Chelation-Assisted Nickel-Catalyzed Oxidative Annulation via Double C-H Activation/Alkyne Insertion Reaction

Article abstract of DOI:10.1002/chem.201504596

A nickel/NHC system for regioselective oxidative annulation by double C-H bond activation and concomitant alkyne insertion is described. The catalytic reaction requires a bidentate directing group, such as an 8-aminoquinoline, embedded in the substrate. Various 5,6,7,8-tetrasubstituted-N-(quinolin-8-yl)-1-naphthamides can be prepared as well as phenanthrene and benzo[h]quinoline amide derivatives. Diarylalkynes, dialkylalkynes, and arylalkylalkynes can be used in the system. A Ni⁰/Ni^II catalytic cycle is proposed as the main catalytic cycle. The alkyne plays a double role as a two-component coupling partner and as a hydrogen acceptor. In two shakes: Oxidative annulation by a double C-H activation/alkyne insertion reaction was achieved by a nickel/NHC system. A Ni⁰/Ni^II catalytic cycle is proposed as the main catalytic cycle. The alkyne plays a double role as a two-component coupling partner and as a hydrogen acceptor.

Full text of DOI:10.1002/chem.201504596

DOI: 10.1002/chem.201504596
Full Paper
&
CÀH Activation
Chelation-Assisted Nickel-Catalyzed Oxidative Annulation via
Double CÀH Activation/Alkyne Insertion Reaction
Luis C. Misal Castro, Atsushi Obata, Yoshinori Aihara, and Naoto Chatani*[a]
Abstract: A nickel/NHC system for regioselective oxidative
annulation by double CÀH bond activation and concomitant
alkyne insertion is described. The catalytic reaction requires
a bidentate directing group, such as an 8-aminoquinoline,
embedded in the substrate. Various 5,6,7,8-tetrasubstituted-
N-(quinolin-8-yl)-1-naphthamides can be prepared as well as
phenanthrene and benzo[h]quinoline amide derivatives. Dia-
rylalkynes, dialkylalkynes, and arylalkylalkynes can be used in
the system. A Ni0/NiII catalytic cycle is proposed as the main
catalytic cycle. The alkyne plays a double role as a two-com-
ponent coupling partner and as a hydrogen acceptor.
Introduction
of these compounds through double CÀH bond activation,
these methodologies all have weak points. First, most of the
systems reported to date involve the use of expensive precious
metal catalysts such as palladium, rhodium, and iridium.
Second, the necessary addition of stoichiometric amounts of
a terminal oxidant such as a copper salt, make this reactions
less appealing for industrial scaling. In the case of the chela-
tion-assisted method (Scheme 1c), [Cp*RhCl2]2 is mainly used
as the catalyst.[17]
An aromatic homologation reaction with internal alkynes leads
to the direct formation of polysubstituted condensed arenes,
which are attractive compounds for material sciences due to
their physical properties and large potential applications in the
area of organic electronics.[1] Alternative approaches are avail-
able for their preparation, in addition to the traditional
method that uses difunctionalized starting materials.[2] One in-
volves the use of catalytic CÀH bond activation which has
emerged during the last decade as a more sustainable ap-
proach (Scheme 1).[3] Among them, the most extensively stud-
ied methods, mainly developed by the Satoh and Miura
groups, uses monofunctionalized arenes (halides,[4] acyl chlor-
ides,[5] carboxylic acids,[6] alcohols,[7] boronic acids,[8] and
Grignard reagents[9]) with internal alkynes, where the CÀH acti-
vation is directed by the functional group towards the ipso
and ortho positions (Scheme 1a). The other two methods in-
volve the successive activation of CÀH bonds. These strategies
have been less studied and only few examples have appeared
in the literature. For example, a nonchelation-assisted strategy
(Scheme 1b) was first reported by Wu[10] in 2008 and further
developed by Cramer,[11] by taking advantage of the different
steric environment around the molecule to selectively activate
the most accessible CÀH bonds. The chelation-assisted method
using a pyrazole-directing group (Scheme 1c), developed by
Satoh and Miura in 2008, directs a sequential functionalization
towards ortho and meta CÀH bonds with the assistance of
a pyrazole group.[12] Later, 2-phenol,[13] pyridine,[14] and amide
moieties[15,16] were successfully applied in this strategy by sev-
eral groups. However, despite the straightforward preparation
In recent years, earth-abundant first-row transition-metals
have emerged as potential surrogates of expensive metal cata-
lysts in direct CÀH functionalization reactions.[18] In our ongo-
ing research directed at developing new catalytic systems that
use nickel,[19,20] we hypothesized that a bidentate-directing
group could assist the synthesis of polycondensed aromatic
ring derivatives in one step. Herein, we describe the first regio-
selective aromatic homologation reaction catalyzed by a nickel
catalyst by double CÀH bond activation that is directed by an
8-aminoquinolinyl moiety (Scheme 2).
Results and Discussion
Various reaction parameters were screened in order to deter-
mine the optimal conditions for the reaction (Table 1 and
Table S1 in the Supporting Information). The reaction of 1a
with diphenylacetylene in the presence of [Ni(cod)2]/PCy3 did
not give 2a and 81% of 1a was recovered (entry 1). The use
of [NiBr2(dme)] as the nickel catalyst gave 2a in 28% NMR
yield (entry 2). A curious effect of KOtBu was seen by compar-
ing the results for entries 1 and 3. The yield of 2a was also dra-
matically improved when KOtBu was used as the additive (en-
tries 2 and 4). The use of ICy as the ligand also improved the
product yield (entry 5). The product 2a was obtained in 86%
isolated yield under the optimized reaction conditions:
[NiBr2(dme)] (10 mol%) as the catalyst, SIMes·HCl (20 mol%) as
the ligand, KOtBu (22 mol%) as the base for the in situ genera-
tion of the NHC ligand with 5 equiv of diphenylacetylene in
[a] Dr. L. C. Misal Castro, A. Obata, Y. Aihara, Prof. N. Chatani
Department of Applied Chemistry, Faculty of Engineering
Osaka University, Suita, Osaka 565-0871 (Japan)
E-mail: chatani@chem.eng.osaka-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504596.
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Scheme 1. Aromatic homologation reactions by transition-metal-catalyzed CÀH activation/functionalization.
Table 1. Ni-catalyzed CÀH aromatic homologation of benzamides.
Scheme 2. Ni-catalyzed CÀH aromatic homologation of benzamides.
toluene (0.25 mL) at 1608C for 1 h under a nitrogen atmos-
phere (entry 8). The base, KOtBu plays an important role for
the efficiency of the reaction, such as not only for the genera-
tion of the NHC ligand, but also to accelerate the reaction. An
alkenylation product 3 was not obtained in all cases.[21]
It was found that the use of an 8-aminoquinoline as the di-
recting group was essential for the success of the reaction,
while other directing groups, which are known to promote CÀ
H bond activation reactions, did not afford the desired prod-
ucts (Scheme 3 and Table S3 in the Supporting Information).
Various benzamides were examined under the optimized re-
action conditions (Scheme 4). While the 2-, and 3-methyl-, and
Entry
[Ni]
Ligand
KOtBu
[mol%]
NMR yield
2a [%]
1
2
3
4
5
6
7
8
9
[Ni(cod)2]
[NiBr2(dme)]
[Ni(cod)2]
[NiBr2(dme)]
[NiBr2(dme)]
[NiBr2(dme)]
[NiBr2(dme)]
[NiBr2(dme)]
[Ni(cod)2]
PCy3
PCy3
PCy3
PCy3
Icy·HCl
SiCy·HBF4
IMes·HCl
SlMes·HCl
SlMes·HCl
–
–
0
28
58
65
77
10
10
22
22
22
22
22
80
70
90 (86)[a]
62
[a] Isolated yield.
2,3-dimethyl-substituted benzamides gave excellent conver-
sions in each case (2b 88%, 2c 95%, and 2e 92%), the 4-
methyl and 2,4-dimethyl substrates afforded 2d in 53% and
2 f in 68%, respectively. The 2-phenyl-substituted benzamides
gave 2g in 58%. These results indicate that the presence of
a substituent at the 4-position and a sterically demanding
phenyl group at the 2-position caused a slight decrease in the
product yields. Interestingly, when a dimethylamino group is
Scheme 3. Screening of directing groups.
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present in the substrate, the expected product 2i was accom-
panied by a small amount (2%) of the side reaction product
2a. In the case of 3-(2-tolyloxy)- and 3-benzyloxy-substituted
benzamides, 2j (82%) and 2k (81%) were obtained in good
yields, along with a small amount of 2a. The formation of 2a
provided a clue that a Ni0 species is involved in the catalytic
cycle, because a Ni0 species is known to activate such types of
“hard” bonds.[22–24] Surprisingly, with the 3-SMe-substituted
amide 1p, the product 2p was obtained in 65% yield, and no
side product 2a was detected, although there are some exam-
ples of Ni0-catalyzed CÀS activation reactions in the litera-
ture.[25] This reaction is also applicable to the condensed ring
system in compounds with more than one aromatic ring, as in
2s, 2t, and 2u. The reaction of 1a with bis(4-methoxyphenyl)-
ethyne and bis(4-(trifluoromethyl)phenyl)ethyne gave the cor-
responding naphthalene products 2v and 2w in 87 and 63%,
respectively.
Scheme 5. Switching the outcome with the presence of a proton source.
and stability of polyarenes in organic solvents, which is an im-
portant issue for potential applications.[26] The reaction of 1a
with 4-octyne and 8-hexadecyne afforded good yields (4a
60%, and 4b 45%), along with small amounts of ortho-alkeny-
lated products 5a (7%) and 5b (trace; Scheme 6).
In the case of unsymmetrical alkynes, the reaction proceeds
in a regioselective manner (Scheme 7). The regioselectivity of
the major isomers 6a and 6d was confirmed by NOE spectros-
copy.
We next tested the scalability of the reaction. The reaction
of 1a with diphenylacetylene in a 4.5 mmol scale (1.12 g) gave
2a in 89% isolated yield (2.41 g).
The use of 2 equiv of diphenyacetylene and 0.5 equiv of piv-
alic acid in a 2 h reaction did not give 2a, but instead the alke-
nylation product 3 was obtained in 47% isolated yield, with
the starting material 1a being recovered (34%; Scheme 5).[21]
We next examined the use of dialkylalkynes as the coupling
partner because such alkyl chains could increase the solubility
Scheme 6. Dialkylalkynes as the coupling partner in the aromatic homologa-
tion.
Scheme 7. Reaction with unsymmetrical alkynes. [a] The ratio of regioisom-
ers was 30 (6a):5:4:1. [b] The ratio of a major regioisomer 6b and the other
three regioisomers was >3.0:1. [c] The ratio of a major regioisomer 6c and
the other three regioisomers was >3.7:1. [d] The ratio of regioisomers was
150 (6d):20:10:1.
To better understand the electron preferences of the system,
intermolecular competition experiments using two sets of elec-
tronically different benzamides were carried out (Scheme 8).
The results show that an electron-donating group facilitates
the reaction, but that an electronic effect was not significant
(OMe/Me/CF3 =2.1:1.3:1).
To gain insights into the mechanism, deuterium-labeling ex-
periments were conducted (Scheme 9). Compounds 1b and
[D7]-1b were treated separately under the optimized condi-
tions and the reaction was terminated after 15 min. The KIE
calculated is close to one, indicating that CÀH bond activation
Scheme 4. Scope of benzamides and alkynes. Reaction conditions:
1 (0.25 mmol), alkyne (1.25 mmol), [NiBr2(dme)] (0.025 mmol), SIMes·HCl
(0.05 mmol), KOtBu (0.055 mmol), toluene (0.25 mL), N2, 1608C, 1 h (other-
wise indicated).
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turn toward the meta-CÀH bond which is activated through
a second cleavage of CÀH bond to afford the complex V via IV.
A second molecule of the alkyne is then inserted in complex V
to form the complex VIa or VIb which then undergoes reduc-
tive elimination to form the product 2a with the formation of
a Ni0 species,[30] the generation of which suggests that the cat-
alytic cycle is not completed when the NiII complex initiates
the catalysis. The generated Ni0 species then initiates the main
catalytic cycle. The Ni0 species reacts with 1a followed by oxi-
dative addition of the NÀH bond in 1a to give the nickel(II) hy-
dride complex VII. The insertion of the alkyne into the NiÀH
bond of VII affords the vinylnickel complex VIII. The complex II
can be generated in two ways: 1) the cleavage of the ortho CÀ
H bond with the concomitant formation of cis/trans stilbene
through s-bond metathesis, or 2) the insertion of another
alkyne molecule into complex VIII to form IX, which also un-
dergoes cleavage of the ortho CÀH bond to generate II with
the concomitant generation of stereoisomers of the 1,2,3,4-tet-
raphenyl-1,3-butadiene. The rest of the catalytic cycle from
complex II is the same as described before (II!III!IV!V!
VI!2a and Ni0). Hence, the Ni0/NiII manifold is the main cata-
lytic cycle. In fact, alkenes were isolated by column chromatog-
raphy from the reaction of 1a (0.25 mmol) with diphenylacety-
lene (1.25 mmol) under the optimized reaction conditions to
form 2a in 86% (0.215 mmol) isolated yield, and a mixture of
cis- and trans-stilbene (0.08 mmol) and a mixture of stereoiso-
mers of the tetraphenyl-1,3-butadiene (0.16 mmol) were ob-
tained. Alkenes were obtained in a total of 0.24 mmol, which is
in coincident with the amount (0.215 mmol) of 2a.
Scheme 8. Intermolecular competition experiments.
Conclusions
We report on the development of a catalytic system for the ar-
omatic homologation of benzamides using a nickel catalyst
where an 8-aminoquinoline directing group is fundamentally
required for the success of the reaction.[31] The reaction toler-
ates various functional groups. A Ni0/NiII catalytic cycle is pro-
posed as the main manifold.
Scheme 9. Deuterium-labeling experiments.
is not involved in the rate-determining step. The recovered
1
starting material [D7]-1b was analyzed by H NMR spectrosco-
Experimental Section
py, and the results show that H/D exchange occurred only at
the ortho CÀH bonds.
To an oven-dried 5 mL screw-capped vial inside a glovebox SI-
Mes·HCl (17.2 mg, 0.05 mmol), [NiBr2(dme)] (7.9 mg, 0.025 mmol),
KOtBu (6.3 mg, 0.055 mmol), and toluene (0.25 mL) were added
and the solution stirred for 10 min at room temperature. N-(Quino-
lin-8-yl)benzamide (1a; 62 mg, 0.25 mmol) and diphenylacetylene
(227 mg, 1.25 mmol) were then added and the mixture was stirred
for 1 h at 1608C, followed by cooling. The mixture was filtered
through a Celite pad and the filtrate concentrated in vacuo. The
residue was purified by column chromatography on silica gel
(hexane/EtOAc 10:1) and further purified by recycle HPLC (GPC) to
afford the desired product 2a.
Based on the analysis of the experimental data obtained and
our previously reported studies on nickel-catalyzed functionali-
zation of CÀH bonds,[19b] a proposed mechanism is shown in
Scheme 10. Coordination of the benzamide 1a with NiIIX2
forms complex I, in which the CÀH bonds are cleaved though
a concerted metallation–deprotonation (CMD) mechanism to
generate the NiII complex II. The complex II then undergoes an
alkyne insertion to form III. Unlike previous works with
nickel[27] and others with cobalt[28] and ruthenium[29] for the
synthesis of isoquinolones, complex III does not undergo re-
ductive elimination, probably because the amide nitrogen is Acknowledgements
not sufficiently nucleophilic to undergo reductive elimination.
Rather, the dissociation of both the quinoline and the amide
nitrogen atoms from the catalyst causes the nickel center to
This work was supported, in part, by a Grant-in-Aid for Scientif-
ic Research on Innovative Areas “Molecular Activation Directed
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Scheme 10. Proposed mechanism.
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toward Straightforward Synthesis” from Monbusho (The Minis-
try of Education, Culture, Sports, Science and Technology) and
by JST Strategic Basic Research Programs “Advanced Catalytic
Transformation Program for Carbon Utilization (ACT-C)” from
the Japan Science and Technology Agency.
Keywords: alkynes · aromatic homologation · CÀH activation ·
chelate-assistance · nickel
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[30] An alternative mechanism involves a direct insertion of a second mole-
cule of alkyne into IV leading to VIa.
[31] For reported examples of the conversion of an 8-aminoquinoline
moiety to other functional groups, see Scheme S1 in the Supporting In-
formation of our paper: K. Shibata, N. Chatani, Chem. Sci. 2015, DOI:
10.1039/c5sc03110a.
[21] Ni-catalyzed alkenylation of CÀH bonds using an 8-aminoquinoline di-
recting group with internal alkynes have been reported; a) M. Li, Y.
Yang, D. Zhou, D. Wan, J. You, Org. Lett. 2015, 17, 2546–2549; b) S.
Maity, S. Agasti, A. M. Earsad, A. Hazra, D. Maiti, Chem. Eur. J. 2015, 21,
11320–11324.
[22] For recent papers on the Ni0-catalyzed CÀN bond activation, see: a) M.
Received: November 16, 2015
Published online on December 21, 2015
Tobisu, K. Nakamura, N. Chatani, J. Am. Chem. Soc. 2014, 136, 5587–
Chem. Eur. J. 2016, 22, 1362 – 1367
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