Mechanistic insight into the nickel-catalyzed cross-coupling of aryl phosphates with arylboronic acids: Potassium phosphate is not a spectator base but is involved in the transmetalation step in the Suzuki-Miyaura reaction

Article abstract of DOI:10.1002/asia.201300688

Spectator or actor? Density functional theory calculations were performed to examine the role of the base in the nickel-catalyzed cross-coupling of aryl phosphates with arylboronic acids. Potassium phosphate was found to not act as a spectator base but was involved in the transmetalation step, as shown by a lower barrier than that of a base-free process, owing to the activation of the carbon-boron bond by the base. Further experimental observations support the theoretical findings. Copyright

Full text of DOI:10.1002/asia.201300688

COMMUNICATION
DOI: 10.1002/asia.201300688
Mechanistic Insight into the Nickel-Catalyzed Cross-Coupling of Aryl
Phosphates with Arylboronic Acids: Potassium Phosphate is Not a Spectator
Base but is Involved in the Transmetalation Step in the Suzuki–Miyaura
Reaction
[
a]
[a]
[c]
[a]
[b]
[a]
Liu Liu, Shuangyan Zhang, Hu Chen, Ye Lv, Jun Zhu,* and Yufen Zhao*
CarbonÀcarbon (CÀC) and carbonÀheteroatom (CÀX)
bond-forming reactions are highly important for the con-
struction of structurally sophisticated compounds in the syn-
thesis of functional molecules, such as organic electronic ma-
terials and biologically active compounds. Among these re-
actions, nickel-catalyzed Suzuki–Miyaura cross-coupling
showed that potassium phosphate (4.5 equiv) was the most-
effective base to generate the desired product, 2-phenyl-
naphthalene (93%). No obvious change in yield was ob-
served when the loading of K PO was increased to
3
4
[1]
5.0 equivalents. Note that this reaction performed well with
an even stronger base, such as KOH (4.5 equiv, 80% yield).
Interestingly, in the reaction system, aryl phosphates could
(
SMC) reactions have attracted increasing attention for
[2]
À
their high efficiency and inexpensive catalyst systems. In
most of these nickel-catalyzed SMC reactions, an appropri-
ate and excessive base is particularly important. However,
be dehydrated to form PÀO complexes in the presence of
[
4]
[5]
KOH.
Previously, Suzuki, Miyaura, and co-workers,
[6]
[7]
Percec and co-workers, and Han and co-workers pro-
posed that the phosphate anion may coordinate to the metal
center of the oxidative-addition product through a ligand-
exchange step. Shi and co-workers reported the formation
[2r,3]
its function in the reaction is controversial.
Recently, we
reported the nickel-catalyzed cross-coupling of aryl phos-
phates with arylboronic acids to give a variety of biaryl com-
[2q]
[8]
pounds (Scheme 1). In an initial study, we found that dif-
ferent bases significantly affected this reaction. For example,
when 2-naphthyl phosphates and phenylboronic acid were
chosen as the model substrates, a screening of various bases
of borate from aryloxylate and boronic acid derivatives.
Moreover, our ongoing interest in phosphoric-carboxylic
[9]
mixed anhydride reactivities led us to propose a new boro-
[10]
phosphate species (Scheme 2) that may be involved in the
transmetalation step.
Scheme 1. Nickel-catalyzed cross-coupling of aryl phosphates with aryl-
boronic acids.
Scheme 2. Proposed active species (Ar=aryl).
Herein, we report a detailed theoretical and experimental
study on the reaction mechanism of the nickel-catalyzed
SMC reaction of aryl phosphates with arylboronic acids. In
the density functional theory (DFT) calculations, the model
[
a] L. Liu, S. Y. Zhang, Y. Lv, Prof. Dr. Y. F. Zhao
Key Laboratory for Chemical Biology of Fujian Province
College of Chemistry and Chemical Engineering
Department of Chemistry, Xiamen University
Xiamen, 361005 (China)
catalyst Ni ACHTGNUTRNEUNG( PCy ) , diethyl phenyl phosphate, and phenylbor-
3 2
E-mail: yfzhao@xmu.edu.cn
Homepage: http://chem.xmu.edu.cn/group/yfzhao/zhao-home.html
onic acid were used. Relative free energies are employed to
analyze the reaction mechanism.
[
b] Dr. J. Zhu
In general, three basic steps (oxidative addition, transme-
talation, and reductive elimination) are included to study
the mechanisms of the SMC reactions, as exemplified by
State Key Laboratory of Physical Chemistry of
Solid Surfaces, Fujian Provincial Key Laboratory of
Theoretical and Computational Chemistry
College of Chemistry and Chemical Engineering
Xiamen University, Xiamen, 361005 (China)
E-mail: jun.zhu@xmu.edu.cn
[11]
a number of recent theoretical studies. Two possible path-
ways for the oxidative addition step in our model reaction
are considered in Figure 1 (paths A and B), based on the
palladium- and nickel-catalyzed CÀX (X=O, Cl, Br, I) acti-
[
c] Dr. H. Chen
Department of Chemistry and Chemical Engineering
Hefei Normal University
[
11b,12]
vation reactions.
Both processes start with a bis-ligated
0
Ni species, which is generated from the catalyst precursor
Ni(PCy ) Cl ] in the presence of arylboronic acid and
K PO . Path A involves the direct coordination of the bi-
Hefei, 230601 (China)
[
ACHTUNGTRENNUNG
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300688.
3
2
2
3
4
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Jun Zhu, Yufen Zhao et al.
puted to be 51.6 (TS3) and
À1
4
9.8 kcalmol
(TS3’). There-
fore, the base-free pathway is
not energetically feasible, in
line with the experimental ob-
servations that this reaction
cannot occur without the pres-
ent of the base.
Next, we turned our attention
to the base-assisted transmeta-
lation step. In the previous cal-
culations of Ni-catalyzed SMC
reactions (Scheme 3), Liu and
[
11b]
co-workers
co-workers
and Houk and
proposed that
[11e]
Figure 1. Gibbs energy profile for the oxidative addition step in the nickel-catalyzed cross-coupling of diethyl
the active species that partici-
pated in the transmetalation
À1
phenyl phosphate with phenylboronic acid; L=PCy
3
(in kcalmol ).
sphosphine NiL complex to PhOP(O) CAHTUNGTERNN(UGN OEt) to generate an
2
2
À1
unstable intermediate, IN1 (29.4 kcalmol ). Subsequent ox-
idative addition from IN1 proceeds through a three-mem-
bered transition state, TS1, thus leading to a four-coordinat-
ed cis-Ni complex, IN2, which can easily isomerize into the
more-stable trans-Ni complex, IN3.
II
II
[11d]
Path B starts with
2
a monophosphine h complex, IN4, which is formed by the
removal of one phosphine ligand from NiL , followed by
2
a coordination to PhOP(O) ACHTUGNTERNN(UNG OEt) . Immediate IN5 is
2
formed through five-centered transition state TS2, which
II
can isomerize into a four-coordinated Ni Àmonophosphine
complex, IN6, which is more stable than IN3 by 28.4 kcal
À1
mol , thus suggesting that path B is kinetically and thermo-
dynamically favorable for the oxidative addition step.
To investigate the role of the potassium phosphate in the
transmetalation step, we first calculated the base-free path-
way (Figure 2). Coordination of the phenylboronic acid to
Scheme 3. Mechanism of the nickel-catalyzed Suzuki–Miyaura reaction.
À
step were borate anions (ArB(OH) ) that were formed
3
II
species IN6 can generate two types of Ni intermediates,
in situ from arylboronic acid and K PO . However, the only
3
4
IN7 and IN7’, which undergo six- (TS3) and four-membered
transition states (TS3’) to form IN8 and IN8’, respectively.
The overall barriers for this transmetalation step were com-
experimental evidence for the involvement of the borate
anions in the transmetalation step was their presence in the
[13]
reaction mixture.
a new Ni-catalyzed SMC reaction and successfully isolated
In 2011, Shi and co-workers reported
[8]
a key intermediate borate species. In addition, a clear
change in yield was reported by using different bases in a va-
riety of nickel-catalyzed SMC reactions. For most of these
reactions, the optimal reaction conditions favored potassium
[2]
phosphate as the base. To the best of our knowledge, no
solid evidence has been reported to address the effect of po-
tassium phosphate. Herein, we propose a new transmetala-
tion pathway that is mediated by a reactive anhydride spe-
cies, borophosphate (Figure 3). This transmetalation reac-
tion begins with the interaction of phosphate with phenyl-
boronic acid, which generates an active species, borophos-
II
phate. This species coordinates with the Ni complex, IN6,
to form a slightly stable intermediate, IN9. In species IN9,
three potassium cations are coordinated to P-bonded
oxygen atoms and the BÀOH groups. The distances between
Figure 2. Gibbs energy profile for the base-free transmetalation step in
+
K and oxygen atoms range from 2.627–2.908 ꢁ. The subse-
the nickel-catalyzed cross-coupling of diethyl phenyl phosphate with phe-
À1
nylboronic acid; L=PCy
3
(in kcalmol ).
quent dissociation of KOP(O) ACHTUNEGRTNNUNG( OEt) leads to the formation
2
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Jun Zhu, Yufen Zhao et al.
complex, IN15, was formed.
The corresponding transition
state, TS6, was only 5.0 kcal
À1
mol
higher in energy than
complex IN15. The formation
of the CÀC bond completes the
catalytic cycle by regenerating
NiL2.
To test the reasonability of
our model reactive species, we
examined the interactions be-
tween the phosphate and phe-
3
1
nylboronic acid by P NMR
Figure 3. Gibbs energy profile for the base-mediated transmetalation step in the nickel-catalyzed cross-cou-
À1 [14]
pling of diethyl phenyl phosphate with phenylboronic acid (in kcalmol ). PCy was used as a ligand in the spectroscopy. A mixture of po-
3
calculations. For clarity, the cyclohexyl groups on the ligand and the CÀH hydrogen atoms are not shown.
tassium phosphate (0.15 mmol)
and phenylboronic acid
0.15 mmol) in water was fol-
(
II
31
of tetracoordinated Ni complex IN10, which is converted
lowed by P NMR spectroscopy (Figure 5). Initially, there
was only one signal in the spectrum, which was assigned to
into the transmetalation product through a four-membered
transition state, TS4. The overall barrier of this borophos-
À1
phate-assisted transmetalation step is 30.2 kcalmol , in line
with the experimental observation that this reaction was car-
[15]
ried out at 1108C. Notably, this barrier is much smaller
than those for the base-free transmetalation reaction
(
Figure 2), which can mainly be attributed to the activation
of the CÀB bond by the borophosphate. For instance, the
CÀB bond length (1.645 ꢁ) in IN10 is much longer than
those in IN7 and IN7’ (1.618 and 1.571 ꢁ, respectively). Ac-
cordingly, the bond order of the CÀB bond in IN10 (0.73) is
smaller than those in IN7 and IN7’ (0.81 and 0.88, respec-
tively). Moreover, the CÀB bond length (2.056 ꢁ) in TS4 is
also shorter than those in TS3 and TS3’ (2.138 and 2.112 ꢁ,
respectively). Therefore, breaking the CÀB bond in the bor-
ophosphate-assisted transmetalation step is much easier
than that in the base-free process.
Finally, the reductive elimination step shown in Figure 4
was not energy-demanding. This process was completed
through a monophosphine pathway to avoid steric interac-
tions between the two bulky phosphine ligands. After re-
3
1
Figure 5. P NMR study of the interaction between phosphate and phe-
nylboronic acid.
II
moval of the K PO B(OH) species, a tricoordinated Ni
2
4
2
K PO (d =5.2 ppm). Then, a new phosphorus-containing
3
4
a
species was formed over time (d =3.1 ppm). Although this
b
species was not stable enough to be characterized by HRMS
and could not be crystallized, thus leading to uncertainty
À
over its structure, a very similar species, B(OH) CO , was
reported by McElligott and Byrne.
2
3
[16]
In summary, we have studied the mechanism of the
nickel-catalyzed cross-coupling of aryl phosphates with aryl-
boronic acids both experimentally and computationally. Our
results reveal that, in the oxidative addition and reductive
elimination steps, the monophosphine-ligand pathway is pre-
ferred to the bis-ligated one. More importantly, potassium
phosphate was found not to act as a spectator base but
rather was involved in the transmetalation step. Thus, our
findings provide key insight into the mechanism of the
nickel-catalyzed Suzuki–Miyaura reactions and open a new
avenue to the design of efficient catalysts.
Figure 4. Gibbs energy profile for the reductive elimination step in the
nickel-catalyzed cross-coupling of diethyl phenyl phosphate with phenyl-
À1
boronic acid; L=PCy
3
(in kcalmol ).
Chem. Asian J. 2013, 00, 0 – 0
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Jun Zhu, Yufen Zhao et al.
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°
k=(k
B
T/h)exp
A
H
U
G
R
N
U
G
B
is the
°
Boltzmann constant, T is the temperature, DG is the free energy of
activation, R is the gas constant, and h is Planck’s constant. Assum-
ing that the half-life of this reaction is 24 h and that the concentra-
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equal to 1.2ꢅ10 Lmol s, we obtained that at 1108C, DG is
1
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À1
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[
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prize.org/nobel_prizes/chemistry/laureates/2010/suzuki-lecture.html.
Received: May 20, 2013
Revised: June 12, 2013
Published online: && &&, 0000
À
4] The PÀO species in the hydrolysate were confirmed by MS (ESI)
and the data are provided in the Supporting Information.
&
&
Chem. Asian J. 2013, 00, 0 – 0
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

COMMUNICATION
Spectator or actor? Density functional
theory calculations were performed to
examine the role of the base in the
nickel-catalyzed cross-coupling of aryl
phosphates with arylboronic acids.
Potassium phosphate was found to not
act as a spectator base but was
Reaction Mechanisms
Liu Liu, Shuangyan Zhang, Hu Chen,
Ye Lv, Jun Zhu,*
Yufen Zhao*
&&&& — &&&&
Mechanistic Insight into the Nickel-
Catalyzed Cross-Coupling of Aryl
Phosphates with Arylboronic Acids:
Potassium Phosphate is Not a Spectator
Base but is Involved in the Transmeta-
lation Step in the Suzuki–Miyaura
Reaction
involved in the transmetalation step, as
shown by a lower barrier than that of
a base-free process, owing to the acti-
vation of the carbonÀboron bond by
the base. Further experimental obser-
vations support the theoretical find-
ings.
Chem. Asian J. 2013, 00, 0 – 0
5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ