Intermediacy of Ni-Ni Species in sp2 C-O Bond Cleavage of Aryl Esters: Relevance in Catalytic C-Si Bond Formation

Article abstract of DOI:10.1021/jacs.8b04479

Monodentate phosphine ligands are frequently employed in the Ni-catalyzed C-O functionalization of aryl esters. However, the extensive body of preparative work on such reactions contrasts with the lack of information concerning the structure and reactivity of the relevant nickel intermediates. In fact, experimental evidence for a seemingly trivial oxidative addition into the C-O bond of aryl esters with monodentate phosphines and low-valent nickel complexes still remains elusive. Herein, we report a combined experimental and theoretical study on the Ni(0)/PCy₃-catalyzed silylation of aryl pivalates with CuF₂/CsF additives that reveals the involvement of unorthodox dinickel oxidative addition complexes in C-O bond cleavage and their relevance in C-Si bond formation. We have obtained a mechanistic picture that clarifies the role of the additives and demonstrates that dinickel complexes act as reservoirs of the propagating monomeric nickel complexes by disproportionation. We believe this study will serve as a useful entry point to unravelling the mechanistic underpinnings of other related Ni-catalyzed C-O functionalization reactions employing monodentate phosphines.

Full text of DOI:10.1021/jacs.8b04479

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Article
Intermediacy of Ni–Ni species in sp2 C–O bond cleavage
of aryl esters: relevance in catalytic C-Si Bond Formation
Rosie Somerville, Lillian V. A. Hale, Enrique Gomez-Bengoa, Jordi Burés, and Ruben Martin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04479 • Publication Date (Web): 18 Jun 2018
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Intermediacy of Ni–Ni Species in sp2 C–O Bond Cleavage of
Aryl Esters: Relevance in Catalytic C–Si Bond Formation
Rosie J. Somerville,† Lillian V. A. Hale,¶ Enrique GómezꢀBengoa,*‡ Jordi Burés,*£ and Ruben Marꢀ
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tin†§*
†Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Av. Països Cataꢀ
lans 16, 43007 Tarragona, Spain
‡Department of Organic Chemistry I, Universidad País Vasco, UPV/EHU, Apdo. 1072, 20080, San Sebastian, Spain
£The University of Manchester, School of Chemistry Oxford Road, M13 9PL Manchester (UK)
¶Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States
§ICREA, Passeig Lluïs Companys, 23, 08010, Barcelona, Spain
ABSTRACT: Monodentate phosphine ligands are frequently employed in the Niꢀcatalyzed C–O functionalization of aryl esters.
However, the extensive body of preparative work on such reactions contrasts with the lack of information concerning the structure
and reactivity of the relevant nickel intermediates. In fact, experimental evidence for a seemingly trivial oxidative addition into the
C–O bond of aryl esters with monodentate phosphines and lowꢀvalent nickel complexes still remains elusive. Herein, we report a
combined experimental and theoretical study on the Ni(0)/PCy3ꢀcatalyzed silylation of aryl pivalates with CuF2/CsF additives that
reveals the involvement of unorthodox dinickel oxidative addition complexes in C–O bond cleavage and their relevance in C–Si
bond formation. We have obtained a mechanistic picture that clarifies the role of the additives and demonstrates that dinickel comꢀ
plexes act as reservoirs of the propagating monomeric nickel complexes by disproportionation. We believe this study will serve as a
useful entry point to unravelling the mechanistic underpinnings of other related Niꢀcatalyzed C–O functionalization reactions based
on monodentate phosphines.
Prompted by the broad accessibility, thermal stability and
costꢀefficiency of phenol derivatives, C–O electrophiles have
recently gained momentum as alternatives to aryl halides in
crossꢀcoupling reactions.1 Although activated aryl sulfonates
have been widely adopted in the crossꢀcoupling arena, much
less attention has been devoted to much simpler aryl esters
(Scheme 1). Unlike aryl sulfonates, aryl esters are not cleaved
with Pd catalysts; however, aryl ester C–O bondꢀcleavage
readily occurs with cheap and Earthꢀabundant Ni catalysts,
thus providing the opportunity to employ this functional group
in orthogonal crossꢀcoupling reactions in the presence of aryl
halide partners.
Despite the advances realized in Niꢀcatalyzed crossꢀ
coupling reactions of aryl esters,1 these processes are poorly
understood in mechanistic terms, and progress in the field is
typically based on empirical discoveries.2 At present, there
seems to be consensus that sp2 C–O activation is initiated by
oxidative addition to wellꢀdefined Ni(0)Ln. To the best of our
knowledge, Itami and coꢀworkers isolated and characterized
the first direct oxidative addition complex of an aryl ester to
Ni(0)Ln (Ni-1) (Scheme 2, left).3 Specifically, this reaction
used bidentate dcype (1,2ꢀbis(dicyclohexylphosphino)ꢀethane)
as the supporting ligand and required elevated temperatures
(100 ºC).3–5 However, experimental evidence for direct oxidaꢀ
tive addition complexes bearing monodentate phosphine ligꢀ
ands (PR3), the most commonly employed ligands in catalytic
C–O functionalization reactions, is altogether absent in the
literature (Scheme 2, right). Indeed, access to aryl–Ni(II)–
OCOR species supported by PR3 has only been reported via
indirect pathways, rather than direct oxidative addition.6 The
paucity of oxidative addition complexes supported by PR3 is
tentatively attributed to (a) the formation of unstable shortꢀ
lived entities; (b) the less rigid coordination sphere of PR3
compared to their bidentate analogues; (c) their tendency for
rapid ligand dissociation to form fluxional intermediates;
and/or (d) the possibility for unproductive disproportionation.
Scheme 1. C–O electrophiles in cross-coupling reactions.
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Scheme 2. Oxidative addition into the sp2 C–O bond.
functionalization,8,9,17 we now report a combined experimental
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and computational study on the C–O silylation of aryl esters.
Our work not only sheds light on the role of both Cu and fluoꢀ
ride ions but also demonstrates, for the first time, the role of
unorthodox dinickel oxidative addition complexes in C–O
cleavage. These rather stable offꢀcycle species evolve via
disproportionation into monomeric arylꢀNi(II) intermediates
that ultimately lead to C–Si bond formation. We anticipate that
these results will both provide an entry point to further studies
on the mechanistic intricacies of related C–O functionalizaꢀ
tions and highlight the importance of offꢀcycle intermediates,18
which may be relevant to improving the efficiency and generꢀ
ality of Niꢀcatalyzed reactions.
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RESULTS AND DISCUSSION
We and others have recently demonstrated that the crossꢀ
coupling of aryl esters via sp2 C–O cleavage is by no means
confined to C–C bond formations, and that aminations,7 silylaꢀ
tion,8 stannylation,9 borylations,10 and phosphorylations11 are
within reach. In 2014, our group described a mild Ni/Cuꢀ
catalyzed C–O silylation of aryl esters using PCy3 as the ligꢀ
and (Scheme 3, top).8 Notably, the coupling of regular arenes
posed no problems, which contrasts with a number of C–O
cleavage protocols that are limited to extended π systems.12,13
It was hypothesized that Cu was acting as a silyl transfer cataꢀ
lyst,14 and that fluoride ions from CsF/CuF2 were probably
activating the Si–B bond prior to reaction with the Cu complex
(scheme 3, middle). The Ni/Cu couple has also been employed
in related silylation or borylation events, among others
(scheme 3, bottom).15,16
Synthesis and characterization of oxidative addition
complexes of aryl pivalates to Ni/PCy3. Although oxidative
addition has been invoked as the first step in the catalytic
cycle of Ni(0)/PR3ꢀcatalyzed crossꢀcoupling reactions of aryl
pivalates, the resulting putative Ar–Ni(II)–OPiv species have
not been confirmed experimentally.2 With this in mind, our
investigations began by reacting 1ꢀnaphthyl pivalate (1a) with
[Ni(COD)2]/PCy3 (1:2) in tolueneꢀd8 at room temperature
(Scheme 4).19 After 5 h, a new 31P NMR signal had appeared
at δP = 24.8 ppm, and 1a had undergone 10% conversion as
judged by the appearance of a new set of naphthyl signals in
the 1H NMR spectrum. Specifically, the downfield signal at δH
= 10.4 ppm paralleled that of “classical” square planar Ni(II)
oxidative addition species such as [Ni(PCy3)2(σꢀ1ꢀ
=
10.2, CDCl3)20 or [Ni(PCy3)2(σꢀ1ꢀ
Scheme 3. Ni/Cu-catalyzed C–O silylation(borylation).
naphthyl)Cl] (δH
naphthyl)F] (δH = 9.9, THFꢀd8).21 However, the upfield signal
at δH = 5.6 ppm also suggested that η2ꢀarene coordination to
nickel was present.22 At first glance, one might suspect that
these signals correspond to different nickel complexes; howꢀ
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ever, a H–31P HMBC spectrum showed correlations between
the new 31P NMR signal at δP = 24.8 ppm and both δH = 5.6
ppm and δH = 10.4 ppm, indicating the presence of a single
Cy3P–Ni–naphthyl fragment.
Scheme 4. 1H-NMR evidence for Ni–naphthyl bonding.
Optimization of the reaction conditions allowed us to isolate
a green airꢀsensitive solid (2a) from the reaction mixture in a
50% yield (Scheme 5, top), and crystallization from toluꢀ
ene/pentane at –30 ºC furnished crystals suitable for Xꢀray
diffraction (Figure 1). The most striking feature of 2a is the
presence of a Ni–Ni bond, the length of which (2.3949(9) Å)
is consistent with related Ni–Ni complexes supported by
phosphine ligands, Nꢀheterocyclic carbenes, or a naphthyꢀ
ridineꢀdiimine ligand.23 In line with our spectroscopic data, the
naphthyl fragment interacts with the Ni–Ni core via both a σꢀ
bond to Ni2 (1.857(6) Å) and an η2 interaction between the
C1–C2 bond and Ni1. Completing the dinickel structure is a
bridging pivalate ligand having identical Ni1–O1 and Ni2–O2
bond distances within 3σ (1.951(4) and 1.948(4) Å). Magnetic
Due to the lack of experimental evidence for a classical
(PR3)Ni(0)/(II) cycle with aryl esters and the enigmatic role
played by CuF2 and CsF in many crossꢀcoupling reactions, we
sought to elucidate the mechanistic details of the C–O silylaꢀ
tion of aryl pivalates. At the outset of our investigation, it was
unclear whether it would be possible to unravel the mechanisꢀ
tic underpinnings of this reaction. If successful, we recognized
that this would be a significant step forward in the C–O funcꢀ
tionalization arena. As part of our efforts in the field of C–O
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measurements confirmed that 2a is diamagnetic with a singlet
ground state, consistent with its wellꢀresolved NMR spectra.24
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NBO analysis of 2a showed a symmetrical distribution of
charge at the Ni–Ni core, with the two Ni atoms bearing simiꢀ
lar positive charges (+0.16 and +0.15) and the two oxygen
atoms of the pivalate fragment bearing similar negative chargꢀ
es (ꢀ0.69 and ꢀ0.68). As mentioned above, the room temperaꢀ
ture 31P NMR spectrum of 2a contained one signal (δP = 24.8
ppm), suggesting that the two distinct Ni–C interactions are
rapidly exchanged between the Ni centers to place the PCy3
ligands in a single averaged environment. Indeed, lowering the
temperature ultimately resulted in this signal splitting into two
signals at δP = 16.3 and 32.8 ppm, with coalescence occurring
at approximately –40 ºC.24
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Scheme 5. Synthesis of dinickel oxidative addition
complex.
Figure 1. Xꢀray crystal structure of 2a (thermal ellipsoids at
the 50% probability level). Hydrogen atoms have been omitted
for clarity. Selected bond lengths (Å) and angles (º): Ni1–Ni2
= 2.3949(9), Ni2–C1 = 1.857(6), Ni1–C1 = 2.019(6), Ni1–C2
= 2.102(6), Ni1–O1 = 1.951(4), Ni2–O2 = 1.948(4). C10–1–
Ni2 = 127.4(5), C2–C1–Ni2 = 117.6(4).
Interestingly, 1,1’ꢀbinaphthalene was produced when 1a
was reacted with [Ni(COD)2]/PCy3 at 100 ºC (Scheme 6).25
This homocoupled product was accompanied by dark green
crystals of an airꢀstable compound. Xꢀray crystallographic
analysis showed the formation of Ni(II) paddlewheel complex
[Ni2(µꢀOPiv)4(PCy3)2] (3) with a Ni–Ni distance of 2.7364(2)
Å.25,26 Notably, binaphthalene and broad signals belonging to
The formation of 2a requires the dissociation of COD, and
thus may be hindered by competing ligand substitution proꢀ
cesses. Using [Ni(PCy3)2]2N2 as an alternative Ni(0) source,
we found that 2a formed much more rapidly in the absence of
COD, with full conversion to a 1:1 mixture of 2a and PCy3
obtained within 15 minutes at room temperature (Scheme 5,
bottom). The facile oxidative addition of 1a was further supꢀ
ported when the reaction was monitored by variable temperaꢀ
ture NMR spectroscopy, with signals for 2a appearing at 0 ºC
when the reaction was warmed from –40 ºC. This result is of
particular importance, as it not only shows that C–OPiv cleavꢀ
age via oxidative addition to Ni(0)/PCy3 is facile, but also
argues against oxidative addition being rateꢀdetermining under
our reaction conditions. These results are in contrast with the
conditions described by Itami (100 ºC) for the synthesis of Ni-
1 via C–OPiv cleavage with otherwise related dcype (Scheme
2, left).3,4 However, it is worth noting that Love4 showed that
oxidative addition complexes supported by bidentate dtbpe
(1,2ꢀbis(ditertbutylphosphino)ꢀethane) could also be prepared
at lower temperatures to those shown by Itami.3,5
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3 were also detected by H NMR upon heating 2a to 100 ºC.
Undesirable homocoupling pathways can negatively affect
crossꢀcoupling reactions of aryl esters that are conducted at
high temperatures. Therefore, this finding suggests that preꢀ
venting the formation of Ni(II) dimers such as 3 may help
maintain productive catalyst activity for sp2 C–O functionaliꢀ
zation methods based on Ni(0)/PR3.27
Scheme 6. Formation of 1,1’–binaphthalene and Ni(II)
OPiv dimer 3 at elevated temperatures.
The formation of 2a can be explained by invoking an oxidaꢀ
tive addition of the aryl pivalate to Ni(0)/PCy3, a pathway that
was first described computationally by Liu and coꢀworkers.2f,28
This results in a monomeric Ni(II) species [Ni(PCy3)(σꢀ
aryl)(ꢀ2ꢀOPiv)] (4). In the presence of Ni(0)Ln – likely to be
formulated
as
[(PCy3)Ni(toluene)],29
or
[Ni(0)(PCy3)(ArOPiv)]30 – comproportionation could occur to
form a Ni(I)–Ni(I) bond. A similar comproportionation reacꢀ
tion at a Ni(II) center supported by a diphosphine ligand has
been proposed by Balcells and Hazari.31 It is worth highlightꢀ
ing that complex 2a represents the first experimental and
structural evidence for an oxidative addition of an aryl ester to
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Ni(0) supported by a monodentate phosphine. Additionally, 2a
contains a rare case of a σꢀarene ligand having a πꢀinteraction
to a second metal center and is a new member of the small
family of dinickel complexes exhibiting such coordination.23,32
Considering the prevalence of PCy3 in C–O cleavage reacꢀ
tions, we speculate that 2a is relevant to other crossꢀcoupling
reactions of aryl ester derivatives and may offer a useful entry
point to studying the mechanisms of these reactions.
and that a small amount (7.5% at 16.7 h) of homocoupled 1a
(1,1’ꢀbinaphthalene) was observed (Figure 2ꢀright).
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Scheme 7. Accessing dinickel species from ArOPiv.
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Figure 2. H NMR monitoring of the CsF/CuF2ꢀfree C–O
silylation of 1a with Et3SiBPin. Spectra collected every 38
min. [1a]: 0.094 M.
Unravelling the role of dinickel complexes as reaction
intermediates. Although the above data provided compelling
evidence for the formation of dinickel complex 2a during the
Ni(0)/PCy3ꢀcatalyzed silylation of 1a, it was unclear how 2a
participated in the catalytic cycle. Indeed, the Ni–Ni bond
might remain intact throughout the reaction with Et3SiBPin or
might be cleaved to release mononickel species in solution
(Scheme 8).35 In the latter case, three scenarios are conceivaꢀ
ble for Ni–Ni cleavage: (a) heterolytic cleavage, in which a
twoꢀelectron transfer from one nickel atom results in the forꢀ
mation of a pair of charged nickel species (path a, Scheme 8);
(b) homolysis of the Ni–Ni bond, leading to a pair of neutral
Ni(I) species (path b); or (c) disproportionation of the Ni–Ni
bond to generate a Ni(0) and a Ni(II) species (path c).
Prompted by these results, we anticipated that the facile oxꢀ
idative addition of [Ni(PCy3)2]2(N2) with 1a would not be
limited to the formation of 2a. As shown in Scheme 7, this
turned out to be the case, and aryl pivalates 1b–f provided
dinickel oxidative addition complexes 2b–f in moderateꢀtoꢀ
excellent NMR yields after 2 h.33 Unfortunately, complexes
2b–f were considerably more soluble than 2a, and although no
crystals suitable for Xꢀray diffraction were obtained, the
dinickel(µꢀη2ꢀarene) core was identified by comparing the
13C{1H} NMR signals of the Ni–C environment (δ = 143–150
ppm (2JPC triplet)) and the C=O signal of the bridging pivalate
ligand (δ = 195–196 ppm (3JPC triplet)) of 2b–f, with those of
2a. Given that extended πꢀsystems are typically more reactive
in C–O functionalization than regular arenes,12,13 we wondered
whether a dinickel oxidative addition complex would also
form with simple phenyl pivalate (1f). As anticipated, the
reaction of 1f with [Ni(PCy3)2]2(N2) was rather sluggish when
compared with naphthyl pivalates 1a–e. After 2 h at room
temperature, the 31P NMR spectrum showed that although
[(PCy3)Ni(toluene)]28 and free PCy3 were the major species in
solution, a small signal had appeared that was consistent with
a dinickel oxidative addition complex (2f, 18% yield).
Scheme 8. Accessing catalytic relevant species via Ni–Ni
cleavage from dinickel complexes.
Identification of dinickel complex 2a as the catalyst
resting state. The data outlined above leave no doubt about
the importance of dinickel complexes in the C–O cleavage of
aryl esters. Therefore, we next investigated the relevance of 2a
in the C–O silylation of 1ꢀ(triethylsilyl)naphthalene (5a).
Notably, control experiments revealed that 2a could be used as
the precatalyst to afford 5a in an almost identical yield to that
obtained using [Ni(COD)2]/PCy3 or [Ni(PCy3)2]2N2, demonꢀ
strating its ability to participate in the silylation reaction.24 We
next monitored the silylation of 1a by NMR spectroscopy to
determine whether 2a was present during the reaction.34 Due
to the heterogeneity introduced by CsF and CuF2, we omitted
these additives in our NMR monitoring studies. Although this
resulted in a reduced yield of 5a, the early formation of 2a and
its persistence throughout the reaction was evident in both the
In order to clarify the role of dinickel oxidative addition
complexes as reaction intermediates, we carried out stoichioꢀ
metric experiments with 2a. Importantly, reaction of 2a with
Et3SiBPin resulted in no conversion to 5a after 5 h (Scheme 9,
top). This suggested that silylation does not occur directly
from the dinickel oxidative addition complex. Intriguingly, the
addition of 1a (0.90 equiv) gave 5a in a 27% yield after 5 h,
together with unreacted 2a (Scheme 9, bottom). These results
suggested that the presence of aryl pivalate facilitates C–Si
bondꢀformation, thus arguing against a heterolytic or homolytꢀ
ic cleavage pathway (Scheme 8, left pathways). Indeed, reacꢀ
tion of 1a with the monomeric Ni species derived from homoꢀ
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31P and H NMR spectra of the reaction (Figure 2 – H NMR)
suggesting that 2a is indeed relevant to the catalytic C–O
silylation reaction. It is worth noting that the concentration of
2a decreased slightly throughout the course of the reaction,
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lytic or heterolytic cleavage of the Ni–Ni bond would not be
expected to aid formation of 5a in the presence of Et3SiBPin.36
Scheme 11. Disproportionation with dcype.
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Scheme 9. Reaction of dinickel complex 2a with Et3SiBpin.
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Scheme 12. Reactivity of 2a with Et3SiBPin and ArOPiv.
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The combined data from our stoichiometric experiments
suggested that a disproportionation pathway comes into play,
where 2a is in equilibrium with Ni(II) complex [Ni(PCy3)(σꢀ
naphthyl)(ꢀ2ꢀOPiv)] (4) and Ni(0) (Scheme 10). The addition
of ArOPiv would then intercept the Ni(0) and in doing so,
increase the concentration of 4. Indeed, disproportionation
reactions for group 10 metal–metal dimers have been reported,
providing precedent for the viability of Scheme 8 (path c) with
intermediates such as 2a.35 For example, Pfaltz observed Pd–
Pd dimers and their corresponding Pd(0) and Pd(II) disproporꢀ
tionation products by ESIꢀmass spectroscopy.37
In order to validate the conclusions extracted from the stoiꢀ
chiometric studies, kinetic studies were performed. First, the
kinetic order in catalyst was determined. If an equilibrium
with 2a to form active monomeric species affects the reaction
rate, a half order in [Ni] may be expected. Therefore, 1a was
reacted with Et3SiBPin in the presence of [Ni(PCy3)2]2(N2)
(2.08 mM–6.25 mM) and CsF/CuF2 at 50 ºC.39 Due to the
stirring difficulties that occur after 90 minutes (vide infra),
kinetic orders were obtained using data collected from the first
90 minutes of the reactions. The order in catalyst was extractꢀ
ed from the resulting data by variable time normalization
analysis (VTNA) – a method that uses reaction profiles (inteꢀ
gral data) to determine the kinetic orders of reaction compoꢀ
nents (Figure 3).40 In this analysis, the time axes of the conꢀ
centration profiles were normalized to remove the effect of the
catalyst on the catalytic profile. Specifically, normalizing by
t[cat]order allowed us to determine that the three profiles best
overlay at t[cat]0.5. A halfꢀorder dependence is consistent with
the presence of an offꢀcycle equilibrium lying towards the
catalytically inactive species 2a. This conclusion contrasts
with a recent disclosure, which suggested that similar ꢁꢀη2ꢀ
arene dinickel complexes supported by Nꢀheterocyclic carꢀ
benes remain intact during the crossꢀcoupling of an aryl halide
with a Grignard reagent.32a To what extent the nature of the
supporting ligand is responsible for this behavior remains to
be elucidated.
Scheme 10: Perturbation of disproportionation equilibri-
um of 2a in the presence of ArOPiv.
It is worth noting that disproportionation of 2a is also the
microscopic reverse of the comproportionation mechanism
proposed for its formation. In line with this notion, we investiꢀ
gated whether disproportionation could be triggered by addiꢀ
tion of dcype, a bidentate phosphine ligand that was shown by
Itami to stabilize the mononuclear Ni(II) species [Ni(dcype)(2ꢀ
naphthyl)OPiv] (Ni-1).3,38 Indeed, exposure of 2b to dcype (3
equivalents) at room temperature resulted in the formation of
Ni-1 and [Ni(0)(dcype)2] (6) (Scheme 11). As expected, reꢀ
peating the experiment with 2a gave rise to 6 and
[Ni(dcype)(1ꢀnaphthyl)OPiv] (Ni-2), the structure of which
was confirmed by NMR and Xꢀray crystallography.24 The
presence of Ni(0) species was further assessed by reacting 2a
with 2ꢀnaphthyl pivalate (1b) in the presence of Et3SiBPin
(Scheme 12). We hypothesized that if Ni(0) is generated in
situ, then isomeric oxidative addition products – dinickel
complex 2b and silylated 5b – would be detected alongside
5a. As shown in Scheme 12, this was indeed the case, with
both 5a and 5b observed in 25% and 46% yields, respectively.
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ble compounds were obtained depending on whether excess
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CsF (2 equiv) or a combination of CsF (1 equiv) and CuF2 (0.3
equiv) was utilized. In the former, we obtained a 79% yield of
5b after 9 h, demonstrating that removing the Cu source enꢀ
tirely and replacing it with an extra equivalent of CsF gives a
comparable yield to the standard conditions (85%, see Figure
5ꢀleft for a comparison of the reaction profiles). In this case,
we isolated CsOPiv and Cs[F2Bpin] from the reaction mixture
1
(Scheme 13, bottom left). The CsOPiv was identified by H
NMR, and the fluoroborate by a wellꢀresolved triplet in the 11
B
NMR spectrum (δB = 4.78 ppm (1JB–F = 21 Hz)) and a multiꢀ
plet in the 19F NMR spectrum (δF = ꢀ138.9 ppm (1JBꢀF = 20.5
Hz)). If both CsF and CuF2 were employed, Cs[F2Bpin] was
accompanied by a new set of fluoroborate signals (δB = 5.34
ppm, δF = ꢀ138.3 ppm) (Scheme 13, right). These signals
presumably correspond to [F(PivO)Bpin]ꢀ, as only 1.6 equivaꢀ
lents of fluoride are available under these conditions, thus
preventing the complete formation of [F2Bpin]ꢀ.
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Scheme 13. Effect of additive on byproduct identity.
Figure 3. VTNA for order in catalyst showing the greatest
overlay at 0.5 order. [Ni(PCy3)2]2(N2) (2.08 mM–6.25 mM).
Additionally, the kinetic orders of 1a and Et3SiBPin were
determined. As shown in Figure 4, the reaction exhibited zeroꢀ
order dependence in 1a, suggesting that oxidative addition is
not rate determining. In contrast, a positive order dependence
on [Et3SiBPin] was determined, with VTNA analysis giving a
range of orders between 0.4 and 1, providing evidence for the
participation of this species in a rateꢀdetermining step of the
reaction.41
Puzzled by the reduced yields of 5a and 5b obtained in the
absence of additives, the formation of 5b under CsF/CuF2ꢀfree
conditions was monitored and compared to the standard protoꢀ
col employing CsF/CuF2. As shown in Figure 5ꢀleft, similar
behavior was observed for both reactions during the first 45
minutes. However, in the absence of additives, the formation
of 5b slowed dramatically after this time. This disparity could
be due to catalyst deactivation in the absence of additives,
and/or a mechanism based on Cu–SiEt3 intermediates becomꢀ
ing operative at later stages. However, it is important to note
that, as shown in Figure 5ꢀleft, CuF2 alone did not significantly
increase the yield of 5b, and its replacement with an extra
equivalent of CsF furnished 5b in an almost identical yield to
that obtained with CsF/CuF2 (83% vs 89%, red vs dark grey).
The addition of [Ni(PCy3)2]2(N2) (5 mol%) to the additiveꢀfree
reaction after product formation had stalled (3.75 h) resulted in
a slight increase in 5b (Figure 5ꢀright). Strikingly, when 1
equivalent of CsF was added at this time, product formation
resumed without the need for extra Ni(0) (Figure 5ꢀright).
Therefore, we propose that the generation of PivOBPin in the
additiveꢀfree reaction might inhibit the reaction.42 The presꢀ
ence of fluoride ions allows catalytic activity to be restored by
removing PivOBpin from solution as fluoroborate salts.
Figure 4. Kinetic study of order in 1a (left) and Et3SiBpin
(right).
Role of the CuF2 and CsF. To summarize, we have deterꢀ
mined that oxidative addition of aryl pivalates to Ni(0)/PCy3
readily forms dinickel complexes. These release mononickel
complexes that participate in the rateꢀdetermining transmetaꢀ
lation with Et3SiBPin. Still, however, the role of CuF2 and CsF
remained unclear. Specifically, control experiments revealed a
threeꢀfold increase in yield for the silylation of 1b when both
CsF and CuF2 were present, affording 5b in 90% isolated yield
after 16 h at 50 ºC. Prompted by these results, we next turned
our attention to studying the influence of CuF2 and CsF
(Scheme 13). In the absence of both CsF and CuF2, the forꢀ
mation of PivOBpin was observed by 11B NMR spectroscopy
(δB = 23 ppm, tolueneꢀd8, 50 ºC), and the reaction mixture
remained homogeneous (Scheme 13, top left); however, the
addition of CsF, with or without CuF2, resulted in the forꢀ
mation of insoluble byproducts. In particular, different insoluꢀ
Taken together, these experiments indicated that our initial
assumption of cooperative Ni/Cu catalysis was premature. In
light of our experimental data, we currently support a mechaꢀ
nism in which fluoride anions provided by CsF and CuF2 help
to prevent catalyst deactivation by sequestering the boronꢀ
containing byproducts as fluoroborates. At present, we have
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Journal of the American Chemical Society
and C–Si bond reductive elimination (Figure 6). The calculaꢀ
not obtained evidence that supports the intermediacy of a
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8
CuSiEt3 species. Whether a similar conclusion can be drawn
for recent C–O functionalization scenarios based on CuF2
remains to be investigated.
tions were performed in the absence of fluoride sources due to
the fact that (a) the first 45 minutes of the reaction profiles
were similar regardless of whether fluoride sources were preꢀ
sent or not, and (b) the observation that fluoride sources play a
role in preventing catalyst deactivation by sequestering offꢀ
cycle intermediates. As expected, initial πꢀcoordination of the
arene to the Ni(0) center is triggered by dissociation of PCy3
from either Ni(PCy3)2 or [(PCy3)Ni(toluene)],28 and leads to a
facile oxidative addition that gives rise to [Ni(PCy3)(σꢀ
naphthyl)(ꢀ2ꢀOPiv)] (II) with a low activation Gibbs free
energy of 14.7 kcal molꢀ1 (TSI-II). The reaction further evolves
to III, from which transmetalation with Me3SiBPin leads to
[Ni(PCy3)(σꢀnaphthyl)(SiMe3)] (IV) through a sixꢀmembered
transition state (TSIII-IV) with ꢀG‡ = 22.4 kcal molꢀ1. Subseꢀ
quently, an energetically downhill dissociation of PivOBPin
sets the scene for an effectively barrierless C–Si bond reducꢀ
tive elimination (TSV-VI, ꢀG‡ = 3.2 kcal molꢀ1). This is folꢀ
lowed by ligand exchange with 1a that releases 2a and recovꢀ
ers I. As a result, it becomes apparent that the transmetalation
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Figure 5. Left: Effect of additives on the silylation of 1b over
9 h. Right: Effect of adding [Ni(PCy3)2]2(N2) (5 mol%) (grey),
or CsF (2 equiv) (red) to the additiveꢀfree silylation of 1b.
Addition took place at 3.75 h. Aliquots were removed from the
reactions every 45 minutes and analyzed by GCꢀFID.
process with the Si–B species is turnoverꢀlimiting (IIꢀTSIII–V
=
22.4 kcal molꢀ1). This observation is consistent with the zeroꢀ
order dependence in 1a and the positiveꢀorder dependence in
Et3SiBPin.24 Our experimental data, however, showed that the
mononickel oxidative addition complex is in equilibrium with
dinickel 2a. Gratifyingly, the thermodynamic stability of 2a
compared with the mononickel complex was corroborated by
calculations, with the equilibrium Gibbs free energy found to
be –10.6 kcal molꢀ1 in favor of 2a (Scheme 14).
DFT calculations: Although our experimental data providꢀ
ed empirical evidence for the formation of dinickel complexes
and for their role within the catalytic cycle of the C–O silylaꢀ
tion of aryl esters, we turned our attention to DFT in order to
gather indirect evidence about other elementary steps within
the catalytic cycle. We performed DFT calculations with the
Gaussian 09 suite of programs.43 Optimizations were carried
out using 6ꢀ31G(d,p) basis sets for C, H, O, P, B, Si and the
Stuttgart Dresden (SDD) basis including an effective core
potential for nickel. The energies were refined by singleꢀpoint
calculations at the M06/def2tzvpp level,44 with the inclusion of
a solvent model system (IEFPCM, dichloromethane, toluꢀ
ene).45 We used Me3SiBPin in place of Et3SiBPin to avoid the
conformational complexity of the latter.
Scheme 14. Calculated equilibrium
We initially focused our attention on computing a monoꢀ
nickel mechanism consisting of an initial oxidative addition of
1a to Ni(0)PCy3, followed by transmetalation with Et3SiBpin
PCy3
O
Ni
TSIꢀII
14.7
O
tBu
[Ni(PCy3)(toluene)]
PCy3
11.0
O
tBu
Ni
Si
O
B
TSIIIꢀIV
1.8
PCy3
Ni
O
O
Me
O
tBu
Me
Me
Me
O
B
0.0
PCy3
Si
O
Me
Me
Me
O
III
Me
Ni
O
-10.3
O
tBu
I
PCy3
Ni
in
out
Me3Si-BPin
TSVꢀVI
-27.3
Me
-20.6
PCy3
Ni
PivO-BPin
Si
Me
-22.1
PCy3
Me
O
SiMe3
Ni
Si
O
tBu
O
tBu
-30.5
PCy3
Ni
O
OPiv
PCy3
Ni
II
B
out
O
O
Me
Me
Me
Si
Me
SiMe3
Me
Me
Me
in
IV
VI
V
-45.0
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Figure 6. Free energy profile for C–O silylation via a mononickel pathway. Energies are in kcal molꢀ1 and are calculated at the
M06/def2tzvpp level with the inclusion of a solvent model system (IEFPCM, dichloromethane, toluene).
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This result reinforces the role of 2a as an offꢀcycle species
and is consistent with the experimental halfꢀorder dependence
on 2a. In line with our observation that silylated product 5a is
not formed from 2a upon exposure to Et3SiBPin in the absence
of 1a, we were unable to locate a transition state for direct
transmetalation of 2a with the corresponding Si–B species. It
is worth highlighting that we computed the mechanism in the
presence of CsF, confirming that similar conclusions can be
drawn to those without CsF.24
Taking into consideration that monodentate phosphines are
frequently utilized as ligands in a wide variety of C–O funcꢀ
tionalization reactions, it is tempting to speculate that dinickel
complexes play an important role in these processes. More
importantly, we believe our study provides longꢀawaited inꢀ
sights into the mechanistic aspects of sp2 C–O functionalizaꢀ
tion in aryl esters and will serve as a useful entry point to
unravelling the mechanistic intricacies of related, yet not fully
understood, Niꢀcatalyzed crossꢀcoupling reactions.
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Scheme 15. Final mechanistic proposal.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Experimental procedures, spectral and crystallographic data
(PDF)
Computational details and Cartesian coordinates (PDF)
Data for 3 (CCDCꢀ1839593) (CIF)
Data for 2a (CCDCꢀ1839592) (CIF)
Data for Ni-2 (CCDCꢀ1839591) (CIF)
AUTHOR INFORMATION
Corresponding Authors
Overall, our experimental and computational data support a
mechanism where oxidative addition forms an offꢀcycle
dinickel complex that is in equilibrium with a mononickel
Ni(II) complex and Ni(0) (Scheme 15). Transmetalation ocꢀ
curs from the mononickel complex and forms PivOBpin as a
byproduct, which is intercepted by fluoride ions to form very
strong B–F bonds and insoluble fluoroborates, thus ensuring
catalyst turnover.
* rmartinromo@iciq.es
* jordi.bures@manchester.ac.uk
* enrique.gomez@ehu.es
Funding Sources
The authors declare no competing financial interest.ꢀ
ACKNOWLEDGMENT
R.J.S and R.M. thank ICIQ, MINECO (CTQ2015ꢀ65496ꢀR), &
Severo Ochoa Accreditation SEVꢀ2013ꢀ0319 for support. We also
thank the ICIQ XꢀRay Diffraction Unit and the ICIQ Mass Specꢀ
trometry unit. We sincerely thank Dr C. Sáenz and Prof J. R.
GalánꢀMascarós for the magnetic susceptibility analysis of 2a and
3. R. J. S. thanks “La Caixa” Severo Ochoa programme for a
predoctoral fellowship. L. V. A. H. acknowledges financial supꢀ
port through the Global Engagement of Doctoral Education Proꢀ
gram funded by Rackham Graduate School. We also acknowledge
insightful discussions with Drs. M. PérezꢀTemprano and F. Miꢀ
loserdov. E.G.ꢀB. thanks MINECO (CTQ2016ꢀ78083ꢀP) and
IZOꢀSGI SGIker of UPVꢀEHU for human and technical support.
CONCLUSION
The preparative potential of the Niꢀcatalyzed sp2 C–O funcꢀ
tionalization of aryl esters generally stands in contrast with the
dearth of experimental evidence available for the sp2 C–O
cleavage step or for the roles of the reaction intermediates
within the catalytic cycle. This is particularly evident when
monodentate phosphines are employed, an observation that is
typically associated with the formation of lowꢀcoordinated
entities that are prone to ligand dissociation. Indeed, experiꢀ
mental evidence for a seemingly trivial oxidative addition into
the sp2 C–O bond with monodentate phosphines was elusive.
Our study of the Ni/PCy3ꢀcatalyzed silylation of aryl esters via
sp2 C–O cleavage provides compelling evidence for a mechaꢀ
nism containing an unusual dinickel(µꢀη2ꢀarene) oxidative
addition complex. We have shown that decomposition pathꢀ
ways come into play upon heating, leading to a dinickel
pivalateꢀbridged species and homocoupling products, and
suggesting that such dimetallic complexes might be applicable
to other sp2 C–O functionalizations. Spectroscopic data, stoiꢀ
chiometric experiments and VTNA kinetic studies provide a
detailed picture of dinickel complexes as offꢀcycle reservoirs
of the active mononickel oxidative addition complex via disꢀ
proportionation. The products of disproportionation were
trapped by reacting isolated dinickel complex with bidentate
dcype. We have also revealed the importance of fluoride ions
in maintaining active catalysis.
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6) For nickel(II) aryl carboxylate complexes synthesized via anꢀ
ion metathesis, see: (a) Batten, M. P.; Canty, A. J.; Cavell, K.
J.; Rüther, T.; Skelton, B. W.; White, A. H. Synthesis of nickꢀ
el(II) Complexes Containing Neutral N,Nꢀ and Anionic N,Oꢀ
Bidentate Ligands, and Their Behaviour as ChainꢀGrowth
Catalysts; Structural Characterisation of Complexes Containꢀ
ꢀ
ing (mim)2CO, mimCO2 , and mimCPh2Oꢀ (Mim = 1ꢀ
Methylimidazolꢀ2ꢀyl). Inorganica Chim. Acta 2006, 359,
1710–1724. (b) Desjardins, S. Y.; Cavell, K. J.; Hoare, J. L.;
Skelton, B. W.; Sobolev, A. N.; White, A. H.; Keim, W. Sinꢀ
gle Component NꢀO Chelated Arylnickel(II) Complexes as
Ethene Polymerisation and CO/ethene Copolymerisation Catꢀ
alysts. Examples of Ligand Induced Changes to the Reaction
Pathway. J. Organomet. Chem. 1997, 544, 163–174. (c)
Isaeva, L. S.; Drogunova, G. I.; Peregudov, A. S.; Petrovskii,
P. V; Kravtsov, D. N. Aryl Bis(tricyclohexylꢀ
phosphine)nickel Carboxylates. Organomet. Chem. USSR
1988, 1, 482–485. (d) Isaeva, L. S.; Drogunova, G. I.; Pereꢀ
gudov,
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S.;
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Arꢀ
ylbis(triphenylphosphinyl)nickel Carboxylates. B. Acad. Ussr,
Div. Chem. Sci. 1988, 37, 153–157.
7) (a) Yue, H.; Guo, L.; Liu, X.; Rueping, M. NickelꢀCatalyzed
Synthesis of Primary Aryl and Heteroaryl Amines via CꢀO
Bond Cleavage. Org. Lett. 2017, 19, 1788–1791. (b)
MacQueen, P. M.; Stradiotto, M. NickelꢀCatalyzed Crossꢀ
Coupling of Ammonia or Primary Alkylamines with (Hetꢀ
ero)Aryl Sulfamates, Carbamates, or Pivalates. Synlett 2017,
28, 1652–1656. (c) Shimasaki, T.; Tobisu, M.; Chatani, N.
16) For selected examples of Niꢀcatalyzed Sonogashira reactions,
see: (a) Gallego, D.; Brück, A.; Irran, E.; Meier, F.; Kaupp,
M.; Driess, M.; Hartwig, J. F. From Bis(silylene) and
Bis(germylene) PincerꢀType nickel(II) Complexes to Isolable
Intermediates of the NickelꢀCatalyzed Sonogashira Crossꢀ
Coupling Reaction. J. Am. Chem. Soc. 2013, 135, 15617–
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15626. (b) Vechorkin O.; Barmaz D. NiꢀCatalyzed Soꢀ
Microwave and Solvothermal Methods for the Synthesis of
nogashira Coupling of Nonactivated Alkyl Halides. J. Am.
Chem, Soc. 2009, 131, 12078–12079. (c) Beletskaya I.P.;
Latyshev G. V.; Tsvetkov A. V.; Lukashev N. V. The Nickelꢀ
Catalyzed SonogashiraꢀHagihara Reaction. Tetrahedron Lett.
2003, 44, 5011–5013.
Nickel and Ruthenium Complexes with 9ꢀAnthracene Carꢀ
boxylate Ligand. Inorganica Chim. Acta 2015, 424, 176–185.
and references therein.
1
2
3
4
5
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7
8
27) 18% yield of 5a was observed when 3 was used as a precataꢀ
lyst (10 mol% Ni). See Supporting Information for details.
28) Under catalytic conditions, a dinuclear oxidative addition
scenario would be kinetically indistinguishable from that of a
mononuclear oxidative addition followed by formation of 2a.
However, the transition state for the oxidative addition inꢀ
volving dimeric nickel species was never located. For an exꢀ
ample of a carboxylate ligand aiding dinuclear oxidative addiꢀ
tion: Powers, D. C.; Ritter, T. A Transition State Analogue for
the Oxidation of Binuclear palladium(II) to Binuclear palladiꢀ
um(III) Complexes. Organometallics 2013, 32 (7), 2042–
2045.
17) Selected references: (a) Zarate, C.; Nakajima, M.; Martin, R.
A Mild & LigandꢀFree NiꢀCatalyzed Silylation via C–OMe
Cleavage. J. Am. Chem. Soc. 2017, 139, 1191–1197. (b) Zaraꢀ
te, C.; Manzano, R.; Martin, R. IpsoꢀBorylation of Aryl
Ethers via NiꢀCatalyzed C−OMe Cleavage. J. Am. Chem. Soc.
2015, 137, 6754–6757. (c) Cornella, J.; GómezꢀBengoa, E.;
Martin, R. Combined Experimental and Theoretical Study on
the Reductive Cleavage of Inert C–O Bonds with Silanes:
Ruling out a Classical Ni(0)/Ni(II) Catalytic Couple and Eviꢀ
dence for Ni(I) Intermediates. J. Am. Chem. Soc. 2013, 135,
1997–2009. (d) ÁlvarezꢀBercedo, P.; Martin, R. NiꢀCatalyzed
Reduction of Inert C−O Bonds: A New Strategy for Using
Aryl Ethers as Easily Removable Directing Groups. J. Am.
Chem. Soc. 2010, 132, 17352–17353.
18) For a recent example of an investigation of offꢀcycle intermeꢀ
diates in Niꢀcatalysis: Nett, A. J.; Zhao, W.; Zimmerman, P.
M.; Montgomery, J. Highly Active Nickel Catalysts for C−H
Functionalization Identified through Analysis of OffꢀCycle
Intermediates. J. Am. Chem. Soc. 2015, 137, 7636–7639.
19) Formation of complex 2b was observed with 2ꢀnaphthyl
pivalate (1b).
20) Jezorek, R. L.; Zhang, N.; Leowanawat, P.; Bunner, M. H.;
Gutsche, N.; Pesti, A. K. R.; Olsen, J. T.; Percec, V. Airꢀ
Stable Nickel Precatalysts for Fast and Quantitative Crossꢀ
Coupling of Aryl Sulfamates with Aryl Neopentylglycolꢀ
boronates at Room Temperature. Org. Lett. 2014, 16, 6326–
6329.
21) Jover, J.; Miloserdov, F. M.; BenetꢀBuchholz, J.; Grushin, V.
V; Maseras, F. On the Feasibility of NickelꢀCatalyzed Triꢀ
fluoromethylation of Aryl Halides. Organometallics 2014, 33,
6531–6543.
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29) Zhu, S.; Shoshani, M. M.; Johnson, S. A. Versatile (η6ꢀ
arene)Ni(PCy3) Nickel Monophosphine Precursors. Chem.
Commun. 2017, 53, 13176–13179.
30) Preꢀcoordination of the aryl pivalate by Ni(0) before oxidative
addition has been invoked in computational studies (see ref.
2).
31) Beromi, M.; Nova, A.; Balcells, D.; Brasacchio, A.; Brudvig,
G.; Guard, L.; Hazari, N.; Vinyard, D. Mechanistic Study of
an Improved Ni Precatalyst for SuzukiꢀMiyaura Reactions of
Aryl Sulfamates: Understanding the Role of Ni(I) Species. J.
Am. Chem. Soc. 2016, 139, 922–936.
32) (a) Matsubara, K.; Yamamoto, H.; Miyazaki, S.; Inatomi, T.;
Nonaka, K.; Koga, Y.; Yamada, Y.; Veiros, L. F.; Kirchner,
K. Dinuclear Systems in the Efficient NickelꢀCatalyzed Kuꢀ
madaꢀTamaoꢀCorriu CrossꢀCoupling of Aryl Halides. Organ-
ometallics 2017, 36, 255–265. (b) Beck, R.; Johnson, S. A.
Structural Similarities in Dinuclear, Tetranuclear, and Pentaꢀ
nuclear Nickel Silyl and Silylene Complexes Obtained via
Si−H and Si−C Activation. Organometallics 2012, 31, 3599–
3609. (c) Eisch, J. J.; Piotrowski, A. M.; Han, K. I.; Krüger,
C.; Tsay, Y. H. Oxidative Addition of Nickel(0) Complexes
to CarbonꢀCarbon Bonds in Biphenylene: Formation of Nickꢀ
elole and 1,2ꢀDinickelecin Intermediates. Organometallics
1985, 4, 224–231.
22) For examples of η2ꢀNi(0)/bidentate phosphine coordination to
naphthyl or quinoline systems (a) Li, T.; García, J. J.; Brenꢀ
nessel, W. W.; Jones, W. D. C−CN Bond Activation of Aroꢀ
matic Nitriles and Fluxionality of the η2ꢀArene Intermediates:
Experimental and Theoretical Investigations. Organometallics
2010, 29, 2430–2445. (b) Garcia, J. J.; Brunkan, N. M.; Jones,
W. D. Cleavage of CarbonꢀCarbon Bonds in Aromatic Niꢀ
triles Using nickel(0). J. Am. Chem. Soc. 2002, 124, 9547–
9555.
23) (a) Pal, S.; Zhou, Y. Y.; Uyeda, C. Catalytic Reductive Vinylꢀ
idene Transfer Reactions. J. Am. Chem. Soc. 2017, 139,
11686–11689. (b) Pal S.; Uyeda C. Evaluating the Effect of
Catalyst Nuclearity in NiꢀCatalyzed Alkyne Cyclotrimerizaꢀ
tions. J. Am. Chem. Soc. 2015, 137, 8042–8045. (c) Wu, J.;
Nova, A.; Balcells, D.; Brudvig, G. W.; Dai, W.; Guard, L.
M.; Hazari, N.; Lin, P. H.; Pokhrel, R.; Takase, M. K. Nickꢀ
el(I) Monomers and Dimers with Cyclopentadienyl and Inꢀ
denyl Ligands. Chem. Eur. J. 2014, 20, 5327–5337. (d) Beck,
R.; Johnson, S. A. Mechanistic Implications of an Asymmetꢀ
ric Intermediate in Catalytic C−C Coupling by a Dinuclear
Nickel Complex. Chem. Commun. 2011, 47, 9233–9235. (e)
Velian, A.; Lin, S.; Miller, A. J. M.; Day, M. W.; Agapie, T.
Synthesis and C−C Coupling Reactivity of a Dinuclear
Ni(I)−Ni(I) Complex Supported by a Terphenyl Diphosphine.
J. Am. Chem. Soc. 2010, 132, 6296–6297.
33) Previously unreported aryl pivalates 1c and 1d can be silylatꢀ
ed under the standard silylation conditions (see supporting inꢀ
formation for details).
34) The isomeric complex 2b was observed by 31P NMR when 2ꢀ
naphthyl pivalate 1b underwent additiveꢀfree silylation. Howꢀ
ever, integration was not possible due to overlapping signals.
See Supporting Information for details.
35) For selected reviews of metalꢀmetal bonds in similar systems,
see: (a) Inatomi, T.; Koga, Y.; Matsubara, K. Dinuclear Nickꢀ
el (I) and Palladium (I) Complexes for Highly Active Transꢀ
formations of Organic Compounds. Molecules 2018, 23, 140.
(b) Powers, I. G.; Uyeda, C. MetalꢀMetal Bonds in Catalysis.
ACS Catal. 2017, 7, 936–958.
36) For selected references concerning mononickel(I) complexes:
see ref. 28 and (a) Mohadjer Beromi, M.; Banerjee, G.; Bruꢀ
dvig, G. W.; Hazari, N.; Mercado, B. Q. Nickel(I) Aryl Speꢀ
cies: Synthesis, Properties, and Catalytic Activity. ACS Catal.
2018, 8, 2526–2533. (b) Bajo, S.; Laidlaw, G.; Kennedy, A.
R.; Sproules, S.; Nelson, D. J. Oxidative Addition of Aryl
Electrophiles to a Prototypical Nickel(0) Complex: Mechaꢀ
nism and Structure/Reactivity Relationships. Organometallics
2017, 36, 1662–1672. (c) Dürr, A. B.; Fisher, H. C.; Kalvet,
I.; Truong, K.ꢀN.; Schoenebeck, F. Divergent Reactivity of a
Dinuclear (NHC) Nickel (I) Catalyst versus Nickel (0) Enaꢀ
bles Chemoselective Trifluoromethylselenolation. Angew.
Chem. Int. Ed. 2017, 56, 1–6.
24) See Supporting Information for details.
25) This finding parallels the result of Itami et al. (see ref. 3)
when the conditions for the synthesis of Ni-1 were employed
with PCy3. The balanced equation for the formation of 3 from
[Ni(COD)2] is 2[Ni(COD)2] + 2PCy3 + 4 1a ꢀ 2 (1,1’ꢀ
binaphthalene) + [Ni2(PCy3)2(µꢀOPiv)4] + 4 COD.
26) For a related carboxylateꢀbridged structure, see Cortijo, M.;
DelgadoꢀMartinez, P.; GonzalezꢀPrieto, R.; Herrero, S.;
JimenezꢀAparicio, R.; Perles, J.; Priego, J. L.; Torres, M. R.
37) Markert, C.; Neuburger, M.; Kulicke, K.; Meuwly, M.; Pfaltz,
A. PalladiumꢀCatalyzed Allylic Substitution: Reversible Forꢀ
mation of AllylꢀBridged Dinuclear palladium(I) Complexes.
Angew. Chemie Int. Ed. 2007, 46, 5892–5895.
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38) For an example where Pd–Pd disproportionation products are
results suggest that PivOBPin might react with Ni(0) to preꢀ
vent catalyst turnover.
43) For a full reference of Gaussianꢀ09, See Supporting Inforꢀ
mation.
trapped with chelating alkenes, see: Hruszkewycz, D. P.; Balꢀ
cells, D.; Guard, L. M.; Hazari, N.; Tilset, M. Insight into the
Efficiency of CinnamylꢀSupported Precatalysts for the Suzuꢀ
ki–Miyaura Reaction: Observation of Pd(I) Dimers with
Bridging Allyl Ligands During Catalysis. J. Am. Chem. Soc.
2014, 136, 7300–7316.
1
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3
4
5
6
7
8
44) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionꢀ
als for Main Group Thermochemistry, Thermochemical Kiꢀ
netics, Noncovalent Interactions, Excited States, and Transiꢀ
tion Elements: Two New Functionals and Systematic Testing
of Four M06ꢀClass Functionals and 12 Other Function. Theor.
Chem. Acc. 2008, 120, 215–241.
45) (a) Cances, E.; Mennucci, B.; Tomasi, J. A New Integral
Equation Formalism for the Polarizable Continuum Model:
Theoretical Background and Applications to Isotropic and
Anisotropic Dielectrics. J. Chem. Phys. 1997, 107, 3032–
3041. (b) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Ab
Initio Study of Ionic Solutions by a Polarizable Continuum
Dielectric Model. Chem. Phys. Lett. 1998, 286, 253–260. (c)
Tomasi, J.; Mennucci, B.; Cances, E. The IEF Version of the
PCM Solvation Method: An Overview of a New Method Adꢀ
dressed to Study Molecular Solutes at the QM Ab Initio Levꢀ
el. J. Mol. Struct. THEOCHEM 1999, 464, 211–226.
39) Aliquots were analyzed by GCꢀFID. See Supporting Inforꢀ
mation for further details.
40) (a) Burés, J. Variable Time Normalization Analysis: General
Graphical Elucidation of Reaction Orders from Concentration
Profiles. Angew. Chemie 2016, 128, 16318–16321. (b) Burés,
J. A Simple Graphical Method to Determine the Order in Catꢀ
alyst. Angew. Chemie Int. Ed. 2016, 55, 2028–2031.
41) Although speculative, this range of normalization factors is
probably due to the formation of insoluble fluoroborates durꢀ
ing the reaction (vide infra), which result in stirring difficulꢀ
ties and may both affect the concentration of additives and inꢀ
fluence processes that are sensitive to the concentration of
Et3SiBpin. See Supporting information for further details.
42) Upon carrying out the CsF/CuF2ꢀfree silylation of 1b in the
presence of 10 mol% PivOBpin, a decrease in the yield of 5b
was obtained. See Supporting Information for details. These
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Ni/PCy3 catalyst
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Ni-Ni
intermediate
OPiv
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SiEt3
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+
O
O
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Et3Si
B
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role of CsF, CuF2 additives
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