluene
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
0
2
Et3N
Et3N
EtN(i-Pr)2
(i-Pr)2NH
pyrrolidine
A-1
24
34
23
37
19
50
57
52
54
58
4
3
4
5
6
7
8
A-2
a
Reaction conditions: 1a (1 mmol), 2 (0.5 mmol), MnBr(CO)5 (0.05
9
A-3
b
mmol), Cy2NH (0.1 mmol), Et2O (1.2 mL), 80 °C, 6 h. Isolated
yields of product 3 are shown. 3ac/6ac = 9:1. 100 °C, 12 h. 3aj/
6aj = 9:1. 3ap/5ap = 9:1. 3ar/5ar = 8:1.
10
11
12
13
14
A-4
c
d
e
A-2
f
g
A-2
A-2
8
a
,b
Scheme 3. Scope of Arylpyridines
A-2
0
d
e
15
MnBr(CO)5
A-2
77(58)
a
Reaction conditions unless otherwise noted: 1g (0.2 mmol), 2a (0.2
mmol), catalyst (0.02 mmol), base (0.04 mmol), solvent (0.5 mL),
b
1
100 °C, 6 h. Yields of 3ga determined by H NMR analysis with
c
d
1,3,5-trimethoxybenzene as an internal standard. No base. 1g/2a =
2:1. Isolated yield of 3ga on 0.5 mmol scale with 0.5 mL of Et2O;
e
3ga/6ga = 17:1; 48% of 1g was recovered after reaction.
variations on metal catalysts showed that Mn2(CO)10 worked as
well, while other metal carbonyls, such as ReBr(CO)5 and
Ru3(CO)12, and noncarbonyl manganese catalysts exhibited
much lower activity, if any at all (entries 11−14).12 Finally, the
optimal reaction conditions were obtained by tuning the ratio of
1g/2a (entry 15). It should be noted that only a small amount of
Z-isomer 6ga was detected (3ga/6ga = 17:1) and neither
dialkenylated product 4ga nor Markovnikov adduct 5ga was
observed in the reaction, which reflects a high level of chemo-,
regio-, and stereocontrol in this protocol.
a
Reaction conditions: 1 (1 mmol), 2 (0.5 mmol), MnBr(CO)5 (0.05
b
mmol), Cy2NH (0.1 mmol), Et2O (1.2 mL), 80 °C, 6 h. Isolated
yields of 3 are shown. Single product.
c
With the optimized conditions in hand, we next explored the
scope of alkynes (Scheme 2). Both electro-donating and
-withdrawing groups on aromatic alkynes are well tolerated
affording the expected products (3aa−f) in good yields. It is
noteworthy that halogen functionalities, in particular Br and I,
remain intact after reaction, providing easy handles for further
synthetic elaborations (3ag−j). Ortho- and meta-substituents on
the benzene moiety show no obvious influence on the reaction
outcome (3ak−m). 2-Ethynylnaphthalene containing an ex-
tended conjugated system gives the corresponding product 3an
as well. As an example of a heteroaromatic alkyne, 2-
ethynylthiophene is also amenable to this protocol (3ao).
Aliphatic alkynes with varied functionalities are compatible with
the reaction conditions (3ap−r), albeit displaying relatively
sluggish reactivities. Finally, no reaction took place with internal
alkynes, which hints at the reaction mechanism.
steric hindrance for C−H activation. Also, a wide array of
functional groups on the benzene rings, again including fragile
halogens, are compatible in the reaction despite their different
electronic properties (3hk−nk). An ortho-substituent, however,
has a profound influence on the reaction outcome (3og), which
echoes the fact that no dialkenylated products are observed in the
reaction for all cases. A perfect regioselectivity was found when
two adjacent C−H bonds exist in the meta-substituted benzene
moiety and single regioisomers of less steric bulkiness were
obtained in high yields (3pk−qk). Interestingly, completely
reversed regioselectivity was observed when a second-directing
m-fluoro group exists affording 3ra′ and 3rk′ as the sole products.
It proved that the m-methoxyl group had less directing effect
resulting in the formation of a mixture of 3sk and 3sk′, which is
consistent with Liebeskind’s study.8a 2-Thiophenylpyridine was
smoothly alkenylated on the 3-position of the thiophene ring
though the C−H bond adjacent to the S-atom is often
susceptible (3tg). 1-Phenylpyrazole is also amenable to this
protocol affording 3uk in good yield.
Next, a variety of arylpyridines were further examined
(Scheme 3). Substituents on pyridine rings with disparate
electronic properties have no obvious bias on the formation of
products (3bk−fk). Arylpyridine 1g bearing a 3-positioned
methyl group gives rise to 3gk smoothly despite the increased
B
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Journal of the American Chemical Society
Communication
To probe the possible reaction mechanism, we conducted a
series of experimental investigations. A stoichiometric reaction of
MnBr(CO)5 with alkyne 2b was first examined in the presence of
Cy2NH (Scheme 4a, left). No alkynylmanganese species Mn-I
Note that a related alkynylmanganese species was found by
Nicholson et al. as a byproduct in a stoichiometric reaction of
ortho-manganated triphenylphosphate with phenylacetylene.8g
The second C−H olefination of Mn-VI would finally lead to the
formation of 7 in a stoichiometric reaction, while ligand exchange
of Mn-VI with 1a would deliver Mn-VII and release
monoalkenylated product 3ab in the catalytic reaction (Scheme
5). To further verify the involvement of Mn-VII, alkynylmanga-
nese Mn-I was prepared from MnBr(CO)5 and 2b via a
lithiation−trapping sequence (Scheme 4c).14a We envisioned
that Mn-I and 1a would undergo a ligand-exchange releasing CO
to afford Mn-VII, which subsequently transforms into Mn-III via
C−H activation, or further leads to Mn-II with recoordination of
CO (Scheme 5). To our delight, Mn-II was isolated in 22% yield
when Mn-I was treated with 1a (Scheme 4d), which correlated
with our hypothesis. Importantly, it was demonstrated that both
Mn-II and Mn-I could catalyze the alkenylation of 1a with 2b
affording 3ab in comparable yields (Scheme 4e).
Scheme 4. Mechanistic Experiments
Based on these results, a plausible reaction mechanism is
proposed in Scheme 5. Density functional theory (DFT)
calculations were then conducted to shed light on the role of
base in the initial C−H activation of 2-phenylpyridine 1a and the
profile of the whole catalytic cycle. First, it is seen from Scheme 5
that five-member manganacycle Mn-II can be easily generated
from [Mn(1a)(CO)4]+ Mn-0 with the aid of Cy2NH base by an
activation energy of 12.5 kcal/mol. This relatively low barrier is
mainly due to the stability of Mn-II, which leads to an exothermic
C−H activation process by 14.4 kcal/mol. This is in line with the
experimental isolation of Mn-II as shown before. After
replacement of one binding CO by an alkyne, Mn-III is reached
as the starting point of the entire catalytic cycle. The subsequent
insertion of alkyne into the Mn−Caryl bond only needs to
overcome a barrier of 9.4 kcal/mol, resulting in seven-member
manganacycle Mn-IV. We speculate that the relatively crowded
octahedron configuration of manganese in manganacycle Mn-III
and Mn-IV accounts for the high chemo-, regio-, and
stereoselectivity of this protocol. Then, Mn-IV reacts with
another alkyne to protonate the alkenylmanganese group via a
ligand-to-ligand H-transfer process and generates Mn-VI
containing the final product, which is liberated from the catalytic
cycle by substitution with 1a affording alkynylmanganese Mn-
VII. It can regenerate Mn-III via C−H activation assisted by the
alkynyl ligand in Mn-VII with a barrier of 15.5 kcal/mol.
Collectively, the whole catalytic cycle is exothermic by 28.7 kcal/
mol.
was observed, which is in line with previous reports that the
formation of Mn-I necessitated the existence of strong alkynyl
nucleophiles or silver salts.14 In contrast, MnBr(CO)5 reacted
with 2-phenylpyridine 1a affording five-member manganacycle
Mn-II (structure in Scheme 5) in 31% isolated yield (Scheme 4a,
a
Scheme 5. Proposed Reaction Mechanism with DFT Study
To gain more details on the nature of Mn-catalyzed C−H
activation, deuterium-labeling experiments were next explored.
The reaction of pentadeuterated arylpyridine 1a-d5 and alkyne
2q resulted in the incorporation of deuterium at both olefinic
positions of 3aq-d (Scheme 6a). Similarly, the scrambling of
deuterium on the double bond of 3aq-d was also observed when
deuterated alkyne 2q-d was treated with 1a under the same
conditions (Scheme 6b). These results are in good agreement
with the reaction mechanism shown in Scheme 5. In addition, it
was found that the sum of deuterium decreased after reactions
and D/H were scrambled in the recovered starting materials
(Scheme 6a,b), which raised the concern that D/H exchange
might exist in the reaction. We therefore subjected mere 1a-d5 to
the reaction, and the partial loss of deuterium was detected
(Scheme 6c, left), indicating the C−H activation step might be a
reversible deprotonation/protonation equilibrium. Also, the
degree of deuteration in 2q-d decreased upon treatment with
base (Scheme 6c, right). Finally, the reaction of 1a and 2q in the
a
DFT computed relative energies and barriers (kcal/mol) with zero-
point energy (ZPE) corrections are marked in red (for R = Ph).
right). Remarkably, only a trace amount of Mn-II was detected in
the absence of Cy2NH, which underlies the key role of Cy2NH in
this C−H activation step. Subsequently, Mn-II was treated with
alkyne 2b and bis-olefin 7 was formed in 29% isolated yield
(Scheme 4b). It is widely accepted8 and confirmed by
́
Nicholson8g and Suarez8j that seven-member manganacycles
are formed by a regioselective insertion of alkyne into Mn−Caryl
bonds of five-member manganacycles. Therefore, we surmised
that Mn-IV were formed from Mn-II and 2b and would further
react with 2b to afford alkynylmanganese Mn-VI (Scheme 5).
C
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Journal of the American Chemical Society
Communication
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In conclusion, the Mn-catalyzed C−H alkenylation with
terminal alkynes was successfully developed for the first time.
The reaction highlights not only a practical catalyst system
consisting of commercially available MnBr(CO)5 and Cy2NH
but also high levels of control in chemo-, regio-, and
stereoselectivity. Experimental and computational investigations
show that the reaction is initiated by a deprotonative C−H
activation step via the synergy of manganese and base.
Remarkably, key intermediates such as manganacycle and
alkynylmanganese species were identified for the reaction.
Moreover, the ligand-to-ligand H-transfer and alkynyl-assisted
C−H activation steps are probed as key pathways to furnish the
entire catalytic cycle. Further studies to explore Mn-catalyzed
novel reactions using the combination of Mn catalysts and weak
bases are underway in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
Experimental and computational procedures and full spectro-
scopic data for all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
wangcy@iccas.ac.cn; chenh@iccas.ac.cn
■
Nicholson, B. K. J. Organomet. Chem. 2001, 633, 162. (j) Suar
Faraldo, F.; Vila, J. M.; Adams, H.; Fernandez, A.; Lopez-Torres, M.;
Fernandez, J. J. J. Organomet. Chem. 2002, 656, 270. These
́
ez, A.;
́
́
Notes
́
The authors declare no competing financial interest.
stoichiometric reactions can be applied to the syntheses of indenols
when carbonyls serve as directing groups; see refs 8a−d.
ACKNOWLEDGMENTS
■
(9) Bruce, M. I.; Liddell, M. J.; Pain, G. N. Inorg. Synth. 1989, 26, 171.
(10) (a) Matsuura, Y.; Tamura, M.; Kochi, T.; Sato, M.; Chatani, N.;
Kakiuchi, F. J. Am. Chem. Soc. 2007, 129, 9858. (b) Cheng, K.; Yao, B.;
Zhao, J.; Zhang, Y. Org. Lett. 2008, 10, 5309.
Generous financial support from the Natural Science Foundation
of China (21002103, 21290194), the Major State Basic Research
Development Program (2012CB821600), and Institute of
Chemistry, CAS are gratefully acknowledged.
(11) For dimerizations, see: (a) Trost, B. M.; Toste, F. D.; Pinkerton,
A. B. Chem. Rev. 2001, 101, 2067. (b) Jahier, C.; Zatolochnaya, O. V.;
Zvyagintsev, N. V.; Ananikov, V. P.; Gevorgyan, V. Org. Lett. 2012, 14,
2846 and references therein. For trimerizations, see: (c) Domínguez,
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dx.doi.org/10.1021/ja311689k | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX