Nickel-catalyzed cyanation of phenol derivatives activated by 2,4,6-trichloro-1,3,5-triazine

Article abstract of DOI:10.1039/c8ob01034j

A nickel-catalyzed cyanation of phenol derivatives activated by 2,4,6-trichloro-1,3,5-triazine (TCT) using aminoacetonitrile as the cyanating agent is described. This catalytic system delivered the desired products in moderate to good yields with good substrate compatibility. The readily available starting materials, cost-effective nickel catalyst and metal-free cyanating agent are the major features of the present method.

Full text of DOI:10.1039/c8ob01034j

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Nickel-Catalyzed Cyanation of Phenol Derivatives Activated by
2,4,6-Trichloro-1,3,5-triazine
Liang Wang,a* Yaoyao Wang,a Jun Shen,a Qun Chen,a and Ming-Yang Hea*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A nickel-catalyzed cyanation of phenol derivatives activated by various organic cyanating agents have been developed, these
2,4,6-trichloro-1,3,5-triazine (TCT) using aminoacetonitrile as the reactions still remain in their infancy in synthetic chemistry.
cyanating agent is described. This catalytic system delivered the Moreover, the existing cyanation reactions always require the
desired products in moderate to good yields with good substrate use of expensive palladium catalysts. Particularly, the use of
compatibility. The readily available starting materials, cost- aryl halides as the aryl source also generates halogen-
effective nickel catalyst and the metal-free cyanating agent are containing waste.
the major features of the present method.
The use of phenol derivatives instead of aryl halides as the
aryl source in cross-coupling reaction has received much
attention since phenols derivatives are usually inexpensive and
readily available, and moreover, no halogen-containing waste
is generated (Scheme 1, a).16 Notably, nickel catalysts showed
excellent catalytic activity toward the cleavage of C-O
bonds,14,17 thereby providing a cost-effective and powerful
tool to construct C-C and C-X bonds. Recently, Iranpoor
group17l,17p,17q, 17s demonstrated that phenols could be readily
activated by TCT to give 2,4,6-triaryloxy-1,3,5-triazines (TAT),
which acted as pseudohalides in cross-coupling reactions.
Compared with phenols, the C-O bonds in TAT were relatively
longer, thus making it a better electrophilic center. These
reagents were also readily available and all of the three aryl
groups in TAT could be utilized.17 Inspired by these
contributions, herein we report a nickel-catalyzed cyanation of
phenol derivatives activated by TCT using aminoacetonitrile as
the cyanating agent (Scheme 1, b).
Aryl nitriles are one of the most important compounds in
organic chemistry. They are widely existed as key motifs in
numerous products including natural products, agrochemicals,
pharmaceuticals, dyes, and materials.1 The nitrile moiety also
serves as a versatile functional group and can be readily
converted into other functional groups. Therefore, the
preparation of aryl nitriles is of great importance and also of
long-standing interest to organic chemists.
Although much effort has been made for the synthesis of
aryl nitriles, the direct introduction of cyano group into the
aromatic rings is the most straightforward approach. Over the
past decades, significant progress has been achieved in the
transition-metal-catalyzed cyanation reactions.2 Metal
5
cyanides such as CuCN,3 KCN,4 Zn(CN)2 and K4[Fe(CN)6]),6 and
organic reagents such as TMSCN,7 acetone cyanohydrin,8
benzyl cyanide,9 benzoyl cyanide,10 2,3-dichloro-5,6-dicyano-p-
benzoquinone
(DDQ),11
N-cyano-N-phenyl-p-
toluenesulfonamide (NCTS),12 azobisisobutyronitrile (AIBN),13
and aminoacetonitrile14 have been successfully employed as
efficient cyanating agents. The combination of ammonium salt
and DMF or DMSO also turned out to be promising cyanide
surrogate.15 Despite the significant progress, there are still
some issues to be addressed. For example, hydrogen cyanide
gas and metal-containing waste are usually generated when
metal cyanides are utilized as the cyanating agents. Although
(a) Cross-coupling of activated phenol derivatives
AG(Activating Group):
O
FG
AG Tf, Ms,Ts, Piv, Boc, CONMe2, TCT,etc
Transition metal catalysts
FG(Functional Group)
(b) Thiswork
Ar
O
O
Ni-catalysis
N
N
+
ArCN
N
Ar
Ar
CN
O
N
O
Scheme 1 Transition metal-catalysed cross-coupling of activated phenol derivatives.
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The 2,4,6-triphenoxy-1,3,5-triazine
(
1a
)
was readily bis(diphenylphosphino)ferrocene (dppf) as the ligand provided
moderate yield (entry 2), and the use of 1,2-
prepared by grinding method under solvent-free condition
a
within short time (See ESI). Initially, 1a and 2- bis(dicyclohexylphosphino)ethane (dcype) delivered the
morpholinoacetonitrile (2a) were chosen as the model desired product 3a in 79% yield (entry 9). To explore a more
substrates to optimize the reaction conditions (Table 1).
simple and cost effective catalytic system, the combination of
Ni(II) precatalysts and Zn dust were tested (entries 10~13).
Pleasingly, the yield was increased to 84% by using NiBr2/Zn
system (entry 10). Among the surveyed Ni catalysts, NiBr2 was
the ideal one and other Ni(II) catalysts gave poorer yields
(entries 11~13). Other external reducing agents including Mn,
Cr, Et3SiH and PhSiH3 were also tested (entries 14~17). Only
moderate yield (62%) was obtained when Mn was used, while
other reductants were not effective. Switching the bases to
K2CO3, KHCO3, and Cs2CO3 also led to unsatisfactory results,
and the reaction was completely shut down without base
(entries 18~21). The solvent showed significant influence on
the reaction. Toluene also proved to be an appropriate
solvent, while reaction in THF, DMF, DMSO, DCE, DMA as well
as ethanol led to poor results (entries 22~28). Other
aminoacetonitriles were also checked. The use of 2-
(diethylamino)acetonitrile (2b), 2-pyrrolidin-1-yl acetonitrile
Table 1. Optimization of reaction conditions.a
Entry
[Ni]
Ligand
Additive
Base
Solvent
Yield
(%)b
34
45
0
1
2
3
4
5
6
7
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2
Ni(COD)2 Xantphos
Ni(COD)2
Ni(COD)2
dppp
dppf
PCy3
Ph3P
-
-
-
-
-
-
-
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
0
0
0
0
IPr•
HCl
1,10-
phen
8
9
Ni(COD)2 2,2’-bipy
-
-
Zn
Zn
Zn
Zn
Mn
Cr
Et3SiH
PhSiH3
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K2CO3
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
0
79
(
2d) and 2-piperidin-1-ylacetonitrile (2e) afforded the desired
Ni(COD)2
NiBr2
NiCl2
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
dcype
products in 38%, 45% and 64% yields, respectively. The use of
(diisopropyl)acetonitrile (2c), however, led to the failure of the
reaction. Finally, the survey of the reaction temperature
suggested that 130 oC was the optimum.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
84(81)
40
Ni(OAc)2
Ni(acac)2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
NiBr2
27
21
62
11
23
0
32
trace
23
0
72
26
trace
19
trace
trace
0
Encouraged by the successful cyanation of 1a, a series of
phenol derivatives (A~E) were evaluated under the standard
reaction conditions for reactivity comparison (Scheme 2). No
reaction took place when phenol was directly used. Only
phenol derivatives with ortho-nitrogen atoms could promote
the reaction, which was attributed to the fact that nitrogen
atoms could coordinate to the catalyst center, thereby
facilitating the oxidative addition step.
KHCO3 dioxane
Cs2CO3 dioxane
-
dioxane
toluene
THF
DMF
DMSO
DCE
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
Zn
Zn
Zn
DMA
EtOH
dioxane 62c, 85d
aReaction conditions: 1a (0.10 mmol = phenyl 0.3 mmol), 2a (0.6 mmol), Ni
catalyst (0.03 mmol, 10 mol%), ligand (bidentate: 20 mol %, monodentate: 40
mol %), additive (60 mol%, for entries 10~29), base (0.6 mmol), solvent (1.5 mL),
N2, 130 oC, 24 h. bGC yield. The value in parentheses is the isolated yield after
purification. c120 oC. d140 oC.
Pleasingly, tentative reaction using Ni(cod)2 as the nickel
source, 1,3-bis(diphenylphosphino)propane (dppp) as the
o
ligand and K3PO4 as the base in dioxane at 130 C for 24 h
Scheme 2 Substrate activity comparison.
Employing the optimized reaction conditions, the reaction
scope was explored (Table 2). Results showed that a series of
TAT substrates bearing either electron-donating or electron-
withdrawing groups on the aromatic ring could be efficiently
transformed into the corresponding nitriles in moderate to
afforded the desired product 3a in 34% yield (Table 1, entry 1).
The reaction did not occur by switching the ligands to PCy3,
Ph3P,
(Xantphos), IPr
(entries 3~8).
4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene
HCl, 1,10-phenanthroline or 2,2’-bipyridine
In contrast, employing 1,1’-
•
2 | J. Name., 2017, 00, 1-4
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a
Reaction conditions: 1 (0.1 mmol), 2a (0.6 mmol), NiBr2 (10 mol%), dcype (20
mol %), Zn (60 mol%), K3PO4 (0.6 mmol), dioxane (1.5 mL), N2, 130 oC, 24 h,
isolated yields.
good yields. Diverse functional groups such as methyl,
methoxyl, halogen, nitro, cyano, acetyl, methylthiol, and ester
were all well tolerated. Notably, the C-Cl bond in the products 3e
and 3m were not significantly affected, enabling the further
modification of the initial products. Expectedly, substrates with
substitution group at the ortho position showed less activities
owing to the hindrance effect, affording the products 3l and
3m in relatively lower yields. Interestingly, product 3q was
obtained in 73% yield without affecting the ester group. 1-
naphthyl and 2-naphthyl TATs also provided the corresponding
products in 70% and 68% yields, respectively. Particularly,
product 3t which was a crucial intermediate for preparation of
L692, 429 (non-peptidyl growth hormone secretagogue), was
obtained in 46% yield. Heterocyclic substrates including
pyridine, thiophene, and furan bearing hydroxyl group also
survived the reaction conditions to deliver the desired
Furthermore, one-pot synthesis aryl nitriles was performed
(Scheme 3). Generally, the TAT substrates could be readily
prepared in the presence of a base. Thus phenols 1a and 1b
was firstly treated with NaH at room temperature, then
reacted with TCT in dioxane at 100 oC to afford the
corresponding TAT substrates. The intermediates, without
isolation, further reacted with 2a under standard reaction
conditions to give products 3a and 3b in 80% and 76% yields,
respectively.
products 3u~3y in moderate yields. Reactions of TAT
substrates derived from resorcinol and hydroquinone were
also achieved, providing the expected bis-cyanation products
3g and 3z in moderate yields. However, the bis-cyanation
reaction was not suitable for pyrocatechol-TCT substrate since
the both two hydroxyl groups could not couple with TCT
simultaneously due to the large hindrance effect. It also should
be pointed out that TAT substrate bearing alkyl group was not
compatible with the present reaction conditions.
Scheme 3 One-pot cyanation of phenol/TCT derivatives.
Table 2. Reaction scope.a
To demonstrate the synthetic application of this method,
substrate 1e was utilized for sequential transition metal–
catalyzed cross-coupling reaction and present cyanation
(Scheme 4).18 Encouragingly, the initial transition metal–
catalyzed Stille coupling, Heck reaction, N-arylation and
alkoxycarbonylation from 1e successfully occurred. After
cyanation reaction under the present conditions, a series of
products could be obtained in moderate yields.
NiBr2 (10 mol%)
dcype (20 mol%)
Zn (60 mol%)
Ar
N
O
O
N
+
Ar CN
N
Ar
Ar
K3PO4, dioxane
130 oC, 24 h
O
N
O
CN
3
2a
1
R
CN
CN
CN
R
3a, R = H, 81%
3l, R = OMe, 62%
3m, R = Cl, 65%
3k, 84%
3b, R = CH3, 83%
3c, R = OMe, 85%
3d, R = F, 75%
3e, R = Cl, 70%
3f, R = NO2, 74%
3g, R = CN, 72%
3h, R = Ac, 78%
3i, R = CHO, 70%
3j, R = SMe, 80%
MeO
CN
MeO
MeO
CN
MeO
OMe
3n, 76%
3o, 72%
CN
CN
O
O
O
O
Ph
CN
3p, 75%
3q, 73%
3r, 70%
NC
CN
CN
N
3u, 2-CN, 42%
3v, 3-CN, 73%
3w, 4-CN, 65%
3t, 46%
3s, 68%
Scheme 4 Sequential functionalization of 4-Cl phenol/TCT derivative.
CN
X
A plausible mechanism was also proposed (Scheme 5).
CN
NC
Initially, the nickel catalyst coordinates with ortho nitrogen
atom of substrate
1
. Then C−O oxidaꢀve addiꢀon of
1
to Ni(0)
3x, X=O, 41%
3y, X=S, 50%
3g, 53%, from hydroquinone
3z, 48%, from resorcinol
catalyst forms intermediate II 17f,17l,17j
.
Subsequently,
intermediate III is generated via anion exchange with CN-,
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which is released from 2a 14
reductive elimination to give the desired product
.
Finally, intermediate III undergoes
and
1
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Guide to Functional Group Preparations; Wiley-VCH:
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intermediate IV with simultaneous regeneration of Ni(0)
species. The produced intermediate IV can undergo another
cycle through other two aryloxy groups. Thus, all the three aryl
groups were utilized. However, we still can’t determine the
2
(a) G. Yan, Y. Zhang and J. Wang, Adv. Synth. Catal., 2017,
359, 4068; (b) J. Yu, F. Teng and J. Cheng, Adv. Synth. Catal.,
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side products released from
1 after reaction currently. The
detailed mechanism will be further studied in our laboratory.
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Conclusions
In summary, an efficient Ni-catalyzed cyanation of phenol
derivatives activated by TCT using organic cyanating agents,
aminoacetonitrile has been developed. The TAT substrates can
be readily prepared and utilized as aryl source. It should be
pointed out that all the three aryl groups in TATs could be
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,
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge financial support from the
National Natural Science Foundation of China (21302014 and
21676030), China Postdoctoral Science Foundation funded
project, the Jiangsu Key Laboratory of Advanced Catalytic
Materials and Technology (BM2012110), the Priority Academic
Program Development (PAPD) of Jiangsu Higher Education
Institutions, and the Advanced Catalysis and Green
Manufacturing Collaborative Innovation Center of Changzhou
University.
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Organic & Biomolecular Chemistry
Page 6 of 6
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DOI: 10.1039/C8OB01034J
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