General and Mild Nickel-Catalyzed Cyanation of Aryl/Heteroaryl Chlorides with Zn(CN)2: Key Roles of DMAP

Article abstract of DOI:10.1021/acs.orglett.7b00732

A new and general nickel-catalyzed cyanation of hetero(aryl) chlorides using less toxic Zn(CN)₂ as the cyanide source has been developed. The reaction relies on the use of inexpensive NiCl₂·6H₂O/dppf/Zn as the catalytic system and DMAP as the additive, allowing the cyanation to occur under mild reaction conditions (50-80 °C) with wide functional group tolerance. DMAP was found to be crucial for successful transformation, and the reaction likely proceeds via a Ni(0)/Ni(II) catalysis based on mechanistic studies. The method was also successfully extended to aryl bromides and aryl iodides.

Full text of DOI:10.1021/acs.orglett.7b00732

Letter
pubs.acs.org/OrgLett
General and Mild Nickel-Catalyzed Cyanation of Aryl/Heteroaryl
Chlorides with Zn(CN)2: Key Roles of DMAP
Xingjie Zhang, Aiyou Xia, Haoyi Chen, and Yuanhong Liu*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of
Sciences, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: A new and general nickel-catalyzed cyanation of
hetero(aryl) chlorides using less toxic Zn(CN)2 as the cyanide source
has been developed. The reaction relies on the use of inexpensive
NiCl2·6H2O/dppf/Zn as the catalytic system and DMAP as the
additive, allowing the cyanation to occur under mild reaction
conditions (50−80 °C) with wide functional group tolerance. DMAP
was found to be crucial for successful transformation, and the reaction likely proceeds via a Ni(0)/Ni(II) catalysis based on
mechanistic studies. The method was also successfully extended to aryl bromides and aryl iodides.
romatic nitriles are common structural motifs found in
organic materials, natural products, pharmaceuticals, and
substrate scope,8 poor reproducibility,9 with carcinogenic HMPA
as the solvent,8b,d,e under microwave irradiation at high reaction
temperatures,8c etc. In addition, the cyanation of heteroaryl
chlorides remains problematic and examples are quite rare.8d
These situations prompted us to explore a mild, robust, and
efficient Ni-based catalytic system for cyanation reactions,
especially using less toxic cyanating agents. In this letter, we
report the first example of Ni-catalyzed cyanation of aromatic
chlorides with Zn(CN)2. The reaction proceeds at moderate
temperatureswithair-stableandinexpensivenickel(II)saltsasthe
precatalyst, allowingthecyanationofarylandheteroarylchlorides
with widefunctional groupcompatibility (Scheme 1). DMAPwas
found to be crucial for the reaction efficiency, and the reaction
likely proceeds via a Ni(0)/Ni(II) catalysis based on mechanistic
studies. The method was also successfully extended to aryl
bromides and aryl iodides.
A
agrochemicals.1 Theyalsoserveasversatilebuildingblocksforthe
synthesis of heterocycles, as well as precursors for amides, amines,
carboxylic acids, aldehydes, ketones, and alcohols.2 The classical
methods for preparing aryl nitriles involve diazotization of
anilines followed by the Sandmeyer reaction,3 and the Rose-
nmund−von Braun reaction where a stoichiometric amount of
copper(I) cyanide is heated with an aryl iodide at high
temperatures (≥150 °C).4 All these reactions occur either
under harsh reaction conditions and/or with limited scope.
Transition-metal-catalyzed cyanation of aryl halides represents
one of the most promising methods for the synthesis of aryl
nitriles. Extensive research has been devoted to Pd-catalyzed
cyanation of aryl halides, and remarkable improvements in terms
of efficiency and practicality have been achieved in this field over
thepast30years.1 Despitemuchprogress, Pd-catalyzed cyanation
ofarylchlorides,5 theleastexpensiveandthemostwidelyavailable
aryl halides, still remains as a formidable task. The known
methods generally require high reaction temperatures (120−160
°C),5d,e,g−i and only a few of them are capable to catalyze the
cyanation of nonactivated aryl chlorides. Recently, Pd complexes
of electron-rich, bulky phosphine ligands such as Buchwald biaryl
ligand (binaphthyl)P(t-Bu)25f or indole-based phosphine ligand
CM-phos5c were found topromote thecyanation of aryl chlorides
under milder reaction conditions. Nevertheless, the high cost of
Pd metal coupled with the ancillary ligands led chemists to
consider the first-row metal complex for such reactions.6 In this
context, nickel-basedcatalysts arehighlyattractive notonlydueto
their low cost but also because of their high reactivity toward the
activation of relatively inert substrates such as aryl chlorides and
sulfonates. However, nickel-catalyzed cyanation reactions have
far less been developed. In 1973, Cassar reported the first nickel-
catalyzed cyanation of aryl halides with NaCN in EtOH,7 since
then, only very limited reports are precedent.8 To date, most Ni-
catalyzed cyanation reactions use highly toxic KCN or NaCN as
the cyanide source with certain drawbacks, such as limited
Scheme 1. Transition-Metal-Catalyzed Cyanation of Aryl/
Heteroaryl Chlorides
Received: March 11, 2017
© XXXX American Chemical Society
A
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The challenge in Pd- or Ni-catalyzed cyanations is to avoid
catalyst poisoning. The use of zinccyanide has been widelyspread
in Pd-catalyzed cyanation reactions since it is significantly less
toxic compared to a sodium or potassium cyanide source.5a,f,i It
also exhibits low solubility in most organic solvents, which is
beneficial for maintaining a low cyanide concentration and
preventing catalyst deactivation. We initially investigated the
nickel-catalyzed reactions of 4-chlorobenzaldehyde 1a with
Zn(CN)2. However, either no reaction or small amounts of the
desired nitrile was observed in most of the experiments, possibly
due to the poor solubility of Zn(CN)2. After tremendous effort in
the optimization studies, including the effects of the nickel source,
reducing agents, base, additives, reaction temperature, and
solvents,10 wefoundthattheadditive4-(dimethylamino)pyridine
(DMAP)playedkeyroles intheeffective transformation. Thus, in
the presence of 1.0 equiv of DMAP, the desired 4-formyl-
benzonitrile 2a could be obtained in 89% yield catalyzed by 5 mol
% NiCl2·6H2O, 6 mol % dppf, and 20 mol % activated zinc flakes
in CH3CN at 80 °C (Table 1, entry 1). Interestingly, no cyanide
higher yield of 92% was achieved by using 0.8 equiv of Zn(CN)2
(entry 14). Interestingly, Ni(COD)2 also showed excellent
catalytic reactivity (entry 15). Control experiments run without
either nickel or zinc catalyst resulted in no formation of 2a.
We chose the reaction conditions shown in Table 1, entry 14 as
the standard catalytic system for further development. A series of
aryl chlorides were first usedasthesubstrates totest thegenerality
of thereaction (Scheme 2). Duringthis process, we foundthatthe
a
Scheme 2. Scope of Ni-Catalyzed Cyanation of Aryl Halides
a
Table 1. Optimization of the Reaction Conditions
b
entry
deviation from conditions shown in eq 1
yield (%)
1
none
89
2
no DMAP
12 (75)
2 (86)
81
3
no dppf
4
Nil2 instead of NiCl2·6H2O
Ni(acac)2 instead of NiCl2·6H2O
10 mol % PMePh2 instead of 6 mol % dppf
10 mol % PPh3 instead of 6 mol % dppf
dppb instead of dppf
5
− (78)
67
6
7
10 (61)
45
8
9
Xantphos instead of dppf
84
10
11
12
13
14
15
DMF instead of CH3CN
74
K2CO3 instead of DMAP
64
60 °C instead of 80 °C
22
50 mol % DMAP was used
0.8 equiv of Zn(CN)2 was used
Ni(COD)2 instead of NiCl2·6H2O, and without Zn
51
92
a
Isolated yields. All reactions were carried out on 1 mmol scale in
c
b
c
90
CH3CN (5 mL) in a sealed Schlenk tube. NMR yields. 10 mol %
a
All reactions were carried out on 1.0 mmol scale in a sealed Schlenk
NiCl2·6H2O, 12 mol % dppf, 40 mol % Zn and CH3CN (10 mL) were
b
c
d
tube. Isolated yields. Yields of recovered 1a are in parentheses. 0.8
used. 2,4-Dichlorobenzonitrile was used as the substrate. 10 mol %
equiv of Zn(CN)2 was used.
NiCl2·6H2O, 12 mol % dppf, 40 mol % Zn, 2.0 equiv of DMAP, 1.6
equiv of Zn(CN)2, and CH3CN (10 mL) were used.
ion-induced benzoin condensation was observed, indicating that
the current process is mild and selective. Without DMAP, only
12% of 2a was obtained (entry 2). In the absence of dppf, only a
trace of 2a was observed (entry 3). NiI2 also displayed efficient
catalyticactivity, whileNi(acac)2 failedtogivethedesiredproduct
(entries 4−5). The effects of ligands on the reaction course were
also examined (entries 6−9), among them, a bidentate ligand
Xantphos with a large bite angle (β = 111°) showed comparable
activity with that of dppf. Notably, a monodentate ligand PPh3,
which has been used in Ni-catalyzed cyanation of aryl halides with
NaCN or KCN,7,8b,d−g was ineffective in this reaction (entry 7).
As the bases are often used to promote cyanide dissociation,5b
differentbaseswereexamined. ItwasfoundthatK2CO3 couldalso
be used for effective reactions to give 2a in moderate yield (entry
11). The results suggest that DMAP may have a similar impact on
cyanideiondissociation. Lowering thereaction temperature to60
°C ledtoamarked decrease intheyieldof 2a(entry12). Aslightly
reaction was influenced significantly by the nature of aryl
substituents. For example, deactivated aryl chlorides such as 1-
chloro-4-methoxybenzene underwent cyanation smoothly at 80
°C to afford 2b in 80% yield. However, when 1-chloro-4-
methylbenzene was used as the substrate, only 37% nitrile 2c was
obtained at 80 °C, and the side products of 1-p-tolylethanone
derived from the addition of aryl nickel intermediate to CH3CN
(4%) as well as homocoupled biaryl (1%) were also observed. It
appeared likely that, at higher reaction temperature, the side
reactions became competitive with the desired transformation in
some electron-rich aryl chlorides, thereby deactivating the nickel
catalyst. This led us to consider performing the reaction under
milderreactionconditions. Gratifyingly, ahighyieldofthedesired
aryl nitrile 2c could be achieved by lowering the reaction
temperature to 60 °C. This result contrasted with the reaction of
activated aryl chlorides such as 4-chlorobenzaldehyde, in which a
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poor product yield was observed at 60 °C (Table 1, entry 12).
These observations also indicated that oxidative addition of the
Ar−Cl bond may not be the rate-determining step since activated
aryl chlorides are typically more reactive toward oxidative
addition.11 The differences in reactivity of these substrates
might be attributed to the effectiveness of the subsequent
transmetalation/reductive elimination to form Ar−CN with
electron-donating aryl groups, thus allowing the cross-coupling of
electron-rich aryl chlorides to proceed at a lower reaction
temperature. Cyanation of 1-chloro-4-methoxybenzene at 60 °C
was then examined; as expected, a good product yield (2b, 6 h,
88%) could also be achieved at this temperature. This is in line
withourassumption. ArecentstudybyHartwigetal.revealedthat
an arylpalladium cyanide complex containing an electron-
donating aryl group underwent reductive elimination much faster
than that containing an electron-withdrawing aryl group.12
Similar electronic effects may also be involved in our system.
Similarly, n-Bu, t-Bu, TBSOCH2, and 3,5-di(MeO)-substituted
arylchloridesgavethecorrespondingnitriles2d−2ginhighyields
at 60 °C. In the case of sterically hindered 1-chloro-2-
methylbenzene, the reaction appeared to work well only by
doubling the amounts of NiCl2·6H2O, dppf, and Zn under dilute
conditions (2h). Trimethylsilyl-substituted aryl chloride was
cyanatedsuccessfully(2i). Thehighlychallenging4-chloroaniline
with a free amino group could also be cyanated (2j). A series of
electron-withdrawing group-substituted aryl chlorides under-
went the cyanation efficiently (2k−2q). Common functional
groups such as keto, ester, amide, sulfonamide, nitrile, aldehydo
groups were well tolerated under the standard reaction
conditions. Especially, aryl chlorides with a hindered ortho-
substituent were well suited (2o−2q). Double cyanation of 2,4-
dichlorobenzonitrile could also be achieved (2r). Interestingly,
efficient cyanation of 2-chloronaphthalene could occur at a lower
temperature of 50 °C (2s). Aryl chlorides featuring a heterocycle
ring at the para-position were well accommodated (2t and 2u). In
addition to aryl chlorides, aryl bromides and iodides also coupled
well with Zn(CN)2, and both electron-donating and -with-
drawing substituents on the aryl rings were tolerated.
Scheme 3. Scope of Ni-Catalyzed Cyanation of Heteroaryl
Chlorides
a
a
Isolated yields. All reactions were carried out on 1 mmol scale in
b
CH3CN (5 mL) in a sealed Schlenk tube. 2-Chloronicotinonitrile was
used as the substrate. 10 mol % NiCl2·6H2O, 12 mol % dppf, 40 mol
c
% Zn, 2.0 equiv DMAP, 1.6 equiv Zn(CN)2 and CH3CN (10 mL)
d
were used. CH3CN (10 mL) was used.
present method can be used for the late-stage functionalization of
medicinally relevant compounds.
Scheme 4. Synthesis of Letrozole
Given the prevalence and wide utility of heterocycles in
pharmaceuticals, we then turned our attention to exploring the
cyanation of heteroaryl chlorides. We were pleased to see that the
current nickel-based catalytic system is highly efficient and
general for cyanation of a wide variety of heteroaryl chlorides
(Scheme 3). For example, reactions of 2-chloropyridine (4a) or
pyridines bearing MeO (4b) or cyano (4c) functionalities with
Zn(CN)2 gave the desired nitriles in 75−90% yields. Both
chlorides on 3,5-dichloropyridine could be cyanated (4d).
Heterocycles with two nitrogen atoms such as pyridazine (4e),
pyrazine (4f), and pyrimidine (4g) turned out to be also perfect
substrates. Chloro quinolines or isoquinoline reacted efficiently
(4h−4l). Unprotected indole bearing a free N-H group was well
tolerated (4m). Quinoxaline (4n and 4o), imidazo[1,2-a]-
pyridine (4p), and benzo[d]oxazole (4q) were also suitable.
Sulfur-containing heterocycles such as benzo[d]thiazole (4r) and
thiophene (4s) worked efficiently, indicating that the catalyst did
not become deactivated. Interestingly, N-oxides such as 7-chloro-
2-methylquinoline 1-oxide could also be cyanated (4t).
For nickel-catalyzed cross-coupling reactions, both Ni(0)/
Ni(II)andNi(I)/Ni(III)redoxcycles14 havebeenproposedtobe
possible reaction pathways. To understand the mechanism, the
complexes of air stable [(dppf)Ni(II)(Cl)(o-MeC6H4)] 715 and
[(dppf)Ni(I)Cl] 814a were prepared. It was found that Ni(II)
complex 7 showed excellent catalytic activity to form 2d in 94%
yield, whereas Ni(I) complex 8 failed to catalyze the reaction
(Scheme 5, eqs 2 and 3). These results strongly support the
Ni(0)/Ni(II) reaction pathway. It should be noted that, in the
absence of DMAP, only a trace of 2d was observed. The results
emphasized the crucial role of DMAP. The stoichiometric
reactions of Ni(II) complex 7 with Zn(CN)2 were also
investigated. The reaction proceeded efficiently in the presence
of 10 equiv of DMAP to give 2h in 82% yield (Scheme 5, eq 4).
However, in the absence of DMAP, only 25% of 2h was obtained,
together with small amounts of side products (Scheme 5, eq 5).
The results imply that Ni(II) complex 7 serves as the reaction
intermediate, and DMAP may effect the activation of Zn(CN)2
through coordination to form a DMAP−Zn(CN)2 adduct and
facilitating the transmetalation step.16 To test whether the DMAP
could be a coligand,17 the catalytic activity of NiCl2(DMAP)4·
2H2O was evaluated.18 Cyanation of 1d catalyzed by this complex
The practicality of this method was demonstrated by a gram-
scalestudy(4c, 80%yield, seeScheme3). Inaddition, Letrozole, a
nonsteroidal aromatase inhibitor used for the treatment of
hormonally responsive breast cancer,13 was prepared in good
yield by this Ni-catalyzed cyanation (Scheme 4). Thus, the
C
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Scheme 5. (A) Examination of the Catalytic Activity of Ni(II)
versus Ni(I); (B) Stoichiometric Reactions
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precatalyst, normal phosphine as the ligand in conjunction with
wide functional group tolerance, and mild reaction conditions
makes this approach one of the most attractive and practical
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serves as the reaction intermediate but also displays high catalytic
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(10) For optimization details, see the Supporting Information.
ASSOCIATED CONTENT
* Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b00732.
■
S
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yhliu@sioc.ac.cn.
ORCID
̈
Yuanhong Liu: 0000-0003-1153-5695
Notes
The authors declare no competing financial interest.
(17) (a) Xiao, Y.-L.; Min, Q.-Q.; Xu, C.; Wang, R.-W.; Zhang, X. Angew.
Chem., Int. Ed. 2016, 55, 5837. (b)Wang, X.;Wang, S.;Xue, W.;Gong, H.
J. Am. Chem. Soc. 2015, 137, 11562.
ACKNOWLEDGMENTS
■
We thank the National Key Research and Development Program
(2016YFA0202900) and the Strategic Priority Research Program
of the Chinese Academy of Sciences (Grant No. XDB20000000),
the National Natural Science Foundation of China (Grant No.
21421091) for financial support.
(18) See the Supporting Information for details.
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