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Nickel-Catalyzed Cross-Coupling Reaction of Aryl Sulfoxides with Arylzinc Reagents: A Case that Leaving Group is an Oxidant Keita Yamamoto, Shinya Otsuka, Keisuke Nogi, and Hideki Yorimitsu ACS Catal., Just Accepted Manuscript • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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Nickel-Catalyzed Cross-Coupling Reaction of Aryl Sulfoxides with Arylzinc Reagents: A Case that Leaving Group is an Oxidant Keita Yamamoto, Shinya Otsuka, Keisuke Nogi, and Hideki Yorimitsu* Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 6068502, Japan

ABSTRACT: Nickel-catalyzed Negishi-type cross-coupling of aryl methyl sulfoxides with arylzinc reagents has been developed. By consuming catalyst-oxidizing methanesulfenate anion through oxidative homo-coupling of the arylzinc reagent, smooth catalyst turnover could be executed. Arylzinc reagents prepared from arylmagnesium bromide, zinc bromide, and lithium bromide were optimal to afford the products in good to high yields, while arylzinc reagents prepared through other procedures showed lower reactivities. The reactivity of aryl methyl sulfoxide was compared with that of typical aryl (pseudo)halides.

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KEYWORDS: Negishi coupling, Nickel catalyst, Aryl sulfoxide, Arylzinc, Alkanesulfenate anion

Introduction In research horizons of transition-metal-catalyzed cross-coupling,1 expansion of the range of usable electrophilic coupling partners is an important and valuable theme. Conventionally, highly reactive electrophiles such as aryl iodides, bromides, and sulfonates have been employed. Transformations of less reactive aryl chlorides,2 fluorides,3 carbonates or esters, and even aryl methyl ethers4 are emerging by means of electron-rich and/or coordinatively unsaturated transition metal catalysts. Recently, cross-coupling of organosulfur compounds as electrophiles has gained much attention owing to their ubiquity and versatility in synthetic organic chemistry.5 Indeed, a number of C–C bond formations with organosulfur compounds, even unactivated aryl sulfides, have now been accomplished.6 In sharp contrast, cross-coupling of aryl sulfoxides has rarely been explored. Due to the electrondeficiency of the sulfur atom in the sulfoxide unit, their C–S bonds can be considered easily cleavable and more reactive than those of aryl sulfides. As a seminal work, in 1979, Wenkert developed cross-coupling of aryl sulfoxides by employing arylmagnesium reagents and a nickel catalyst.7a Recently, Enthaler reported a similar transformation.7b However, due to the high reactivity of organomagnesium reagents, the transformations suffer from poor functional group compatibility. In addition, although the coupling reaction of diaryl sulfoxides proceeded smoothly, more general and easily accessible alkyl aryl sulfoxides such as methyl phenyl sulfoxide afforded the corresponding coupling products in much lower yields.8,9


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O R S Ar1 Mn Cycle 1 Ar1 Ar2

CrossCoupling

O Ar1 R S Mn+2

RSm + O

M n+2 or Ar1 Om Ar2 m 2 Ar2 m n+2 Ar2 M n+2 RS M step a RSm Cycle 2 O Om2 RS Om Consumption of R S m step b sulfenate anion alkanesulfenate anion 2 Ar Mn 2 Ar Mn+2 Ar2 Ar2

step c

Scheme 1. Working Hypothesis The lack of cross-coupling of aryl sulfoxide is counterintuitive to the seemingly high reactivity of electron-deficient sulfoxides. We surmised that generation of alkanesulfenate anions during the course of the reaction of alkyl aryl sulfoxides would be problematic. As shown in Cycle 1 in Scheme 1, alkanesulfenate anion R–S(=O)–m would be formed on the transmetalation step. The anion is potentially oxidizing since it is in equilibrium to RS–Om, which is valence-isoelectronic with a peroxide anion RO–Om. Low-valent transition metal catalysts would be sensitive to and incompatible with such oxidizing species which deactivate the catalyst. To remove potentially oxidizing alkanesulfenate anion from the reaction system, we focused on oxidative homocoupling of arylmetal reagents (Scheme 1, Cycle 2). Although low-valent metal species is oxidized by an alkanesulfenate anion (step a), subsequent transmetalation with two equivalents of arylmetal reagents would liberate alkanethiolate anion and a metal oxide (step b), thus alkanesulfenate anion being consumed. The subsequent reductive elimination would furnish lowvalent transition metal species that would continue the catalytic cycle (step c).


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With this hypothesis in mind, here we report nickel-catalyzed Negishi-type cross-coupling of aryl methyl sulfoxides with arylzinc reagents that show promising reactivity as well as high functional group compatibility.

Results and Discussion We started our investigation by utilizing methyl phenyl sulfoxide (1a) as a model substrate and 5 mol % of NiCl2(dppe) (dppe = 1,2-bis(diphenylphosphino)ethane). Fortunately, desired biaryl 3aa was provided in 41% yield by means of one equivalent of 4-methoxyphenylzinc reagent prepared from the corresponding arylmagnesium bromide and zinc bromide (Table 1, entry 1). As we expected, 4,4'-dimethoxybiphenyl (4a) derived from the arylzinc reagent was formed in almost the same molar amount as 3aa (molar ratio: 3aa/4a = 1.2). This result is consistent with our working hypothesis as shown in Scheme 1. By using two and three equivalents of the arylzinc reagent, the yields of 3aa were improved to 66% and 67%, respectively (entries 2 and 3). In both cases, the same amounts of 4a as 3aa were obtained. Addition of lithium bromide significantly increased the yield of 3aa (entry 4). Eventually, 3aa was provided in 88% yield by employing 1.5 equivalents of diarylzinc reagent that was prepared from three equivalents of the arylmagnesium bromide and 1.5 equivalents of zinc bromide and lithium bromide (entry 5). These conditions are highly efficient and a 68% yield of 3aa was generated with only 0.1 mol% of NiCl2(dppe). (entry 6). The reaction proceeded even at 25 °C albeit with a longer reaction time (entry 7). When the reaction was terminated in 6 h, the yield of 3aa was 49% and a major part, 38%, of 1a was recovered. This would exclude a reaction pathway wherein reduction of 1a to the corresponding aryl sulfide is followed by C–S bond cleavage.


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Although other palladium or nickel complexes also catalyzed the coupling reaction, NiCl2(dppe) was found to be optimal (see Supporting Information for more details). Notably, preparation method of arylzinc reagent is crucial for smooth conversion. Indeed, arylzinc iodide lithium chloride complex reported by Knochel10 gave lower yields of the product even in the presence of an external magnesium salt (entries 8 and 9). In these cases, oxidative homo-coupling of arylzinc preferentially proceeded, and 4a was formed almost twice as much as 3aa. Although the mechanism is unclear, aryl sulfoxide 1a also worked just as an oxidant.11 Gosmini’s arylzinc bromide12 furnished 3aa in less than 15% yield regardless of the presence or absence of magnesium and/or lithium salts (entries 10–12). We assume that the validity of the arylmagnesium-derived arylzinc reagent for the cross-coupling would arise from (1) the existence of Lewis acidic magnesium cation accelerating elimination of sulfur fragments from a nickel center, (2) LiBr-assisted formation of more nucleophilic arylzincate species to facilitate transmetalation.13 Instead of arylzinc reagents, arylmagnesium bromide furnished 3aa in only 61% yield probably because arylmagnesium reagents would degrade the methylsulfinyl moiety on 1a (entry 13).14


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Table 1. Screening of Arylzinc Reagents

entry ArZn 2 yield of 3aa (%)a molar ratio 3aa/4aa 41 1.2 1 1 ArMgBr + 1 ZnBr2 2 2 ArMgBr + 2 ZnBr2 66 1.0 3 3 ArMgBr + 3 ZnBr2 67 1.0 4 3 ArMgBr + 3 ZnBr2 + 3 LiBr 83 1.0 b 5 3 ArMgBr + 1.5 ZnBr2 + 1.5 LiBr 88 (88) 1.0 c 3 ArMgBr + 1.5 ZnBr2 + 1.5 LiBr 68 0.92 6 d 7 3 ArMgBr + 1.5 ZnBr2 + 1.5 LiBr 77 1.1 e 8 3 ArZnI·LiCl 48 0.52 e 53 0.64 9 3 ArZnI·LiCl + 3 MgBr2 f 10 3 ArZnBr 14 0.42 f 11 3 ArZnBr + 3 MgBr2 15 0.60 f 12 3 ArZnBr + 3 MgBr2 + 3 LiBr 10 0.17 13 3 ArMgBr 61 0.82 a 1 b c Determined by H NMR. Isolated yield. 0.1 mol % of catalyst was used. Performed for 48 h. d

Performed at 25 °C for 24 h. eSee ref 10. fSee ref 12.

Having identified optimal reaction conditions (Table 1, entry 5), we next investigated the scope of substrates (Table 2). Both electron-rich and -deficient aryl methyl sulfoxides 1b–1j were smoothly involved in the cross-coupling (entries 1–9). Owing to the moderate reactivity of the arylzinc reagents, esters, amides, benzoyl, and cyano groups were well tolerated to provide desired biaryls 3ea–3ja in moderate to high yields (entries 4–9). These results demonstrate superiority of the present system over the arylmagnesium-based previous works.7 The coupling proceeded smoothly without loss of methoxy and pivaloyloxy moieties although their C–O bonds


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are potentially reactive under nickel catalysis (entries 2 and 5). As for the coupling of 1i and 1j, Pd-PEPPSI-SIPr

([1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene](3-

chloropyridine)palladium(II) dichloride)15 showed higher catalytic activity than the nickel complex (entries 8 and 9). 2-(Methylsulfinyl)naphthalene (1k) also furnished the product 3ka in 86% yield (entry 10). The reaction of methyl o-tolyl sulfoxide (1l) with bis[4-(N,Ndimethylamino)phenyl]zinc 2c smoothly proceeded (entry 12). In this case, NiCl2(depe) (depe = 1,2-bis(diethylphosphino)ethane) showed better catalytic activity than NiCl2(dppe), and afforded a 77% yield of the coupling product 3lc. More sterically hindered 2-(methylsulfinyl)-1,1'biphenyl (1m) underwent the reaction with 2c to afford 3mc in 50% yield by means of NiCl2(depe) (entry 13). Heteroaryl sulfoxides 1n–1p were applicable to this system; the reactions with 2a provided the corresponding biaryls 3na–3pa in good to excellent yields (entries 14–16). The scope of arylzinc reagents was also investigated with 2-(methylsulfinyl)pyridine (1p). 4Fluorophenyl- and 2-naphthylzinc reagents 2d and 2e afforded the products 3pd and 3pe in excellent yields (entries 17 and 18). ortho-Substituent on the arylzinc reagent did not diminish the yield of the product; biaryl 3pf was obtained in 88% yield (entry 19).


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Table 2. Scope of Substrates

entry aryl sulfoxide 1

diarylzinc 2

product 3 (%)a

1

2

3

4

2a

5

2a

6

2a


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7

2a

8

2a

9

2a

10

2a

11

12

2c

13

2c

14


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15

2a

16

2a

17

1p

18

1p

19

1p

a

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Isolated yield. b5 mol % of Pd-PEPPSI-SIPr and 3.0 equivalents of monoarylzinc reagent

derived from ArMgBr, ZnBr2, and LiBr were used. c10 mol % of NiCl2(depe) and 2.0 equivalents of 2c was used.

Besides aryl methyl sulfoxides, other alkyl aryl sulfoxides could be employed in this crosscoupling (Scheme 2). Indeed, dodecyl phenyl sulfoxide (1q) and phenethyl phenyl sulfoxide (1r) were smoothly converted into the coupling products without deterioration of the yields. On the other hand, the reaction of t-butyl p-tolyl sulfoxide (1s) was not so efficient maybe due to the steric hindrance of the t-butyl moiety. The cross-coupling of diphenyl sulfoxide (1t) was also


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examined, and to our surprise, the yield of the desired biaryl exceeded 100%. This indicates that the eliminated benzenesulfenate anion was again involved in the coupling with 2a (vide infra). Eventually, by employing two equivalents of 2a, 3aa was obtained in 194% NMR yield, and in 156% isolated yield. This nickel-catalyzed coupling could be applied to thioanisole (5a), and biaryl 3aa was obtained in 93% yield. The coupling of methyl phenyl sulfone (6a) was sluggish; only a 43% yield of 3aa was obtained and 57% of 6a was recovered. By elongating the reaction time,

the

yield

of

6a

was

increased

to

62%.

Scheme 2. Cross-Coupling of Other Organosulfur Compoundsa a

NMR yield. bIsolated yield is shown in parenthesis. c2.0 equiv of 2a was used. dFor 14 h.

To verify the proposed reaction mechanism shown in Scheme 1, we attempted to trap sulfurcontaining fragments derived from leaving groups (Scheme 3). The coupling reaction of 1a with 2a was conducted for 5 min, and the resulting mixture was then treated with two equivalents of benzyl bromide before aqueous workup. As a result, benzyl methyl sulfide (7) was obtained in


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33% yield accompanied by 94% and 81% yields of 3aa and 4a, respectively (Scheme 3a). In this case, benzyl methyl sulfoxide (8) was not observed at all. These observations imply that in situ generated methanesulfenate anion would be reduced to methanethiolate as shown in Scheme 1. In contrast, after the reaction of diphenyl sulfoxide (1s) with 2a, treatment of the reaction mixture with benzyl bromide afforded benzyl phenyl sulfoxide (10) mainly (Scheme 3b). Moreover, an only 14% yield of homo-coupling product 4a was observed while the crosscoupling smoothly proceeded to afford 3aa in 74% yield. These contrastive results shown in Scheme 3a and 3b clearly demonstrate that methanesulfenate anion would be more labile and promote oxidative homo-coupling of arylzinc reagents more efficiently than benzenesulfenate anion.16

Scheme 3. Electrophilic Trap of Anionic Sulfur Fragments: Fate of Leaving Group Next, we conducted competition experiments to gain insight into the effect of electronic properties of aryl sulfoxides (Scheme 4). First, the intramolecular competition reaction of electronically unsymmetrical diaryl sulfoxide 1u was conducted with arylzinc reagent 2c


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(Scheme 4a). The electron-deficient aryl moiety preferentially reacted to afford 4-(N,Ndimethylamino)-4'-trifluoromethyl-1,1'-biphenyl (3dc) in 73% yield accompanied by a 10% of 4(N,N-dimethylamino)-4'-methoxy-1,1'-biphenyl (3cc) as a minor product.17 A similar trend was found in the intermolecular version. In the same vessel, 0.5 mmol of electron-deficient and -rich aryl sulfoxides 1c and 1d were treated with 0.5 mmol of arylzinc reagent 2c in the presence of NiCl2(dppe) (Scheme 4b). To observe the reaction profile at the early stage, the reaction was quenched in 5 min before full conversion of sulfoxides. As a consequence, the reaction predominantly proceeded with electron-deficient 3d providing 3dc in 64% yield. In contrast, a different trend was observed when the coupling reactions of aryl sulfoxide 1c and 1d were conducted separately (Scheme 4c). While the reaction of electron-deficient aryl sulfoxide 1d afforded the product 3dc in 42% yield, a comparable amount, 32% yield of the coupling product 3cc was obtained from electron-rich aryl sulfoxide 1c. These results indicate that oxidative addition of the C–S(=O) bond to the nickel catalyst would be the site-selecting step, but not rate-determining step.


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Scheme 4. Competition Experiments (1) To investigate reactivity of the C–S bond of aryl sulfoxide, we then conducted the reactions involving an intermolecular competition between 1a and other aryl (pseudo)halides such as ptolyl bromide, chloride, triflate, tosylate,18 mesylate, or methyl sulfide (Figure 1). In the presence of the nickel catalyst, 0.5 mmol of 1a and aryl (pseudo)halide were treated with 0.5 mmol of arylzinc reagent 2a, and the reaction was quenched in 5 min before full conversion of substrates.


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In all cases, 3aa derived from sulfoxide 1a was obtained as a major product (entries 1–6, See Table S2 for detailed data). These indicate that aryl methyl sulfoxide would be comparable to or more reactive than aryl halides or sulfonates at 80 °C. On the other hand, at 25 °C, results were drastically changed. The ratio of 3ba/3aa significantly increased compared to those at 80 °C (entries 7–11).

Figure 1. Competition Experiments (2) aCatalyst: 5 mol % of Pd-PEPPSI-SIPr.

We inferred that these distinctions would be derived from the difference in the ratedetermining step between the reaction of aryl methyl sulfoxides and that of aryl halides.


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Koszinowski reported that the rate-determining step of the Negishi coupling of aryl halides would be oxidative additions.13c Indeed, for the coupling of bromobenzene (PhBr) at 23 °C, plotting the initial reaction rate (r0) against initial concentration of PhBr ([PhBr]0) showed a positive dependence (Figure 2, blue), and the reaction would be approximately first-order in [PhBr] (See Supporting Information for more details).

2.0

1.5

r0 / (10–5 M s-1)

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1.0

0.5

0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

[substrate]0 / M

Figure 2. Plots of initial reaction rate (r0) against [1a]0 (red) and [PhBr]0 (blue).

On the other hand, the initial rate of the reaction of 1a was less affected by [1a]0 (Figure 2, red), and the reaction was estimated to 0.17th-order in [1a]. This result also indicates that the oxidative addition step in the reaction of 1a would be not the rate-determining step.


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Even after these mechanistic experiments, it is still unclear which step, transmetalation or reductive elimination, is the actual rate-determining step. We should also pay additional attention to the possibility that the consumption of methanesulfenate anion (Scheme 1, Cycle 2) can determine the rate of the overall reaction. The coupling of 1a was faster than methyl p-tolyl sulfide regardless of the reaction temperature (Figure 1, entries 6 and 12). However, surprisingly, when a NHC-ligated palladium catalyst, PdPEPPSI-SIPr, was used instead of NiCl2(dppe), the coupling of the sulfide preceded that of 1a (Figure 1, entry 13). Biaryl 3ba was obtained as a major product, and small amount of 3aa was formed. These results offer a possibility for selective transformation of targeted C–S bonds by choosing an appropriate metal catalyst.

Conclusion We have developed nickel-catalyzed cross-coupling of aryl methyl sulfoxides with arylzinc reagents. Although in situ generated methanesulfenate anions would be a catalyst-oxidizer, oxidative homo-coupling of arylzinc reagent efficiently consumed the anions resulting in steady catalyst turnover. Arylzinc reagents prepared from arylmagnesium bromides, zinc bromide, and lithium bromide were crucial to obtain the products in good to high yields. Not only aryl methyl sulfoxides, a variety of organosulfur compounds such as alkyl aryl sulfoxides, diaryl sulfoxide, aryl sulfide, and aryl sulfone reacted efficiently. Competition experiments and kinetic measurements provided information about difference in reactivity between aryl methyl sulfoxides and aryl (pseudo)halides. From a synthetic point of view, the inevitable formation of one equivalent of the homo-coupling byproducts diminishes the utility of this coupling reaction. However, the present work has shown


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the importance of the removal of oxidizing leaving groups, alkanesulfenate anions, with external reducing reagents, thus being the first step to establish the use of aryl sulfoxides as coupling partners in the cross-coupling arena. Further research on catalytic transformations of aryl sulfoxides, including the pursuit of a more efficient and atom-economical way with other suitable reducing agents, is underway in our group. AUTHOR INFORMATION Corresponding Author *[email protected] ORCID Hideki Yorimitsu: 0000-0002-0153-1888 Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information. Detailed experimental procedures, kinetic measurements, and full spectroscopic data for all new compounds (PDF)

ACKNOWLEDGMENT


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This work was supported by JSPS KAKENHI Grant Numbers JP16H01019, JP16H04109, JP16H06887, as well as JST ACT-C Grant Number JPMJCR12ZE, Japan. H.Y. thanks Japan Association for Chemical Innovation, Tokuyama Science Foundation, and The Naito Foundation for financial support. S.O. acknowledges JSPS Predoctoral Fellowship. DEDICATION Dedicated to Professor Teruaki Mukaiyama in celebration of his 90th birthday (Sotsuju).

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ACS Catalysis

(9) We recently developed palladium-catalyzed borylation of diaryl sulfoxides: Saito, H.; Nogi, K.; Yorimitsu, H. Synthesis 2017, in press. DOI: 10.1055/s-0036-1588848. (10) Krasovskiy, A.; Malakhov, V.; Gavryushin, A.; Knochel, P. Angew. Chem. Int. Ed. 2006, 45, 6040–6044. (11) When arylzinc 2a was treated with dialkyl sulfoxide in the presence of a catalytic amount of NiCl2(dppe), the homo-coupling of 2a proceeded with the concomitant formation of the corresponding dialkyl sulfide. (12) Fillon, H.; Gosmini, C.; Périchon, J. J. Am. Chem. Soc. 2003, 125, 3867–3870. (13) (a) Hunter, H. N.; Hadei, N.; Blagojevic, V.; Patschinski, P.; Achonduh, G. T.; Avola, S.; Bohme, D. K.; Organ, M. G. Chem. Eur. J. 2011, 17, 7845–7851. (b) McCann, L. C.; Hunter, H. N.; Clyburne, J. A. C.; Organ, M. G. Angew. Chem. Int. Ed. 2012, 51, 7024–7027. (c) Böck, K.; Feil, J. E.; Karaghiosoff, K.; Koszinowski, K. Chem. Eur. J. 2015, 21, 5548–5560. (d) Thapa, S.; Kafle, A.; Gurung, S. K.; Montoya, A.; Riedel, P.; Giri, R. Angew. Chem. Int. Ed. 2015, 54, 8236–8240. (14) (a) Oda, R.; Yamamoto, K. J. Org. Chem. 1961, 26, 4679–4681. (b) Manya, P.; Sekera, A.; Rumpf, P. Tetrahedron 1970, 26, 467–476. (c) Nokami, J.; Kunieda, N.; Kinoshita, M. Chem. Lett. 1977, 6, 249–252. (15) Reviews of Pd-PEPPSI-NHC catalysts: (a) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Aldrichim. Acta 2006, 39, 97–111. (b) Organ, M. G.; Chass, G. A.; Fang, D.-C.; Hopkinson, A. C.; Valente, C. Synthesis 2008, 2776–2797. (c) Valente, C.; Çalimsiz, S.; Hoi, K. H.; Mallik, D.; Sayah, M.; Organ, M. G. Angew. Chem. Int. Ed. 2012, 51, 3314–3332.


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(16) It has been suggested that alkanesulfenate anions are more labile than arenesulfenate anions: (a) Soderman, S. C.; Schwan, A. L. Org. Lett. 2011, 13, 4192–4195. (b) Jia, T.; Zhang, M.; Jiang, H.; Wang, C. Y.; Walsh, P. J. J. Am. Chem. Soc. 2015, 137, 13887–13893. (17) Even with a longer reaction time of 4 h, the ratio of 3cc/3dc changed only little (3cc/3dc = 0.14, 3cc: 9%, 3dc: 64%) because most of arylzinc 2c would be consumed within 5 min. (18) Although the C(sp2)–S bond of the tosyl moiety is potentially cleavable, no such cleavage was observed when p-ethylphenyl tosylate was exposed to the identical conditions.


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Nickel-Catalyzed Cross-Coupling Reaction of Aryl ... - ACS Publications

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