Suzuki-Miyaura Csp2-Csp2Cross-Couplings Employing Nickel(II) Pincer Precatalysts: Mechanistic Investigations

Article abstract of DOI:10.1021/acs.organomet.1c00033

This study investigates the mechanism of Suzuki-Miyaura Csp²-Csp²cross-couplings facilitated by PCP-type nickel(II) pincer complexes. By employing a combination of standard experimental and theoretical methods, it is proposed that a nickel(I)/(III) cycle is involved in this reaction and single-electron pathways likely operate in this transformation.

Full text of DOI:10.1021/acs.organomet.1c00033

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Article
Suzuki−Miyaura Csp2−Csp2 Cross-Couplings Employing Nickel(II)
Pincer Precatalysts: Mechanistic Investigations
Curtis C. Ho, Angus Olding, Rebecca O. Fuller, Allan J. Canty, Nigel T. Lucas, and Alex C. Bissember*
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ABSTRACT: This study investigates the mechanism of Suzuki−
Miyaura Csp2−Csp2 cross-couplings facilitated by PCP-type nickel(II)
pincer complexes. By employing a combination of standard experimental
and theoretical methods, it is proposed that a nickel(I)/(III) cycle is
involved in this reaction and single-electron pathways likely operate in
this transformation.
mechanistic studies to be undertaken in nickel-catalyzed
reactions.8b Although it has been demonstrated that nickel(II)
pincers can promote SM cross-couplings,11 relatively few
studies have explored the underlying mechanisms that
underpin these processes in detail.5h Consistent with
investigations of SM reactions employing other families of
well-defined nickel catalysts,5 the nature of precatalyst
activation and the implication/competence of catalytically
active nickel(I) species are important factors to consider and
explore.3b,4,5 To this end, we sought to investigate the capacity
for PCP-type nickel pincers to promote SM cross-couplings
and better understand how such species may facilitate these
reactions employing a combination of experimental and
theoretical methods.
INTRODUCTION
■
The Suzuki−Miyaura (SM) reaction is a particularly powerful,
efficient, and reliable tool for forming carbon−carbon bonds
that has found an array of applications across organic and
materials synthesis.1 However, with the exception of cross-
couplings featuring Csp3 electrophiles and/or Csp3 nucleo-
philes,2 the majority of SM transformations arguably continue
to rely on palladium catalysis rather than cheaper, more earth
abundant alternatives such as nickel.3 The subtleties of the
mechanisms of nickel-catalyzed SM Csp2−Csp2 couplings are
not as well understood as the analogous palladium-catalyzed
processes.3b,4,5 This may be due to factors including the greater
propensity for fundamental nickel intermediates to speciate
under the reaction conditions, the increased viability of both
heterolytic and homolytic activation pathways involving nickel,
and the inherent paramagnetism of many nickel species that
can complicate reaction monitoring by NMR spectroscopy.4b,6
To improve the efficiency and extend the scope of reactions
within this manifold, detailed investigations of the mechanisms
of nickel-catalyzed SM Csp2−Csp2 couplings are still required.
Nickel pincers are a readily accessible class of complexes that
have found various applications in catalysis.7 In addition to
their ease of preparation, PCP-type nickel pincers such as
molecules 1 and 2 are air- and moisture-stable (Figure 1).8,9
This class of complexes typically also features high thermal
stability,10 which is important in enabling more tractable
RESULTS AND DISCUSSION
■
In preliminary experiments, we demonstrated for the first time
that nickel complexes 1 and 2 can facilitate SM reactions
(Table 1, entries 1−3). While precatalyst 1 was superior to 2,
each of these processes delivered biaryl 5 in moderate yields
with incomplete conversion. Minimal nucleophile homocou-
pling was observed under these conditions {<5% yield of
biphenyl (6); see Supporting Information}. Unsurprisingly,
performing the reaction at 80 °C afforded an even lower yield
and poorer conversion (entry 4). Little or no cross-coupling
occurred in the presence of either water or air and in the
absence of either catalyst or base (entries 5−8). While product
Special Issue: Organometallic Solutions to Challenges
in Cross-Coupling
Received: January 19, 2021
Figure 1. Examples of PCP-type nickel(II) pincer complexes.
https://doi.org/10.1021/acs.organomet.1c00033
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phenylboronic acid pinacol ester and potassium phenyl-
trifluoroborate were not viable coupling partners either under
the standard conditions or in toluene/water solvent mixtures
(entries 5−8). The poor reactivity of the aryl triflate is
consistent with an SM coupling mechanism that features
predominant single-electron pathways.5h This was reinforced
by an experiment demonstrating reaction inhibition in the
presence of the radical scavenger TEMPO (entry 9). These
observations, the above-mentioned influence of nucleophile
loading on reaction efficiency, and the dearth of studies
concerning the mechanism of SM cross-couplings mediated by
nickel(II) pincers,5h when taken together, prompted us to
explore the reaction mechanism further.
Table 1. Nickel-Catalyzed Suzuki−Miyaura Reactions:
a
Preliminary Experiments
b
entry
variation
yield (%)
1
2
3
4
5
6
7
8
9
none
5% Ni 1
10% Ni 2 instead of Ni 1
80 °C instead of reflux
2:1 PhMe/H2O instead of PhMe
air instead of Ar
no Ni 1
no K3PO4
1.7 equiv of PhB(OH)2
5% Ni 1, 1.7 equiv of PhB(OH)2
2.2 equiv of PhB(OH)2
55 (65)
49 (61)
41 (56)
23 (35)
2 (5)
0 (<1)
0 (<1)
0 (<1)
77 (87)
83 (89)
Nickel complex 1 is a d8 low-spin 16-electron species with
square planar coordination geometry confirmed by X-ray
crystallography.14 We employed continuous shape measure-
ments to assess the coordination environment of the d8 center
in complex 1.15 These calculations indicated that the nickel
center approximates a square planar geometry, with little
distortion from the D4h symmetry noted.16,17 Thus, we
anticipated that these properties would render this molecule
diamagnetic, allowing us to readily monitor the SM reaction
via NMR spectroscopy. In this way, we determined that the
predominant resting states of the catalyst are [Ph(PCP)Ni-
(Ph)] (7a) and [Ph(PCP)Ni(p-tol)] (7b) during the early
stages of the reaction (eq 1).18 In a competition experiment
10
11
88 (100)
a
b
Reactions performed employing 100 μmol of 3a. Yield of 5
determined by gas chromatography (GC) with the aid of a calibrated
internal standard (average of 2 experiments); consumption of 3a is
shown in parentheses.
5 was not formed in the control experiment without
[Ph(PCP)NiBr] (1) (Table 1, entry 7), we observed the
formation of biphenyl in 9% yield (see Supporting
Information).12 This suggested that raising the nucleophile
loading would be necessary for developing an enhanced SM
process. Accordingly, we found that increasing the stoichiom-
etry of phenylboronic acid was crucial to establishing an
efficient reaction (Table 1, entries 9−11; Table 2, entry 1).
This allowed us to identify optimal conditions that allowed for
a high-yielding cross-coupling employing only 1% of
precatalyst 1 (Table 2, entry 2).13
While we could couple aryl bromides efficiently, reactions
employing 4-chlorotoluene or p-tolyl triflate proceeded at very
low conversion (Table 2, entries 3 and 4). In addition,
Table 2. Nickel-Catalyzed Suzuki−Miyaura Reactions:
a
Influence of Reaction Parameters and Scope
b
entry
variation
yield (%)
1
2
3
4
5
6
none
1% Ni 1
90 (100)
95 (98)
12 (38)
10
0 (<1)
0 (<1)
0 (<1)
0 (<1)
<1 (2)
p-tol−CI instead of 3a
p-tol−OTf instead of 3a
PhBpin instead of 4a
PhBF3K instead of 4a
PhBpin instead of 4a
PhBF3K instead of 4a
+ 1 equiv of TEMPO
between 4-bromo- and 4-chlorotoluene, the catalyst effectively
differentiated between these electrophiles (eq 2). However, in
the analogous competition experiment involving 4-bromo- and
4-iodotoluene, this differentiation was less pronounced (eq 3).
In order to study the precatalyst activation step, [Ph(PCP)-
NiBr] (1) and 4-bromotoluene were heated together and the
reaction was monitored by 31P NMR spectroscopy (Figure
2A). No reaction occurred under these conditions, with similar
results obtained in the presence of base. Next, we heated
[Ph(PCP)NiBr] (1) with p-tolylboronic acid, which also
afforded no reaction (Figure 2B). However, we observed the
formation of [Ph(PCP)Ni(p-tol)] (7b) as the major product in
c
7
8
c
9
a
b
Reactions performed employing 100 μmol of 3a. Yield of 5
determined by GC with the aid of a calibrated internal standard
(average of 2 experiments); electrophile consumption is shown in
parentheses. Experiment performed using a 2:1 PhMe/H2O solvent
mixture.
c
B
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Figure 2. Stoichiometric reactions of [Ph(PCP)NiBr] (1) with (A) 4-
bromotoluene and (B) p-tolylboronic acid.
the presence of K3PO4.19 Analysis of the reaction mixture by
GCMS indicated that homocoupling also occurred under these
conditions. No reaction occurred when the same experiment
was performed in the presence of TEMPO. Taken together,
these data are consistent with the reaction of [Ph(PCP)NiBr]
(1) with the arylboronate via a single-electron pathway, and we
anticipate that this process is responsible for the activation of
precatalyst 1. These observations are generally consistent with
the results of a study exploring the mechanism of an SM
reaction promoted by a PNP-type nickel pincer.5h Cyclic
voltammetry experiments performed on [Ph(PCP)NiBr] (1)
revealed irreversible electrochemical behavior consistent with
irreversible reduction (and oxidation) of this species across a
range of scan rates (25−250 mV/s) (see Supporting
Information).20
Figure 3. Stoichiometric reactions of [Ph(PCP)Ni(p-tol)] (7b) with
(A) 4-bromobenzene and (B) phenylboronic acid. (C) Demonstrat-
ing the competence of [Ph(PCP)Ni(p-tol)] (7b) under catalytic
conditions. aYield of 5 determined by GC with the aid of a calibrated
internal standard (average of 2 experiments).
In order to better understand the C−C bond-forming
process, [Ph(PCP)Ni(p-tol)] (7b) and 4-bromobenzene were
heated together (Figure 3A). Biaryl 5 was not formed in this
experiment or when this reaction was performed in the
presence of base. These observations suggest that oxidative
addition precedes transmetalation in the catalytic cycle. Next,
we heated [Ph(PCP)Ni(p-tol)] (7b) with phenylboronic acid,
which delivered biaryl 5 in 51% yield (Figure 3B).21
Interestingly, we found that this stoichiometric reaction was
less efficient in the presence of base. When the results of these
stoichiometric experiments are taken together with the
observations shown in eqs 1−3, it is unclear which
fundamental step (oxidative addition or transmetalation) is
turnover-limiting in these nickel-catalyzed SM couplings of aryl
bromides.
To investigate the involvement of single-electron pathways
in the transmetalation step, we examined the reaction of
[Ph(PCP)Ni(p-tol)] (7b) with arylboronic acid radical probe
11 (Figure 4A). Because it is established that radical 12a
rapidly cyclizes {rate constant (k) = 9.6 × 109 s−1 in
DMSO},25 we anticipated that this reaction might lead to
the formation of cyclized products in preference to biaryl
13a.26 In accordance with this, we observed dihydrobenzofur-
ans 13b and 13c and we did not identify cross-coupled
products 13a in the crude reaction mixture via GCMS.27
However, when we performed this reaction employing
bromobenzene and 5% [Ph(PCP)NiBr] (1), we observed the
formation of allyl-coupled product 14 in addition to
dihydrobenzofurans 13b and 13c (Figure 4B). Allyl-coupled
products 13b and 14 may derive from competing nickel-
mediated allylation reactions involving two molecules of
substrate 11.28,29 Finally, we reacted bromide 15 with
phenylboronic acid in the presence of [Ph(PCP)NiBr] (1)
(Figure 4C). This reaction proceeded at low conversion, and
analysis of the crude reaction mixture via GCMS indicated that
the predominant organic molecules present were unreacted
electrophile 15 and biphenyl (6) in addition to dihydrobenzo-
furan 13c. We did not detect possible cross-coupled product 2-
allyloxybiphenyl in this reaction by GCMS; however, we did
isolate dihydrobenzofuran 16 in low yield.30
We demonstrated that [Ph(PCP)Ni(p-tol)] (7b) serves as a
viable precatalyst for this reaction (Figure 3C).22,23 Taken
together, the experiments shown in Figure 3 confirm the
chemical competence of species 7b in the SM cross-coupling.
We used Density Functional Theory (DFT) to model
transmetalation from complex 7b via reaction with a
phenylboronic-acid-derived free phenyl radical 8 and found
that the transformation leading to nickel(III) adduct 9 is
exergonic (ΔG = −14.3 kcal/mol) (eq 4). DFT calculations
indicated that reductive elimination from intermediate 9 to
afford biaryl product 5 and nickel(I) species 10 is also
energetically favorable (ΔG = −33.8 kcal/mol) and should
proceed by a low barrier (ΔG⧧ = +6.9 kcal/mol) (eq 5).24
C
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This recently reported study also invokes a key on-cycle Ni(I)
dimer. However, we did not obtain any experimental evidence
suggesting the involvement of an analogous intermediate in
this chemistry employing PCP-type nickel pincer catalysts.
CONCLUSIONS
■
We have demonstrated for the first time that PCP-type nickel
pincers 1 and 2 can facilitate SM reactions. The findings from
our associated mechanistic studies are consistent with a Ni(I)/
(III) cycle in which single-electron pathways are operative in
this transformation. Our results suggest that elevated temper-
atures are necessary for an efficient cross-coupling. This
indicates that alternative strategies for substrate activation are
probably required to develop milder, more practical SM Csp2−
Csp2 couplings and related processes with PCP-type nickel
pincer catalysts, and this will be the subject of future work in
our laboratory.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00033.
Figure 4. (A) Stoichiometric reaction of [Ph(PCP)Ni(p-tol)] (7b)
with boronic acid 11. Investigation of the Ni-catalyzed SM cross-
coupling employing: (B) nucleophile 11; (C) electrophile 15.
Additional experimental information and compound
characterization data (PDF)
Cartesian coordinates (XYZ)
On the basis of the above-mentioned experiments, we
propose that a Ni(I)/(III) cycle is operative in this cross-
coupling reaction. It is feasible that the catalytically active
Ni(I) species derives from the one-electron reduction of
precatalyst [Ph(PCP)NiBr] (1) by the arylboronic acid in the
presence of base (Figure 5). We anticipate that oxidative
Accession Codes
CCDC 2057111 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Alex C. Bissember − School of Natural Sciences − Chemistry,
University of Tasmania, Hobart, Tasmania 7001, Australia;
orcid.org/0000-0001-5515-2878;
Email: alex.bissember@utas.edu.au
Authors
Curtis C. Ho − School of Natural Sciences − Chemistry,
University of Tasmania, Hobart, Tasmania 7001, Australia;
orcid.org/0000-0002-7555-0635
Angus Olding − School of Natural Sciences − Chemistry,
University of Tasmania, Hobart, Tasmania 7001, Australia
Rebecca O. Fuller − School of Natural Sciences − Chemistry,
University of Tasmania, Hobart, Tasmania 7001, Australia;
orcid.org/0000-0003-3926-8680
Allan J. Canty − School of Natural Sciences − Chemistry,
University of Tasmania, Hobart, Tasmania 7001, Australia;
orcid.org/0000-0003-4091-6040
Figure 5. Outline of a possible catalytic cycle for nickel-catalyzed SM
cross-couplings.
addition of the electrophile to the ensuing Ni(I) species
possibly via a single-electron pathway delivers a Ni(III)−aryl
intermediate. It is possible that the subsequent transmetalation
step also proceeds by a single-electron pathway to afford a
Ni(III) species from which reductive elimination regenerates a
Ni(I) species that is capable of reacting with the electrophile to
continue the process. We suggest that, while [Ph(PCP)-
NiII(aryl)] species, such as complex 7b, are chemically
competent intermediates in this reaction, they do not feature
on the primary catalytic cycle (Figure 5). This mechanistic
proposal differs from the catalytic cycle that has been suggested
for an SM reaction promoted by a PNP-type nickel pincer.5h
Nigel T. Lucas − Department of Chemistry, University of
Otago, Dunedin, Otago 9054, New Zealand; orcid.org/
0000-0002-4944-9875
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00033
Notes
The authors declare no competing financial interest.
D
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a Ni(II) PNP pincer complex: Scope and mechanistic insights. Inorg.
Chim. Acta 2020, 504, 119457.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the University of Tasmania School
of Natural Sciences − Chemistry and an Australian Research
Council (ARC) Discovery Project Grant (DP200101714) for
funding, the University of Tasmania Central Science
Laboratory for providing access to NMR spectroscopy services,
and the Australian National Computing Infrastructure. R.O.F.’s
contributions were supported by an ARC Discovery Early
Career Researcher Award (DE180100112). N.T.L. acknowl-
edges financial support by the MacDiarmid Institute for
Advanced Materials and Nanotechnology, and the Marsden
(6) (a) Ananikov, V. P. Nickel: The “Spirited Horse” of Transition
Metal Catalysis. ACS Catal. 2015, 5, 1964−1971. (b) Beromi, M. M.;
Brudvig, G. W.; Hazari, N.; Lant, H. M. C.; Mercado, B. Q. Synthesis
and reactivity of paramagnetic Nickel polypyridyl complexes relevant
to C(sp2)-C(sp3) coupling reactions. Angew. Chem., Int. Ed. 2019, 58,
6094−6098. (c) Bridgeman, A. J. On the origin of paramagnetism in
planar nickel(II) complexes. Dalton Trans. 2008, 1989−1992.
(d) Pelties, S.; Carter, E.; Folli, A.; Mahon, M. F.; Murphy, D. M.;
Whittlesey, M. K.; Wolf, R. Influence of Ring-Expanded N-
Heterocyclic Carbenes on the Structures of Half-Sandwich Ni(I)
Complexes: An X-ray, Electron Paramagnetic Resonance (EPR), and
Electron Nuclear Double Resonance (ENDOR) Study. Inorg. Chem.
2016, 55, 11006−11017.
Fund Council, managed by the Royal Society Te Aparangi,
New Zealand. We thank reviewers for helpful suggestions.
̅
(7) (a) Valdes, H.; Garcia-Eleno, M. A.; Canseco-Gonzalez, D.;
Morales-Morales, D. Recent Advances in Catalysis with Transition-
Metal Pincer Compounds. ChemCatChem 2018, 10, 3136−3172.
(b) Gafurov, Z. N.; Kagilev, A. A.; Kantyukov, A. O.; Balabaev, A. A.;
Sinyashin, O. G.; Yakhvarov, D. G. Classification and synthesis of
nickel pincer complexes. Russ. Chem. Bull. 2018, 67, 385−394.
(8) (a) Rimml, H.; Venazi, L. M. The facile cyclometallation
reaction of 1,3-BIS[(diphenyl-phosphino)methyl]benzene. J. Organo-
met. Chem. 1983, 259, C6−C7. (b) Shih, W.-C.; Ozerov, O. V. One-
Pot Synthesis of 1,3-Bis(phosphinomethyl)arene PCP/PNP Pincer
Ligands and Their Nickel Complexes. Organometallics 2015, 34,
4591−4597.
(9) Moulton, C. J.; Shaw, B. L. Transition Metal-Carbon Bonds. Part
XLII. Complexes of Nickel, Palladium, Platinum, Rhodium and
Iridium with the Tridentate Ligand 2,6- Bis[(di-t-butylphosphino)-
methyl]phenyl. J. Chem. Soc., Dalton Trans. 1976, 1020−1024.
(10) Peris, E.; Crabtree, R. H. Key factors in pincer ligand design.
Chem. Soc. Rev. 2018, 47, 1959−1968.
(11) For reports of SM reactions facilitated by PXP-type nickel
pincer complexes, see: (a) Estudiante-Negrete, F.; Hernández-Ortega,
S.; Morales-Morales, D. Ni(II)-POCOP pincer compound [NiCl-
{C10H5-2,10-(OPPh2)2}] an efficient and robust nickel catalyst for the
Suzuki-Miyaura coupling reactions. Inorg. Chim. Acta 2012, 387, 58−
63. (b) Di Franco, T.; Boutin, N.; Hu, X. Suzuki-Miyaura Cross-
Coupling Reactions of Unactivated Alkyl Halides Catalyzed by a
Nickel Pincer Complex. Synthesis 2013, 45, 2949−2958. (c) Mastalir,
M.; Stöger, B.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Air-Stable
Triazine-Based Ni(II) PNP Pincer Complexes As Catalysts for the
Suzuki-Miyaura Cross-Coupling. Org. Lett. 2016, 18, 3186−3189.
(d) Favela-Mendoza, R.; Rufino-Felipe, E.; Valdés, H.; Toscano, R.
A.; Hernández-Ortega, S.; Morales-Morales, D. Synthesis and
characterization of non-symmetric Ni(II)- and Pd(II)-POCOP pincer
complexes derived from 1,7-naphthalenediol. Evaluation of their
catalytic activity in Suzuki-Miyaura couplings. Inorg. Chim. Acta 2020,
512, 119920.
REFERENCES
■
(1) For selected reviews on the Suzuki−Miyaura cross-coupling
reactions, see: (a) Suzuki, A. Carbon-carbon bonding made easy.
Chem. Commun. 2005, 2005, 4759−4763. (b) Hussain, I.; Capricho,
J.; Yawer, M. A. Synthesis of Biaryls via Ligand-Free Suzuki-Miyaura
Cross-Coupling Reactions: A Review of Homogeneous and
Heterogeneous Catalytic Developments. Adv. Synth. Catal. 2016,
358, 3320−3349.
(2) Choi, J.; Fu, G. C. Transition metal-catalyzed alkyl-alkyl bond
formation: Another dimension in cross-coupling chemistry. Science
2017, 356, No. eaaf7230.
(3) (a) Hazari, N.; Melvin, P. R.; Beromi, M. M. Well-defined nickel
and palladium precatalysts for cross-coupling. Nat. Rev. Chem. 2017,
1, 0025. (b) Han, F.-S. Transition-metal-catalyzed Suzuki-Miyaura
cross-coupling reactions: a remarkable advance from palladium to
nickel catalysts. Chem. Soc. Rev. 2013, 42, 5270−5298.
(4) For recent reviews of nickel-catalyzed SM reactions and related
cross-couplings, see for example: (a) Tasker, S. Z.; Standley, E. A.;
Jamison, T. F. Recent advances in homogeneous nickel catalysis.
Nature 2014, 509, 299−309. (b) Balcells, D.; Nova, A. Designing Pd
and Ni Catalysts for Cross-Coupling Reactions by Minimizing Off-
Cycle Species. ACS Catal. 2018, 8, 3499−3515.
(5) For recent reports investigating the mechanism of nickel-
catalyzed Csp2−Csp2 SM cross-couplings, see for example: (a) Guard,
L. M.; Mohadjer Beromi, M.; Brudvig, G. W.; Hazari, N.; Vinyard, D.
J. Comparison of dppf-Supported Nickel Precatalysts for the Suzuki-
Miyaura Reaction: The Observation and Activity of Nickel(I). Angew.
Chem., Int. Ed. 2015, 54, 13352−13356. (b) Mohadjer Beromi, M.;
Nova, A.; Balcells, D.; Brasacchio, A. M.; Brudvig, G. W.; Guard, L.
M.; Hazari, N.; Vinyard, D. J. Mechanistic Study of an Improved Ni
Precatalyst for Suzuki-Miyaura Reactions of Aryl Sulfamates:
Understanding the Role of Ni(I) Species. J. Am. Chem. Soc. 2017,
139, 922−936. (c) Manzoor, A.; Wienefeld, P.; Baird, M. C.;
Budzelaar, P. H. M. Catalysis of Cross-Coupling and Homocoupling
Reactions of Aryl Halides Utilizing Ni(0), Ni(I), and Ni(II)
Precursors; Ni(0) Compounds as the Probable Catalytic Species
but Ni(I) Compounds as Intermediates and Products. Organometallics
2017, 36, 3508−3519. (d) Bajo, S.; Laidlaw, G.; Kennedy, A. R.;
Sproules, S.; Nelson, D. J. Oxidative Addition of Aryl Electrophiles to
a Prototypical Nickel(0) Complex: Mechanism and Structure/
Reactivity Relationships. Organometallics 2017, 36, 1662−1672.
(e) Payard, P.-A.; Perego, L. A.; Ciofini, I.; Grimaud, L. Taming
Nickel-Catalyzed Suzuki-Miyaura Coupling: A Mechanistic Focus on
Boron-to-Nickel Transmetalation. ACS Catal. 2018, 8, 4812−4823.
(f) Mohadjer Beromi, M.; Banerjee, G.; Brudvig, G. W.; Hazari, N.;
Mercado, B. Q. Nickel(I) Aryl Species: Synthesis, Properties, and
Catalytic Activity. ACS Catal. 2018, 8, 2526−2533. (g) Barth, E. L.;
Davis, R. M.; Mohadjer Beromi, M.; Walden, A. G.; Balcells, D.;
Brudvig, G. W.; Dardir, A. H.; Hazari, N.; Lant, H. M. C.; Mercado, B.
Q.; Peczak, I. L. Bis(dialkylphosphino)ferrocene-Ligated Nickel(II)
Precatalysts for Suzuki-Miyaura Reactions of Aryl Carbonates.
Organometallics 2019, 38, 3377−3387. (h) Madera, J.; Slattery, M.;
Arman, H. D.; Tonzetich, Z. J. Suzuki-Miyaura coupling catalyzed by
(12) In the control experiment performed in the absence of K3PO4·
H2O (Table 1, entry 8), we did not observe the formation of biphenyl.
(13) The conditions shown in Table 2, entries 1 and 2, provided
biphenyl in 11% and 13% yields, respectively.
(14) Bachechi, F. X-Ray Structural Analysis of NiII, PdII, and PtII
Complexes with the Potentially Tridentate Ligand 1,3-Bis-
(diphenylphosphinomethyl)benzene, 1,3-C6H4(CH2PPh2)2. Struct.
Chem. 2003, 14, 263−269.
(15) Alvarez, S.; Avnir, D.; Llunell, M.; Pinsky, M. Continuous
symmetry maps and shape classification. The case of six-coordinated
metal compounds. New J. Chem. 2002, 26, 996−1009.
(16) The distortion parameter from SHAPE (D4h) is 1.281 for
[Ph(PCP)NiBr] (1).
(17) Ligand field is well-known to have an impact on the electronic
properties of nickel(II) complexes. Square planar low spin d8
complexes tend to be diamagnetic, as a result of overlap between
2
2
the ligand s orbital and the metal dx −y . The s* is unoccupied, so that
the eight d electrons pair up in the remaining d orbitals, resulting in
observed diamagnetism of the complex.
E
https://doi.org/10.1021/acs.organomet.1c00033
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
(18) When we performed the experiment shown in eq 1 employing
[Ph(PCP)Ni(p-tol)] (7b) in place of [Ph(PCP)NiBr] (1), we obtained
the same results.
(19) In addition to the presence of [Ph(PCP)Ni(p-tol)] (7b) and
[Ph(PCP)NiBr] (1) in this experiment, we observed the presence of a
minor signal (δP 33.3 ppm) that we were not able to assign to a
complex.
(20) Diccianni, J. B.; Katigbak, J.; Hu, C.; Diao, T. Mechanistic
Characterization of (Xantphos)Ni(I)-Mediated Alkyl Bromide
Activation: Oxidative Addition, Electron Transfer, or Halogen-Atom
Abstraction. J. Am. Chem. Soc. 2019, 141, 1788−1796.
(21) Cyclic voltammetry experiments performed on [Ph(PCP)Ni(p-
tol)] (7b) revealed irreversible electrochemical behavior consistent
with irreversible reduction (and oxidation) of this species across a
range of scan rates (25−250 mV/s) (see Supporting Information).
(22) This reaction proceeded with complete conversion of 4-
bromotoluene, and biphenyl was formed in 6% yield.
(23) When we performed the experiment shown in Figure 3C in the
absence of base, cross-coupled product 5 was only formed in trace
amounts.
(24) The geometry of [Ph(PCP)Ni(p-tol)(Ph)]• is distorted square-
pyramidal where axial P−Ni−P and Cax−Ni−Ceq angles are 142.0°
and 108.7°, respectively. These angles are decreased to 136.7° and
65.3°, respectively, in the transition state for p-tol···Ph coupling (see
Supporting Information).
(25) Annunziata, A.; Galli, C.; Marinelli, M.; Pau, T. Determination
of Rate Constants for the Reaction of Aryl Radicals with Enolate Ions.
Eur. J. Org. Chem. 2001, 2001, 1323−1329.
(26) Lockner, J. W.; Dixon, D. D.; Risgaard, R.; Baran, P. S. Practical
Radical Cyclizations with Arylboronic Acids and Trifluoroborates.
Org. Lett. 2011, 13, 5628−5631.
(27) When we performed the experiment shown in Figure 4A in the
presence of the K3PO4·H2O, similar results were observed and
dihydrobenzofurans 13b and 13c were the major products formed.
Compound 13c is volatile, and we were not able to isolate this
molecule. However, we could identify this species via GCMS analysis
of the reaction mixture and comparison to an authentic sample.
(28) See, for example: (a) Wang, G.; Gan, Y.; Liu, Y. Nickel-
Catalyzed Direct Coupling of Allylic Alcohols with Organoboron
Reagents. Chin. J. Chem. 2018, 36, 916−920. (b) Nazari, S. H.;
Bourdeau, J. E.; Talley, M. R.; Valdivia-Berroeta, G. A.; Smith, S. J.;
Michaelis, D. J. Nickel-Catalyzed Suzuki Cross Couplings with
Unprotected Allylic Alcohols Enabled by Bidentate N-Heterocyclic
Carbene (NHC)/ Phosphine Ligands. ACS Catal. 2018, 8, 86−89.
(29) The existence of this competing nickel-mediated allylation
reaction from substrate 11 represents a limitation of this system as a
radical probe for this nickel-catalyzed SM cross-coupling (in addition
to the issue discussed in ref 30).
(30) The formation of dihydrobenzofuran 16 does not provide
conclusive evidence for a radical pathway. For example, the possibility
that product 16 results from the concerted oxidative addition of
bromide 11 to a nickel species, followed by β-migratory insertion,
transmetalation, and reductive elimination, cannot be excluded.
F
https://doi.org/10.1021/acs.organomet.1c00033
Organometallics XXXX, XXX, XXX−XXX