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Atom Transfer Radical Addition to Alkynes and Enynes: a Versatile Gold/Photoredox Approach to Thio-Functionalized Vinylsulfones Haoyu Li, Zengrui Cheng, Chen-Ho Tung, and Zhenghu Xu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02194 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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ACS Catalysis

Atom Transfer Radical Addition to Alkynes and Enynes: a Versatile Gold/Photoredox Approach to Thio-Functionalized Vinylsulfones Haoyu Li,† Zengrui Cheng,† Chen-Ho Tung† and Zhenghu Xu*,†,‡ †

Shandong University, No. 27 Shanda South Road, Jinan, Shandong 250100, China

‡

State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry,

Chinese Academy of Sciences, Shanghai 200032, China

ABSTRACT An efficient intermolecular atom transfer addition reaction of alkynes via the combination of visible-light photoredox catalysis and gold catalysis has been developed, affording diverse trifluoromethylthio-

and

difluoromethylthio-functionalized

vinylsulfones

with

high

stereoselectivity in good yields. Thiosulfonylation reaction of enyne can also be realized for constructing functionalized carbo- and heterocycles through radical cascade cyclization process. These reactions proceeds through a gold-assisted sulfonyl radical addition pathway.

KEYWORDS Radical, alkyne difunctionalization, vinylsulfone, gold, photoredox

INTRODUCTION

ACS Paragon Plus Environment

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Atom transfer radical addition (ATRA) to unsaturated carbon-carbon bonds is an attractive and

straightforward

synthetic

method

with

which

to

realize

alkene

or

alkyne

difunctionalization,1,2 and provide vicinal functionalized molecules in an atom economic pathway. In recent years, the ATRA addition of functionalized activated halides to alkenes has been well developed using transition-metal or photoredox catalysis3, and we have also devloped an ATRA reaction of alkenes with benzenesulfonothioate with a gold participated photoredox catalysis,3b but the atom transfer radical addition of alkynes has been much less developed.3h,4 The major reason for this is that radical addition to alkynes generates vinyl radicals, which are much more reactive than the alkyl radicals generated by radical addition to alkenes. More importantly, the vinyl radicals have two configurations (E and Z) and E/Z isomerization is very facile, being typically associated with very low activation barriers (Scheme 1a). Generally the E/Z selectivity was controlled by steric effects and also the relative reaction rate between radical isomerization and the radical trapping step. Consequently, the stereoselectivity control of the final products is a fundamental problem in the ATRA of alkynes. Recently, transition-metalcatalysis has been developed as a very important strategy to tune the reactivities and selectivities of radical reactions.5 It is known that gold catalysts can activate triple bonds and allow nucleophilic attacks from the back side, generating a trans-vinyl gold intermediate. We questioned whether a radical would also react in this parthway and whether the possible interaction between gold catalyst and a vinyl radical formed in situ may stabilize this vinyl radical and deliver good stereoselectivity as shown in Scheme 1b. In this communication, we report preliminary results from our investigation of these questions. The construction of C-S bonds is a basic and important transformation in synthetic chemistry.6 Sulfur-containing compounds exist widely in biological systems and they also serve as significant synthetic intermediates in organic synthesis7 Vinyl sulfones are a type of functional group of interest in both organic synthesis and biological research. They have found wide application in bioconjugation by acting as Michael acceptors to react with nucleophiles.8 Direct alkyne difunctionalization involving the incorporation of a sulfonyl group with concomitant formation of another C-hetero,9 or C-C10 bond represents a convenient route to functionalized vinylsulfones.11 However, thiosulfonylation of alkynes involving the formation of C-SO2 and another C-S bond simultaneously has not been realized to date.


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Scheme 1. Atom transfer radical addition to alkynes Recently, the incorporation of trifluoromethylthio groups (SCF3)12 and difluoromethylthio groups (SCF2H)13 has experienced a strong acceleration of research interest in organic synthesis. However, methods to access trifluoromethylthiolated olefins14 are limited and methods to access difluoromethylthiolated olefins are even rarer. In this context, we sought to develop a gold/photoredox-cocatalyzed15 thiosulfonylation reaction of alkynes using the difunctionalization reagents

PhSO2SCF3

and

PhSO2SCF2H.16

(Scheme

1c).

Compounds

containing

a

trifluoromethylthio group (SCF3) or a difluoromethylthio group (SCF2H) and vinylsulfones could be synthesized by this unified strategy. More importantly, this strategy has been successfully applied in enyne system to construct functionalized carbo- and heterocycles through multiple bond formation in one step in the most atom economic parthway.

RESULTS AND DISCUSSION Table

1

shows

the

results

of

various

gold-catalyzed

visible-light

associated

difunctionalization of phenylacetylene (1a) with PhSO2SCF3 (2a). A single E-isomer of the


ACS Catalysis

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target vinyl sulfone substituted with a trifluoromethylthio group (3a) was obtained in the presence of a gold catalyst. Only 8% yield was observed when the gold catalyst was removed from the reaction system (entry 1). Ph3PAuCl failed to generate the products (entry 2), indicating that a cationic gold catalyst is crucial to this reaction. Various gold catalysts were tested and Ph3PAuNTf2 was the best one, giving the desired difunctionalization compound (3a) in 46% yield (entry 5). Control experiments indicated the light irradiation and Ru(II) photocatalyst are both indispensable for the reaction to proceed. Very low yield was observed even at 70 oC without light irradiation in the presence of gold catalyst. (For details, see table S4 in Supporting Information (SI)). Use of other solvents such as CH3CN, THF and DMSO led to decreased yields (entries 6-9). It was found that a cleaner reaction and higher yield were obtained when the loading of gold catalyst was decreased (entry 10); 3a could be isolated in 82% yield when using only 0.5 mol% of gold catalyst together with 0.1 mol% of photocatalyst (entry 11). Replacing gold catalyst with copper or silver catalyst, almost no products were observed (entries 12, 13). Table 1. Optimization of Reaction Conditionsa

Entry

[Au]

Solvent

Yield (%)d

1

none

Dioxane

8

2

PPh3AuCl

Dioxane

8

3

IPrAuCl/AgSbF6

Dioxane

10

4

PPh3AuCl/AgSbF6

Dioxane

29

5

PPh3AuNTf2

Dioxane

46

6

PPh3AuNTf2

THF

25

7

PPh3AuNTf2

CH3CN

20

8

PPh3AuNTf2

DMSO

21

9

PPh3AuNTf2

DME

46

10b

PPh3AuNTf2

Dioxane

62

11b,c

PPh3AuNTf2

Dioxane

88(82)

12

CuCl/PPh3

Dioxane

0

13

AgNTf2/PPh3

Dioxane

7


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a

Reaction conditions: A mixture of 1a (0.2 mmol), 2a (0.22 mmol), [Ag] catalyst (15 mol%), [Au] catalyst

(10 mol%), Ru(bpy)3Cl2 (2.5 mol%), solvent (1 mL) was irradiated at r.t. by a blue LED strip for 6 hours under a N2 atmosphere.

b

used. dDetermined by

FNMR using (trifluoromethyl)benzene as the internal standard. Isolated yield was

19

Ru(bpy)3Cl2 (0.1 mol%), [Au] catalyst (0.5 mol%) were used. c2a (0.4 mmol) was

listed in parentheses.

The standard conditions are in Table 1, entry 11 and with these optimal reaction conditions, the alkyne scope of the trifluoromethylthiosulfonylation reaction was investigated. Diverse trifluoromethylthio functionalized vinylsulfones were obtained in good to excellent yields as single E-isomers (Table 2). the reaction proceeded very well for the para-substituted substrates (3a-3f). Electron-donating groups including methoxyl and methyl favored this reaction, and the corresponding products were isolated in 81% and 75% yields (3e,3f), but electron-withdrawing groups disfavor this reaction, substrates containing F atom and ester groups both gave slightly decreased yields (3c, 3d). Substitution at the ortho and meta positions are all compatible (3g-3i), including the very sensitive phenol (3g). In addition, thiophene and indole functionalized alkynes also reacted smoothly with 2a giving the corresponding products in moderate yields (3j, 3k). Aliphatic alkyne 1l also worked well, delivering a single difunctionalization product 3l in 60% yield. With internal alkynes (1m-1o), the reaction also proceeded smoothly under standard conditions, affording the desired tetrasubstituted alkenes (3m-3o) in serviceable yields, however the observed stereoselectivity was not as good as with terminal alkynes. The mild conditions and promising functional group compatibility of this protocol encouraged us to apply the process to late-stage modification of biological important natural compounds. An aryl acetylene derived from tyrosine could be readily difunctionalized under standard conditions in 86% yield (3p) and aryl acetylenes derived from commercially available pharmaceuticals, such as clofibrate and estrone, were all amenable to the transformation (3q, 3r). This approach provides a practical tool for the modification of bioactive compounds and drugs.


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Table 2. Alkyne scope for trifluoromethylthiosulfonylationa

a

Standard conditions were employed. A mixture of 1 (0.2 mmol), 2a (0.4 mmol), PPh3AuNTf2 (0.5 mol%),

Ru(bpy)3Cl2 (0.1 mol%) in dioxane (1 mL) was irradiated at r.t. by a blue LED strip for 6 hours under a N2 atmosphere. Isolated yields are reported.

Table 3. Alkyne scope for difluoromethylthiosulfonylationa

a

Standard conditions were employed. A mixture of 1 (0.2 mmol), 4a (0.4 mmol), PPh3AuNTf2 (0.5 mol%), Ru(bpy)3Cl2 (0.1 mol%) in dioxane (1 mL) was irradiated at r.t. by a blue LED strip for 6 hours under a N2 atmosphere.. Isolated yields are reported.


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Next, we began to realize the difluoromethylthiosulfonylation of alkynes by this strategy (For details, see table S6 in SI). When the reaction was performed under irradiation without gold catalyst, the target difunctionalization product (5a) could be obtained in 58% yield, but the E/Z selectivity is only 6/1. When 0.5 mol% of Ph3PAuNTf2 was introduced into the reaction system, the desired product could be isolated in 86% yield, and more importantly, the E/Z ratio increased to over 25/1, which clearly demonstrate the importance of a gold catalyst. Diverse alkynes were investigated under these optimized conditions and the desired difluoromethylthiovinyl sulfones could be obtained in moderate to excellent yield (Table 3). Essentially single isomers were obtained in all these reactions and the structure of 5c was unambiguously characterized by X-ray crystallographic analysis. Notably, free phenol groups (5g), heterocycles (5j, 5k) and amino acids (5n) are all tolerated. Aliphatic alkyne (5l) and internal alkyne (5m) are all viable in this transformation. This approach offered a general approach to diverse SCF2H substituted alkenes, which has never been reported previously. 1,n-Enynes are readily accessed and highly useful substrates for the construction of various ring structures. Recently, radical cascade cyclization of enynes have attracted tremendous attentions to build up simple cyclic scaffolds and even more complex polycyclic ring systems.17 However, atom economic atom transfer radical addition across enynes are very rare. Herein we investigated the gold/photoredox mediated atom transfer thiosulfonylation across enynes (6), to our delight, the reactions are very successful under the above standard conditions (Table 4). A terminal phenyl subsituted 1,6-enyne 6a reacted smoothly with PhSO2SCF3 (2a) to give a very clean reaction, and the target thio-functionalized dihydropyran 7a was isolated as single regioisomer in 84% yield. Various electron-withdrawing or donating group on the aromatic ring are all viable, giving the corresponding addition products in excellent yields (7a-7e). The structure of a fluorine substitued product 7d was confirmed by X-ray diffraction analysis. For the enynes 6f-6h bearing a terminal alkyne moiety, the sulfonyl group attacked the terminal carbon and subsequent radical cyclization giving the five-membered ring products (7f-7h) in reasonable yields. To our delight, oxygen, nitrogen or carbon-linked enynes all worked well, demonstrating this method applicable in construction of both heterocycles and carbocycles. For the oxygen or nitrogen-linked 1,7-enynes 6i-6j, thio-functionalized six-membered heterocycles 7i-7j were obtained in good yields with sulfonyl group connected to the terminal carbon of alkynes. In this


ACS Catalysis

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highly selective radical cascade reaction, two C-S bonds and one C-C bonds were constructed efficiently in one step. Table 4. Substrate scope of enyne thiosulfonylation reactionsa R1

6

SO2Ph

SO2Ph

Ph3PAuNTf 2 (0.5 mol%) n=1-2 R1 X Ru(bpy)3Cl2 (0.1 mol%) + PhSO2SCF3 hv, rt, N2 F3CS

n=1-2 X

or

X

(R1 = H)

F3CS

2a

7

R

SO2Ph

SO2Ph

R

O

O

O O F3CS

F3CS

7f, 55%

6f

7a, R = H, 84% 7b, R = Me, 81% 7c, R = Ph, 73% 7d, R = F, 86%

6a, R = H 6b, R = Me 6c, R = Ph 6d, R= F

SO2Ph NTs NTs F3CS 7g, 52%

6g

7d

SO2Ph SO2Ph

COOEt

COOEt

COOEt

COOEt

O F3CS

O 7e

F3CS 7e, 84%

SO2Ph

SO2Ph

O

NTs NTs

O F3CS 6i

7h, 51%

6h

7i, 64%

F3CS 6j

7j, 83%

a

Standard conditions were employed. A mixture of 6 (0.2 mmol), 2a (0.4 mmol), PPh3AuNTf2 (0.5 mol%), Ru(bpy)3Cl2 (0.1 mol%) in dioxane (1 mL) was irradiated at r.t. by a blue LED strip for 6 hours under a N2 atmosphere.. Isolated yields are reported.

The sulfonyl group is a versatile functional groups in organic synthesis and further synthetic transformation of the synthesized vinylsulfones was also explored (Scheme 2). For example, the sulfonyl group was easily removed upon treatment with SmI2 to give the α-substituted SCF3- or SCF2H-functionalized disubstituted olefins (8-10) in good yields. Functionalized molecules derived from tyrosine and clofibrate can all be used in this reaction. More importantly, the sulfonyl group can be replaced by an aromatic group or alkyl group by Ni(PPh3)2Cl2-catalyzed coupling reactions by using Grignard reagents, giving multifunctionalized, trisubstituted alkenes (11,12) in very good yields.18 These SCF3- or SCF2H-substituted olefins cannot easily be obtained by other methods.


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Scheme 2. Further functionalization of vinylsulfones a)

Ph3PAuNTf2 (0.5 mol%) Ru(bpy)3Cl2 (0.1 mol%)

Ph Ph

+ PhSO 2SCF3 1a

Ph SO2Ph 3, 0%

hv, rt, N2

Ph 13

2a

SCF3

b) Ph3PAuNTf2 (0.5 mol%)

R

R

Ru(bpy)3Cl2 (0.1 mol%) PhSO2R

• SO2Ph

SO2Ph

hv, N2

1s

3s, 40%

14, 32%

R = SCF2H 5p, 33%

15, 49%

R = SCF3

c)

SCF3 Ph

+ PPh3AuSCF3

+ PhSO2Cl

Ru(bpy)3Cl2 (0.1 mol%)

Ph

hv, N2

16

SO2Ph 3a, 0%

d)

Au(I) Ph

PhSO2SCF3

[Au]/[Ru] hv

PhSO2

SO2Ph

SO2Ph

SCF3

PhSO2SCF3 and Au(I)

3a

Au(II)

Au(I)

SO2Ph

PhSO2SCF3 2a

PhSO2

2a Au(I) 3a

F3CS

Au(III)

SO2Ph

Scheme 3. Control experiments and a tentative mechanism Control experiments were conducted in an attempt to understand the mechanism of this reaction. When 1,1-diphenylethene (13) was added, the reaction was totally inhibited, which indicates its radical nature (Scheme 3a). When a cyclopropyl substituted alkyne (1s) was subjected to the standard reactions, both the normal difunctionalization products (3s, 5s) and allene products (14, 15) were obtained. Such radical clock experiments indicate that the reaction was initiated by addition of a sulfonyl radical to terminal alkynes (Scheme 3b). The quantum yield of this reaction was measured as 1.4, which indicated the reaction went through radical chain propagation process (For details, see SI). The reaction of gold catalyst with PhSO2SCF3


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might generate Ph3PAuSCF3 (16).3b Ph3PAuSCF3 (16) was also prepared and was shown not to be the reaction intermediate (Scheme 3c). Also considering previous observation that Ph3PAuCl could not facilitate the transformation (entry 2, table 1), we concluded that the gold catalyst serve as a π acid to activate alkyne moiety and also help to deliver better stereoselectivity. Based on these experiments and our previous report,3b a tentative mechanism was proposed (Scheme 3d). Here, the phenylsulfonyl radical is generated in the presence of the gold catalyst and the photocatalyst under irradiation. The sulfonyl radical reacts with the triple bond of alkyne, activated by the gold catalyst, giving a vinyl radical. This radical may interact with gold(I) catalyst or perhaps form an Au(II) intermediate, which causes this vinyl radical to favor the E configuration. This E-vinyl radical can then react with 2a to form the trans-difunctionalization product and regenerate a sulfonyl radical and the Au(I) catalyst. The Au(II) intermediate might react with 2a through sing electron oxidation to generate sulfonyl radical and a Au(III) intermediate, which could form the product 3a and regenerate Au(I) catalyst by reductive elimination. This type of transition-metal catalyzed radical interactions have been observed in Cu5a,b, Co5c, Ni5d,e, Pd5fand Fe catalysis5g, but in gold catalysis is very rare. CONCLUSIONS In summary, we have developed a gold and photoredox-cocatalyzed stereoselective thiosulfonylation reaction of alkynes, leading to the highly efficient regio- and stereoselective synthesis of diverse thiofunctionalized vinyl sulfones in good yields. The gold catalyst is crucial for the control of E/Z selectivity of this reaction. This approach could also be utilized in the atom transfer addition across enyne system to construct carbo- and heterocycles. This transformation has many important features include multiple bonds formation in one step, mild conditions, very low catalyst loading. Such “single stone for several birds” strategies are versatile tools in organic synthesis. Further application of this type of gold assisted radical reaction are in progress in our laboratory.

ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (no. 21572118), subject construction funds (104.205.2.5) and Tang scholar award of Shandong University.


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SUPPORTING INFORMATION Detailed experimental procedures, spectroscopic and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION. Corresponding Authors [email protected] (Z.X.)

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