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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 Downloaded from http://pubs.acs.org on June 12, 2017
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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*
†‡
†
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
ABSTRACT: 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 alkenylalkyl 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. KEYWORDS: alkylsulfonylation, boronic acids, gold, Nheterocyclic carbene complexes, sulfones
Although Au(I) has the same d10 configuration as Pd(0), Au(I) complexes have been rarely applied in reactions with conventional 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% tBu3PAuCl was applied, up to 66% yield was achieved; however, the scope was somewhat limited. Notably, this useful threecomponent sulfone synthesis constituted a challenging catalytic transformation,3 even for Pd(0) catalysis. In general, mono-ligated cationic Au complexes were considered the true catalytic species, which require strong electronic and steric stabilization from the ligands.4 N-heterocyclic carbene (NHC) ligands are excellent σdonors and exhibit high activity in various transformations.5-6 However, when the IPrAuCl complex,4 [IPr:1,3-bis(2,6diisoprophyl)-imidazole-ylidene], 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. Recently, we showed that NHC-Pd complexes containing acenaphthoimidazolylidene ligands with a π-extended aromatic ring exhibit higher catalytic activities than the corresponding IPr analogues in several challenging cross-coupling reactions,7 including carbonylation reactions,8 due to their stronger σ-donor and weaker π-acceptor properties. Considering the similarities of the coordination behaviors of CO and SO2 to the metal centers,9 we envisioned that a combination of acenaphthoimidazolylidene
ligands and gold catalysis may provide a practical approach to directly accessing various structurally diverse sulfones. Sulfones are one of the most prominent motifs in functional molecules with applications in agrochemical, pharmaceutical and materials science,10 and simple protocols for their synthesis are in high demand. Herein, following our recent research on the synthesis and potential application of novel NHC-metal complexes,7,8,11 we report the robust acenaphthoimidazolylidene Au(I) complexcatalyzed alkylsulfonylation of various boronic acids by K2S2O5 and diverse C-electrophiles under mild reaction conditions. Furthermore, along with the steric effects,12 we show that strong σdonation of the acenaphthoimidazolylidene is crucial and accelerates this challenging transformation.
Scheme 1. Gold-catalyzed direct sulfone synthesis. To verify our hypothesis, the catalytic efficiencies of NHC-Au complexes 1 and 2 (Scheme 1) were investigated using the direct coupling of 4-methoxyphenylboronic acid, tert-butyl bromoacetate and potassium metabisulfite as a model reaction (Table 1). After a detailed optimization of various reaction conditions (including 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 hydroxide analogue 1b was applied under identical reaction condi-
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tions, a slightly decreased yield was observed (64%, Table 1, 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 confirmed our previous observation from the NHC-Pd catalytic systems that ylidenes derived from acenaphthoimidazolium salts with π-extended NHC systems showed better catalytic activities than their imidazolium analogues due to their stronger σ-donor and weaker π-acceptor properties.7,8,15 Again, the less bulky imidazolylidene catalyst 2c4,16 resulted in an even lower yield (Table 1, entry 6). In addition, the assistance of NHC ligands is crucial for the Au-catalyzed 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 enhancement 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 outcomes not only highlight the strong ligand effect in this challenging transformation but also indicate that Au(I) may constitute the true catalytic species.
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para-phenyl and acetyl substitutes. In the case of a chorocontaining substrate, delightedly, a 65% yield was delivered to the corresponding sulfone (11b). No Suzuki-coupling product was isolated, and this observation not only demonstrates the chemoselectivity 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 outcomes in various Au-catalyzed reactions.18 Nevertheless, the substrate 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. Naphthalen-1-yl and naphthalen-2-yl boronic acids led to satisfactory outcomes (53-67%, 12-13), and especially, a 63% yield was found for the extremely bulky and conjugated phenanthren-9-yl boronic acid (14). Notably, selected heterocyclic aryl boronic acids containing O and S atoms were well tolerated, and moderate to good yields were obtained (15-17). In the case of thiophen-3ylboronic acid (16), 10 mol% of NHC-Au complex 1a was required to reach this goal. In addition to (hetero)-aryl boronic acids, the challenging cyclohexen-1-yl boronic acid was also a suitable substrate for this transformation, delivering the corresponding alkenyl-alkyl sulfone (18) in 55% yield, again reflecting the applicability of this protocol.
Table 1. Optimization of the reaction conditionsa Table 2. Direct alkylsulfonylation of various (hetero)aryl boronic acidsa
Entry
[Cat.]
Yield (%)b
1
1a
75
2
1b
64
3
1c
35
4
2a
36
5
2b
30
6
2c
23
7c
AuCl(tht)
15
8
AuCl(tht)/PPh3
35
9e
/
NR
d
a
In the presence of 5 mol% catalyst, the reaction was carried out with 4methoxyphenylboronic 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 c d on the boronic acid. AuCl(tht) = chloro(tetrahydrothiophene)gold(I). Reaction 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 NHC-Au 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). Furthermore, the relative position of the substituents had a slight influence on the sulfonylation efficiency. Similar outcomes were observed with o-, m-, and p-methylphenylboronic acids (62-72%, 5a-c). Remarkably, the electronic effect was also not obvious. Substrates bearing electron-donating (6-8) and electronwithdrawing 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 2-bromoacetate, benzyl 2bromoacetate and 2-bromo-acetophenone were all compatible Celectrophiles to afford the corresponding sulfones in moderate yields (58%, 62% and 38%, for sulfones 19, 20 and 21, respectively). Pleasingly, less-active tert-butyl 2-chloroacetate also produced the corresponding sulfone 3 in 42% yield, further demonstrating the efficiency of the acenaphthoimidazolylidene gold complex 1a. When benzyl bromides were applied as Celectrophiles, the transformation was also unaffected by the elec-
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tronic nature of the substituent on the phenyl ring. Substrates with electron-withdrawing and electron-donating groups were all processed very well (22-27, Table 3), with an up to 82% yield observed. Again, less-active benzyl chloride, which was considered an unsuitable substrate in the tBu3PAuCl-catalyzed alkylsulfonylation reaction,2 still delivered an acceptable synthetic yield (22). It should be noted the chemoselectivity of the protocol is relatively good: when 4-bromobenzyl bromide was applied, a 65% yield was observed with the alkylsulfonylation product 24b containing an unaffected aryl bromide motif, and no direct Suzuki crosscoupling biaryl product was detected.19 In the case of conjugated 2-(bromomethyl)naphthalene, 10 mol% catalyst loading was required to provide a 48% yield (26). When the inert n-butyl bromide was utilized, 35% yield of sulfone 27 was still obtained with 10 mol% catalyst loading, confirming the efficiency of our protocol.
(30b-c) and electron-withdrawing (30d) substrates, providing moderate to good yields of the desired allylic sulfones. In addition, bulky heterocyclic aryl boronic acids were also selectively coupled to form the alkylsulfonylation products 31 and 32 in 65% and 45% isolated yields, respectively. Combining the results obtained so far and the previous stoichiometric experiments with IPrAuCl by Toste and coworkers,2 a plausible mechanism21 is illustrated in Scheme 2. Initially, the NHC-Au(I) species transmetalates with the aryl boronic acid to produce the intermediate A (NHC-Au-Ar).22 Next, SO2 (generated in situ from K2S2O5 under heating) insertion into the Au-C bond affords a sterically congested sulfonyl Au(I) complex B (NHCAu-SO2-Ar).23 In the presence of a suitable base, along with regeneration of the less-congested mono-ligated NHC-Au(I) catalyst to complete the catalytic cycle, the key intermediate arylsulfinate C is produced, which further reacts with the selected Celectrophile to afford the corresponding sulfone products.
Table 3. Scope of activated or unactivated electrophiles in the Au-NHC-catalyzed sulfone synthesisa
O O O S
O O O S
OEt
MeO
23a: 71% (R = Me); 23b: 68% (R = Ph)
22: 82% (X = Br); 45% (X = Cl) R
MeO 24a: 62% (R = F); 24b: 65% (R = Br); 24c: 57% (R = CF3) O O S
O O S
O O S
26: 48% c
25: 75% CO2Me
O O S
O O S
MeO
MeO 28: 65%
O O S
O O S
29: 79%
O
O O S
O
O 30a: 56% (R = H); 30b: 60% ( R = Me); 30c: 62% (R = OMe); 30d: 58% (R = F)
CO2Me
MeO
MeO
27: 35%c
R
Scheme 2. A plausible mechanism. R
MeO
MeO
MeO
O O S
O O S
OtBu
3: 42% (X = Cl)
Ph
21: 38%
20: 62%
MeO
O O S
O O O S
Ph MeO
MeO
19: 58% O O O S
O
31: 65%
32: 45%
a
Reaction was carried out under the optimized reaction conditions. b Isolated yield. c Reaction was run with 10 mol% catalyst loading.
The utility of this newly developed protocol was further demonstrated by the chemoselective synthesis of unsaturated sulfones 28-32, which feature pendant aryl sulfone groups from a series of vinyl- and allyl- bromides (Table 3). To our delight, methyl 3-(4-bromomethyl)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 direct allylation of aryl boronic acids was reported recently using gold catalysts.20 However, no direct allylation coupling products were found using our gold-catalyzed 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, challenging 3-bromo-2-methylpropene was selected as an allylic electrophile 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 electron-neutral (30a), electron-rich
In summary, using acenaphthoimidazolylidene as a monoligated ligand, the robust NHC-Au(I) complex exhibited high activity in the direct alkylsulfonylation of boronic acids by K2S2O5 and diverse C-electrophiles, constituting a challenging transformation even for Pd(0) catalytic systems.3 Although sulfur has a very high affinity for gold, boronic acids containing thiomethyl, thienyl and benzothienyl groups were all compatible. Furthermore, the chemoselectivity of this protocol was demonstrated by the selective synthesis of vinyl- and allyl-sulfones, 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 imidazolylidene analogue (IPrAuCl), the high activity of the acenaphthoimidazolylidene gold complex confirms that the sterically bulky environment and strong σ-donating ability of the NHC ligand are crucial for this challenging transformation. Further exploitation of acenaphthoimidazolylidene gold(I) complexes toward the catalytic sulfonylation of diverse analogues of boronic acids with other intriguing electrophiles for the direct syntheses of sulfones and sulfonamides are ongoing in our laboratories.
ASSOCIATED CONTENT Supporting Information 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 *
[email protected] Notes The authors declare no competing financial interest.
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ACKNOWLEDGMENT 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 Department of Chemistry, Fudan University is gratefully acknowledged.
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