Acenaphthoimidazolylidene Gold Complex-Catalyzed Alkylsulfonylation of Boronic Acids by Potassium Metabisulfite and Alkyl Halides: A Direct and Robust Protocol to Access Sulfones

Article abstract of DOI:10.1021/acscatal.7b00860

A robust acenaphthoimidazolylidene gold complex is demonstrated as a highly efficient catalyst in the direct alkylsulfonylation of boronic acids. Remarkably, a wide range of highly reactive and unreactive C-electrophiles were well-tolerated to produce various (hetero)aryl-alkyl, aryl-alkenyl, and alkenyl-alkyl sulfones in satisfactory yields with 5 mol % catalyst loading. Along with the steric properties of NHC ligands, the high catalytic activity of this gold complex suggests that the strong σ-donation of acenaphthoimidazolylidene also played a role in promoting this challenging redox-neutral catalytic process.

Full text of DOI:10.1021/acscatal.7b00860

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Letter
Acenaphthoimidazolylidene Gold Complex-Catalyzed Alkylsulfonylation
of Boronic Acids by Potassium Metabisulfite and Alkyl
Halides: A Direct and Robust Protocol to Access Sulfones
Haibo Zhu, Yajing Shen, Qinyue Deng, Jiangbo Chen, and Tao Tu
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017
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Acenaphthoimidazolylidene Gold Complex-Catalyzed Alkylsulfonyl-
ation of Boronic Acids by Potassium Metabisulfite and Alkyl Halides:
A Direct and Robust Protocol to Access Sulfones
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Haibo Zhu,† Yajing Shen,† Qinyue Deng,† Jiangbo Chen† and Tao Tu*†‡
†Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University,
220 Handan Road, Shanghai, 200433, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
ligands and gold catalysis may provide a practical approach to
ABSTRACT: A robust acenaphthoimidazolylidene gold comꢀ
directly accessing various structurally diverse sulfones. Sulfones
are one of the most prominent motifs in functional molecules with
applications in agrochemical, pharmaceutical and materials sciꢀ
ence,10 and simple protocols for their synthesis are in high deꢀ
mand. Herein, following our recent research on the synthesis and
potential application of novel NHCꢀmetal complexes,7,8,11 we
report the robust acenaphthoimidazolylidene Au(I) complexꢀ
catalyzed alkylsulfonylation of various boronic acids by K2S2O5
and diverse Cꢀelectrophiles under mild reaction conditions. Furꢀ
thermore, along with the steric effects,12 we show that strong σꢀ
donation of the acenaphthoimidazolylidene is crucial and accelerꢀ
ates this challenging transformation.
plex is demonstrated as a highly efficient catalyst in the direct
alkylsulfonylation of boronic acids. Remarkably, a wide range of
highly reactive and unreactive Cꢀelectrophiles were well tolerated
to produce various (hetero)arylꢀalkyl, arylꢀalkenyl and alkenylꢀ
alkyl sulfones in satisfactory yields with 5 mol% catalyst loading.
Along with the steric properties of NHC ligands, the high catalytic
activity of this gold complex suggests that the strong σꢀdonation
of acenaphthoimidazolylidene also played a role in promoting this
challenging redoxꢀneutral catalytic process.
KEYWORDS: alkylsulfonylation, boronic acids, gold, Nꢀ
heterocyclic carbene complexes, sulfones
Although Au(I) has the same d10 configuration as Pd(0), Au(I)
complexes have been rarely applied in reactions with conventionꢀ
al coupling reagents, including aryl boronic acids and/or aryl
halides.1 Toste and coworkers reported the first Au(I)ꢀcatalyzed
alkylsulfonylation of aryl boronic acids with a SO2 surrogate,
potassium metabisulfite (K2S2O5, Scheme 1).2 When 10 mol% tꢀ
Bu3PAuCl was applied, up to 66% yield was achieved; however,
the scope was somewhat limited. Notably, this useful threeꢀ
component sulfone synthesis constituted a challenging catalytic
transformation,3 even for Pd(0) catalysis. In general, monoꢀligated
cationic Au complexes were considered the true catalytic species,
which require strong electronic and steric stabilization from the
ligands.4 Nꢀheterocyclic carbene (NHC) ligands are excellent σꢀ
donors and exhibit high activity in various transformations.5ꢀ6
However, when the IPrAuCl complex,4 [IPr:1,3ꢀbis(2,6ꢀ
diisoprophyl)ꢀimidazoleꢀylidene], was used, a lower yield (21%)
was achieved under identical reaction conditions.2 Nevertheless,
due to its robustness, a stoichiometric test with IPrAuCl helped
provide better understanding to a plausible mechanism for this
challenging transformation.
Scheme 1. Gold-catalyzed direct sulfone synthesis.
To verify our hypothesis, the catalytic efficiencies of NHCꢀAu
complexes 1 and 2 (Scheme 1) were investigated using the direct
coupling of 4ꢀmethoxyphenylboronic acid, tertꢀbutyl bromoaceꢀ
tate and potassium metabisulfite as a model reaction (Table 1).
After a detailed optimization of various reaction conditions (inꢀ
cluding bases, solvents, catalysts and additives, for details please
see the Supporting Information), a 75% isolated yield of sulfone 3
was obtained in the presence of 5 mol% complex 1a, 2.0 equiv.
K2S2O5 and 2.0 equiv. Na2CO3 as a base in dioxane at 100 °C
under a N2 atmosphere for 24 h (Table 1, entry 1). When its hyꢀ
droxide analogue 1b was applied under identical reaction condiꢀ
Recently, we showed that NHCꢀPd complexes containing
acenaphthoimidazolylidene ligands with a πꢀextended aromatic
ring exhibit higher catalytic activities than the corresponding IPr
analogues in several challenging crossꢀcoupling reactions,7 inꢀ
cluding carbonylation reactions,8 due to their stronger σꢀdonor
and weaker πꢀacceptor properties. Considering the similarities of
the coordination behaviors of CO and SO2 to the metal centers,9
we envisioned that a combination of acenaphthoimidazolylidene
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tions, a slightly decreased yield was observed (64%, Table 1,
paraꢀphenyl and acetyl substitutes. In the case of a choroꢀ
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8
entry 2). However, an even lower yield was obtained with the less
bulky catalyst 1c (35%, Table 1, entry 3). When the sterically
bulky IPr analogues 2a13 and 2b14 were utilized instead under
identical reaction conditions, the yields dropped to 36% and 30%,
respectively (Table 1, entries 4 and 5). These results further conꢀ
firmed our previous observation from the NHCꢀPd catalytic sysꢀ
tems that ylidenes derived from acenaphthoimidazolium salts with
πꢀextended NHC systems showed better catalytic activities than
their imidazolium analogues due to their stronger σꢀdonor and
weaker πꢀacceptor properties.7,8,15 Again, the less bulky imidazolꢀ
ylidene catalyst 2c4,16 resulted in an even lower yield (Table 1,
entry 6). In addition, the assistance of NHC ligands is crucial for
the Auꢀcatalyzed alkylsulfonylation of aryl boronic acids. When
AuCl(tht)17 was applied directly, only a 15% yield was obtained
(Table 1, entry 7). With additional PPh3 (10 mol%), a slight enꢀ
hancement was achieved (35%, Table 1, entry 8). Moreover, no
reaction occurred in a control experiment without a catalyst under
the optimized reaction conditions (entry 9, Table 1). These outꢀ
comes not only highlight the strong ligand effect in this challengꢀ
ing transformation but also indicate that Au(I) may constitute the
true catalytic species.
containing substrate, delightedly, a 65% yield was delivered to the
corresponding sulfone (11b). No Suzukiꢀcoupling product was
isolated, and this observation not only demonstrates the chemoseꢀ
lectivity of our newly developed protocol but also provides an
opportunity for future synthetic transformations. In general, sulfur
atoms have high affinities to gold, leading to unsatisfactory outꢀ
comes in various Auꢀcatalyzed reactions.18 Nevertheless, the subꢀ
strate containing a thiomethyl group (8) still gave a 54% isolated
yield, clearly indicating the protocol efficiency. Furthermore,
bulky and heterocyclic aromatic boronic acids were well tolerated.
Naphthalenꢀ1ꢀyl and naphthalenꢀ2ꢀyl boronic acids led to satisfacꢀ
tory outcomes (53ꢀ67%, 12ꢀ13), and especially, a 63% yield was
found for the extremely bulky and conjugated phenanthrenꢀ9ꢀyl
boronic acid (14). Notably, selected heterocyclic aryl boronic
acids containing O and S atoms were well tolerated, and moderate
to good yields were obtained (15ꢀ17). In the case of thiophenꢀ3ꢀ
ylboronic acid (16), 10 mol% of NHCꢀAu complex 1a was reꢀ
quired to reach this goal. In addition to (hetero)ꢀaryl boronic acids,
the challenging cyclohexenꢀ1ꢀyl boronic acid was also a suitable
substrate for this transformation, delivering the corresponding
alkenylꢀalkyl sulfone (18) in 55% yield, again reflecting the apꢀ
plicability of this protocol.
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Table 1. Optimization of the reaction conditionsa
Table 2. Direct alkylsulfonylation of various (hetero)aryl bo-
ronic acidsa
Entry
1
[Cat.]
Yield (%)b
1a
75
64
35
36
30
23
15
35
NR
1b
2
1c
3
2a
4
2b
5
2c
6
7c
8d
9e
AuCl(tht)
AuCl(tht)/PPh3
/
a In the presence of 5 mol% catalyst, the reaction was carried out with 4-
methoxyphenylboronic acid (0.37 mmol, 1.0 equiv.), K2S2O5 (2.0 equiv.),
Na2CO3 (2.0 equiv.), and tert-butyl bromoacetate (2.0 equiv.) in 2 mL of
dioxane at 100 °C under an atmosphere of N2 for 24 h. b Isolated yield based
on the boronic acid. c AuCl(tht) = chloro(tetrahydrothiophene)gold(I). d Reac-
tion was run with 5 mol% AuCl(tht) and 10 mol% PPh3. e Reaction was run
without a catalyst.
To confirm the efficiency of our newly established protocol and
the feasibility of employing NHCꢀAu complex 1a, an array of
(hetero)aryl and alkenyl boronic acids were explored (Table 2).
Phenyl boronic acid was well adapted, and a good yield of the
corresponding sulfone 4 was attained (78%, Table 2). Furtherꢀ
more, the relative position of the substituents had a slight influꢀ
ence on the sulfonylation efficiency. Similar outcomes were obꢀ
served with oꢀ, mꢀ, and pꢀmethylphenylboronic acids (62ꢀ72%,
5aꢀc). Remarkably, the electronic effect was also not obvious.
Substrates bearing electronꢀdonating (6-8) and electronꢀ
withdrawing groups (9, 10 and 11a) all underwent smoothly and
delivered the corresponding sulfones in good yields. It should be
emphasized that 10 mol% catalyst loading was required to achieve
the satisfactory yields (6b and 10) with the substrates containing
a Reaction was carried out under the optimized reaction conditions. b Isolated
yield. c Reaction was run with 10 mol% catalyst loading.
With this newly developed direct alkylsulfonylation protocol in
hand, we next evaluated the use of diverse alkylation electrophiles.
Primary alkyl bromides, ethyl 2ꢀbromoacetate, benzyl 2ꢀ
bromoacetate and 2ꢀbromoꢀacetophenone were all compatible Cꢀ
electrophiles to afford the corresponding sulfones in moderate
yields (58%, 62% and 38%, for sulfones 19, 20 and 21, respecꢀ
tively). Pleasingly, lessꢀactive tertꢀbutyl 2ꢀchloroacetate also proꢀ
duced the corresponding sulfone 3 in 42% yield, further demonꢀ
strating the efficiency of the acenaphthoimidazolylidene gold
complex 1a. When benzyl bromides were applied as Cꢀ
electrophiles, the transformation was also unaffected by the elecꢀ
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tronic nature of the substituent on the phenyl ring. Substrates with
(30bꢀc) and electronꢀwithdrawing (30d) substrates, providing
1
2
3
4
5
6
7
8
electronꢀwithdrawing and electronꢀdonating groups were all proꢀ
cessed very well (22ꢀ27, Table 3), with an up to 82% yield obꢀ
served. Again, lessꢀactive benzyl chloride, which was considered
an unsuitable substrate in the tBu3PAuClꢀcatalyzed alkylsulfonylꢀ
ation reaction,2 still delivered an acceptable synthetic yield (22). It
should be noted the chemoselectivity of the protocol is relatively
good: when 4ꢀbromobenzyl bromide was applied, a 65% yield
was observed with the alkylsulfonylation product 24b containing
an unaffected aryl bromide motif, and no direct Suzuki crossꢀ
coupling biaryl product was detected.19 In the case of conjugated
2ꢀ(bromomethyl)naphthalene, 10 mol% catalyst loading was reꢀ
quired to provide a 48% yield (26). When the inert nꢀbutyl broꢀ
mide was utilized, 35% yield of sulfone 27 was still obtained with
10 mol% catalyst loading, confirming the efficiency of our protoꢀ
col.
moderate to good yields of the desired allylic sulfones. In addition,
bulky heterocyclic aryl boronic acids were also selectively couꢀ
pled to form the alkylsulfonylation products 31 and 32 in 65% and
45% isolated yields, respectively.
Combining the results obtained so far and the previous stoichiꢀ
ometric experiments with IPrAuCl by Toste and coworkers,2 a
plausible mechanism21 is illustrated in Scheme 2. Initially, the
NHCꢀAu(I) species transmetalates with the aryl boronic acid to
produce the intermediate A (NHCꢀAuꢀAr).22 Next, SO2 (generated
in situ from K2S2O5 under heating) insertion into the AuꢀC bond
affords a sterically congested sulfonyl Au(I) complex B (NHCꢀ
AuꢀSO2ꢀAr).23 In the presence of a suitable base, along with reꢀ
generation of the lessꢀcongested monoꢀligated NHCꢀAu(I) cataꢀ
lyst to complete the catalytic cycle, the key intermediate arylꢀ
sulfinate C is produced, which further reacts with the selected Cꢀ
electrophile to afford the corresponding sulfone products.
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Table 3. Scope of activated or unactivated electrophiles in the
Au-NHC-catalyzed sulfone synthesisa
O
O
O O
S
O
O O
S
O O
S
Ph
OEt
Ph
O
MeO
MeO
MeO
MeO
MeO
MeO
19: 58%
21
: 38%
20: 62%
Scheme 2. A plausible mechanism.
R
O
O O
S
O O
S
O O
S
In summary, using acenaphthoimidazolylidene as a monoꢀ
ligated ligand, the robust NHCꢀAu(I) complex exhibited high
activity in the direct alkylsulfonylation of boronic acids by
K2S2O5 and diverse Cꢀelectrophiles, constituting a challenging
transformation even for Pd(0) catalytic systems.3 Although sulfur
has a very high affinity for gold, boronic acids containing thiomeꢀ
thyl, thienyl and benzothienyl groups were all compatible. Furꢀ
thermore, the chemoselectivity of this protocol was demonstrated
by the selective synthesis of vinylꢀ and allylꢀsulfones, along with
the preservation of the double bonds from the corresponding aryl
boronic acids and unsaturated halides, and no direct allylation
products were detected. Remarkably, in contrast to its imidazolꢀ
ylidene analogue (IPrAuCl), the high activity of the acenaphꢀ
thoimidazolylidene gold complex confirms that the sterically
bulky environment and strong σꢀdonating ability of the NHC ligꢀ
and are crucial for this challenging transformation. Further exploiꢀ
tation of acenaphthoimidazolylidene gold(I) complexes toward
the catalytic sulfonylation of diverse analogues of boronic acids
with other intriguing electrophiles for the direct syntheses of sulꢀ
fones and sulfonamides are ongoing in our laboratories.
OtBu
23a: 71% (R = Me);
22
: 82% (X = Br);
45% (X = Cl)
3: 42% (X = Cl)
23b
: 68% (R = Ph)
R
CO2Me
O O
S
O O
S
O O
S
MeO
MeO
MeO
c
24a
: 62% (R = F);
24b: 65% (R = Br);
26
: 48%
25: 75%
24c: 57% (R = CF3)
CO2Me
O O
S
O O
S
O O
S
MeO
MeO
MeO
c
28: 65%
29: 79%
27:
35%
O O
S
O O
S
O O
S
O
O
O
R
30a
: 56% (R = H);
31
32: 45%
: 65%
30b: 60% ( R = Me);
30c: 62% (R = OMe);
30d:
58% (R = F)
a Reaction was carried out under the optimized reaction conditions. b Isolated
yield. c Reaction was run with 10 mol% catalyst loading.
ASSOCIATED CONTENT
Supporting Information
The utility of this newly developed protocol was further
demonstrated by the chemoselective synthesis of unsaturated sulꢀ
fones 28-32, which feature pendant aryl sulfone groups from a
series of vinylꢀ and allylꢀ bromides (Table 3). To our delight,
methyl 3ꢀ(4ꢀbromomethyl)cinnamate bearing an alkenyl group
was a suitable substrate, providing a good yield of sulfone 28
along with a preserved double bond. It is noteworthy that the diꢀ
rect allylation of aryl boronic acids was reported recently using
gold catalysts.20 However, no direct allylation coupling products
were found using our goldꢀcatalyzed alkylsulfonylation system,
which further confirms the chemoselectivity of our protocol.
When the simple allylic bromide was applied, a good yield of
allylic sulfone product 29 was observed. Subsequently, challengꢀ
ing 3ꢀbromoꢀ2ꢀmethylpropene was selected as an allylic electroꢀ
phile to examine the scope of the aryl boronic acids. Again, no
direct allylation products were detected in all cases. The protocol
tolerated the coupling of electronꢀneutral (30a), electronꢀrich
The Supporting Information is available free of charge on the
ACS Publications website at DOI:
Experimental details and characterization data (PDF)
AUTHOR INFORMATION
Corresponding Author
*taotu@fudan.edu.cn
Notes
The authors declare no competing financial interest.
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Financial support from the National Key R&D Program of China
(2016YFA0202902), the National Natural Science Foundation of
China (No. 21572036 and 91127041), the Shanghai International
Cooperation Program (14230710600), the External Cooperation
Program of Jiangxi Province (20151BDH80045), and the Deꢀ
partment of Chemistry, Fudan University is gratefully acknowlꢀ
edged.
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