Synthesis of Quinoline-Based NNN-Pincer Nickel(II) Complexes: A Robust and Improved Catalyst System for C-H Bond Alkylation of Azoles with Alkyl Halides

Article abstract of DOI:10.1021/acs.organomet.6b00201

The quinoline-based pincer nickel(II) complexes κ^N,κ^N,κ^N-{R₂N-C₆H₄-(μ-N)-C₉H₆N}NiX ((^R2NNN^Q)NiCl: R = Me, 2a; R = Et, 2b) were synthesized by the reaction of the ligand precursors (^R2NNN^Q)H (R = Me, 1a; R = Et, 1b) with (DME)NiCl₂ in the presence of Et₃N. Similarly, the pincer nickel(II) derivatives (^R2NNN^Q)NiX (R = Me, X = Br, 3a; R = Et, X = Br, 3b; R = Me, X = OAc, 4a) were obtained by treatment of the ligands (^R2NNN^Q)H with the nickel precursor (THF)₂NiBr₂ or Ni(OAc)₂. All of these complexes were characterized by ¹H and ¹³C NMR spectroscopy as well as by elemental analysis. Further, the molecular structures of 2a and 3a,b were elucidated by X-ray crystallography. Complex 2a is found to be an efficient catalyst for the direct C-H bond alkylation of substituted benzothiazoles and oxazoles with various unactivated alkyl halides containing β-hydrogens under mild reaction conditions. The catalyst 2a is very robust and was recycled and reused five times for the alkylation reaction without a decrease in its catalytic activity. Preliminary studies reveal that the catalyst 2a acts as an active catalyst and the alkylation reaction appears to operate via a radical pathway.

Full text of DOI:10.1021/acs.organomet.6b00201

Article
pubs.acs.org/Organometallics
Synthesis of Quinoline-Based NNN-Pincer Nickel(II) Complexes: A
Robust and Improved Catalyst System for C−H Bond Alkylation of
Azoles with Alkyl Halides
Ulhas N. Patel,† Dilip K. Pandey,† Rajesh G. Gonnade,‡ and Benudhar Punji*,†
‡
†Organometallic Synthesis and Catalysis Group, Chemical Engineering Division, and Centre for Material Characterization,
CSIR-National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune 411 008, Maharashtra, India
S
* Supporting Information
ABSTRACT: The quinoline-based pincer nickel(II) complexes κN,κN,κN-
{R2N-C6H4-(μ-N)-C9H6N}NiX ((R2NNNQ)NiCl: R = Me, 2a; R = Et, 2b)
were synthesized by the reaction of the ligand precursors (R2NNNQ)H (R
= Me, 1a; R = Et, 1b) with (DME)NiCl2 in the presence of Et3N. Similarly,
the pincer nickel(II) derivatives (R2NNNQ)NiX (R = Me, X = Br, 3a; R =
Et, X = Br, 3b; R = Me, X = OAc, 4a) were obtained by treatment of the
ligands (R2NNNQ)H with the nickel precursor (THF)2NiBr2 or Ni(OAc)2.
All of these complexes were characterized by 1H and 13C NMR
spectroscopy as well as by elemental analysis. Further, the molecular
structures of 2a and 3a,b were elucidated by X-ray crystallography. Complex 2a is found to be an efficient catalyst for the direct
C−H bond alkylation of substituted benzothiazoles and oxazoles with various unactivated alkyl halides containing β-hydrogens
under mild reaction conditions. The catalyst 2a is very robust and was recycled and reused five times for the alkylation reaction
without a decrease in its catalytic activity. Preliminary studies reveal that the catalyst 2a acts as an active catalyst and the
alkylation reaction appears to operate via a radical pathway.
nickel complex, (MeNN2)NiCl, that shows excellent catalytic
INTRODUCTION
■
activity for the direct C−H bond alkylation of oxazoles and
Pincer-ligated transition-metal complexes have been extensively
pursued as catalysts for various chemical transformations,
thiazoles with unactivated alkyl halides (Figure 1).12 However,
mostly due to their exceptional stability and unusual chemical
reactivity.1 Among the various pincer−metal complexes, noble
metals such as palladium, rhodium, iridium, and ruthenium are
very often employed and explored as catalysts in many
challenging chemical reactions.2 From an economical prospec-
tive, the use of a pincer system with an earth-abundant metal in
these catalytic processes would be highly desirable. Toward this
endeavor, pincer-ligated catalysts based on inexpensive first-row
transition metals,3 particularly pincer nickel complexes, have
successfully been applied to the traditional C−C and C−S
cross-couplings,4 CC bond functionalization,5 hydrosilylation
of aldehydes and ketones,6 hydroamination of nitriles,7 and
CO2 reduction reaction.8 In contrast, the use of pincer nickel
Figure 1. Pincer nickel complexes for the C−H bond alkylation of
azoles.
complexes as catalysts for the relatively challenging C−H bond
functionalization9−11 has rarely been investigated.12
In particular, the C−H bond alkylation of azoles with
unactivated alkyl halides containing β-hydrogen is a promising
yet challenging reaction, due to the undesired β-elimination
from these electrophiles.10b,13 Noble-metal palladium precur-
sors with added ligands,14 as well as earth-abundant nickel14b,15
and copper16 precursors, are known to catalyze the alkylation of
azoles with alkyl halides.17 Though, the reported nickel- and
copper-catalyzed method is significant from an economic
prospective; it uses an in situ generated catalyst, needs a high
reaction temperature, and/or shows poor activity for the sulfur-
containing thiazoles. Hu et al. reported a well-defined pincer
this catalyst system requires an additional copper cocatalyst for
achieving good yields of the desired alkylated products. In
addition, (MeNN2)NiCl acts as a precatalyst and decomposes to
nanoparticles during the catalysis, which might hamper a
mechanistic investigation involving this catalyst. Within our
research program directed toward the development of pincer-
based metal catalysts for C−H bond functionalization18 and
Received: March 9, 2016
© XXXX American Chemical Society
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Scheme 1. Synthesis of (R2NNNQ)H Ligand Precursors and (R2NNNQ)NiX Complexes
inspired by the excellent catalytic behavior of (MeNN2)NiCl,
developed by the Hu group, herein we report the synthesis of a
sterically less demanding and electronically distinct quinolinyl-
based nickel pincer system, (R2NNNQ)NiCl, and its catalytic
activity for the C−H bond alkylation of oxazoles and thiazoles
with unactivated alkyl halides (Figure 1). The present catalyst
system (R2NNNQ)NiCl is highly robust, does not need a
cocatalyst, and acts as an active catalyst; in addition, the
reactions were efficiently carried out at 100−120 °C.
Preliminary studies have been carried out to establish the
status of the catalyst and the pathway of the alkylation reaction.
coordination of amido ligand 1a with a Pt precursor under
similar conditions, with the dimethylamino (−NMe2) group
being unbound.19 However, we have observed the tridentate
amido pincer coordination of the ligand 1a with different nickel
precursors, which could be due to the small binding pocket of
1a that is able to accommodate the small Ni ion rather than the
large Pt ion in a tridentate fashion.
The pincer complexes (Et2NNNQ)NiCl (2b) and
(
Et2NNNQ)NiBr (3b) were synthesized in excellent yields by
the reaction of (Et2NNNQ)H (1b) with (DME)NiCl2 and
(THF)2NiBr2, respectively, in the presence of Et3N in THF at
70 °C. These pincer nickel complexes were obtained as purple
1
RESULTS AND DISCUSSION
solids, and they are highly air and moisture stable. In the H
■
Synthesis and Characterization of R2NNNQ Nickel
Complexes. The tridentate proligand precursors R2N-C6H4-
N(H)-8-quinolinyl ((R2NNNQ)H: R = Me, 1a;19 R = Et, 1b)
were prepared in one step by the palladium-catalyzed amination
reaction of 2-bromo-N,N-dialkylaniline with 8-aminoquinoline
in moderate yields (Scheme 1). These compounds were readily
isolated by column chromatography. Compound 1a was
initially obtained as a viscous liquid and solidified at room
temperature in 3−4 days. The 1H and 13C NMR data of 1a are
in good agreement with those reported in the literature for the
same compound.19 Compound 1b was obtained as an orange
solid, which was fully characterized by 1H and 13C NMR
spectroscopy as well as by HRMS and elemental analysis.
The metalation reaction of the proligand (Me2NNNQ)H (1a)
with (DME)NiCl2 in the presence of Et3N in THF at 70 °C
produced the amido pincer complex (Me2NNNQ)NiCl (2a) in
good yield, whereas the treatment of 1a with (THF)2NiBr2 and
Ni(OAc)2 in the presence of Et3N at room temperature
afforded (Me2NNNQ)NiBr (3a) and (Me2NNNQ)Ni(OAc) (4a),
respectively (Scheme 1). In 1H NMR spectra of complexes 2a−
4a, the disappearance of the N−H signal corresponding to 1a
suggests covalent bonding between the central nitrogen and the
nickel center. Further, the −NMe2 protons in these complexes
resonate as a singlet in the range 2.96−3.20 ppm, which were
significantly deshielded in comparison to those observed for the
same protons in ligand 1a (δ 2.75 ppm). The C−H proton
ortho to the quinolinyl N atom resonates differently from that
in the free ligand 1a. All of these observations clearly suggest
the tridentate amido pincer coordination of the ligand 1a to the
Ni center in complexes 2a−4a. For complex 4a, analysis of the
1H and 13C NMR spectra of the −OCOCH3 group is
consistent with the terminal bonding of −OCOCH3.20 Notably,
in particular, the −NMe2 protons are deshielded and the C−H
proton ortho to the quinolinyl N atom is shielded in the Ni
complexes in comparison to those observed in the free ligand.
Interestingly, Peters and co-workers have shown the bidentate
NMR spectra of complexes 2b and 3b, the −CH2 protons of
the −NEt2 group displayed two sets of multiplets, one in the
shielded region (δH ∼2.55−2.43 ppm) and other in the
deshielded region (δH ∼3.62−3.46 ppm), against one quartet
for the four protons in 1b (δH 3.05 ppm), which could be due
to the diastereotopic −CH2 protons generated upon the
coordination of the −NEt2 group to the nickel center. All these
complexes were further characterized by 13C NMR spectros-
copy, elemental analysis, and HRMS techniques. The molecular
structures of 2a and 3a,b were determined by single-crystal
diffraction studies.
The ORTEP diagrams of complexes 2a and 3a,b are shown
in Figures 2−4. Selected bond lengths and bond angles are
given in Table 1. For these three complexes, the geometry
around the nickel is slightly distorted from the expected square
planar. The Ni−N1amido bond lengths are in the range
1.842(2)−1.845( 2) Å, which are comparable with the similar
bond length in (MeNN2)NiCl (1.835(2) Å).21 Similarly, the
Figure 2. Thermal ellipsoid plot of (Me2NNNQ)NiCl (2a). All
hydrogen atoms are omitted for clarity.
B
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complex (MeNN2)NiCl. The Ni−Cl bond length (2.185 Å) in
complex 2a is slightly shorter than that observed in the complex
(MeNN2)NiCl (Ni−Cl = 2.203 Å), which could be due to the
less steric and more planar structure of former than of the
latter. The N2−Ni−N3 (170.77(8)°) and N1−Ni−Cl
(177.21(6)°) bond angles in complex 2a are significantly
greater than the similar bond angles in the complex
(MeNN2)NiCl (N2−Ni−N3 = 169.73(8)°, N1−Ni−Cl =
174.75(7)°), which is consistent with the more planar geometry
of Ni in complex 2a in comparison to that in (MeNN2)NiCl.
The comparison of bond lengths and bond angles of the
complex 2a with those of (MeNN2)NiCl suggests that complex
2a has a more planar and rigid structure than the latter species,
which might provide extra stability to complex 2a during
catalysis.
Figure 3. Thermal ellipsoid plot of (Me2NNNQ)NiBr (3a). All
hydrogen atoms are omitted for clarity.
Catalytic Activity of (R2NNNQ)NiX Complexes for
Alkylation of Azoles. The newly developed quinolinyl-
based pincer nickel complexes (R2NNNQ)NiX (2a−4a) were
screened and optimized for the direct C−H bond alkylation of
benzothiazole using nonactivated alkyl halides (Table 2).
Initially, the nickel complex 2a was employed and the reaction
parameters were optimized for the alkylation of benzothiazole
(5a) with 1-iodooctane (6a) as electrophile. After a thorough
investigation of the various experimental parameters, we found
that the coupled product 2-n-octylbenzothiazole (7aa) could be
obtained in 91% yield by employing 5 mol % of catalyst 2a, in
the presence of LiOtBu in 1,4-dioxane at 100 °C (Table 2, entry
7). Mild inorganic bases such as Li2CO3, Na2CO3, K2CO3,
LiOAc, and KOAc were found to be ineffective (entries 1−5),
whereas the employment of K3PO4 gave 35% of coupled
product 7aa (entry 6). In addition to the alkylation reaction in
1,4-dioxane, the reaction in other solvents such as 1,2-
dimethoxyethane (DME), THF, and toluene is also competent
(entries 8−10). The nickel complexes 3a and 4a are slightly less
efficient, whereas complexes 2b and 3b are very poor for the
alkylation reaction in comparison with the catalyst 2a (entries
13−16). Notably, the dimethyl derivatives 2a−4a were more
active in comparison to the diethyl derivatives 2b and 3b, which
might be due to the decreased sterics around the Ni center in
dimethyl derivatives in comparison to the latter species.
Catalytic coupling of benzothiazole with 1-iodooctane also
occurs at 80 °C to produce the desired product in 78% yield
(entry 17). The alkylation reaction occurs smoothly even with a
catalyst loading of 2 mol % of 2a, giving the coupled product
7aa in 78% yield (entry 18). More challenging alkyl
electrophiles, such as 1-bromooctane and 1-chlorooctane,
were also coupled with benzothiazole in good yields, by
employing NaI and by performing the reactions at 120 °C
(entries 19 and 20). The presence of a nickel catalyst under the
standard conditions is necessary; without it, alkylation did not
occur (entry 21).
Figure 4. Thermal ellipsoid plot of (Et2NNNQ)NiBr (3b). All
hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 2a
and 3a,b
2a
3a
3b
Ni(1)−N(1)
Ni(1)−N(2)
Ni(1)−N(3)
Ni(1)−Cl(1)
1.8441(18)
1.8990(18)
1.9545(19)
2.1850(8)
1.842(2)
1.902(2)
1.962(2)
1.8447(18)
1.9073(19)
1.9678(18)
Ni(1)−Br(1)
2.3471(4)
84.59(9)
2.3349(4)
84.83(8)
85.91(8)
170.57(8)
N(1)−Ni(1)−N(2)
N(1)−Ni(1)−N(3)
N(2)−Ni(1)−N(3)
N(1)−Ni(1)−Cl(1)
C(1)−N(1)−C(10)
C(1)−N(1)−Ni(1)
C(10)−N(1)−Ni(1)
N(1)−Ni(1)−Br(1)
84.77(8)
86.32(8)
85.41(10)
169.57(9)
170.77(8)
177.21(6)
128.09(18)
115.89(14)
116.02(14)
127.8(2)
128.04(19)
115.32(15)
116.59(14)
173.20(6)
115.45(18)
116.49(18)
171.60(8)
Having the optimized reaction conditions in hand, we
explored the scope of the alkylation of various azoles. As shown
in Table 3, an array of alkyl bromides and alkyl iodides
containing β-hydrogen can be coupled with the benzothiazole
to yield the desired alkylated products. Alkyl bromides with
different chain lengths are coupled in good yields (Table 3,
entries 1−8). Various important functional groups such as
halide, ether, alkene, and heteroarene are well tolerated under
the alkylation conditions. Substrates containing alkene and
carbazole moieties coupled in moderate to good yields (entries
12 and 13). The substituted benzothiazoles reacted with 1-
iodooctane to produce the desired alkylated products in
Ni−N3amine bond lengths (1.954( 2)−1.967( 2) Å) are
comparable with the Ni−NMe2 bond length (1.959 Å) in the
complex (MeNN2)NiCl. However, the Ni−N2quinolinyl bond
lengths (1.899( 2)−1.907( 2) Å) in complexes 2a and 3a,b
are slightly shorter than the Ni−N3amine bond lengths
(1.954( 2)−1.967( 2) Å), which suggest that the quinoline
nitrogen strongly binds to the nickel center than the −NMe2 or
−NEt2 nitrogen. This further emphasizes the greater rigidity of
the current ligand binding to nickel in comparison to that in the
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a
Table 2. Optimization of Catalytic Conditions for the Nickel-Catalyzed Alkylation of Benzothiazole
b
entry
Ni cat.
base
solvent
yield (%)
trace
1
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
3a
4a
2b
3b
2a
2a
2a
2a
Na2CO3
K2CO3
KOAc
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
DME
2
3
4
Li2CO3
LiOAc
K3PO4
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
5
trace
35
6
7
95 (91)
82 (80)
74
8
9
THF
10
11
12
13
14
15
16
toluene
78 (77)
trace
trace
74
diglyme
DMF
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
84
33
35
c
17
78
d
18
78
e
19
85 (81)
64
f
20
21
a
Conditions unless specified otherwise: benzothiazole (0.068 g, 0.503 mmol), 1-iodooctane (0.180 g, 0.750 mmol), base (1.0 mmol), solvent (1.0
b
c
d
e
mL). GC yield (isolated yields are given in parentheses). At 80 °C for 16 h. 2 mol % of catalyst 2a. Employing 1-bromooctane (0.145 g, 0.75
mmol) and NaI (0.112 g, 0.75 mmol) at 120 °C. Employing 1-chlorooctane (0.112 g, 0.75 mmol) and NaI (0.112 g, 0.75 mmol) at 120 °C.
f
excellent yields (entries 14−16). In addition to benzothiazole,
5-arylated oxazoles were alkylated in good yields, whereas the
naphthalenyl- and pyridinyl-substituted oxazoles were moder-
ately coupled with 1-bromooctane. Notably, all of these
alkylation reactions proceeded with the nickel catalyst 2a and
without the use of an external cocatalyst, which makes this
catalyst distinct from the existing well-defined nickel catalyst
(MeNN2)NiCl.12 Furthermore, the reactions were performed at
temperatures as low as 100−120 °C to achieve the alkylated
products of benzothiazoles in excellent yields. Though
undefined and precatalyst systems are known for the alkylation
of relatively activated oxazole substrates, they show very poor
activity for the alkylation of sulfur-containing benzothiazole
derivatives14b,15 and/or need a Cu cocatalyst.12 However, the
catalyst 2a performed uniquely for the alkylation of a variety of
benzothiazoles and azoles under rather mild reaction
conditions. In addition, this catalyst system acts as an active
catalyst and was found to be highly robust for the alkylation
reaction (discussed below).
Considering the well-defined nature of catalyst 2a and its
excellent catalytic activity for the alkylation of azoles under mild
reaction conditions, we performed a preliminary mechanistic
investigation for the 2a-catalyzed alkylation of azoles with alkyl
halides. Hu and co-workers have reported that the well-defined
pincer complex (MeNN2)NiCl decomposes to heterogeneous
nanoparticles, which in turn act as a catalyst for the alkylation of
azoles. In view of that, initially, we examined the status of the
well-defined, newly developed catalyst 2a during the alkylation
reaction of benzothiazole with 1-iodooctane. The standard
alkylation reaction of benzothiazole with 1-iodooctane in the
presence of 200 equiv and 600 equiv (with respect to 2a) of
mercury (Hg) afforded the desired alkylated product 7aa in
88% and 85% yields, respectively, against the 91% yield in the
absence of mercury (Scheme 2a). This indicates the mercury
has no significant effect on the 2a-catalyzed alkylation reaction;
hence, this ruled out the possibility of nickel nanoparticles or
colloids being the active catalyst during catalysis. In addition, a
filtration experiment was conducted to support this finding
(Scheme 2b). Hence, a catalytic reaction mixture of
benzothiazole with 1-iodooctane was heated at 100 °C for 1
h, and the reaction mixture was then filtered into a new reaction
vessel. The reaction was continued further after adding fresh
LiOtBu to the filtrate, wherein the yield of the product 7aa
obtained was 85%. This further supports the probability of a
catalytically active insoluble nanoparticle being unlikely during
the alkylation reaction. Again, the GC analysis of a standard
alkylation reaction, which was performed employing 25 mol %
of the catalyst 2a, does not show the formation of free ligand
1a, which suggests that the dissociation of ligand from complex
2a and generation of soluble Ni particles during the alkylation
reaction is less likely.
Further, the progress of the alkylation reaction was
monitored by gas chromatography (GC) analysis at regular
intervals of time (see section 2 in the Supporting Information
for details). The reaction profile of the benzothiazole alkylation
catalyzed by 2a over a period of 3 h is shown in Figure 5. As
shown, the alkylated product 7aa formed steadily and an
induction period for the reaction is absent. This again confirms
the direct involvement of the catalyst 2a during the alkylation
D
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a
Table 3. Scope for the 2a-Catalyzed Alkylation of Azoles with Alkyl Halides
a
Conditions unless specified otherwise: compound 5 (0.5 mmol), compound 6 (0.75 mmol), LiOtBu (0.081 g, 1.0 mmol), NaI (0.112 g, 0.75
b
c
d
mmol), 1,4-dioxane (1.0 mL), 120 °C, 16 h. Isolated yield. Employing alkyl iodide (NaI was not used). Employing alkyl chloride (NaI was used).
reaction and rules out the generation of a new catalytically
active species.
assume that the catalysis starts with the reaction of
(
Me2NNNQ)NiCl (2a) with benzothiazole in the presence of
LiOtBu to produce the Ni(II) intermediate (Me2NNNQ)Ni-
(benzothiazolyl) (8) (Figure 6). This species might trigger the
formation of an alkyl radical via single-electron transfer,
followed by radical rebound to form a Ni(IV) species,
To probe the probability of a free radical pathway, the 2a-
catalyzed alkylation of benzothiazole with 1-iodooctane was
performed in the presence of 1.5 equiv of TEMPO, a free
radical inhibitor (Scheme 2a). In this reaction, the alkylation
reaction completely shut down and formation of the coupled
product was not observed. This suggests the probable
involvement of a radical intermediate during the 2a-catalyzed
alkylation reaction.
(
Me2NNNQ)Ni(benzothiazolyl)(alkyl)X (9). The reductive
elimination would deliver the desired alkylated product and
regenerate the catalytically active species. The electron-rich
pincer ligand moiety is assumed to stabilize the Ni species in a
higher oxidation state. Though the possibility of a Ni(IV)
species is assumed by the reaction of alkyl electrophiles, a
On the basis of our preliminary mechanistic studies and
literature precedent for the alkylation reaction,14b,15,22,23 we
E
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Scheme 2. Alkylation Reaction under Controlled Conditions
reactivity studies, and isolation and reactivity of the
intermediate species, is currently under investigation.
The robustness and stability of the catalyst 2a was examined
by conducting recycling experiments. As the alkylation reaction
is inhibited by both the substrate (benzothiazole) and product
7aa,24 we performed a recycling experiment by distilling out the
product and other volatiles after each catalytic cycle (Figure 7).
Hence, after the catalytic reaction of benzothiazole with 1-
iodooctane under the standard conditions, the yield of the
product 7aa obtained was 92% in the first cycle. The product
and other volatiles were distilled out under vacuum at 110 °C.
In the same reaction vessel were placed fresh benzothiazole, 1-
iodooctane, LiOtBu, and 1,4-dioxane and the reaction mixture
was heated at 100 °C for the second cycle, wherein the coupled
product 7aa was analyzed to be 90%. Similarly, in the third,
fourth, and fifth cycles, the yields of product 7aa obtained were
89%, 91%, and 85%, respectively. This chain of experiments
clearly suggests that the catalyst 2a is very robust and active for
a long period of time and that the catalyst can be recycled and
reused several times. This is a very significant aspect of this
catalyst, considering the importance of the alkylation of azoles.
Figure 5. Reaction profile for the 2a-catalyzed alkylation of
benzothiazole (5a) with 1-iodooctane (6a).
CONCLUSION
■
In summary, the inexpensive and novel pincer nickel complexes
(R2NNNQ)NiX were synthesized and fully characterized by
various spectroscopic techniques. These complexes were
demonstrated for the direct C−H bond alkylation of important
benzothiazoles and azoles with unactivated alkyl halides
containing β-hydrogen atoms. In particular, the complex
(
Me2NNNQ)NiCl (2a) shows excellent activity for the
alkylation reaction under mild reaction conditions, without
the use of an additional cocatalyst. Catalyst 2a is very robust
and could be recycled and reused several times for the
alkylation reaction without a decrease in its catalytic activity. In
the course of alkylation reactions, catalyst 2a tolerates
numerous functional groups such as halide, ether, alkene,
heteroarene, etc. A preliminary study reveals that complex 2a
acts as an active catalyst during the alkylation and the reaction
follows a free radical pathway. We are currently investigating
the detailed mechanistic pathway for the 2a-catalyzed alkylation
reaction, which will be reported in due course.
Figure 6. Proposed mechanism for 2a-catalyzed alkylation reactions.
EXPERIMENTAL SECTION
General Experimental Considerations. All manipulations were
conducted under an argon atmosphere either in a glovebox or using
probable Ni(I)/Ni(III) mechanism cannot be ruled out at this
moment. The detailed mechanistic pathway for this 2a-
catalyzed alkylation reaction, through kinetic studies, controlled
■
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Figure 7. Catalyst recycling experiment. The yield refers to the GC yield using n-dodecane as internal standard; slight variations in yields might be
due to experimental error. Legend: (a) conditions, 5a (0.5 mmol), 6a (0.75 mmol), LiOtBu (1.0 mmol), 1,4-dioxane (1.0 mL), 100 °C, 16 h; (b)
isolated yield in parentheses.
standard Schlenk techniques in predried glassware. The catalytic
reactions were performed in flame-dried reaction vessels with a Teflon
screw cap. Solvents were dried over Na/benzophenone or CaH2 and
distilled prior to use. Liquid reagents were flushed with argon prior to
use. The compounds 2-bromo-N,N-dimethylaniline,25 2-bromo-N,N-
NMR (100 MHz, CDCl3): δ 147.6 (CH), 141.2 (Cq), 140.1 (Cq),
139.6 (Cq), 139.1 (Cq), 136.0 (CH), 129.1 (Cq), 127.4 (CH), 124.2
(CH), 123.0 (CH), 121.6 (CH), 120.6 (CH), 116.5 (CH), 116.4
(CH), 107.8 (CH), 47.9 (2C, CH2), 13.0 (2C, CH3). HRMS (ESI):
m/z calcd for C19H21N3 + H+ [M + H]+ 292.1808, found 292.1812.
Anal. Calcd for C19H21N3: C, 78.32; H, 7.26; N, 14.42. Found: C,
79.05; H, 6.58; N, 13.59.
diethylaniline,26 Me2NNNQ)H,19 (THF)2NiBr2,5b substituted benzo-
(
thiazoles,27 and 5-arylazoles28 were synthesized according to
previously described procedures. All other chemicals were obtained
from commercial sources and were used without further purification.
Yields refer to isolated compounds, estimated to be >95% pure as
Synthesis of (Me2NNNQ)H (1a). This compound was synthesized
following the literature procedure19 similarly to the synthesis of
(
Et2NNNQ)H. Yield: 0.98 g, 50%. 1H NMR (500 MHz, CDCl3): δ 8.85
1
(dd, 3JH−H = 4.3, 4JH−H = 1.6 Hz, 1H, Ar-H), 8.72 (br s, NH), 8.12 (dd,
determined by H NMR. TLC used silica gel 60 F254, with detection
3JH−H = 8.3, 4JH−H = 1.6 Hz, 1H, Ar-H), 7.67 (dd, 3JH−H = 7.9, 4JH−H
=
under UV light at 254 nm. In chromatography, separations were
carried out on Spectrochem silica gel (0.120−0.250 mm, 100−200
mesh). High-resolution mass spectroscopy (HRMS) was carried out
on a Thermo Scientific Q-Exactive, Accela 1250 pump. Melting points
1.6 Hz, 1H, Ar-H), 7.58 (dd, 3JH−H = 7.9, JH−H = 1.2 Hz, 1H, Ar-H),
7.47−7.39 (m, 2H, Ar-H), 7.24−6.95 (m, 4H, Ar-H), 2.75 (s, 6H,
CH3). 13C NMR (100 MHz, CDCl3): δ 147.7 (CH), 145.1 (Cq), 140.0
(Cq), 139.3 (Cq), 136.2 (CH), 136.0 (Cq), 129.1 (Cq), 127.4 (CH),
123.3 (CH), 121.6 (2C, CH), 119.4 (CH), 117.8 (CH), 116.4 (CH),
107.8 (CH), 44.0 (2C, CH3). HRMS (ESI): m/z calcd for C17H17N3 +
H+ [M + H]+ 264.1495, found 264.1498.
4
were determined on a Buchi 540 capillary melting point apparatus;
̈
values are uncorrected. NMR (1H and 13C) spectra were recorded at
400 or 500 MHz (1H), 100 or 125 MHz (13C, DEPT (distortionless
enhancement by polarization transfer)), and 377 MHz (19F) on Bruker
AV 400 and AV 500 spectrometers in CDCl3 solutions, if not
otherwise specified; chemical shifts (δ) are given in ppm. The 1H and
13C NMR spectra are referenced to residual solvent signals (CDCl3:
δ(H) 7.26 ppm, δ(C) 77.2 ppm).
Synthesis of (Me2NNNQ)NiCl (2a). A solution of (Me2NNNQ)H
(1a; 0.50 g, 1.899 mmol) in THF (15 mL) was added to a suspension
of (DME)NiCl2 (0.50 g, 2.276 mmol) in THF (10 mL) using a
cannula. After the reaction mixture was stirred for 15 min, Et3N (0.53
mL, 3.803 mmol) was added and the resulting reaction mixture was
refluxed for 5 h under an argon atmosphere (N.B.: completion of the
reaction was confirmed by TLC monitoring of the reaction mixture).
At ambient temperature, the reaction mixture was filtered through a
cannula and the filtrate was concentrated to 5 mL. To the
concentrated solution was added n-hexane to precipitate the product
2a in analytically pure form. Compound 2a was further recrystallized
from an n-hexane solution by slow evaporation to obtain X-ray-quality
Synthesis of (Et2NNNQ)H (1b). A Schlenk flask was charged with
Pd2(dba)3 (0.208 g, 0.227 mmol) and 1,1′-bis(diphenylphosphino)-
ferocene (DPPF; 0.253 g, 0.456 mmol), and toluene (30 mL) was
added to this mixture under an argon atmosphere. The resulting
suspension was stirred at room temperature for 10 min. To the
resulting catalyst solution were added 8-aminoquinoline (0.82 g, 5.69
mmol), N,N-diethyl-2-bromoaniline (1.30 g, 5.70 mmol), and NaOtBu
(0.66 g, 6.87 mmol). The reaction mixture was then refluxed for 3
days. At ambient temperature, the reaction mixture was quenched with
water and extracted with ethyl acetate (30 mL × 3). The combined
organic layer was washed with distilled water (30 mL × 3) and dried
over Na2SO4. The volatiles were then evaporated under vacuum, and
the crude mixture was purified by column chromatography on silica gel
(n-hexane/EtOAc 40/1) to obtain an orange solid. Yield: 0.91 g, 55%.
Mp: 89−91 °C. 1H NMR (500 MHz, CDCl3): δ 9.27 (br s, NH), 8.86
1
single crystals. Yield: 0.56 g, 83%. Mp: 150−152 °C. H NMR (500
MHz, CDCl3): δ 8.55 (d, 3JH−H = 4.9 Hz, 1H, Ar-H), 8.08 (d, 3JH−H
=
8.2 Hz, 1H, Ar-H), 7.54 (d, 3JH−H = 8.2 Hz, 1H, Ar-H), 7.41 (d, 3JH−H
3
= 7.9 Hz, 1H, Ar-H), 7.34 (vt, JH−H = 7.9 Hz, 1H, Ar-H), 7.20 (dd,
3
3JH−H = 8.1, JH−H = 5.2 Hz, 1H, Ar-H), 7.13−7.08 (m, 2H, Ar-H),
6.87 (d, 3JH−H = 7.9 Hz, 1H, Ar-H), 6.61 (vt, 3JH−H = 7.5 Hz, 1H, Ar-
H), 3.05 (s, 6H, CH3). 13C NMR (125 MHz, CDCl3): δ 150.6 (CH),
148.1 (Cq), 147.4 (Cq), 147.2 (Cq), 147.1 (Cq), 138.6 (CH), 129.8
(Cq), 129.4 (CH), 128.7 (CH), 121.0 (CH), 120.5 (CH), 117.3 (CH),
115.0 (CH), 113.0 (CH), 110.6 (CH), 51.8 (2C, CH3). HRMS (ESI):
m/z calcd for C17H16ClN3Ni − Cl+ [M − Cl]+ 320.0692, found
3
3
(d, JH−H = 3.9 Hz, 1H, Ar-H), 8.12 (d, JH−H = 8.2 Hz, 1H, Ar-H),
3
3
7.72 (d, JH−H = 7.9 Hz, 1H, Ar-H), 7.66 (d, JH−H = 7.6 Hz, 1H, Ar-
H), 7.46−7.41 (m, 2H, Ar-H), 7.23−7.20 (m, 2H, Ar-H), 7.15 (vt,
3JH−H = 7.6 Hz, 1H, Ar-H), 6.97 (vt, JH−H = 7.5 Hz, 1H, Ar-H), 3.05
3
(q, 3JH−H = 7.0 Hz, 4H, CH2), 1.09 (t, 3JH−H = 7.0 Hz, 6H, CH3). 13
C
G
DOI: 10.1021/acs.organomet.6b00201
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
320.0689. Anal. Calcd for C17H16ClN3Ni: C, 57.28; H, 4.52; N, 11.79.
Found: C, 57.22; H, 4.25; N, 11.69.
n-hexane to precipitate the product, which was then washed several
times with n-hexane to obtain a pure product of 4a. Yield: 0.39 g, 68%.
1
3
Synthesis of (Et2NNNQ)NiCl (2b). This compound was synthe-
sized following a procedure similar to the synthesis of 2a, employing
Mp: 218−220 °C. H NMR (500 MHz, CDCl3): δ 8.10 (d, JH−H =
7.9 Hz, 1H, Ar-H), 7.51 (br s, 1H, Ar-H), 7.43 (d, 3JH−H = 7.9 Hz, 1H,
Ar-H), 7.34−7.28 (m, 2H, Ar-H), 7.23 (br s, 1H, Ar-H), 7.10−7.08
(
Et2NNNQ)H (1b; 0.50 g, 1.716 mmol), (DME)NiCl2 (0.452 g, 2.057
3
3
(m, 2H, Ar-H), 6.83 (d, JH−H = 6.7 Hz, 1H, Ar-H), 6.59 (vt, JH−H
=
mmol), and Et3N (0.48 mL, 3.444 mmol) in THF (20 mL). Yield:
7.1 Hz, 1H, Ar-H), 2.96 (s, 6H, CH3), 2.09 (s, 3H, COCH3). 13C
NMR (125 MHz, CDCl3): δ 179.1 (CO), 147.9 (Cq), 147.4 (CH),
147.2 (Cq), 147.0 (Cq), 146.7 (Cq), 138.5 (CH), 129.9 (Cq), 129.5
(CH), 128.8 (CH), 121.1 (CH), 120.1 (CH), 116.9 (CH), 115.2
(CH), 112.5 (CH), 111.0 (CH), 50.5 (2C, CH3), 24.9 (COCH3).
1
0.58 g, 88%. Mp: 195−198 °C. H NMR (500 MHz, CDCl3): δ 8.58
3
4
3
(dd, JH−H = 5.2, JH−H = 1.2 Hz, 1H, Ar-H), 8.10 (dd, JH−H = 8.2,
4JH−H = 1.5 Hz, 1H, Ar-H), 7.53 (dd, 3JH−H = 8.2, 4JH−H = 0.9 Hz, 1H,
Ar-H), 7.40 (dd, JH−H = 7.3, JH−H = 0.6 Hz, 1H, Ar-H), 7.33 (vt,
3JH−H = 7.9 Hz, 1H, Ar-H), 7.23 (dd, 3JH−H = 8.2, 3JH−H = 5.2 Hz, 1H,
3
4
+
3
4
HRMS (ESI): m/z calcd for C19H19N3O2Ni − OC(O)CH3 [M −
Ar-H), 7.13 (td, JH−H = 7.8, JH−H = 1.2 Hz, 1H, Ar-H), 6.98 (dd,
OC(O)CH3]+ 320.0692, found 320.0692. Anal. Calcd for
C19H19N3O2Ni: C, 60.04; H, 5.04; N, 11.06. Found: C, 60.25; H,
4.52; N, 10.36.
3JH−H = 7.9, JH−H = 1.2 Hz, 1H, Ar-H), 6.86 (d, JH−H = 7.6 Hz, 1H,
4
3
3
4
Ar-H), 6.64 (td, JH−H = 7.4, JH−H = 0.9 Hz, 1H, Ar-H), 3.50−3.43
(m, 2H, CH2), 2.47−2.40 (m, 2H, CH2), 2.20 (t, 3JH−H = 7.0 Hz, 6H,
CH3). 13C NMR (100 MHz, CDCl3): δ 150.8 (Cq), 150.2 (CH), 148.2
(Cq), 147.2 (Cq), 141.7 (Cq), 138.4 (CH), 129.8 (Cq), 129.3 (CH),
128.5 (CH), 120.9 (CH), 120.3 (CH), 117.5 (CH), 114.7 (CH),
112.5 (CH), 110.4 (CH), 57.9 (2C, CH2), 13.3 (2C, CH3). Anal.
Calcd for C19H20ClN3Ni: C, 59.35; H, 5.24; N, 10.93. Found: C,
59.38; H, 4.92; N, 10.38.
Representative Procedure for Alkylation of Azoles: Syn-
thesis of 2-n-Octylbenzo[d]thiazole (7aa). In a flame-dried screw-
cap tube equipped with a magnetic stir bar were introduced catalyst 2a
(0.009 g, 0.025 mmol), LiOtBu (0.081 g, 1.012 mmol), benzothiazole
(5a; 0.068 g, 0.503 mmol), and 1-iodooctane (6a; 0.180 g, 0.750
mmol) inside the glovebox. To the above reaction mixture was added
1,4-dioxane (1.0 mL) under an argon atmosphere, and the resultant
reaction mixture was stirred at 100 °C in a preheated oil bath for 16 h.
At ambient temperature, the reaction mixture was quenched with
distilled water (5 mL) and neutralized with 2 N HCl (0.5 mL). The
crude product was then extracted with ethyl acetate (15 mL × 3) and
the combined organic extract was washed with distilled water (15 mL
× 3). The organic extract was dried over Na2SO4, and the volatiles
were evaporated in vacuo. The remaining residue was purified by
column chromatography on silica gel (n-hexane/EtOAc 100/1) to
yield the compound 7aa as a yellow liquid.
Synthesis of (Me2NNNQ)NiBr (3a). A solution of (Me2NNNQ)H
(1a; 0.35 g, 1.329 mmol) in THF (10 mL) was added dropwise to a
suspension of (THF)2NiBr2 (0.53 g, 1.461 mmol) in THF (20 mL),
and Et3N (0.37 mL, 2.655 mmol) was added to this mixture. The
reaction mixture was stirred at room temperature for 24 h. The
reaction mixture was filtered through a cannula, and the filtrate was
evaporated to dryness under vacuum. The resultant crude product was
washed several times with n-hexane to give compound 3a as an orange
solid. Compound 3a was recrystallized from n-hexane solution by slow
evaporation to obtain X-ray-quality single crystals. Yield: 0.304 g, 57%.
Mp: 147−150 °C. 1H NMR (500 MHz, CDCl3): δ 8.93 (br s, 1H, Ar-
Note: when alkyl bromides and alkyl chlorides were employed as
coupling partners, NaI (1.5 equiv, i.e. same amount as for alkyl
bromide or alkyl chloride) was added to the reaction mixture and the
reactions were carried out at 120 °C.
3
H), 8.05 (d, 3JH−3H = 7.6 Hz, 1H, Ar-H), 7.55 (d, JH−H = 7.6 Hz, 1H,
3
Ar-H), 7.40 (d, JH−H = 6.6 Hz, 1H, Ar-H), 7.34 (vt, JH−H = 7.6 Hz,
1H, Ar-H), 7.19 (br s, 1H, Ar-H), 7.12−7.06 (m, 2H, Ar-H), 6.86 (d,
3
3JH−H = 7.3 Hz, 1H, Ar-H), 6.60 (vt, JH−H = 6.8 Hz, 1H, Ar-H), 3.20
ASSOCIATED CONTENT
(s, 6H, CH3). 13C NMR (125 MHz, CDCl3): δ 153.2 (CH), 147.7
(Cq), 146.9 (Cq), 146.8 (Cq), 146.7 (Cq), 138.6 (CH), 129.8 (Cq),
129.3 (CH), 128.6 (CH), 121.6 (CH), 120.6 (CH), 117.4 (CH),
115.2 (CH), 113.2 (CH), 110.9 (CH), 52.8 (2C, CH3). HRMS (ESI):
m/z calcd for C17H16BrN3Ni − Br+ [M − Br]+ 320.0692, found
320.0688. Anal. Calcd for C17H16BrN3Ni: C, 50.93; H, 4.02; N, 10.48.
Found: C, 49.42; H, 4.15; N, 10.29.
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00201.
Detailed experimental procedures, analytical data for the
1
compounds, and H and 13C NMR spectra of pincer
Synthesis of (Et2NNNQ)NiBr (3b). This compound was synthe-
sized following a procedure similar to the synthesis of 2a, employing
precursors, nickel complexes, and alkylated compounds
(PDF)
Crystallographic data for 2a (CCDC-1457902) (CIF)
Crystallographic data for 3a (CCDC-1457903) (CIF)
Crystallographic data for 3b (CCDC-1457904) (CIF)
(
Et2NNNQ)H (0.10 g, 0.343 mmol), (THF)2NiBr2 (0.137 g, 0.377
mmol), and Et3N (0.096 mL, 0.686 mmol) in THF (20 mL). Yield:
0.124 g, 84%. Mp: 188−191 °C. 1H NMR (400 MHz, CDCl3): δ 8.91
3
4
3
(dd, JH−H = 5.4, JH−H = 1.5 Hz, 1H, Ar-H), 8.09 (dd, JH−H = 8.2,
4JH−H = 1.5 Hz, 1H, Ar-H), 7.57 (dd, 3JH−H = 8.2, 4JH−H = 1.0 Hz, 1H,
Ar-H), 7.45 (dd, JH−H = 7.9, JH−H = 1.0 Hz, 1H, Ar-H), 7.33 (vt,
3JH−H = 7.9 Hz, 1H, Ar-H), 7.20 (dd, 3JH−H = 8.3, 3JH−H = 5.4 Hz, 1H,
3
4
AUTHOR INFORMATION
3
4
■
Ar-H), 7.13 (td, JH−H = 7.7, JH−H = 1.2 Hz, 1H, Ar-H), 6.97 (dd,
3JH−H = 7.9, JH−H = 1.2 Hz, 1H, Ar-H), 6.86 (d, JH−H = 8.1 Hz, 1H,
4
3
Corresponding Author
3
4
Ar-H), 6.65 (td, JH−H = 8.1, JH−H = 1.2 Hz, 1H, Ar-H), 3.66−3.57
(m, 2H, CH2), 2.57−2.49 (m, 2H, CH2), 2.19 (t, 3JH−H = 7.1 Hz, 6H,
CH3). 13C NMR (100 MHz, CDCl3): δ 153.1 (CH), 150.9 (Cq), 148.4
(Cq), 147.2 (Cq), 141.6 (Cq), 138.5 (CH), 129.9 (Cq), 129.3 (CH),
128.6 (CH), 121.3 (CH), 120.5 (CH), 117.7 (CH), 114.7 (CH),
112.7 (CH), 110.4 (CH), 58.9 (2C, CH2), 13.5 (2C, CH3). Anal.
Calcd for C19H20BrN3Ni: C, 53.20; H, 4.70; N, 9.80. Found: C, 52.65;
H, 4.38; N, 9.54.
*B.P.: tel, + 91-20-2590 2733; fax, + 91-20-2590 2621; e-mail,
b.punji@ncl.res.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the SERB, New Delhi,
India (SR/S1/IC-42/2012). We are thankful to the Alexander
von Humboldt Foundation of Germany for an equipment
grant. U.N.P. and D.K.P. thank UGC-New Delhi for research
fellowships. We are grateful to Dr. P. R. Rajmohanan for NMR
facilities, Dr. (Mrs.) Shanthakumari for HRMS analyses, and
Dr. S. P. Borikar for GC-MS analyses.
Synthesis of (Me2NNNQ)Ni(OAc) (4a). A solution of (Me2NNNQ)
H (1a; 0.40 g, 1.519 mmol) in THF (20 mL) was added dropwise to a
mixture of Ni(OAc)2 (0.295 g, 1.671 mmol) and Et3N (0.42 mL, 3.013
mmol) in THF (15 mL). The reaction mixture was stirred at room
temperature for 24 h under an argon atmosphere. The reaction
mixture was then filtered through a cannula, and the filtrate was
concentrated under vacuum. To the concentrated solution was added
H
DOI: 10.1021/acs.organomet.6b00201
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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DOI: 10.1021/acs.organomet.6b00201
Organometallics XXXX, XXX, XXX−XXX