Nickel-catalyzed cyclization of alkyne-nitriles with organoboronic acids involving: Anti -carbometalation of alkynes

Article abstract of DOI:10.1039/c6sc01191h

A nickel-catalyzed regioselective addition/cyclization of o-(cyano)phenyl propargyl ethers with arylboronic acids has been developed, which provides an efficient protocol for the synthesis of highly functionalized 1-naphthylamines with wide structural diversity. The reaction is characterized by a regioselective and anti-addition of the arylboronic acids to the alkyne and subsequent facile nucleophilic addition of the resulting alkenylmetal to the tethered cyano group. Mechanistic studies reveal that a Ni(i) species might be involved in the catalytic process.

Full text of DOI:10.1039/c6sc01191h

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Journal Name
ARTICLE
Nickel-Catalyzed Cyclization of Alkyne-Nitriles with
Organoboronic Acids Involving anti-Carbometalation of Alkynes
Received 00th January 20xx,
Accepted 00th January 20xx
Xingjie Zhang, Xin Xie and Yuanhong Liu*
A nickel-catalyzed regioselective addition/cyclization of o-(cyano)phenyl propargyl ethers with arylboronic acids has been
developed, which provides an efficient protocol for the synthesis of highly functionalized 1-naphthylamines with a wide
range of structural diversity. The reaction is characterized by a regioselective and anti-addition of the arylboronic acids to
the alkyne and subsequent facile nucleophilic addition of the resulting alkenylmetal to the tethered cyano group.
Mechanistic studies reveal that Ni(I) species might be involved in the catalytic process.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cyclizations involving the regioselective formation of

-to the R1 substituent such as syn-
Introduction
alkenylmetal with a metal
B
are quite rare[5] (Scheme 1, eq 2), possibly due to that the
Transition-metal-catalyzed cascade reactions consisting of
multiple carbometalation steps have attracted considerable
attention in organic synthesis since these processes enable the
rapid assembly of complex structures in an efficient, atom-
economical and green manner.[1] Among these reactions,
organoboron compounds are one of the most widely used
reagents not only due to their chemical stability and easy
availability, but also due to that they can undergo a series of
addition reactions to unsaturated compounds such as alkynes,
dienes, enones, aldehydes/ketones, nitriles and isocyanates
etc. in the presence of a transition metal catalyst, especially a
Rh, Pd or Ni complex.[2] The development of cascade reactions
by combining of the different types of these elemental
reactions is undoubtedly important and attractive. In this
regard, the cascade reactions involving the addition of
organoboron compounds to alkynes as the initial step have
been realized mainly through Rh-[1b-c,3] or Pd-catalysis[4]
reported by Murakami, Hayashi, Lu and other groups. These
catalytic reactions generally proceed by syn-1,2-addition of
organometal species generated through transmetalation
between organoboron and metal complex across the carbon-
carbon triple bond, followed by nucleophilic attack of the
resulting alkenylmetal on the remained electrophiles (Scheme
1, eq 1). So far most of the reported reactions proceed via
subsequent cyclization process will evoke a highly strained
transition state. Thus the development of new cyclization
systems with controlled regiochemistry towards
B is highly
challenging. During our studies on nickel-catalyzed reactions,
we found such transformation could be achieved by addition
of organoborons to benzene-tethered alkyne-nitriles utilization
of nickel as the catalyst, possibly through the isomerization of
syn-B to anti-B. Herein, we report the first example of nickel-
catalyzed carboarylative cyclization of alkyne-nitriles with
organoboronic acids involving regioselective and anti-
carbonickelation of alkynes, which provides an efficient
protocol for the synthesis of highly functionalized 1-
naphthylamines. In addition, mechanistic studies revealed that
Ni(I) species[6] rather than Ni(II) species was involved as key
intermediates, which has not been reported in Ni-catalyzed
boron addition reactions.
R1
known reactions:
R1
X
R2
cat. [M]
R2B(OH)2
R1
syn
R2
[M]
syn-addition
exo-alkene
[M]
(1)
M = Rh, Pd
X
X
X= C, N, O
this work:
A
R1
anti
R1
TBSO
OTBS
TBSO
TBSO R2
[M]
cat. [M]
R2B(OH)2
R2
R1
formation of regioisomer
A
in which R2 group of R2B(OH)2
R2
(2)
R1
[M]
M = Ni
CN
CN
syn-B
isomerization
locates on a carbon adjacent to alkyne terminus R1, leading to
N
anti-B
NH2
an exo-alkene upon cyclization[3,4] (Scheme 1, eq 1).
unique regioselectivity
anti-addition
endo-alkene
Scheme 1. Metal-catalyzed cascade addition/cyclization reactions
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032.
E-mail: yhliu@sioc.ac.cn
† Electronic Supplementary Information (ESI) available. CCDC 1440646 and
1440647. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x.
Results and discussion
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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We chose the nickel-catalyzed reaction of o-(cyano)phenyl
catalytic amounts of base were enough. Increasing the catalyst
loading did not improve the yield of the product (entry 10).
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DOI: 10.1039/C6SC01191H
propargyl ether[7] 1a and phenylboronic acid as a model
reaction for the optimization of the reaction conditions.
Initially, we examined the reactions in the presence of Ni(cod)2
and various phosphine ligands such as PPh3 in 1,4-dioxane at
Triarylphosphine ligands and N-heterocyclic carbene ligand IPr
(IPr =1,3-bis(2,6- diisopropylphenyl)imidazole-2-ylidene) were
also effective, while the ligands such as PPh2Me and PCy3 were
less efficient (entries 11-17). Changing the solvent to THF or
toluene afforded 2a in satisfied yields of 64-68% (entries 18-
19). Addition of one equivalent of H2O as a promoter or proton
source did not afford a better result (entry 20). Ni(acac)2 also
catalyze the reaction efficiently (entry 21). When Ni(cod)2 was
used as the catalyst, only trace amounts of 2a were obtained
(entry 22). Without the phosphine ligand, the reaction also
proceeded to afford 2a in 65% yield, however, with a longer
reaction time (entry 23). Without nickel catalyst, no reaction
occurred (entry 24). On the basis of the above optimization
studies, the reaction conditions shown in Table 1, entry 5 were
chosen as the best conditions.
o
90 C. However, only trace of the desired cyclization product
was observed, along with some byproducts (Table 1, entry 1).
Replacing of Ni(cod)2 with Ni(II) complex Ni(acac)2·2H2O ( acac
= acetylacetonate) could afford a cyclized product of 4-OTBS-
substituted 1-naphthylamine 2a, albeit in only 17-18% yields
t
(entries 2-3). To our delight, the addition of 10 mol% of BuOK
as a base improved the yield of 2a dramatically to 66% within a
short reaction time (entry 4). The results suggest that a base is
necessary for this reaction possibly for promoting the
transmetalation step by formation of
a
borate[8] with
organoboronic acid. The structure of 2a also revealed that
arylation in the initial step occurred regioselectively on the
alkyne carbon closer to the OTBS group. Subsequently, the
effects of bases, phosphine ligands and solvents were
evaluated. Among various bases, Cs2CO3 gave the best result
(73%, entry 5). Increasing the catalytic loading of Cs2CO3 to 20
mol% had little effect on the yield of 2a (entry 8). However,
when stoichiometric amount of Cs2CO3 was used, the yield was
reduced rapidly (entry 9). It was remarkable that unlike the use
of more than one equivalent of bases in most of the transition
metal-catalyzed reactions involving organoborons, here only
Next, we proceeded to investigate the scope of this new
cascade
addition/cyclization
reaction
catalyzed
by
Ni(acac)2·2H2O. The reactivity of various organoboronic acids
was first examined using 1a as a reaction partner (Table 2).
During this process, we found that the 5 mol% catalyst loading
was not effective in some cases, thus 10 mol% Ni(acac)2·2H2O,
10 mol% P(p-CF3C6H4)3 and 20 mol% Cs2CO3 were used in most
of the cases to achieve the better product yields. As shown in
Table 2, a wide range of diversely substituted aryl- or
heteroaryl-boronic acids were suitable for this reaction,
Table 1. Optimization studies for the formation of 1-naphthylamine 2a
.
5 mol% catalyst
5 mol% ligand
10 mol% base
TBSO
OTBS
leading to the desired 1-naphthylamines 2a-2s in generally
good to high yields. Arylboronic acids bearing the electron-
donation groups such as p-Me, p-tBu and p-MeO or electron-
withdrawing groups such as p-F, p-Cl, p-CF3, p-CN, p-CO2Et and
p-Ac on the aryl rings underwent the cyclization smoothly to
+
B(OH)2
solvent, 90 o
C
Ph
Ph
CN
NH2
2.0 equiv
1a
2a
Time [h] Yield [%][a]
Solvent
Entry
1
Catalyst
Ligand
Base
-
provide the corresponding 1-naphthylamines 2b-2j in 64-77%
[b]
PPh3
1,4-dioxane
trace
Ni(cod)2
3
yields, and these functional groups were well tolerated during
the reaction. Of note is that a CN and Ac group remained intact
under the reaction conditions, and no nickel-catalyzed boron
additions to these groups were observed. The results indicated
that electron-poor or -rich aryl substituents on arylboronic

-
Ni(acac)2 2H2O
1,4-dioxane
1,4-dioxane
1,4-dioxane
18
17
66
PPh3
2
3
4
5
6
24
24
4

-
Ni(acac)2 2H2O
P(p-CF3C6H4)3
P(p-CF3C6H4)3
P(p-CF3C6H4)3
P(p-CF3C6H4)3
tBuOK
Cs2CO3
CsF

Ni(acac)2 2H2O

Ni(acac)2 2H2O
1,4-dioxane
1,4-dioxane
3
6
73
67

Ni(acac)2 2H2O

7
Ni(acac)2 2H2O
P(p-CF3C6H4)3
P(p-CF3C6H4)3
P(p-CF3C6H4)3
P(p-CF3C6H4)3
PPh3
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
10
3
31
K2CO3
[c]
acids had little influence on the yields of products 2. Sterically

68
8
Ni(acac)2 2H2O
Cs2CO3
[d]

8 (59)
74
9
10[e]
Cs2CO3
10
3
demanding o-MeO substituted arylboronic acid afforded 2k
with a longer reaction time in lower yield of 47%, indicating
that the reaction is markedly influenced by steric effect.
Arylboronic acids with -MeO or -Cl substituents at 3-, 3,4- or
3,5-positions of the phenyl ring, or with a biphenyl or 2-
Ni(acac)2 2H2O

Ni(acac)2 2H2O
Cs2CO3
Cs2CO3

69
11
Ni(acac)2 2H2O
5

Ni(acac)2 2H2O
P(p-MeC6H4)3
Cs2CO3
Cs2CO3
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
THF
4
5
68
63
45
55
64
62
64
68
57
12
13
14
15
16
17
18
19

Ni(acac)2 2H2O
P(C6F5)3

Ni(acac)2 2H2O
PPh2Me
Cs2CO3
Cs2CO3
Cs2CO3
tBuOK
7

naphthyl ring transformed into products 2l-2m and 2o-2r
Ni(acac)2 2H2O
PCy3
17
6

Ni(acac)2 2H2O
IPr
efficiently in good yields. The use of 2-fluorophenylboronic
acid gave 2n in 41% yield. 2-Thienylboronic acid also
participated in this cascade reaction, albeit with a lower yield
of 2s. However, when alkylboronic acid such as n-butylboronic
acid was employed, no desired product was obtained.
The scope of o-(cyano)phenyl propargyl ethers was then
examined (Table 3). A variety of electron-donating and -
withdrawing groups on the aryl rings at the alkyne terminus
were found to be compatible, such as p-Me, p-OMe, p-F, p-Cl,
p-CF3 and p-CO2Me substituents, and the corresponding

Ni(acac)2 2H2O
IPr
7

8
Ni(acac)2 2H2O
P(p-CF3C6H4)3
P(p-CF3C6H4)3
P(p-CF3C6H4)3
P(p-CF3C6H4)3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3

Ni(acac)2 2H2O
toluene
3
6
3
20[f] Ni(acac)2 2H2O
1,4-dioxane
1,4-dioxane

21
Ni(acac)2
Ni(cod)2
72
[b]
P(p-CF3C6H4)3
22
23
24
Cs2CO3
Cs2CO3
Cs2CO3
1,4-dioxane
1,4-dioxane
1,4-dioxane
9
trace
65

-
Ni(acac)2 2H2O
-
5
10
P(p-CF3C6H4)3
(99)
[a] Isolated yields. The yields of the recovered 1a were shown in parentheses. [b]
10 mol% of the ligand was used. [c] 20 mol% of Cs2CO3 was used. [d] 1.0 equiv of
Cs2CO3 was used. [e] 10 mol% Ni(acac)2·2H2O, 10 mol% P(p-CF3C6H4)3 and 20 mol%
Cs2CO3 were used. [f] One equiv of H2O was added.
products 2t
-2y were formed in 61-76% yields. Interestingly, in
contrast to the results of the reaction of 1a with 2-
2 | J. Name., 2012, 00, 1-3
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[a]
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Table 3. Scope of the o-(cyano)phenyl propargyl ethers.
Table 2. Scope of the reaction with respect to arylboronic
DOI: 10.1039/C6SC01191H
acids.[a]

10 mol% Ni(acac)2 2H2O
OTBS
TBSO

10 mol% Ni(acac)2 2H2O
10 mol% P(p-CF3C6H4)3
OTBS
TBSO
20 mol% Cs2CO3
10 mol% P(p-CF3C6H4)3
R
B(OH)2
+
R
20 mol% Cs2CO3
1,4-dioxane, 90 o
C
R
B(OH)2
CN
R
+
1,4-dioxane, 90 o
C
Ph
CN
Ph
NH2
2.0 equiv
TBSO
1
2
NH2
2.0 equiv
1a
TBSO
2
TBSO
TBSO
TBSO
Me
t-Bu
OMe
Ph
Ph
Ph
Ph
TBSO
TBSO
TBSO
NH2
NH2
NH2
NH2
Ph
Ph
Ph
Ph
CH3
OMe
F
Cl
NH2
NH2
NH2
NH2
2t, 5 h, 76%
2u, 4 h, 69%
TBSO
2v, 4 h, 76%
TBSO
2w, 4 h, 61%
TBSO
2a, 3 h, 73%[b]
2b, 8 h, 73%[b]
2c, 4 h, 77%
2d, 6 h, 68%
TBSO
Ph
Ph
Ph
Ph
F
Cl
CF3
CN
TBSO
TBSO
TBSO
TBSO
S
NH2
NH2
NH2
NH2
S
CO2Me
CF3
Ph
Ph
Ph
Ph
2x, 5.5 h, 74%
2y, 4.5 h, 62%
TBSO
2z, 4 h, 80%
TBSO
2za, 4 h, 74%
TBSO
NH2
NH2
NH2
NH2
2e, 5 h, 68%
2f, 3 h, 69%
2h, 3 h, 74%[c]
TBSO
Ph
2g, 4 h, 76%
O
Ph
Ph
Ph
CO2Et
TBSO
TBSO
TBSO
TBSO
OMe
NH2
NH2
2zc, 8 h, 35%
NH2
2zd, 6 h, 23%
NH2
2ze, 3 h, 29%
OMe
Ph
Ph
Ph
Ph
2zb, 4 h, 70%
NH2
NH2
NH2
NH2
2j, 6 h, 70%[c]
[a] The yields given are for the isolated products.
2i, 5 h, 64%
2k, 10 h, 47%
2l, 7.5 h, 75%
OMe
Cl
OMe
TBSO
TBSO
TBSO
TBSO
compound IPrNi(acac) was also isolated in 62% yield.
3
Cl
Cl
Complex
3 is paramagnetic as indicated by the appearance of
F
broad signals in 1H NMR. The X-ray crystal analysis of 3[9]
Ph
Ph
Ph
Ph
NH2
NH2
NH2
NH2
2o, 3 h, 68%[b]
clearly shows a rare, three-coordinate distorted T-shape Ni(I)
2p, 2 h, 65%
2m, 10.5 h, 65%
2n, 11 h, 41%
t
structure.[6e,f] When Cs2CO3 was used instead of BuOK, the
S
same Ni(I) complex was also observed, however, in a low yield
of 14%. In this case, a large amount of precipitate could be
observed. The precipitate was assumed to be PhB(OH)2-Cs2CO3
adduct. It might be due to the formation of this low soluble
adduct, thus preventing their further reaction with the nickel
complex. In fact, stirring a 1:1 mixture of PhB(OH)2 and Cs2CO3
in 1,4-dioxane at 90 oC resulted in the formation of a large
amount of white precipitate. This might also explain why the
use of 1 equivalent of Cs2CO3 provided the cyclized product 2a
TBSO
TBSO
TBSO
Ph
Ph
Ph
NH2
NH2
NH2
2q, 6 h, 73%
2r, 6 h, 70%
2s, 10 h, 35%
[a] The yields given are for the isolated products. [b] 5 mol% Ni(acac)2·2H2O,
5 mol% P(p-CF3C6H4)3 and 10 mol% Cs2CO3 were used. [c] THF was used as
the solvent.
thienylboronic acid, the presence of 2-thienyl or 3-
benzothienyl ring as the alkyne terminals did not give much
influence to the reaction, and the corresponding products 2z
and 2za were obtained in high yields of 80% and 74%,
respectively. A 9H-fluorene substituent, which is a very useful
unit in organofunctional materials, was also successfully
incorporated into the product 2zb in 70% yield. Alkenyl or
alkyl-substituted alkynes, such as cyclohexenyl, cyclopropyl
and propyl-substituted one, however, afforded the desired
in low yield (Table 1, entry 9). Complex
3 might be formed by
the comproportionation of Ni(0) and Ni (II) species.[6c,d] To
confirm this point, the stoichiometric reaction of Ni(COD)2 with
Ni(acac)2
+
IPr
O
O
Ph-Ph
30%
+ IPr Ni
(1)
tBuOK
+
+
PhB(OH)2
2 equiv
1,4-dioxane
90 oC, 5 h
2 equiv
3, 62%
products 2zc
naphthylamine products was unambiguously confirmed by X-
ray crystallographic analysis of 2o [9]
1-Naphthylamines are
-2ze in low yields of 23-35%. The structure of 1-
X-ray of 3
(2)
+
3, 55%
Ni(COD)2
IPr
+ Ni(acac)2
1,4-dioxane
90 oC, 3 h
.
2.0 equiv
important structural motifs found in a variety of biologically
active substances. They are also acted as useful building blocks
in synthetic chemistry and dyestuffs industry. However,
efficient methods for their synthesis are quite limited.[10] Our
reaction provides a convenient route to 1-naphthylamines.
To understand the reaction mechanism, we tried to isolate
the possible reaction intermediates. First, a stoichiometric
reaction of Ni(acac)2 (1 equiv), IPr (1 equiv), PhB(OH)2 (2 equiv)
and tBuOK (2 equiv) was carried out (Scheme 2, eq 1). It was
found that in addition to a biphenyl product, a red crystalline
OTBS
5 mol% 3
10 mol% Cs2CO3
+
PhB(OH)2
2.0 equiv
2a, 53% (3)
Ph
1,4-dioxane, 90 oC, 5 h
CN
1a
10 mol% Ni(acac)2 2H2O
10 mol% P(p-CF3C6H4)3
20 mol% Cs2CO3
1,4-dioxane, 90 oC, 1 h
OTBS
N
OTBS
+
(4)
PhB(OH)2
2.0 equiv
CN Ph
(2a was not detected)
5, 33%
4
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
10 mol% Ni(acac)2 2H2O
10 mol% P(p-CF3C6H4)3
20 mol% Cs2CO3
1,4-dioxane, 90 oC, 5 h
View Article Online
DOI: 10.1039/C6SC01191H
Ph
Ph
ligand, ArB(OH)2
Cs2CO3
Ar
Ar
(5)
+ PhB(OH)2
2.0 equiv

Ph
Ni(acac)2 2H2O
LnNi
+
Cs(acac) + B(OH)2(CsCO3)
CN
O
B
NH2
7, 10%
9
+
Ar
HCsCO3
6
OH
LnNi(0)
+
Ar-Ar
LnNi

10 mol% Ni(acac)2 2H2O
10 mol% P(p-CF3C6H4)3
20 mol% Cs2CO3
ArB(OH)2
Cs2CO3
Ar
TBSO

+
LnNi(acac)
+
Cs(acac)
+
0.5 LnNi(0) 0.5 Ni(acac)2 2H2O
LnNi-Ar
Ph
Ph
2.0 equiv TEMPO
11
10
1a
PhB(OH)2
2.0 equiv
+
+
2a
(6)
B(OH)2(CsCO3)
1,4-dioxane, 90 oC, 3 h
O
1
+
HCsCO3
B
46%
LnNi-Ar
11
HN
OH
Ph
8, 28%
Ar
TBSO
NiLn
R1
insertion
ArB(OH)2
Scheme 2. Mechanistic studies
transmetalation
OTBS
Ar
CN
-
+
LnNi+
CsCO3
12
Ni(acac)2 in the presence of IPr was carried out. To our delight,
the same complex was formed in moderate yield (Scheme 2,
eq 2). This Ni(I) complex was found to catalyze the cyclization
R1
16
3
N
TBSO Ar
H
3
15
NiLn
of 1a with PhB(OH)2 to 2a (Scheme 2, eq 3), implying that Ni(I)
species was likely involved in the catalytic process. Recently,
Ni(I) species was proposed to be relevant in Ni-catalyzed cross-
coupling reactions.[6] Our results demonstrate that Ni(I) may
also be involved in Ni-catalyzed addition reactions of
organoboron reagents to the alkynes. In addition, reaction of
R1
syn-13
HCsCO3
tautomerization
CN
OTBS
cis-to-trans
Ar
R1
NiLn
isomerization
2
TBSO Ar
R1
N
14
NiLn
N
anti-13
allene
4 with PhB(OH)2 under Ni-catalyzed conditions afforded
indeno[1,2-b]quinoline
5
via base-promoted cyano-Schmittel
Scheme 3. Possible reaction mechanism
organoborons to alkynes gave the syn-addition product while
cyclization,[7] while the desired 2a was not observed (Scheme 2,
eq 4), indicating that the reaction does not involve the allene
intermediate.
17]
few reactions produce the anti-addition product.[2r,
The
regio- and stereochemistry for addition process here are
consistent with those observed by cobalt(II)-catalyzed
hydroarylation of propargyl -alcohols or -carbamates with
arylboronic acids.[18] Cyano group may play a role in facilitating
the cis-trans-isomerization by stabilizing the metal species and
directing the subsequent addition reaction. Nucleophilic attack
of alkenylmetal in anti-13 to the cyano group forms a cyclized
intermediate 14. Subsequent protonation of 14 produces the
The origin of the regioselectivity in this reaction is not clear
yet, possibly it is controlled by electronic factors in
12 (see Scheme 3).[11] To understand the role of OTBS group,
the alkyne-nitrile without the OTBS group was synthesized.
The Ni-catalyzed reaction of with PhB(OH)2 afforded only 10%
of the desired naphthylamine , along with a small amounts of
an unidentified byproduct (Scheme 2, eq 5). The results
-complex
6
6
7
indicate that the presence of OTBS group is crucial for this N-H imine 15 and a nickel(I) species 16. Tautomerization of 15
affords the observed product 16 undergoes transmetalation
with ArB(OH)2 to regenerate the arylnickel(I) catalyst 11
2
.
reaction.
In addition, it was found that in the presence of a radical
scavenger such as TEMPO, the desired 2a and an N-arylated
product were formed in 74% combined yield (Scheme 2, eq
6). was possibly formed by Ni-catalyzed oxidative amination Conclusions
of arylboronic acid with amine 2a [12,13]
The results indicated
that the reaction was not inhibited by TEMPO, and this
suggests that radical species is not involved in this system.
Based on the above results, we propose the following
reaction mechanism (Scheme 3). Initially, transmetalation of
arylboronic acid with Ni(II) complex promoted by a base
.
8
8
.
In summary, we have developed
regioselective addition/cyclization of
a
nickel-catalyzed
o-(cyano)phenyl
propargyl ethers with arylboronic acids, which provides an
efficient protocol for the synthesis of highly functionalized 1-
naphthylamines with a wide range of structural diversity. The
reaction is characterized by a regioselective and anti-addition
of the arylboronic acids to the alkyne and subsequent facile
nucleophilic addition of the resulting alkenylmetal to the
tethered cyano group. Mechanistic studies reveal that Ni(I)
species might be involved in the catalytic process. Further
mechanistic studies and the extension to alkynes tethered with
a wide variety of electrophiles are currently ongoing in our
laboratory.
provides diarylnickel(II) species
9
, together with HOBO,
HCsCO3 and Cs(acac).[14]
9
undergoes reductive elimination to
form a Ni(0) species. The observation of biphenyl[15] in the
catalytic reaction of 1a with PhB(OH)2 also supported that Ni(II)
was reduced in the reaction process. This Ni(0) species
comproportionates with Ni(II) to afford Ni(I) complex 10, which
undergoes transmetalation with arylboronic acid to give
arylnickel(I) species 11
.
Regioselective 1,2-addition of
arylnickel(I) species 11 to the alkyne moiety in a syn-fashion
takes place to give an alkenylnickel(I) intermediate syn-13. Cis-
Acknowledgements
We thank the National Natural Science Foundation of China
(Grant Nos. 21125210, 21421091) for financial support.
to-trans isomerization of 13 [16]
possibly through a carbene-like
,
zwitterionic resonance species[17] yields alkenylnickel(I)
intermediate anti-13 with a metal trans-to Ar substituent. It
was noted that most of the metal-catalyzed reactions of
4 | J. Name., 2012, 00, 1-3
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Chemical Science
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Journal Name
ARTICLE
Conda-Sheridan, S. R. Cooke and J. Louie, Organometallics.,
View Article Online
DOI: 10.1039/C6SC01191H
2011, 30, 2546-2552.
Notes and references
7
For our recent results on cyano-Schmittel cyclization of (o-
cyano)phenylpropargyl ethers, see: X. You, X. Xie, H. Chen, Y.
Li and Y. Liu, Chem. Eur. J., 2015, 21, 18699-18705.
Y. Makida, E. Marelli, A. M. Z. Slawin and S. P. Nolan, Chem.
Commun., 2014, 50, 8010-8013.
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CCDC 1440646
(2o
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5
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