Light-Promoted Nickel Catalysis: Etherification of Aryl Electrophiles with Alcohols Catalyzed by a NiII-Aryl Complex

Article abstract of DOI:10.1002/anie.202003359

A highly effective C?O coupling reaction of (hetero)aryl electrophiles with primary and secondary alcohols is reported. Catalyzed by a Ni^II-aryl complex under long-wave UV (390–395 nm) irradiation in the presence of a soluble amine base without any additional photosensitizer, the reaction enables the etherification of aryl bromides and aryl chlorides as well as sulfonates with a wide range of primary and secondary aliphatic alcohols, affording synthetically important ethers. Intramolecular C?O coupling is also possible. The reaction appears to proceed via a Ni^I–Ni^III catalytic cycle.

Full text of DOI:10.1002/anie.202003359

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Accepted Article
Title: Light-Promoted Nickel Catalysis: Etherification of Aryl
Electrophiles with Alcohols Catalyzed by a Ni(II)-Aryl Complex
Authors: Dong Xue, Liu Yang, Huan-Huan Lu, Chu-Hui Lai, Gang Li,
Wei Zhang, Rui Cao, Fengyi Liu, Chao Wang, and Jianliang
Xiao
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Light-Promoted Nickel Catalysis: Etherification of Aryl
Electrophiles with Alcohols Catalyzed by a Ni(II)/Aryl Complex
Liu Yang†, Huan-Huan Lu†, Chu-Hui Lai†, Gang Li†, Wei Zhang†, Rui Cao†, Fengyi Liu†, Chao Wang†,
Jianliang Xiao†,‡ and Dong Xue*†
Abstract: A highly effective C-O coupling reaction of (hetero)aryl
electrophiles with primary and secondary alcohols is reported.
Catalyzed by a Ni(II)-aryl complex under long-wave UV (390-395 nm)
irradiation in the presence of a soluble amine base without any
additional photosensitizer, the reaction allows for the etherification of
aryl bromides, aryl chlorides as well as sulfonates with a wide range
of primary and secondary aliphatic alcohols, affording synthetically
important ethers. The intramolecular C-O coupling is also possible.
The catalysis appears to proceed via a Ni(I)-Ni(III) cycle.
Alkyl aryl ethers are versatile structural components and are
widely found in natural products, agrochemicals and
pharmaceuticals.[1] Effective approaches, such as the
Williamson ether synthesis,[2] the Mitsunobu reaction,[3] and
nucleophilic aromatic substitution,[4] have contributed to the easy
Scheme 1. Nickel-catalyzed C−O coupling of aryl electrophiles with alcohols.
synthesis of alkyl aryl ethers. However, owing to the abundance
of various electrophiles and alcohols, it is more desired to
synthesize such ethers through the metal-catalyzed cross-
electron-rich diphosphine ligand as precatalyst, aryl electrophiles
coupling of these two types of starting materials.[5-7] Pioneering
were coupled with alcohols to afford aromatic ethers under
work based on palladium,[6] copper[7] and nickel[8] catalysis has
been carried out, with the former two metals being most widely
used for etherification at the present time. However, all of these
C-O coupling reactions necessitate the use of strong inorganic
bases or metal alkoxides, which brings about solubility and
compatibility issues and related operational challenges. In the
closely related C-N coupling, these issues have already caught
attention and have been elegantly addressed with amine bases
recently by Buchwald and coworkers.[9] Likewise, using mild,
soluble amines as bases for the coupling of alcohols with aryl
electrophiles, especially inexpensive and abundant (hetero)aryl
chlorides,[10] would provide a practical solution to the long-
standing challenge in the synthesis of alkyl aryl ethers. A
cheaper metal catalyst could make the protocol more attractive.
Ni complexes are prone to undergoing oxidative addition with
CSp2-Cl bonds,[11] which could make them able, economic
catalysts for the C-O cross-coupling of (hetero)aryl chlorides with
alcohols. Recently, two successful strategies for Ni catalyzed
C−O couplings of aryl electrophiles with alcohols have in fact
been developed.[8] Using a Ni(II) complex bearing a bulky,
thermal conditions (Scheme 1, A).[8a] However, highly-basic
metal alkoxides were required, which are moisture-sensitive and
incompatible with base-sensitive functional groups.[8a] The
merger of photoredox and nickel catalysis has provided another
strategy, where the oxidation state of the nickel catalyst is
modulated by the excited photosensitizer (Scheme 1, B).[8b]
Although this dual-catalytic method affords an important
alternative for the synthesis of alkyl aryl ethers, the scope is
limited to the use of (hetero)aryl bromides, and expensive iridium
photocatalysts and insoluble inorganic bases are needed.
Recently, we reported a hydroxylation reaction of aryl halides
with water enabled by the merger of organophotoredox and
nickel catalysis with mildly basic N,N-diisopropylethylamine as a
base.[12] Our study showed that only organic amines can promote
the reaction, with traditional strong bases such as NaOtBu and
inorganic bases being ineffective. In continuing our work, we
report herein a highly effective C-O coupling reaction of aryl
electrophiles with alcohols catalyzed by the dual action of a
Ni(II)-aryl complex and light, with an organic amine as base and
with no use of any external photosensitizers[13] (Scheme 1, C).
In our initial study, we discovered that the isolable, air-stable
Ni(II)-aryl halide complexes A-D[14] could catalyze the cross-
coupling reaction of p-acetylchloro benzene 1 with n-butanol 2
(Table 1, entries 1-4) under the irradiation of long-wave UV light
(390-395 nm, purple LEDs) in the absence of any external
photosensitizers.[15] Notably, light was found to play a crucial role
in the reaction; only the long-wave UV light could afford high
product yields. Whilst irradiation using other light also promoted
the reaction, it was much less effective (Table 1, entries 5–8).
Moreover, only a trace amount of product was obtained when the
reaction was run at 120 oC in the absence of light (Table 1, entry
[a]
L. Yang, H.-H. Lu, C.-H. Lai, Dr. G. Li, Prof. Dr. W. Zhang, Prof.
Dr. R. Cao, Prof. Dr. F. Liu, Dr. C. Wang, Prof. Dr. J. Xiao, Prof.
Dr. D. Xue, Key Laboratory of Applied Surface and Colloid
Chemistry, Ministry of Education, and School of Chemistry and
Chemical Engineering, Shaanxi Normal University, Xi’an, 710062
(China), E-mail: xuedong_welcome@snnu.edu.cn
Prof. Dr. J. Xiao
[b]
Department of Chemistry
University of Liverpool
Liverpool, L69 7ZD, (UK)
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Table 1. Reaction development and optimization[a]
(16,17), benzothiophene (18), benzoxazole (19) and
benzothiazole (20) with good to excellent yields, which are
important building blocks in drug molecules.
As shown in Scheme 2, various alcohols can be used. Thus, the
simple, abundant methanol afforded 21 in good yield, opening a
route to anisole derivatives.[6i] Primary aliphatic alcohols bearing
benzylic and branched structures (23-25) as well as pendant
trifluoroethanol (22), per-fluorobutylethanol (26), alkenyl (27, 28),
amino (30), furan-2-yl (32) and thiophen-2-yl (33) groups were
also good coupling substrates. It is worth noting that the catalyst
was not poisoned by the coordinative amino and thiophen
moieties. To our delight, the carbohydrate alcohol derived from
D-galactopyranose (34) and fructose (35) also worked well. Diols
could be employed (36-39). We note that the mono-substituted
products were dominant, showing sterically less cumbersome
alcohols to be more reactive (37-39). Although secondary
alcohols were typically less reactive for the coupling with
acetylchloro benzene 1, a higher catalytic loading (15 mol %)
[a] 1 (0.5 mmol), 2 (2.0 mmol, 4.0 equiv), catalyst (5.0 mol %), DBU (1,8-
diazabicyclo[5.4.0]undec-7-ene, 0.75 mmol), toluene (1 mL). For details, see
Supporting Information. [b] Yields determined by 1H NMR. [c] Isolated yield.
9), revealing the critical role of the long-wave UV light. The
choice of base also significantly impacts the success of the
reaction, and delightfully, the cheap amine base DBU (1,8-
diazabicyclo[5.4.0]undec-7-ene) outperformed all other bases
tested.[15] In addition, the presence of oxygen dramatically
decreased the reaction efficiency (Table 1, entry 10). Further
optimization demonstrated that a combination of complex A and
DBU in toluene was the best with respect to reaction yield.[15]
Under such conditions, the desired product 3 was obtained with
92% isolated yield at 80 oC for 12 h under the irradiation of long-
wave UV light in argon. Further control experiments revealed that
the reaction did not proceed in the absence of nickel catalyst,
light, or base (Table 1, entries 11-13).
To investigate the scope of this new method, a variety of aryl
electrophiles and alcohols were tested. First, a wide range of aryl
chlorides were explored. As summarized in Scheme 2, electron-
deficient aryl chlorides containing ketone (3), ester (4),
trifluoromethyl (5) and cyano (6) groups show higher efficiency
in coupling with n-butanol. It is noted that functionalities such as
the ester and cyano may not be compatible with the conventional,
strongly-basic conditions. Indeed, owing to the use of strong
bases, transesterification in copper catalysis[7f] and formation of
impurities in palladium catalysis due to the cyano group[16] have
been reported. In contrast, all ester and cyano-containing
electrophiles are suitable for this new protocol, demonstrating
the advantage of using organic amines as base. More
importantly, the challenging electron-neutral aryl chlorides also
worked well, albeit at a higher catalytic loading (15 mol %) (7-9).
However, this was achieved at a lower reaction temperature as
compared to the thermal reaction catalyzed by copper[7f] and
nickel[8a] systems. Unfortunately, electron-rich aryl chlorides
such as 4-chloroanisole, were found inactive under the current
conditions. Notably, for aryl chloride bearing a chloro substituent,
the mono-etherified product (10) was dominant, thus providing
the possibility for subsequent transformation. Of further interest
is that a range of (hetero)aryl chlorides are compatible, delivering
the desired products containing quinolines (12-15), pyridine
Scheme 2. Etherification of aryl chlorides. Yields refer to isolated yields. For
details, see Supporting Information. [a] Ni catalyst A (15 mol %). [b] 24 h.
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and longer reaction time (24 h) afforded the etherified products
in good yields (40-46).
acid, amides, or ortho-substituents gave poor yields or showed
no activities under the standard conditions.[15]
Next, we explored the scope and limitations of aryl bromides
for this C-O coupling reaction. As shown in Scheme 3, aryl
bromides with a variety of functional groups reacted efficiently
with two equivalents of alcohols, delivering the desired products
with good to excellent yields (47-57). Notably, for aryl bromides
bearing both a chloro and bromo substituent, the corresponding
mono-substituted products were obtained with good yields (49,
50).The disubstituted aryl bromides bearing either electron-
Various types of alcohols were next explored. As in the case
of aryl chlorides, the coupling reactions exhibited high efficiency
and functional group compatibility (61-79). To further
demonstrate the application of this new etherification protocol,
intramolecular C-O bond formation[8c,17] was studied (80-90). As
shown in Scheme 3, a series of important five- and six-
membered ring products were obtained with high yields.
Furthermore, compound 88 could be prepared at gram-scale,
which showcases the significant potential of this protocol in
practical synthesis of complex molecules.[18]
donating or withdrawing groups were all applicable, affording the
desired products in high yields (53-57). A wide range of
(hetero)aryl bromides were also compatible. Thus, etherified
heteroarenes containing quinoline, carbazole, benzothiophene
and indole were obtained with good to excellent yields (58-60,
13, 18). However, aryl bromides bearing a methoxy, carboxylic
Finally, the scope of the electrophiles was expanded to
sulfonates. As shown in Scheme 4, mesylates, tosylates and
triflates were all viable in the coupling with n-butanol, affording
the ethers with good yields.
Scheme 4. Etherification of aryl sulfonates. Yields refer to isolated yields. For
details, see Supporting Information.
The detailed mechanism of the etherification remains to be
delineated. The recent, in-depth studies of the photo-physics and
-chemistry of complex B by Doyle and co-workers[14a,d] indicate
that complex A may itself act as a photocatalyst, promoting the
reaction via a Ni(I)-Ni(III) cycle. The results from our preliminary
experiments appear to be in line with this. As shown in Scheme
5, when complex A was irradiated at 390-395 nm for 1 h without
substrates, significant amounts of arenes were observed,
indicating concomitant formation of Ni(I) (Scheme 5, A).[14d]
When the in-situ prepared Ni(0) complex Ni(dtbbpy)(cod)[19] was
used as catalyst under the standard conditions, the reaction
Scheme 5. Reactions aimed to probe the reaction mechanism. [a] Yields
determined by 1H NMR. For details, see Supporting Information. THF =
tetrahydrofuran.
Scheme 3. Etherification of aryl bromides with 2 equivalents of alcohols. Yields
refer to isolated yields. For details, see Supporting Information. [a] Ni catalyst
A (10 mol %). [b] 24 h.
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proceeds with high yields; however, when the light was omitted,
the desired product was not observed (Scheme 5, B),
highlighting that light is required to enable the C-O coupling,
regardless of the pre-catalyst being Ni(II) or Ni(0).[15]
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (21871171) and the 111 project (B14041).
Next, the Ni(II) complex A was irradiated under the long-
wave UV light for 30 min in the absence of substrates and then
the irradiation was stopped. Following introducing the substrates
and carrying out the reaction in the dark for 12 h, the
corresponding product was obtained with a low yield of 35%. A
somewhat higher yield was seen when the initial irradiation was
extended to 60 min (Scheme 5, C). EPR (electron paramagnetic
resonance) analysis of the irradiated A (in the absence of
substrates) revealed a signal with g∥=2.25 and g⊥=2.07 (SI,
Figure 8),[15] which indicates the formation of a d9 Ni(I)
species.[19,20] Moreover, when the Ni(I) complex, prepared in situ
from the comproportionation of catalyst A and Ni(dtbbpy)(cod) by
following the procedure of Nocera and co-workers,[19] was used
as catalyst, the desired product was obtained only in 16% yield
under thermal conditions (Scheme 5, D). The EPR signal of the
resulting Ni species is similar to that of the irradiated Ni(II)
catalyst A (SI, Figure S9).[15]
The observations above suggest that the etherification in
question involves Ni(I) species and light is required throughout
to achieve high product yields. On the basis of these results and
the studies of Doyle, Scholes and Nocera et al,[14,19] a simplified
mechanistic pathway is tentatively suggested for the
etherification under question (Scheme 6). Irradiation of A at 390-
395 nm would excite the Ni(II) complex to a 3MLCT state[21] that
probably decays to a long-lived 3d-d state, which could then
undergo homolysis of the nickel-carbon bond, generating Ni(I)
and aryl radicals.[14d] The former could then react with the aryl
halide to generate a Ni(III)-Ar intermediate, which would undergo
facile reductive elimination upon ligand exchange with the
alcohol (presumably facilitated by DBU),[22] affording the coupled
ether product while regenerating the Ni(I). Continuous irradiation
is necessary. This is probably due to the formation of off-cycle,
catalytically inactive Ni(II) from the comproportionation of Ni(III)
and Ni(I),[19a] which requires irradiation to convert back to the
active Ni(I).[23,24]
Keywords: etherification · aryl electrophiles · Nickel(II)aryl halides ·
nickel catalysis · photocatalysis·
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Scheme 6. A tentative mechanism for the etherification under light-promoted
nickel catalysis (X = halide, X’ = Ar or OR; see reference 24).
In summary, we have developed a light-promoted, highly
effective C-O coupling reaction, which uses an air-stable Ni(II)-
aryl halide complex as catalyst and DBU as base, with no need
for any other photosensitizer. The protocol features a very wide
substrate scope, allowing for primary and secondary aliphatic
alcohols to couple with a wide range of (hetero)aryl electrophiles
with diverse functional groups, including in particular the more
readily available but more challenging (hetero)aryl chlorides. Our
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remain to be answered.[24]
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(Figure S5) and DBU (Figure S6).
[23] The Ni(I) may interact with the Ni(II) species, forming an inactive bimetallic
complex; see reference 19a.
[24] The off-cycle Ni(II) species could not be clearly defined in terms of quantity
and the identity of the anion, and its conversion to Ni(I) is not clear either.
Based on the recent literature, it could be converted back to Ni(I) via
photo-induced disproportionation (reference 14a), photo-induced radical
dissociation (reference 14d), or photo-induced reduction by DBU
(reference 22). In all cases there should be by-product(s), which is not
shown, however.
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10.1002/anie.202003359
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Liu Yang, Huan-Huan Lu, Chu-Hui Lai,
Gang Li, Wei Zhang, Rui Cao, Fengyi Liu,
Chao Wang, Jianliang Xiao,Dong Xue*
Page No. – Page No.
Light-promoted
Nickel
Catalysis:
Etherification of Aryl Electrophiles
with Alcohols by
Halide Complex
a Nickel(II)/Aryl
A highly effective C-O coupling reaction of (hetero)aryl electrophiles with alcohols
catalyzed by a Ni(II)-aryl complex without any additional photosensitizer is reported.
This article is protected by copyright. All rights reserved.