Visible-Light-Induced Ni-Catalyzed Radical Borylation of Chloroarenes

Ya-Ming Tian, Xiao-Ning Guo,* Ivo Krummenacher, Zhu Wu, Jorn Nitsch, Holger Braunschweig,

Article

Visible-Light-Induced Ni-Catalyzed Radical Borylation of

Chloroarenes

Udo Radius,* and Todd B. Marder *

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sı Supporting Information

ABSTRACT: A highly selective and general photoinduced C−Cl borylation protocol

that employs [Ni(IMes) ] (IMes = 1,3-dimesitylimidazoline-2-ylidene) for the radical

borylation of chloroarenes is reported. This photoinduced system operates with visible

light (400 nm) and achieves borylation of a wide range of chloroarenes with B pin at

room temperature in excellent yields and with high selectivity, thereby demonstrating

its broad utility and functional group tolerance. Mechanistic investigations suggest that

the borylation reactions proceed via a radical process. EPR studies demonstrate that

[

Ni(IMes) ] undergoes very fast chlorine atom abstraction from aryl chlorides to give

Ni (IMes) Cl] and aryl radicals. Control experiments indicate that light promotes the

reaction of [Ni (IMes) Cl] with aryl chlorides generating additional aryl radicals and

[

Ni (IMes) Cl ]. The aryl radicals react with an anionic sp −sp diborane

B pin (OMe)] formed from B pin and KOMe to yield the corresponding

−

•

−

borylation product and the [Bpin(OMe)] radical anion, which reduces [Ni (IMes) Cl ] under irradiation to regenerate

[

Ni (IMes) Cl] and [Ni(IMes) ] for the next catalytic cycle.

INTRODUCTION

borylation of aryl chlorides at 90 °C. Jiao et al. reported a

pyridine-catalyzed transition-metal-free radical borylation of

aryl halides at 85 °C, but only two chloroarenes were described

■

Arylboronic esters have been recognized as key building blocks

in organic synthesis, materials science, and drug discovery.

Recently, highly selective transition-metal-catalyzed borylations

in this report.

Photocatalysis, as an alternative to the thermally induced

borylation of aryl halides by transition metals, has recently

emerged as a powerful tool for the synthesis of aryl boronate

of aryl halides, based on Pd, Ni, Fe, Cu, Co, Zn, have

been developed as an eﬃcient method to synthesize

arylboronic esters. Aryl iodides and bromides are frequently

employed as precursors to aryl boronate esters. However,

although widely available and inexpensive, chloroarene

borylation is more challenging due to the relatively high C−

Cl bond energy. Ishiyama et al. ﬁrst achieved the borylation of

C−Cl bonds by using a Pd(0)-tricyclohexylphosphine complex

esters due to the great enhancement of catalytic eﬃciency.

For example, Li et al. and Larionov et al. showed that aryl

halides, mesitylates, and ammonium salts can be photochemi-

cally borylated (Scheme 1). Aryl radicals, produced by

photoinduced homolytic or heterolytic cleavage of the

C(arene)−X bond, react with a boron source to form the

target aryl boronates. However, the necessary high-intensity

UV irradiation poses potential limitations to the applicability of

this method. Visible light photoredox catalysts can also be

employed to generate aryl boronates from aryl halides. Fu et al.

developed visible-light photoredox borylation of aryl halides

with B pin using fac-Ir(ppy) as the photocatalyst (Scheme

as a catalyst in 2001. The groups of Yamakawa and

Nakamura showed that [NiCl (PMe ) ] and [Fe(acac) ] as

3 2

catalyst eﬃciently borylate chloroarenes in the presence of

cesium 2,2,2-triﬂuoroethoxide and KO Bu at 100 and 130 °C,

respectively. Our groups recently reported an eﬃcient catalytic

procedure for the C−Cl borylation of chloroarenes using

NHC-Ni (NHC = N-heterocyclic carbene) complexes as

catalysts and bis(pinacolato)diboron (B pin ) as the boron

). The method displays high tolerance for various functional

groups; however, it is limited to aryl iodides and bromides.

source at 100 °C. In 2016, we showed that the selective C−F

borylation of polyﬂuoroaromatic compounds can be catalyzed

Received: August 17, 2020

by [Ni(IMes) ] (IMes = 1,3-dimesitylimidazoline-2-ylidene)

in the presence of [NMe ]F or CsF at 80 °C. Geetharani et al.

also reported an eﬃcient catalytic system based on a Co(II)-

NHC precursor for the borylation of chloroarenes at 50 °C,

aﬀording the aryl boronates in good to excellent yields.

Recently, our groups developed an eﬃcient Cu-catalyzed

XXXX American Chemical Society

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Scheme 1. Examples of Photoinduced C−X Borylation of Aryl Halides

Table 1. Photoinduced Borylation of 1a

The yields were determined by GC-MS analysis vs a calibrated internal standard and are averages of two runs.

The photocatalytic conversion of widely available aryl

pseudohalides, carboxylic acids, aryldiazonium salts, and

aromatic amines into versatile boronic esters has been reported

by Li, Glorius, Aggarwal, Yan and Ranu et al. Very recently,

Jiao et al. reported a visible-light-induced organocatalytic

method for the borylation of unactivated chloroarenes, and

even ﬂuorides, by photoactivation of the super electron donor

halide abstraction to form Ni complexes plus aryl radicals.

Louie et al. and Nelson et al. further conﬁrmed that the

products of the reactions of [Ni(NHC) ] with aryl halides are

mainly dominated by the steric impact of the NHC ligand,

based on experimental and computational investigations,

respectively. With this information, and the fact that aryl

radicals are well-known intermediates in some catalytic

borylation processes, we hoped to achieve the activation of

chloroarenes to form corresponding arylboronic esters utilizing

(

SED) complex generated in situ in a diboron/methoxide/

pyridine reaction system (Scheme 1). Our groups recently

reported a photocatalytic C−F borylation protocol that

employs a Rh biphenyl complex ([Rh]) as a triplet sensitizer

[

Ni(IMes) ] as the catalyst (Scheme 1).

and the nickel catalyst [Ni(IMes) ] for the borylation of C−F

RESULTS AND DISCUSSION

■

bonds in aryl ﬂuorides (Scheme 1). Further mechanistic

studies showed that the exceptionally long-lived triplet excited

state of the Rh biphenyl complex allows for eﬃcient triplet

energy transfer to the oxidative addition product trans-

We ﬁrst investigated the eﬃciency of the borylation reaction of

-chloroanisole (1a) with B pin in the presence of KOMe

using 10 mol % [Ni(IMes) ] as the catalyst at 25 °C, which

gave a 12% yield of 1-Bpin-4-methoxybenzene (1b) after 2 h

(Table 1, entry 1). When the reaction time was extended to 6

h, the yield of 1b was almost the same, suggesting that the

protocol is ineﬃcient.

Our previous work showed that the 5 mol % [Rh]/10 mol %

[Ni(IMes) ] dual photocatalytic system exhibited excellent

activity for selective C−F borylation of aryl ﬂuorides (Scheme

1). Therefore, the same system was examined for the

[

Ni F(Ar )(IMes) ] to greatly enhance the rate constant for

16,17

the transmetalation step with B pin at room temperature.

Cavell et al. found that small NHC ligands with less steric

hindrance, such as 1,3,4,5-tetramethylimidazol-2-ylidene

(

tmiy), permit oxidative addition to form Ni complexes,

while Matsubara et al. found that larger NHC ligands, such as

,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IPr), favor

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Table 2. Optimization of the Reaction Conditions for the Photoinduced Borylation of 4-Chloroanisole

Yield (%)

Entry

Base (1.5 equiv)

Solvent (2 mL)

KOMe

NaOMe

hexane

methylcyclohexane

toluene

THF

THF-d₈

MTBE

trace

Et O

hexane

KO Bu

Li CO

Cs CO

KOAc

NaOAc

CsF

−

trace

The yields were determined by GC-MS analysis vs a calibrated internal standard and are averages of two runs. The ratio of anisole-4-d to anisole

was ca. 1:1.

borylation of 1a. First, a reaction carried out in the dark for 2 h

yielded ca. 13% of 1b (Table 1, entry 3), indicating that the

entry 5), suggesting a radical process. The yield is signiﬁcantly

inﬂuenced by the amount of both B pin and KOMe, with 1.5

[

Rh] complex has no eﬀect on the thermal borylation reaction.

equiv of each providing an excellent yield (Table S1 ).

The yield of 1b reached 72% after only 2 h of irradiation of the

reaction mixture with a 400 nm LED (Table 1, entry 4),

suggesting that the dual photocatalytic system works eﬃciently

for the borylation of aryl chlorides. However, the photo-

catalytic process yielded more dechlorohydrogenated by-

product anisole (1d, 15%) compared with that obtained

when the reaction was performed in the dark. Interestingly,

highly eﬃcient and selective borylation of 1a under visible light

We further examined the scope of chloroarene substrates for

the photoinduced borylation under the optimized conditions,

and the results are summarized in Table 3. Chloroarenes with

either electron-donating (1a−13a) or electron-withdrawing

(14a−18a) groups including esters, ketones, and nitriles

reacted smoothly to aﬀord the corresponding arylboronic

esters in good to excellent yields, highlighting the excellent

functional group tolerance of this method. Ortho-substitution

with MeO (2a) or Me (4a, 7a, 9a) groups did not signiﬁcantly

lower the yield of the product, while substitution with two

ortho-methyl groups (10a, 11a) somewhat reduced the yield.

(

[

400 nm) irradiation at 25 °C was observed using only

Ni(IMes) ] as the photocatalyst without [Rh] complex,

giving 1b in 86% yield after 6 h (Table 1, entry 2). Our group

developed a Zn-catalyzed 1,2-selective dual C−H/C−X

borylation of aryl iodides and bromides in 2015. Then,

Larionov et al. reported an additive- and metal-free photo-

induced regioselective 1,2- and 1,3-C−H/C−X diborylation of

aryl halides, including three examples of chloroarenes, under

batch and continuous ﬂow conditions. In this work, we also

observed both C−Cl and C−H borylation of chloroarenes, and

the yield of 1,2-(Bpin) -4-methoxybenzene (1c) in the

Importantly, both strong π-donor (NMe , 13a) and π-acceptor

(CO CH , 14a; COCH , 15a) substituents, as well as the

inductively withdrawing CF group (16a) at the para-position,

resulted in excellent yields of the product. 4-Chlorobiphenyl

(20a), naphthyl chlorides (21a, 22a), and several chloroheter-

oarenes (23a−27a) are also compatible substrates. Notably,

the protocol worked well with 5-chloroindole bearing an NH

group (28a), enabling convenient borylation of such substrates

without the need for a protecting group. High yields of

diborylation products were also achieved with dichloroben-

zenes as substrates (29a and 30a). Not only does the reaction

show wide scope with respect to B pin , but bis(neopentyl

12b

photoinduced reaction, is slightly higher compared to that

from the reaction without irradiation (Table 1, entries 1−4). It

is worth pointing out that this is the ﬁrst report of Ni-catalyzed

C−X/C−H diborylation of arenes. The reaction did not occur

under light irradiation in the absence of [Ni(IMes) ] (Table 1,

glycolato)diboron (B neop ) can be substituted for B pin as

2 2 2 2

entries 5 and 6).

the diboron reagent to give the corresponding aryl neo-

pentylglycolato boronate ester products (31b−34b) in similar

yields to those observed with B pin .

Preliminary mechanistic studies were conducted to provide

insight into the photoinduced borylation process. First, using

allyl o-chlorophenyl ether (1aa) as the substrate gave the

We screened a range of solvents and bases, with hexane and

KOMe proving to be optimal (Table 2). Importantly, THF

gave more of the hydrodehalogenation product ArH (Table 2,

entry 4) than other solvents and there was signiﬁcant

deuterium incorporation when THF-d was used (Table 2,

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Table 3. Scope of the Photoinduced Borylation of Chloroarenes

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Table 3. continued

Reaction conditions: B pin (0.3 mmol), chloroarene (0.2 mmol), KOMe (0.3 mmol), [Ni(IMes) ] (10 mol %), hexane (2 mL), at room

temperature under irradiation with a 400 nm-LED for 6 h unless otherwise stated. The yields are based on aryl chlorides and were determined by

GC-MS analysis vs a calibrated internal standard and are averages of two runs. Isolated yields based on 1 mmol of aryl chloride are given in

parentheses. The reaction time was 24 h. B pin (0.6 mmol) and KOMe (0.6 mmol) were used. B pin (3 mmol) and KOMe (3 mmol) were

used. B neop (0.3 mmol) was used. B neop (1.5 mmol) was used.

Scheme 2. Preliminary Mechanistic Studies: (a) Aryl Radical Trapping; (b) Hydrogen Atom Transfer from THF-d₈

monoborylated product 1ab, but the cyclized (borylmethyl)

indane (1ac) was the main product, indicating an intra-

molecular trapping of the aryl radical by the oleﬁn moiety,

followed by coupling with the boryl species (Scheme 2a).

Second, when the reaction of 1a with B pin was conducted in

hydrogen atom donor 9,10-dihydroanthracene as a radical trap,

both borylation products were obtained in lower yields (1b:

73%; 1c: 7%) with an increased yield of 1d (17%). Increasing

the amount of the dihydroanthracene radical trap to 10 equiv

signiﬁcantly reduced the yield of 1b to 49% and 1c to 1%, and

increased the yield of 1d to 42%. The presence of 0.4 equiv of

2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) as a radical

trap shut down the reaction almost completely. The above

intra- and intermolecular trapping reactions clearly indicate the

generation of aryl radicals from the aryl chlorides.

THF-d instead of hexane, the borylation product 1b was

observed in a lower yield (61%), together with a signiﬁcant

amount of dehalogenation product anisole (1d, 10%) and

anisole-4-d (1e, 10%), due to hydrogen atom transfer from

•

THF to p-MeO-C H₄(Table 2, entry 5, Scheme 2b, and

Figure S1 ). Thus, the hydrogen atom donor THF competes

with the boryl species to react with the aryl radical. When the

reaction of 1a was performed in the presence of 2 equiv of the

In situ EPR spectra at 70 K in frozen hexane after 1, 2, 3, and

5 min were recorded to conﬁrm the formation of radical

species. From Figure 1A, it can be seen that the signals increase

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hexane (see SI for details).

Article

Figure 1. (A) Changes in the low-temperature CW EPR spectra of the reaction of [Ni(IMes) ] and 1a in hexane with the reaction time at 70 K and

(

B) the simulated spectrum shown in red; (C) simulated (red) and experimental (black) CW EPR spectra of isolated [Ni (IMes) Cl] in frozen

Figure 2. (A) UV−visible absorption of [Ni(IMes) ], [Ni (IMes) Cl], [Ni (IMes) Cl ], K[B pin (OMe)], and IMes in degassed hexane solution

at room temperature; (B) emission (excited at 400 nm) and excitation spectra of solid [Ni (IMes) Cl ] under air at room temperature.

•

with time. Comparison of the in situ EPR spectrum at 70 K

[Ni (IMes) Cl]. No signal for Ar was detected in the in situ

(

Figure 1B) with the EPR spectrum of isolated [Ni (IMes) Cl]

for synthesis and molecular structure, see Supporting

EPR spectra, and neither benzene nor biphenyl compounds

were detected by GC-MS. However, from our in situ NMR

spectra and HRMS measurements of the reaction of

Information (SI)) in frozen hexane (Figure 1C) revealed

that they are essentially identical. The similarity of in situ EPR

chlorobenzene with [Ni(IMes) ], the aryl complex

spectra of the reaction of [Ni(IMes) ] with chlorobenzene also

[Ni (IMes) Ph] was detected in addition to [Ni (IMes) Cl]

•

conﬁrmed the formation of the complex [Ni (IMes) Cl]

(Figure S140 ). Therefore, we speculated that the Ar species

during the reaction (Figure S2 ). While the main contribution

likely reacts very quickly with [Ni(IMes) ] to form

•

to the observed EPR spectrum is from [Ni (IMes) Cl], a

[Ni (IMes) Ar], whereas, in the catalytic reaction system, Ar

second small signal which is attributed to [Ni (IMes) Ph] was

observed. The result is consistent with literature reports,

that steric impact of the IMes ligands prevents coordination of

is more likely to react with B pin or with the more

2 2

19,20

−

nucleophilic [B pin (OMe)] to form the borylation product.

2 2

To clarify the role of light, we investigated the photophysical

properties of the diﬀerent compounds in the reaction mixture.

From the UV−vis spectra (Figure 2A), both IMes and

[

Ni(IMes) ] to the chloroarene and leads to chlorine atom

•

abstraction to form an aryl radical Ar and the complex

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−0.94 V (Figure 3 and Figure S5A ), which is not suﬃcient to

Article

K[B pin (OMe)] (the product of the direct reaction of KOMe

Figures S147 −S154 ). The reaction of [Ni (IMes) Cl] with 19a

h,i

with B pin )

have almost no absorption at wavelengths

300 nm in hexane. [Ni(IMes) ] has a very broad absorption

yielded 5% of [Ni (IMes) Cl ] in the dark after 2 h (Scheme 3

and Figures S155 and S156 ). However, almost complete

spectrum with two absorption maxima at 508 and 616 nm,

conversion of [Ni (IMes) Cl] to [Ni (IMes) Cl ] in the

2 2 2

while [Ni (IMes) Cl] has an absorption band between 300−

presence of 19a was observed under 400 nm LED irradiation

for the same time (Scheme 3 and Figures S157 and S158 ), and

biphenyl was detected by GC-MS from the photoinduced

reaction, indicating that a rapid photochemical reaction of

50 nm with a maximum at 340 nm. However, [Ni (IMes) Cl]

and [Ni(IMes) ] do not show any detectable emission,

preventing us from obtaining the energies of their lowest

singlet or triplet states via emission spectroscopy. We know

[Ni (IMes) Cl] with 19a generates [Ni (IMes) Cl ] and a

phenyl radical.

Furthermore, the redox potentials of the participating and

temperature, and therefore we focused on the role of

isolated metal complexes were investigated. The onset

[

Ni (IMes) Cl] in subsequent photochemical processes.

oxidation of [Ni (IMes) Cl] was found to occur at E

The extinction coeﬃcient of [Ni (IMes) Cl] at 400 nm is ca.

−

−1

320 cm

(Figure 2A). Independently prepared

[

(

Ni (IMes) Cl] was employed as catalyst instead of [Ni-

IMes) ] for the photoinduced reaction of chlorobenzene

19a) with B pin under otherwise identical conditions, giving

8% and 17% yields of 19b and 19c, respectively (Table 4,

Table 4. Photoinduced Borylation of 19a with

[

Ni (IMes) Cl] and [Ni (IMes) Cl ] as Catalysts or

2 2 2

Catalyst Precursors

Figure 3. Comparison of redox potentials of the Ni species,

K[B pin (OMe)], MeO-Bpin, and chlorobenzene potentially

involved in the photocatalytic C−Cl borylation reaction. Potentials

are referenced to the ferrocene/ferrocenium couple. Solvent eﬀects on

the redox potentials have not been considered.

The yields were determined by GC-MS analysis vs a calibrated

internal standard and are averages of two runs.

entry 1). When the reaction was carried out in the dark, the

yield of 19b was only 15% (Table 4, entry 2), indicating that

reduce chlorobenzene to its corresponding radical anion

•

−

light is necessary for an eﬃcient reaction. Further control

[Ered(PhCl/PhCl ) = −2.78 V]. Therefore, [Ni (IMes) Cl]

experiments showed that [Ni (IMes) Cl] had good stability

is expected to react with chlorobenzene via chlorine atom

under continuous irradiation (Figures S145 and S146) for 2 h

with 400 nm LEDs, and it did not react with B pin in either

abstraction to generate a phenyl radical and the

[Ni (IMes)

] complex. Light irradiation greatly enhances

the presence or absence of KOMe or light (Scheme 3 and

the eﬃciency of this reaction achieving almost 100% yield of

[

Ni (IMes) Cl ].

[

2 2

Ni (IMes) Cl ] dissolved in degassed hexane has an

2 2

Scheme 3. Control Reactions of [Ni (IMes) Cl] with B pin ,

absorption band at 305 nm and a very weak and broad

absorption band between 350 to 600 nm, and its extinction

KOMe, and Chlorobenzene, Respectively, without (left) and

with (right) Irradiation

−1

coeﬃcient at 400 nm is only 66 cm

(Figure 2A).

Although there was no emission detected spectroscopically

from [Ni (IMes) Cl ] in hexane, we observed that the

“

solution” appeared to be cloudy under UV irradiation and

an orange-red luminescence was observed by eye. This

indicates the presence of very ﬁne [Ni (IMes) Cl ] particles

which did not dissolve in hexane. We therefore recorded the

spectroscopic properties of [Ni (IMes) Cl ] in the solid state

under air at room temperature (Figures 2B and S4).

Interestingly, in contrast to the absorption spectra in dilute

solution, a broad excitation between 320 and 500 nm is visible

with a maximum at 365 nm. A broad solid-state emission with

λ_m_a_x= 625 nm was observed for [Ni (IMes) Cl ], which is

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Article

absent in solution, probably due to rapid nonradiative decay

processes.

V = −1.20 V, E(Ni* /Ni ) = E(Ni /Ni ) + E = −2.82 V +

1.98 V = −0.84 V, E is the emission energy]. Thus, electron

We then employed [Ni (IMes) Cl ] as a catalyst for the

transfer would be expected to occur from the reactive

[Bpin(OMe)]^•species, formed by the in situ reaction of the

−

photocatalytic reaction of 19a with B pin , which resulted in a

−

9a,11a

5% yield of 19b and an 11% yield of 19c, respectively, after 6

aryl radical with [B pin (OMe)] ,

[Ni (IMes) Cl ]* [E(Ni* /Ni ) = −0.84 V] under irradiation,

reducing it to [Ni (IMes) Cl] and then [Ni(IMes) ].

to the excited

h. No reaction was detected without irradiation. In situ NMR

experiments showed that [Ni (IMes) Cl ] is stable under

irradiation with 400 nm LEDs for 2 h (Figures S159 and S160 )

In light of these ﬁndings, a plausible mechanism for the

borylation of chloroarenes under irradiation is shown in

and did not react with 19a, B pin , or KOMe alone either in

the dark or under irradiation at 400 nm (Scheme 4 and Figures

Scheme 5. [Ni(IMes) ] undergoes very fast chlorine atom

Scheme 4. Control Reactions of [Ni (IMes) Cl ] with

Scheme 5. Plausible Mechanism for the Photoinduced

Borylation of Aryl Chlorides by Transformations between

Ni Species in the 0, I, and II Oxidation States

B pin , KOMe, Chlorobenzene, and B pin /KOMe,

Respectively, without (left) and with (right) Irradiation

abstraction from aryl chlorides to give [Ni (IMes) Cl] and aryl

S161 −S172 ). Very recently, Doyle et al. reported that a metal-

radicals. Under further irradiation, [Ni (IMes) Cl] reacts with

aryl chlorides to form [Ni (IMes) Cl ] and another aryl

to-ligand charge transfer (MLCT) state of [Ni( bpy)(Ar)X]

(

bpy = substituted 2,2′-bipyridine) complexes, generated

radical. The reaction of the generated aryl radicals with

initially upon excitation, rapidly decays into a long-lived d−d

state, which features a weak Ni-aryl bond. This results in

excited Ni complexes undergoing Ni−C bond homolysis to

generate aryl radicals and Ni . In our case, when

−

nucleophilic [B pin (OMe)] yields the ﬁnal borylated

•

−

product and the [Bpin(OMe)]

(OMe)] is a strong reducing agent, capable of converting

[Ni (IMes) Cl ] to [Ni (IMes) Cl] and [Ni(IMes) ].

2 2 2 2

radical anion. [Bpin-

•

−

[

Ni (IMes) Cl ] was employed together with B pin and

KOMe under irradiation for 2 h, the formation of

CONCLUSIONS

[

Ni (IMes) Cl] and [Ni(IMes) ] in 23% and 5% yield,

■

respectively, was observed accompanied by the formation of

MeO-Bpin (Scheme 4 and Figures S175 and S176 ); both

reduced Ni compounds were not detected when the reaction

was performed in the dark (Scheme 4 and Figures S173 and

S174 ).

In conclusion, we have developed an eﬃcient photocatalytic

procedure for the C−Cl borylation of chloroarenes using

[Ni(IMes) ] as a catalyst operating with visible light. The

reaction proceeds under mild conditions, displays broad scope

and functional group tolerance, and furnishes arylboronates in

good to excellent yields. Our investigations of the mechanism

suggest that abstraction of a chlorine atom from an aryl

chloride by [Ni(IMes) ] is facile at room temperature, forming

an aryl radical and [Ni (IMes) Cl]. Aided by light, a similar

process is expected to occur for [Ni (IMes) Cl] producing

additional aryl radicals and [Ni (IMes) Cl ]. The aryl radicals

are believed to react with nucleophilic [B pin (OMe)] to

form the ﬁnal borylation product and the [Bpin(OMe)]

Aryl radicals, as key intermediates, should react in our

process with the nucleophilic sp −sp diborane anion

−

9h,i

[

B pin (OMe)] formed from B pin and KOMe

generate the corresponding borylated product and the

Bpin(OMe)]^•radical anion.

−

9a,11a

We collected the

[

NMR spectra of MeO-Bpin using C D , CD CN, and THF-d

−

as solvent, respectively (Figures S181 −S183). No obvious shift

was found from the NMR spectra, indicating that MeO-Bpin is

stable in these solvents and no adducts are formed. The redox

behavior of MeO-Bpin was also investigated. No reduction

peak was observed within the electrochemical window of the

THF solvent (Figure 3 and Figure S5B), indicating that MeO-

2 2

•

−

radical anion, which in turn reduces [Ni (IMes) Cl ] to

regenerate [Ni (IMes) Cl] and [Ni(IMes) ] for the next

catalytic cycle.

Bpin has a very negative reduction potential [E (MeO-Bpin)

ASSOCIATED CONTENT

•

−

■

sı Supporting Information

−3.4 V], making [Bpin(OMe)] a particularly strong

reducing agent. [Ni (IMes) Cl ] undergoes a reversible one-

The Supporting Information is available free of charge at

electron oxidation and an irreversible one-electron reduction at

E = 0.78 V and E = −2.82 V in CH CN and THF solution,

respectively (Figure 3 and Figures S5C and S5D). In the solid

Details of synthetic methods, characterization (GC-MS,

NMR and HRMS data) of products, and investigations

of the reaction mechanism (PDF)

state, [Ni (IMes) Cl ] exhibits a broad emission with λ =

max

25 nm; therefore, the oxidation and reduction potentials of

excited [Ni (IMes) Cl ]* were calculated to be −1.20 V and

−

III

Crystallographic data for [Ni (IMes) Cl] (CIF)

0.84 V [E(Ni /Ni* ) = E(Ni /Ni ) − E = 0.78 V − 1.98

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Westcott, S. A.; Marder, T. B. Diboron(4) Compounds: from

Article

AUTHOR INFORMATION

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Todd B. Marder − Institute of Inorganic Chemistry and Institute

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Authors

Ya-Ming Tian − Institute of Inorganic Chemistry and Institute

for Sustainable Chemistry & Catalysis with Boron, Julius-

Germany; orcid.org/0000-0001-5083-137X

Julius-Maximilians-Universitat

Ivo Krummenacher − Institute of Inorganic Chemistry and

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Germany; orcid.org/0000-0001-9537-1506

Wurzburg, 97074 Wurzburg,

̈ ̈

Zhu Wu − Institute of Inorganic Chemistry and Institute for

Sustainable Chemistry & Catalysis with Boron, Julius-

Maximilians-Universitat

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rn Nitsch − Institute of Inorganic Chemistry and Institute for

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Julius-Maximilians-Universitat Wurzburg, 97074 Wurzburg,

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Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

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i) Billingsley, K. L.; Buchwald, S. L. An Improved System for the

This work was supported by funds from the Julius-Max-

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Scholarship Council for providing a Ph.D. scholarship to Y.-

M.T. and to the Alexander von Humboldt Foundation for

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DEDICATION

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Dedicated to Prof. Pierre H. Dixneuf in recognition of his

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