Reactivity of (NNN)-pincer nickel(II) aryl complex towards oxidative carbon-heteroatom bond formation

Article abstract of DOI:10.1016/j.tet.2018.04.029

The reactivity of an (NNN)-Ni(II) aryl complex towards C–X bond formation upon exposure to a panel of one- and two-electron oxidants is reported. High selectivity for C(sp²)–N bond formation is observed in all cases, except under conditions when C–C bond formation is accessible. Preliminary mechanistic investigations indicate access to either a Ni(III) or Ni(IV) intermediate dependent on oxidant used and that C–N bond formation is more efficient via Ni(IV) pathway due to geometry of reactive species.

Full text of DOI:10.1016/j.tet.2018.04.029

Accepted Manuscript
Reactivity of (NNN)-pincer nickel(II) aryl complex towards oxidative carbon-
heteroatom bond formation
Felipe C. Sousa e Silva, Brandon J. Bloomer, Sarah E. Wengryniuk
PII:
S0040-4020(18)30408-3
10.1016/j.tet.2018.04.029
TET 29444
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 16 January 2018
Revised Date: 10 April 2018
Accepted Date: 11 April 2018
Please cite this article as: Sousa e Silva FC, Bloomer BJ, Wengryniuk SE, Reactivity of (NNN)-pincer
nickel(II) aryl complex towards oxidative carbon-heteroatom bond formation, Tetrahedron (2018), doi:
10.1016/j.tet.2018.04.029.
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Reactivity of (NNN)-pincer Ni(II) aryl
complex towards oxidative carbon-
heteroatom bond formation
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Felipe C. Sousa e Silva, Brandon J. Bloomer, Sarah E. Wengryniuk*
th
Temple University, Department of Chemistry, 1901 North 13 Street, Philadelphia, PA 19122, USA

1
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Tetrahedron
journal homepage: www.elsevier.com
Reactivity of (NNN)-pincer nickel(II) aryl complex towards oxidative carbon-
heteroatom bond formation
a
b
a*∗
Felipe C. Sousa e Silva , Brandon J. Bloomer , and Sarah E. Wengryniuk
a
th
Temple University, Department of Chemistry, 1901 North 13 Street, Philadelphia, PA, 19122, USA
b
Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, USA
ARTICLE INFO
ABSTRACT
Article history:
Received
The reactivity of an (NNN)-Ni(II) aryl complex towards C–X bond formation upon exposure to a
panel of one- and two-electron oxidants is reported. High selectivity for C(sp )–N bond
2
Received in revised form
Accepted
Available online
formation is observed in all cases, except under conditions when C–C bond formation is
accessible. Preliminary mechanistic investigations indicate access to either a Ni(III) or Ni(IV)
intermediate dependent on oxidant used and that C–N bond formation is more efficient via
Ni(IV) pathway due to geometry of reactive species.
Keywords:
nickel
2
009 Elsevier Ltd. All rights reserved.
pincer
high-valent
carbon-heteroatom
oxidative
carbon-nitrogen
∗
Corresponding author. Tel.: +1-215-204-0360; e-mail: sarahw@temple.edu

2
1
. Introduction
Over the last decade, the field of high-valent metal catalysis
ACCEPTED MANUSCRIPT
has emerged as a new and powerful tool in organic synthesis. In
particular, high oxidation state metal centers can facilitate
challenging C–X bond forming reductive eliminations, including
carbon-halogen, carbon-oxygen, and carbon-nitrogen bond
1,2
formation. While palladium catalysis has played a central role
in this field over the last 20 years, there has been a growing
interest in C–X bond forming reactions from high-valent nickel
3-8
intermediates. The development of analogous nickel-catalyzed
methods would be particularly attractive as nickel is more
economical and sustainable, however significant challenges still
9
remain. Firstly, nickel can undergo facile one- or two-electron
redox events allowing access to both Ni(II)/Ni(III) or
Ni(II)/Ni(IV) manifolds, hindering reaction development due to
5,6
limited mechanistic understanding. Second, the relatively low
stability of high-valent nickel centers makes the design of ligand
scaffolds capable of stabilizing these intermediates and
facilitating efficient catalyst turnover challenging, particularly at
10
the Ni(IV) oxidation state. Thus, there are continued efforts
towards the design, synthesis and characterization of high-valent
nickel complexes to probe their reactivity towards C–X bond
formation, providing key insights for the development of
11-13
catalytic manifolds.
In this area, seminal work from the Hillhouse group has
shown that cyclic Ni(II) complexes possessing alkyl and amido
3
ligands undergo facile C(sp )–N reductive elimination upon
exposure to one-electron oxidants including O2, I , or
2
+
14-16
[
(AcCp)Fe] , via a Ni(III) intermediate.
Recent mechanistic
investigations from the Sanford group have reported the synthesis
and isolation of several stable Ni(II) precursors and the study of
their reactivity upon exposure to a range of one- and two-electron
Scheme 1. A. Oxidative C–Br bond formation via proposed Ni(III)
intermediate B. Preferential oxidative C–C coupling along with trace C–Br
and C–N products from bis(phpy)Ni(II) complex. C. Selective C–C coupling
from Ni(IV) intermediate D. First example of catalytic C–N bond formation
via proposed Ni(III) intermediate.
3,5,10,17,18
oxidants under stoichiometric conditions.
In 2009 they
reported that (phpy)(picoline)Ni(II)Br complex 1 (phpy=2-
phenylpyridine) underwent efficient C–Br bond formation upon
In an effort to explore new ligand scaffolds that could enable
catalytic, oxidative C–X bond formation via either Ni(III) or
Ni(IV), we were drawn to the amido bis(amine) pincer Ni-
complexes recently developed by Hu (17, 18, Scheme 2). By
replacing the phosphorous atoms of established PNP-pincer
ligands with nitrogens, Hu aimed to increase the relative
treatment with either Br2 or CuBr , via a proposed Ni(III)
intermediate (Scheme 1A). Subsequently, bis(phpy)Ni(II)
2
3
species 7, was found to undergo preferential C–C bond formation
to give 4 along with trace C–Br (3) and C–N (6) products upon
oxidation with N-bromosuccinimide (Scheme 1B). Although no
high-valent nickel species were characterized, an analogous
Pd(IV) species gave a similar product distribution, suggesting the
“
hardness” in order to better stabilize high-oxidation state metal
4
,19
complexes. To date Hu has successfully utilized (NNN)-NiCl
possible intermediacy of a Ni(IV) species.
In a recent high
3
II
complex 17 for a wide variety of nickel-catalyzed C(sp ) Kumada
profile report, Sanford employed [(bpy)Ni (CH CMe -o-C H )]
2
2
6
4
21-31
–
couplings.
A detailed mechanistic investigation led to the
complex 7 and found that upon oxidation by several 2e oxidants
10-13), 7 underwent selective C–C bond formation with no
evidence of competitive C–X reductive elimination products.
Significantly, use of more stabilizing tridentate tris(2-
conclusion that the Kumada coupling proceeds via a Ni(III)
(
5
intermediate (19), accessed via radical oxidation of the Ni(II)
2
9
center by an aryl iodide after transmetallation (Scheme 2B).
a
However, despite its utility in catalytic C–C bond formation,
there have been no reports examining the viability of the (NNN)-
pincer scaffold to enable analogous C–X bond forming processes
via high-valent nickel intermediates. Herein, we report a study of
the reactivity of (NNN)-NiPh complex 18 upon treatment with a
range of both one and two-electron oxidants (Scheme 2C). Our
pyridyl)methane (tppp) ligand system enabled the isolation and
full characterization of key Ni(IV) intermediates for the first
time, suggesting the feasibility of this redox manifold for C–X
bond formation. Subsequent studies from the Sanford lab have
utilized 7, the (tppp)-ligand system and a closely related
trispyrazolylborate scaffold to examine a range of C–X and C–C
bond forming processes, providing key mechanistic insights
2
studies have found that 18 will undergo preferential C(sp )–N
5,10,17,18
bond formation over competitive carbon-oxygen or carbon-
halogen pathways, however carbon-carbon bond formation will
predominate upon exposure to an appropriate oxidant.
Preliminary mechanistic studies indicate the role of either
Ni(II)/Ni(III) or Ni(II)/Ni(IV) redox manifolds depending on the
oxidant used and our results indicate that C–N bond formation is
more efficient through the proposed Ni(IV) pathway. The
findings indicate the viability C–N bond reductive elimination
from high-valent nickel complexes, most notably Ni(IV), and
reveal the significant role that oxidant selection plays in reaction
towards the feasibility of catalytic manifolds.
Ritter has
reported an oxidative radiofluorination method employing
stoichiometric Ni(II)–aryl complexes, which upon exposure to a
hypervalent iodine oxidant and nucleophilic fluoride, underwent
2
smooth C(sp )–F reductive elimination, presumably via a Ni(IV)
8
intermediate. The Muniz group reported the first example of
3
catalytic C(sp )–N bond formation from a proposed Ni(III)
intermediate in an approach to oxidative olefin diamination
20
(Scheme 1D).

3
development, providing valuable insights for
development.
A
f
C
urt
C
he
E
r
P
ca
T
ta
E
ly
D
tic MAre
N
po
U
rte
S
d
C
f
R
orIP
C–N bond forming reductive eliminations
T
2
,12-14,18
previously.
These results indicate that in our system, a
potential Ni(II)/Ni(IV) pathway was more efficient at promoting
of C–N bond formation (see later for further discussion).
Table 1. Oxidation of 18 with both one- and two-electron
oxidants
a
Product
Distribution
c
Entry
Conditions
b
Yield 23 (%)
1
2
3
4
5
6
(tolyl)I(OAc)
(tolyl)ICl
[PhI(Py) ]2OTf
2
23 only
23 only
23 only
23 only
23 only
23 only
94
99
99
98
96
98
2
2
NCS
NBS
TDTT
1
:0.42
c
7
8
Fe(Cp
2
)PF
6
--
Scheme 2. A. (NNN)-pincer Ni(II) complexes developed by Hu. B. Use of 17
for alkyl-alkyl Kumada coupling via Ni(III) intermediate. C. This work:
examining the reactivity of 18 towards oxidative C–X bond formation.
(23:18)
1
:0.91
CuCl
2
52
(23:17)
2
. Results and Discussion
a
Reactions were carried out in dry THF with exception of the entry 8:
b
1
THF/ACN 1:1 v/v. Initial product distributions determined via H-NMR of
crude reaction mixtures. Isolated yield of 23 determined after column
chromatography. Isolated yield of 23 not obtained as remaining 18 degrades
to 23 upon purification.
Our studies focused on the reactivity of (NNN)-NiPh complex
8, which was readily prepared according to the procedure
c
d
1
32
reported by Hu. We hypothesized that, depending on the nature
of the oxidant, this could give rise to either a Ni(III) (20) or
Ni(IV) (21) intermediate, stabilized by the strong σ-donor
nitrogen-ligands (Scheme 2C). These intermediates could then
either undergo reductive C–X bond formation, where X would
arise from the oxidant, to give 22, or reductive C–N bond
formation onto the ligand scaffold (23). Significantly, complex
Having established the tendency of 18 to undergo highly
selective C–N over competitive C–X pathways, we wished to
examine C–N vs. C–C coupling (Scheme 3). C–C bond
formation has been established as the major pathway in prior
studies of high-valent nickel species
Hu’s studies have revealed a Ni(III) intermediate for his highly
efficient Kumada coupling using 17. We would therefore
predict C–C coupling to predominate over C–N coupling upon
exposure to 2e oxidant such as an aryl halide, aryldiazonium, or
3–5,10,17,18
(vide supra) and
2
3 does not have the ability to undergo a competitive C–C bond
29
reductive elimination, a pathway found to outcompete C–X bond
formation in previous studies of high-valent nickel
intermediates.
–
3-5,8,10,17,18
18
diaryliodonium salt.
diaryliodonium salt 24 gave biaryl 25 as the major product in
30% yield, along with 9% yield of C–N product 23, with
In fact, treatment of 18 with
–
We began by exposing 18 to a panel of well-established 2e
3
electron oxidants, including λ -iodanes (tolyl)I(OAc)2,
3
9
(tolyl)ICl , and [PhI(Py) ]2OTf, as well as NCS and TDTT
remaining mass balance accounted for by free (NNN)-ligand.
2
2
3
-5,8,33,34
(
Table 1, entires 1-6).
In each case we observed exclusive
While the C–C coupling proceeded in only moderate yield, this
result supports the previous observations that in high-valent
nickel intermediates where C–X vs. C–C coupling pathways are
competitive, C–C coupling is preferred and thus this should be
avoided for the design of future ligand scaffolds for C–X bond
formation.
formation of 23 in near quantitative yield, the product of clean
C(sp )–N bond formation. Surprisingly, no products arising
from competitive C–X reductive elimination onto the arene were
observed (i.e. 22) either by GCMS or H-NMR of the crude
reaction mixtures. While there are reports of C(sp )–N bond
formation via Ni(III),
been reported via Pd(II)/Pd(IV), to the best of our knowledge
there exists only one report of C(sp )–N reductive elimination
from a high-valent nickel species, and our finding is in contrast
to Sanford’s observation in which only trace C–N bond
formation was observed upon oxidation of 5 with NBS (see
Scheme 1B). We then examined both outer-sphere ((FeCp )PF )
and inner-sphere (CuCl ) 1e oxidants (entries 7, 8), both of
which proved less efficient at promoting C–N bond formation
than the 2e reagents. (FeCp )PF gave lower conversion to 23,
with a 1:0.42 ratio of 23 with starting complex 18 (entry 7). In
contrast, CuCl gave full consumption of starting complex 18,
2
35
1
36
3
14-16,20
2
and C(sp )–N bond formation has
37
2
3
8
4
2
6
–
Scheme 3. Preferential C–C coupling, leading to biaryl formation upon
oxidation with diaryliodonium salt 24.
2
–
At this stage we wished to probe the mechanistic
2
6
–
underpinnings for the drop in C–N bond efficiency between 2e
oxidants (Table 1, entries 1-7) and 1e oxidant (CuCl
–
, entry 8),
2
2
however produced a 1.0:0.91 ratio of 23 to (NNN)-NiCl complex
with the hypothesis that this may result from a switch in reaction
manifold from Ni(II)/Ni(IV) to Ni(II)/Ni(III). The differential
pulse voltammogram of 18 revealed two oxidative peaks, at
1
7. We were intrigued by this drop in reaction efficiency upon
use of one-electron oxidants as Ni(II)/Ni(III) pathways had been

4
+
+
0.75V and +1.39V versus Ag/Ag , which
A
we
C
a
C
ss
E
ig
P
n
T
to
E
t
D
he MAgi
N
ve
U
s
S
a
C
N
R
i(III) species (20) which must undergo an
I
P
T
Ni(II)/(III) and Ni(III)/(IV) redox couple, respectively. Both
oxidations display quasi-reversible behavior, however complex
degradation appears much more significant upon oxidation to
Ni(IV) (see Supporting Information for details and further
discussion). We then attempted the use of various additives to
probe potential one-electron pathways, unfortunately, addition of
TEMPO or BHT resulted in no change in reaction outcome and
no detectable formation of adducts in the case of any of the
isomerization to access a species (26) capable of C–N bond
formation. However, prior to isomerization, 20 can undergo
homolysis of the labile Ni–Ar bond, giving rise to Ni(II)–Cl
species 17 and an aryl radical gives benzene after H-atom
abstraction, presumably from THF solvent. We believe these
findings could provide valuable insights to address issues of
reaction efficiency and selectivity in the continued development
of nickel-mediated oxidative C–X bond formations.
44
18
oxidants screened. Furthermore, attempts to isolate or observe
any high-valent nickel intermediates by NMR were unsuccessful
due to instability and rapid rate of C–N bond formation from
these species even at low temperature. We therefore attempted to
gain insights into the reaction pathways by altering the
equivalents of respective oxidants added (Scheme 4). In order to
confirm the CuCl result, we turned again to (tolyl)ICl , which is
2
2
–
commonly viewed as a 2e oxidant, however use of 0.5 equiv.
can result in each equivalent performing two 1e oxidations, as
–
demonstrated by Nocera in the oxidation of a (dppe)Ni(II)Cl
2
40
complex to (dppe)Ni(III)Cl3. Treatment of 18 with 0.5 equiv. of
tolyl)ICl resulted in complete consumption of starting material
(
2
1
and analysis of the H-NMR of the crude reaction revealed a
1
4
1
.0:1.01 ratio of C–N product 23 to (NNN)-NiCl (17) (Scheme
A). This result is significantly different than the oxidation with
41
equiv. (Table 1, entry 2) and analogous to the results obtained
–
with CuCl . Next, to probe the 2e oxidation, we treated 18 with
2
0
1
1
.5 equiv. of [PhI(Py) ]2OTf and in this case we observed a
Scheme 5. Role of complex geometry on efficiency of C–N bond formation.
Detected via GC analysis of the crude reaction mixture under conditions
shown in Scheme 4A. See Supporting Information for details.
2
a
.03:1.0 ratio of C–N product (23) to unreacted starting complex
42
8 (Scheme 4B). This result would indicate that each equivalent
–
of [PhI(Py) ]2OTf is performing a net 2e oxidation of a single
Ni(II) center to Ni(IV) as 1 equiv. gave near quantitative yield of
2
supportive of divergent reaction pathways dependent on the
oxidation state of the reactive nickel species and provide
preliminary evidence for a Ni(II)/Ni(IV) redox couple being
more efficient for the formation of C–N bond products.
2
3. Conclusion
3. While the above results are not definitive evidence, they are
In conclusion, we report the reactivity of an (NNN)-NiPh
complex towards oxidative C–X bond formation. Our studies
2
have shown that this complex undergoes highly selective C(sp )–
N bond formation over competitive carbon-halogen or carbon-
oxygen reductive elimination pathways, resulting in
functionalization of the ligand scaffold. Alternatively, (NNN)-
NiPh will undergo preferential C–C bond formation upon
treatment with a diaryliodonium salt, in line with previous reports
on the reactivity of high-valent nickel complexes. Preliminary
mechanistic studies indicate that C–N bond formation is more
facile via a Ni(II)/Ni(IV) pathway than Ni(II)/Ni(III), which we
hypothesize is due to the differences in geometries of the
resultant high-valent complexes. This transformation represents
2
one of the first examples of C(sp )–N reductive elimination from
a proposed Ni(IV) intermediate and we believe that these
findings could aide in the continued pursuit of catalytic C–X
bond forming manifolds via high-valent nickel intermediates.
4
. Experimental section
Scheme 4. Mechanistic probe into the efficiency of Ni(III) vs. Ni(IV)
intermediates in C–N bond formation. Ratio confirmed by isolated yields
after chromatography. 17 isolated as free (NNN)-ligand. See SI for details.
a
4
.1. General Information
1
13
H and C NMR spectra were recorded at 500 MHz and 125
We hypothesize that the divergence in C–N bond efficiency
between the proposed Ni(IV) and Ni(III) pathways can be
explained based on the geometries of the two nickel species upon
oxidation (Scheme 5). Due to the relative instability of the high-
valent nickel species, the initial geometry obtained upon
oxidation could play a central role in the favored reaction
MHz on a Bruker Advance 500 or 400 MHz and 100 MHz on a
1
Bruker Advance 400. H NMR chemical shifts were reported in
part per million (ppm) from the solvent resonance (CDCl 7.26
3
ppm, C D6 = 7.16 ppm). The data was reported as follows:
6
chemical shift number, multiplicity (s = singlet, d = doublet, t =
triplet, sept = septet, dd = doublet of doublets, td = triplet of
–
pathway. Treatment with a 2e oxidant results in cis-addition of
5
doublets, m = multiplet). Proton decoupled attached proton test
two X-groups to the nickel center to give 21, forcing the phenyl
13
(
APT) C NMR shifts were reported in ppm from the solvent
group into an axial position, cis to the nitrogen, and facilitating
43
–
resonance (CDCl 77.16 ppm). The reaction solvents used were
3
rapid C–N bond reductive elimination. In contrast, 1e
anhydrous (HPLC-grade solvent passed through an activated-
alumina column). All other reagents were used without further
oxidation

5
purification. [(Py) IPh]2OTf and [Tol I]OTf
A
we
C
re
C
s
E
yn
P
th
T
es
E
iz
D
ed MAob
N
ta
U
ine
S
d
C
fo
R
llo
I
wing purification by flash chromatography (5%
P
T
2
2
46,47
according to the literature,
and used without further
EtOAc/Pentane).
purification. Flash chromatography was carried out using Sorbent
Technologies silica gel 60 6 (40 63 pm) in the solvent system
—
4.3.1. Compound 23
1
listed in the individual experiments. The reactions were
monitored using analytical thin-layer chromatography (TLC) on
Merck silica gel (60 F2s4) plates. Accurate mass spectra were
recorded on an Agilent 6520 Accurate-Mass Q-TOF LC/MS. All
reactions were carried out inside glovebox unless specified.
Isolated as a pale yellow crystalline solid. H NMR (500
MHz, CDCl ): δ 7.11 – 7.07 (m, 2H), 7.05 – 7.01 (m, 4H), 6.97 –
3
6.94 (m, 2H), 6.88 – 6.82 (m, 3H), 6.81 – 6.77 (m, 2H), 2.45 (s,
13
12H). C NMR (125 MHz, CDCl ): δ 148.04, 138.86, 128.02,
3
127.35, 123.98, 121.65, 120.12, 119.38. HRMS (ESI) m/z:
+
+
C H N [(M + H) ] calcd: 332.2127, found: 332.2122. X-ray
22
25
3
4
.2. Synthesis of (NNN)-NiPh Complex (18)
crystallographic data for 23 is provided in Supporting
4
.2.1. Synthesis of Bis[dimethylamino)phenyl]amine
Information (SI-12)
.
M e
(
H
NN )(SI-1)
2
Inside a glovebox, a 250 mL reaction vessel was charged
4
.4. C–C bond formation: Oxidation with diaryliodonium salt 24:
Inside a glovebox, (NNN)-NiPh (15.0 mg, 0.04 mmol) was
with Pd (dba)3 (1.36 g, 1.49 mmol), bis(diphenylphosphino)-
2
ferrocene (DPPF) (1.65 g, 2.97 mmol), NaOt-Bu (9.84 g, 0.098
mol) and toluene (100 mL). 2-Bromo-N,N-dimethylaniline (14.6
added to a 20 mL scintillation vial and dissolved in a 1:1 mixture
of THF/CH CN (1 mL total volume, 0.04 M). [Tol I]OTf was
added in one portion and the solution was then stirred overnight
at room temperature. The vial was brought outside the glovebox
and the solvent was removed in vacuo. The reaction was purified
by flash chromatography (5% EtOAc/Pentane) to yield 25 (2.0
40
g, 0.073 mol) and 2-amino-N,N-dimethylaniline (9.95 g, 0.073
mol) were degassed and added to the reaction mixture. The
brown solution was then stirred for 3 days at 100 °C. The
solution was then cooled to room temperature and filtered
through Celite. Removal of the solvent yielded a dark liquid that
was taken in dichloromethane (20 mL) and filtered through a
silica plug. Removal of the solvent gave SI-1 as a brown solid
3
2
mg, 0.012 mmol, 29%), 23 (1.6 mg, 0.004 mmol, 9.2%), and
Me
H
NN (27) (5.4 mg, 0.024 mmol, 60%). Spectral data for 25
2
45
(12.0 g, 0.048 mol, 66%) that was used without further
was consistent with that reported in the literature.
purification. Spectral data was consistent with that reported in the
48
literature.
.2.2. Compound 17
In a flame dried flask, under Ar, H NN (SI-1) (0.50 g, 2.0
4
4
.5. Mechanistic Investigation
4
Me
.5.1. Oxidation with 0.5 equiv. of (tolyl)ICl2
2
mmol) was dissolved in THF (6 mL) and the solution cooled to –
8 °C. n-BuLi (0.80 mL, 2.1 mmol, 2.5 M in hexanes) was added
to the solution dropwise. The solution was allowed to warm
naturally and stir for 1 h at room temperature. The resultant
solution was then transferred to a THF (6 mL) suspension of
Inside a glovebox, (NNN)-NiPh (18) (10 mg, 0.025 mmol, 1
7
equiv.) and (tolyl)ICl (3.6 mg, 0.0125 mmol, 0.5 equiv.) were
2
added to a 20 mL scintillation vial. THF (1 mL, 0.025 M) was
added and the mixture was allowed to stir overnight at room
temperature. The solvent was removed in vacuo inside the glove
NiCl (dme) (0.44 g, 2.0 mmol) at –78 °C. The reaction was
2
box. Still inside the glovebox, the crude mixture was dissolved in
allowed to warm naturally and was left stirring overnight at room
1
26
C
6
D
6
, filtered and added to a NMR tube. Analysis of the H-
temperature . The solvent was then removed and the dark brown
oil was taken in benzene and filtered through celite. The brown
oil was purified by recrystallization (layer diffusion of pentane in
benzene at room temperature) to give (NNN)-NiCl (17) as a
brown solid (0.52 g, 1.5 mmol, 74%). Spectral data was
NMR spectra of the crude reaction mixture revealed 23:17 in a
.0:0.91 ratio, along with trace impurities relating to (tolyl)ICl2
see Supporting Information for full details).
1
(
32
4.5.2. Oxidation with 0.5 equiv. of [PhI(Py) ]2OTf
2
consistent with that reported in the literature.
.2.3. Compound 18
Inside a glovebox, PhMgBr (0.72 mL, 1 M solution in THF)
4
Inside a glovebox, (NNN)-NiPh (18) (10 mg, 0.025 mmol, 1
equiv.) and [PhI(Py) ]2OTf (8.3 mg, 0.0125 mmol, 0.5 equiv.)
2
was added dropwise to a THF (5 mL) solution of (NNN)-NiCl
17) (0.25 g, 0.72 mmol) at –78 °C. The resultant solution was
were added to a 20 mL scintillation vial. THF (1 mL, 0.025 M)
was added and the mixture was allowed to stir overnight at room
temperature. The solvent was removed in vacuo inside the glove
box. Still inside the glovebox, the crude mixture was dissolved in
(
allowed to warm naturally to room temperature and stirred
overnight. The solvent was removed in vacuo and the dark
residue was then dissolved in benzene (30 mL) and filtered
through Celite. The solvent was removed in vacuo to afford 18 as
a yellow solid, which was purified by recrystallization (layer
diffusion of pentane in THF) to give 18 (0.17 g, 0.45, 62%).
1
C D , filtered and added to a NMR tube. Analysis of the H-
6
6
NMR spectra of the crude reaction mixture revealed only 23:17
in a 1.03:1.0 ratio, along with trace impurities relating to
[PhI(Py) ] 2OTf (see Supporting Information for full details).
2
32
Spectral data was consistent with that reported in the literature.
.3. General procedure for oxidation of (NNN)-NiPh 18:
Inside a glovebox, (NNN)-NiPh (15.0 mg, 0.04 mmol, 1
4
Acknowledgments
The authors would like to acknowledge the Donors of the
American Chemical Society Petroleum Research Fund for partial
support of this research (DNI-56603) as well the Temple
University Department of Chemistry. B.J.B. was supported in
part by the Temple University Undergraduate Research Program.
Support for the NMR facility at Temple University by a CURE
grant from the Pennsylvania Department of Health is gratefully
acknowledged. We also thank Dr. Charles DeBrosse (Temple
equiv.) and the appropriate oxidant (1.0-1.2 equiv.) were added to
a 20 mL scintillation vial. THF(1.0 mL) was added to give a 0.04
M solution and the mixture was allowed to stir overnight at room
temperature. The vial was brought outside the glovebox and the
solvent was removed in vacuo. Initial product ratios of 23:22:17
1
were evaluated via analysis of the crude H-NMR spectra (see
Supporting Information for details). Isolated yields of 23 were

6
University) for NMR spectroscopic assistance, the Zdilla group
26. Hu, X., Chem Sci. 2011;2:1867.
2
ACCEPTED MANUSCRIPT
7. Vechorkin, O., Godinat, A., Scopelliti, R.., Hu, X. Angew Chem
Int Ed. 2011;50:11777.
8. Garcia, P.M.P., Di Franco, T., Orsino, A., Ren, P., Hu,X. Org Lett.
(
Temple University) for use of their glovebox and Prof.
Dobereiner (Temple University) for helpful discussions.
2
2
012;14:4286.
29. Breitenfeld, J., Ruiz, J., Wodrich, M.D., Hu, X. J Am Chem Soc.
013;135:12004.
0. Breitenfeld, J., Wodrich, M.D., Hu, X. Organometallics. 2014;33:
708.
Appendix A. Supplementary Material
2
3
3
Supplementary material associated with this manuscript can
be found in the online version at [insert link]. This file contains
more detail on experimental procedures as well as NMR spectra
used for key analyses discussed herein.
5
1. Di Franco, T., Stojanovic, M., Keller, S., Scopelliti, R.., Hu, X.
Helv.Chim. Acta. 2016;99:830.
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2
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