Nickel-catalysed direct C2-arylation of N-heterocyclic carbenes

Article abstract of DOI:10.1039/c7dt03099a

A highly efficient nickel catalysed method for the direct C2-arylation of N-heterocyclic carbenes (NHCs) is reported. This protocol enables a facile access to C2-arylated imidazolium salts (NHC-Ar)X (X = Cl, Br, I or OTf). Experimental and theoretical studies suggest the viability of a Ni(i)/Ni(ii) catalytic pathway in which the dinuclear Ni(i) species [(NHC)NiX]₂ plays a key role.

Full text of DOI:10.1039/c7dt03099a

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2017, DOI: 10.1039/C7DT03099A.
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Nickel-Catalysed Direct C2-Arylation of N-Heterocyclic Carbene†
Nga Kim T. Ho,a Beate Neumann,a Hans-Georg Stammler,a Vitor H. Menezes da Silva,b Daniel G.
Watanabe,b Ataualpa A. C. Bragab and Rajendra S. Ghadwal*, a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A highly efficient nickel catalysed method for the direct C2- alkylation
arylation of N-heterocyclic carbenes (NHCs) is reported. This
R
R
R
R
protocol enables a facile access to C2-arylated imidazolium salts
N
N
N
N
Base
Base
R'
X
R'
R'
(NHC-Ar)X (X = Cl, Br, I or OTf). Experimental and theoretical
(R' = H)
(R' = Ar)
N
N
N
N
studies suggest the viability of a Ni(I)/Ni(II) catalytic pathway in
R
R
R
R
which the dinuclear Ni(I) species [(NHC)NiX]2 plays a key role.
(R = alkyl or aryl)
(C2-Carbene)
(C4- or C5-Carbene)
I: R' = H
II: R' = aryl
A
B
(
)
(
)
N-Heterocyclic carbenes (NHCs) (A, Scheme 1) are very
versatile ligands in organometallic chemistry and catalysis,1
which can be readily prepared by the deprotonation of 1,3-
Scheme 1 NHC (
A
), aNHC (
B
), and 1,3-imidazolium salts
I
and II
.
I
with alkenes (Scheme 2, a).13 We recently devised a
Imidazolium salts (
subclass of ionic liquids (ILs).3 Abnormal NHCs (aNHCs,
I I are also known as a highly promising
).2
of
catalytic method for the direct C2-arylation of NHCs with
Pd2(dba)3 (Scheme 2, c).9a The C2‒H arylation of
is also
4
B
)
I
feature the carbene atom at the imidazole-backbone (C4- or
C5) instead of the C2-position. They are exceptionally strong σ-
donors5 and therefore have enormous potentials in synthesis
and catalysis.6 The use of C2-arylated imidazolium salts (II) is
the most reliable strategy for the synthesis of aNHCs4b and
metal-complexes.4 Moreover, II exhibit superior alkaline
feasible with a copper catalyst (Scheme 2, b).9b However, the
scope of both Pd9a and Cu9b catalysis protocols is limited to aryl
iodides. In comparison with palladium, nickel complexes
exhibit higher reactivity towards C(sp2)‒X (X = Cl or Br) bond and
are considered as important candidates in sustainable
synthesis.14 This encouraged us to reason that by employing a
suitable nickel precatalyst the C2-arylation of NHCs (Scheme 2,
d) should be conceivable.15 Herein we report the nickel
catalysed direct C2-arylation of NHCs with aryl halides to C2-
arylated imidazolium salts (Scheme 3).
stability compared to I, rendering them attractive candidates
for their applications in ILs7 and in high-performance hydroxide
conducting membranes.8 Therefore, the significance of II
prompted interests in the development of their facile synthetic
methods.9
We commenced our study with a standard reaction9a of IPr
Transition metal catalysed direct C‒H functionalization is a
powerful strategy in organic synthesis.10 The C‒H arylation of
azole heterocycles has been well explored.11 The C2-arylation
(
1a) and PhI with 5 mol% of Ni(cod)2, which gave 65%
(IPrPh)I(
2)
of
A or I is however considered difficult because of the stability
(a) C2-H alkylation of imidazolium salts: (b) C2-H arylation of imidazolium salts:
Me
of the CNHC‒M bond, making the reductive elimination at the
12
R1
R1
metal atom challenging.5, Cavell et al., reported the C2‒H
+
R3
+ Ar-I
Cu2O
R
N
N
N
N
N
H
X
Ar
I
N
Ni(cod)2/ PPh3
(R3 = Bu, Ph, H; X = BF4, Br)
X
Pr
R2
R2
( X = BF4, I)
I
(c) Pd-catalyzed C2-arylation of NHCs: (d) Ni-catalyzed C2-arylation of NHCs:
This work
Ar1
Ar1
Ar1
+
Ar2-X
Ar2-I
N
N
N
N
N
+
Ar2
I
Ar2
X
[Ni]
N
Pd2(dba)3
Ar1
Ar1
(X = Cl, Br, I, OTf)
Ar1
Scheme 2 Catalytic synthesis of C2-alkylated (a), and C2-arylated (b, c, d) 1,3-
imidazolium salts. [Ni] = Ni(0) or Ni(I) precatalyst.
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4-chlorotoluene. Interestingly, addition of one1 eq of PhBr in
the same reaction mixture, containing
9
and 1a, led to the
Table 1 Ni(I) mediated C2-arylation of IPr (1a) with ArX.
[Ni] 2.5 mol%
IPr + ArX
1a
(IPrAr)X
oꢀxylene, 140 °C
3a: Ar = Ph; 3h: Ar = 4ꢀEtOOCꢀC6H4)
Entry
Ar
X
[Ni]
Yield (h)a
75 (3)
77 (2)
12 (3)
79 (2)
67 (3)
1
2
3
4
5
Ph
Ph
Ph
Ph
Br
Br
Cl
Br
Br
[(IPr)Ni(µ-Cl)]2 (10-Cl)
[(IPr)Ni(µ-Br)]2 (10-Br)
[(IPr)Ni(µ-Cl)]2 (10-Cl)
(IPr)2NiCl (9)
4-EtOOC-C6H4
[(IPr)Ni(µ-Br)]2 (10-Br)
ayield, %; reaction time, h
formation of (IPrPh)Br (3a) in 45% yield. This suggests the
potential involvement of Ni(I) species in the catalytic C2-
arylation of 1a 18
.
Scheme 3 (a) Nickel catalysed C2-arylation of 1a with PhX. (b) C2-Arylated 1,3-
imidazolium bromides: 3a-3j obtained by the arylation of 1a; 6a and 6b by SIPr
Therefore, we sought to probe the catalytic activity of well-
defined Ni(I) complexes [(IPr)Ni(µ-Cl)]2 (10-Cl),17 [(IPr)Ni(µ-Br)]2
(
1b); and 7a-7c by Me2-IPr (1c).
(
10-Br),19 and ([IPr)2NiCl ( ).16 Indeed, all three were active and
9
after 2h (Scheme 3, a). Encouraged by this outcome, we then
employed aryl bromides as coupling partners. Interestingly,
under similar experimental conditions PhBr underwent
reaction with 1a to give (IPrPh)Br (3a) in 75% yield. Having
optimized the procedure (see Table S1 of ESI), we examined
the scope of this protocol to other aryl bromides and NHCs
(Scheme 3, b). A variety of functional groups on aryl bromides
afforded 3a in a high yield (Table 1, entries 1, 2, and 4) even
with a lowered catalyst loading (2.5 mol% instead of 5 mol%).
Moreover, the reaction of PhCl and 1a (entry 3, Table 1) with
2.5 mol% of 10-Cl appeared to be faster compared to that with
5 mol% of Ni(cod)2, indicating higher activity of the former.
Similarly, the coupling of 1a and 4-EtOOCC6H4 with 10-Cl (entry
5) gave a higher yield of 3h in comparison with Ni(cod)2.
were tolerated, including MeO- (3d
Me2N- (3j), except that a small amount of (IPrH)Br salt was
formed with aryl bromides featuring a carbonyl group (3f 3i).
SIPr (1b) (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-
ylidene) and Me2-IPr 1c (Me2-IPr 1,3-bis(2,6-
, 3e), Me(O)C- (3f, 3g), and
Reaction of 1a and Ni(cod)2 with ArCl gives stable dinuclear
Ni(I) complexes {(IPr)Ni(µ-Cl)(µ-Ar)Ni(IPr)} (11-Cl) (Ar = 4-
MeC6H4 or 4-MeOC6H4).20 In order to shed more light on the
mechanism of C2-arylation reactions of NHCs, we sought to
prepare analogues bromo-derivatives {(IPr)Ni(µ-Br)(µ-Ar)-
Ni(IPr)} (11-Br) by treatment of 1a and Ni(cod)2 with PhBr or 4-
bromotoluene (Scheme 4). Under similar reaction conditions,20
this however led to the formation of C2-arylated products
[2{IPr(4-R-C6H4)}]NiBr4 (14-R) (R = H or Me) as crystalline solids
(see Supporting Information for further details), suggesting the
-
(
)
=
diisopropylphenyl)-4,5-dimethyl-imidazol-2-ylidene) carbenes
underwent C2-arylation with aryl bromides to afford the
corresponding imidazolium salts (6a, 6b, and 7a-7c) (Scheme 3,
b).
Chlorobenzene reacted rather sluggishly and gave only 7%
(IPrPh)Cl ( ) after 4h (Scheme 3, a), whereas PhOTf furnished
(IPrPh)OTf ( ) in 16% yield. 4-Chloroanisol also gave the
coupling product {IPr(4-anisyl)}Cl (
(17% after 2h). Compounds 3a 3j
4
anticipated compounds {(IPr)Ni(µ-Br)(µ-4-R-C6H4)Ni(IPr)} (11
-
5
Br) to be unstable. The formation of 14-R indicates the
potential involvement of Ni(I) species 12-Br (the monomer of
10-Br) and 13-R.16-17 Compound 13-R (or its dimer) further
undergoes reaction with ArBr to give 12-Br and biaryls via SET
reactions.20 Reaction of 10-Br with ArBr and subsequent
reductive elimination rationalize the formation of 14-R (see
Figure 1). To confirm this, we also carried out a separate
8
) on treatment with 1a
6a 6b 7a 7c, and
2,
-
,
4,
5,
,
,
-
8
are air-stable solids and have been characterized by NMR
spectroscopy and mass spectrometry (see Supporting
Information). Molecular structures of 3a, 3b, and 3j (Figures
S1-S3) were determined by single crystal X-ray diffraction
studies.
reaction of 10-Br with 4-bromotoluene, which readily gave 14
-
However, no coupling of 1a with 4-chlorotoluene was
observed, instead Matsubara’s16 T-shaped three-coordinated
Me.
Ni(I) complex (IPr)2NiCl (
product 4,4’-bitolyl was formed via single electron transfer
(SET) reaction (eq 1). Compound
exhibits an equilibrium17
with [(IPr)Ni(µ-Cl)]2 (10-Cl) and IPr (eq 1) at 25 ºC.
9) along with the homocoupling
9
Compound
9 was found to be inactive and did not afford
the expected product {IPr-(4-tolyl)}Cl on treatment of 1a with
2 | J. Name., 2012, 00, 1-3
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R
Scheme 4 Stoichiometric reaction of Ni(cod)2, IPr and ArBr to 14-R.
Ar
Ar
Ar
Ar
Ar
Ar
+ 2 Ni(cod)2
N
N
N
N
N
N
+ RC6H4Br
Ni
R
2
+
Ni
Ni
Ni
Br
N
THF, rt
N
N
N
Br
Ar
Ar
Ar
Ar
13
-R
12
-Br
1a
)
11
-Br (R = H or Me)
IPr (
+ RC6H4Br
(Ar = 2,6-iPr2C6H3)
- 0.5 (RC6H4
)
2
Ar
N
Ar
Ar
(R = H or Me)
Br
N
N
N
+ RC6H4Br
2
R
Ni
Ni
N
N
Br
−
Ar
Ar
Ar
[NiBr4]2
10-
Br
14 (R = H or Me)
-R
Fig. 1 Calculated free energy profile (in kcal mol-1 at 298.15 K) for the model Ni(I)/Ni(II) catalytic cycle for the C2-arylation of an NHC (1,3-dimethyl-imidazol-2-ylidene)
with PhBr. Selected bond lengths are given in Å. All relative free energies were calculated at solution ( -xylene) with respect to the Ni(I) complex 15M
o
.
The findings listed in Table 1 and the outcome of reactions the CPh‒Br bond and the formation of two new Br‒Ni and CPh‒
of 1a and Ni(cod)2 with ArBr (Scheme 4) clearly emphasize the Ni covalent bonds. The transition state TS17M-18M presents a
significance of Ni(I) species 10-Br in the catalytic C2-arylation relative free energy barrier of 19.4 kcal mol-1, which is the
of NHCs. We carried out quantum chemical calculations using rate-determining step (RDS) for the overall reaction. The
density functional theory (DFT) for a model reaction NHC + formation of the four-membered intermediate 18M was
PhBr → (NHC-Ph)Br) (where NHC is 1,3- dimethylimidazol-2- showed to be thermodynamically irreversible, with a relative
ylidene) to shed light on the potential reaction pathways energy calculated as -26.0 kcal mol-1. The elongation of the Ni‒
(Figure 1). The dinuclear Ni(I) compound 15M (which mimics Ni distance, going from 2.55 Å in 17M to 3.55 Å in 18M
,
10-Br, where M stands for model) was considered as the probably means a break of the bimetallic coordination, thus
precursor complex. At this point, the approach of the PhBr to facilitating the formation of Ni(II) complexes 19M and 20M
15M gives 16M, only 2.3 kcal mol-1 below the separated which are important to the next steps of the reaction.
,
reactants. The formation of the η2-complex 17M occurs
The formation of two square-planar Ni(II) complexes 19M
through the transition state TS16M-17M. The Ni(I)-Ni(I) bond and 20M from 18M is very exergonic (-80.6 kcal mol-1). The last
length in 15M was found to be 3.20 Å. The developing of the step is the reductive elimination from 20M via the transition
η2-coordination between the PhBr and one of the Ni(I) atoms state TS20-21M to release (NHC-Ph)Br (21M). The NHC‒Ph
shortens the Ni(I)-Ni(I) distance to 2.55 Å in 17M. The second bond distance of 1.48 Å in 21M is a typical value of a C‒C
stage concerning the oxidative addition step is the breaking of covalent bond, confirming that the product is already formed.
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In conclusion, calculations corroborate experimental evidences
on the existence of Ni(I)/Ni(II) catalytic cycle involving a
dinuclear Ni(I) intermediate as the active species.
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‒
Ar
‒
N
+ CuBr
R
+ K{N(SiMe3)2}
‒
3b 3j
or
N
Cu
‒
Ar
: R = 4ꢀMeꢀC6H4
23: R = 4ꢀMe2NꢀC6H4
Br
(Ar = 2,6ꢀiPr2C6H3)
22
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In summary, we have demonstrated the direct C2-arylation
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Br)]2 react with aryl halides to form Ni(II) compounds, which
undergo reductive elimination to deliver the coupling
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‒
‒
We gratefully acknowledge the support from the Deutsche
Forschungsgemeinschaft (GH 129/4-1). We thank Fundação de
Amparo à Pesquisa do Estado de São Paulo for financial
support. The authors are grateful to Professor Norbert W.
Mitzel for his generous support and encouragement.
‒
‒
‒
‒
‒
Notes and references
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Dalton Transactions
Page 6 of 6
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DOI: 10.1039/C7DT03099A
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Direct C2-arylation of NHCs with various aryl electrophiles to C2-arylated imidazolium salts is achieved
using Ni-catalysis. The dinuclear Ni(I) species I undergoes oxidative addition with ArX via SET reaction to
give the Ni(II) intermediate II. Reductive elimination delivers the arylation product (III) and regenerates
the catalyst.