Nickel-Catalyzed Decarboxylative Alkenylation of Anhydrides with

Mar 25, 2019 - 1d). Compared to specialized esters (Scheme 1b) as active substrates .... modify high-value aliphatic acids by converting them into mix...
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Nickel-Catalyzed Decarboxylative Alkenylation of Anhydrides with Vinyl Triflates or Halides Hui Chen, Shuhao Sun, and Xuebin Liao* School of Pharmaceutical Sciences, Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Beijing Advanced Innovation Center for Structural Biology, Tsinghua University, Beijing 100084, China

Org. Lett. Downloaded from pubs.acs.org by WESTERN UNIV on 05/07/19. For personal use only.

S Supporting Information *

ABSTRACT: Decarboxylative cross-coupling of aliphatic acid anhydrides with vinyl triflates or halides was accomplished via nickel catalysis. This methodology works well with a broad array of substrates and features abundant functional group tolerance. Notably, our approach addresses the issue of safe and environmental installation of methyl or ethyl group into molecular scaffolds. The method possesses high chemoselectivity toward alkyl groups when aliphatic/aromatic mixed anhydrides are involved. Furthermore, diverse ketones could be modified with our strategy.

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Scheme 1. Recent Development of Decarboxylative Alkenylation and Our Strategy

lkene moieties are presented widely among the vast array of natural products and biologically active molecules.1 Olefin, as the versatile group, can often be used in numerous transformations such as reduction, oxidation, hydrolysis, and pericyclic reaction as well as Heck and related coupling reactions.2 The development of efficient strategies for incorporating this functional group is therefore of great significance.2c−i During the past decades, methods for preparation of olefin-containing molecular scaffolds have achieved great success,3 especially with transition-metalcatalyzed vinylation.3a−r The most intensively application area is the use of alkenylmetal reagents to readily react with aryl electrophiles for the construction of new C(sp2)−C(sp2) bonds.3 Although there have been elegant works on the production of C(sp2)−C(sp3) bonds by Fu3a−c and others,4 it remains a relatively underexplored area owing in part to βhydride elimination from the alkyl metal intermediates and the lack of alkyl coupling partners as compared to aryl coupling reagents. Aliphatic carboxylic acids as striking alkyl building blocks have received wide attention recently.5−28 In contrast to alkyl halides, aliphatic acids are more stable and nearly nontoxic coupling partners. In 2014, the MacMillan group reported the coupling of vinyl halides with secondary alkyl carboxylic acids via nickel catalysts with light assistance to construct Csp2−Csp3 bonds (Scheme 1a).5b Subsequently, Baran’s pioneering works on the coupling of alkenylzinc reagents with “redox-active” esters by a single nickel catalyst has come to the fore (Scheme 1b).6e Although these approaches for constructing theC(sp2)−C(sp3) bonds are attractive, the decarboxylative process required the assistance of precious photocatalyst5,7 (Scheme 1a) or the demands that aliphatic acids be prepared as “redox-active” esters in advance6,7,13,14,17,27,28 and preformation of alkenylzinc regents6e (Scheme 1b). © XXXX American Chemical Society

Inspired by Stephenson’s elegant work on trifluoromethylation of arenes via photocatalysis (Scheme 1c),7b herein, we report the first nickel-catalyzed decarboxylative cross-couplings of aliphatic acid anhydrides with vinyl triflates or halide (Br Received: March 25, 2019

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DOI: 10.1021/acs.orglett.9b01048 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters and I) under mild, operationally simple conditions (Scheme 1d). Compared to specialized esters (Scheme 1b) as active substrates for extruding carbon dioxide, aliphatic acid anhydrides are also common building blocks that are either commercially available (e.g., acetic anhydride, propionic anhydride) or can be conveniently generated in situ from the corresponding carboxylic acids. In our strategy, vinyl halides are directly applied to be coupled with alkyl groups, avoiding the tedious process of preparing organometal reagents as described by many other groups.3 ,6 More importantly, we show that vinyl triflates are appealing alkenyl coupling partners (readily accessible by treating aliphatic ketones with commercially available sulfonated reagents) and thereby significantly expand the scope of decarboxylative olefination. In our initial investigation, we attempted to apply the methods directly developed by other groups and our recent efforts to realize the decarboxylative cross-coupling of vinyl triflates with anhydrides.29 Unfortunately, the desired product cannot be obtained or formed in an unsatisfactory yield. We therefore started to explore the optimized reaction conditions for this new type of cross-coupling reaction. As illustrated in Table 1, 4-phenylcyclohex-1-enyl trifluoromethanesulfonate (1a) and lauric anhydride (2a) were chosen as model substrates. The stable and inexpensive NiCl2 was used as a precatalyst. A variety of ligands were then tested (entry 1−7); to our delight, the less sterically hindered and electron-neutral 1,10-phenanthroline performed best. When we replaced NiCl2

with NiBr2 or NiI2, the yields of coupling products decreased slightly (entry 8 and 9). The control reactions further demonstrated that both catalysts and ligands were indispensable (entry 10). As reported that zinc powder acted as reductant,35 reactions did not proceed in its absence (entry 11). Further studies revealed that either LiCl (entry 11) or pyridine N-oxide (entry 12) was also essential. Of note, in the absence of KF (entry 13), slightly lower activity of the reactions was observed. Substituting pyridine N-oxide with 4phenylpyridine N-oxide led to a significant decrease in yield (entry 14). In addition, the reactions proceeded in lower yields when conducted at higher temperatures (entry 15−17). Different solvents were screened (entry 18−20), and only DMF performed a little better and afforded coupling product in 52% yield. With optimized conditions in hand, decarboxylative crosscoupling of a variety of aliphatic anhydrides, either commercially available or generated in situ, with 4-phenylcyclohex-1-enyl trifluoromethanesulfonate proceeded efficiently, furnishing the desired products in good yields (Scheme 2, 3a−j). Among them, chain (3a, 3b) and cyclic (3e−3i) aliphatic anhydrides were all well coupled. Heterocontaining anhydrides (3h−3j), such as tetrahydro-2H-pyran-4-carboxylic Scheme 2. Scope of Ni-Catalyzed Coupling of Alkyl Acid Anhydrides with Vinyl Triflates*

Table 1. Optimization of Ni-Catalyzed Cross-Coupling of Alkyl Acid Anhydrides with Vinyl Triflatesa

*

Reaction conditions: reactions were run on 0.5 mmol scale at 0 °C for 72 h. aAlkyl acid anhydrides were commercially available. bAlkyl acid anhydrides were prepared after simple filtration of impurities. c Reaction was run at 25 °C for 48 h. Yields are isolated yields (see the Supporting Information).

a

Reactions were run on 0.5 mmol scale. Yields are isolated yields (see the Supporting Information). B

DOI: 10.1021/acs.orglett.9b01048 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters acid anhydride (3h), were tolerated, resulting in the coupling product in 60% yield. A broad array of vinyl triflates were then investigated (3k−3q). Intriguingly, the large ring sized vinyl triflate (3l) was a suitable coupling partner and formed the corresponding coupling product in a good yield (87%). Furthermore, vinyl triflates possessing diverse functional groups, such as oxygen substituent (3m), ketal (3n), protected nitrogen (3o), fluorine (3p), and aromatic rings (3q), were all compatible with the reaction conditions. More importantly, this method could be applied in methylation and ethylation of vinyl triflates30−35 (Scheme 2, 3r, 3s). Traditionally, these short alkyl groups were introduced via the use of toxic and highly volatile methyl and ethyl iodides or difficult-to-manipulate organometallic reagents relevant to medical chemistry or other fields. This limitation was overcome with the method using commercially available acetic anhydride or propionic anhydride (3r, 3s). Next, we aimed to extend the reaction to other important electrophiles including vinyl bromides and iodides. After slight modification of reaction conditions, we found that replacing 1,10-phenanthroline with 2,2′-bipyridine and stirring the reaction at 25 °C gave the best results (see the Supporting Information). As depicted in Scheme 3, a range of aliphatic anhydrides were initially attempted (5a−5f); we found that reactions proceeded smoothly regardless of the primary or secondary aliphatic anhydrides involved (5a−5l). Under the optimized conditions, vinyl bromides were also converted into the heterocontained products in good yields (5b, 5d−5f). Notably, a diverse set of vinyl bromides were coupled well under the standard conditions (5g−5k). For example, 2bromopropene containing the short chain was a competent coupling partner, pleasingly providing the coupling product in 81% yield (5i). Functional groups, including ester (5g), lactone (5k), and conjugated aromatic rings (5j), were all tolerated. In addition, when (Z)-vinyl bromide (5l) was used as substrate, the coupling product was obtained with high regioselectivity. Compared to vinyl bromides, vinyl iodides as the coupling partners afforded the decarboxylative products with a much higher ratio of E/Z configurations (5m−5q). Note that our approach also featured high chemoselectivity toward alkyl group migration with mixed aliphatic/aromatic anhydrides (Scheme 4). When mixed acid anhydride generated from aliphatic and aryl carboxylic acid was used to react with vinyl halides, only the alkyl segment was involved in the decarboxylative coupling process, affording the desired products in modest to good yields (Scheme 4, 8a−8l). To our delight, we could easily modify the dehydrocholic acids by transforming them into mixed anhydride with the cheap and commercially available anisic acids (8e, 8f). Considering that ketone groups are quite common in natural products, we envisioned that these natural products could be simply converted into the vinyl triflates (Scheme 4, 8m), and then these vinyl triflates could be coupled with aliphatic anhydrides by using our chemistry. As anticipated, the desired product was achieved in a modest yield (Scheme 4, 8n). This further demonstrated the utility of this decarboxylative alkenylation method. More practically, aliphatic acids employed directly by generating the corresponding anhydrides in situ and coupled with vinyl triflates or halides were demonstrated. As depicted in Scheme 5, different substrates proceeded smoothly and afforded decarboxylative products in modest yields. For example, when 4-phenylcyclohex-1-enyl trifluoromethanesulfo-

Scheme 3. Scope of Ni-Catalyzed Coupling of Alkyl Acid Anhydrides with Vinyl Halides (Br, I)*

*

Reaction conditions: Reactions were run on 0.5 mmol scale at 25 °C for 48 h. aAlkyl acid anhydrides were commercially available. bAlkyl acid anhydrides were prepared after simple filtration. cReaction was run at 0 °C for 72 h. dLigand was 1,10-phenanthroline. Yields are isolated yields (see the Supporting Information).

nate reacted with isolated cyclohexanecarboxylic anhydride (Scheme 5, 11b), the yield of coupled product was 75%; there was only a 5% decrease in the yield when it was run in a flask without removal of the impurities. The results indicated that the reaction conditions were robust and practical. Finally, we turned our attention to elucidate the coupling mechanism (Scheme 6). As reported by other groups and us,6,29 the decarboxylative process involved a key step of singleelectron transfer. As shown in Scheme 6, when TEMPO was added to this reaction, the reaction was completely suppressed. Product 12 was detected by GC−MS. In addition, when we used cyclopropylacetic acid anhydride as the substrate, the ring-opening product was achieved in 51% yield (Scheme 6b, 14); this strongly supports the SET process hypothesis. Our data collectively suggests a plausible mechanism for the decarboxylative coupling of vinyl triflate and halide (Br, I) (in Scheme 7) with anhydrides. The precatalyst NiCl2 is reduced by zinc to generate Ni0 I. Subsequent oxidative addition of vinyl halide or triflates can generate the NiII species II. Then the species is reduced to the NiI complex III. Next, complex III provides an electron to the aliphatic carboxylic anhydride adduct VII, thus producing the radical of adduct VIII with C

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Organic Letters Scheme 6. Mechanistic Studiesa

Scheme 4. Chemical Selectivity of Reactions and Natural Products Modification

Reaction conditions: reactions were run on 0.5 mmol scale at 0 °C for 72 h. Yields are isolated yields (see the Supporting Information).

a

Scheme 7. Proposed Mechanism

Reactions were run on 0.5 mmol scale at 25 °C for 48 h. bReactions were run on 0.5 mmol scale at 0 °C for 72 h. cAryl acids were anisic acids. dAryl acids were p-toluic acids. eLigand was 2,2′-bipyridine. Yields are isolated yields (see the Supporting Information).

a

Scheme 5. Ni-Catalyzed Coupling of Alkyl Acid Anhydrides (Generated in Situ) with Vinyl Halide

Interception of alkyl radical by NiII complex IV would then form NiIII intermediate V.6a This intermediate underwent reductive elimination to afford the decarboxylative product and NiI species VI. At this point, NiI was reduced to regenerate Ni0 with zinc powder.29,36 In conclusion, we report a nickel-catalyzed decarboxylative alkenylation of aliphatic anhydrides with vinyl triflates or halides. Direct use of vinyl triflates or halides avoided the tedious preparation of alkenylmetal reagents. More importantly, vinyl triflates are appealing alkenyl coupling partners (readily accessible by treating aliphatic ketones with commercially available sulfonated reagents) and thereby significantly expand the scope of decarboxylative olefination. It is also worth mentioning that our approach addresses the issue of safe and environmental installation of the methyl or ethyl group into alkene moieties by using commercially accessible acetic anhydride or propionic anhydride. Owing to its chemoselectivity, this method also provides a robust tool to modify high-value aliphatic acids by converting them into mixed acid anhydrides by using cheap aryl carboxylic acids. Additional mechanistic investigations are underway.

Reactions were run on 0.5 mmol scale at 0 °C for 72 h. b2,2′Bipyridine as ligand and reactions run at 25 °C for 48 h. cR = corresponding alkyl group of aliphatic acids. dR = 4-methoxy phenyl. [isolated] = anhydrides were prepared after simple filtration of related impurities. [in situ] = anhydrides were prepared without any purification and reaction was taken in one flask. Yields are isolated yields (see the Supporting Information). a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01048.

concomitant generation of NiII complex IV. With the fragmentation of VIII, the desired alkyl radical was produced. D

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Organic Letters

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Detailed experimental procedure and spectral data (1H, 13 C, MS, HRMS) for compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xuebin Liao: 0000-0002-0290-894X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the Beijing Municipal Science & Technology Commission (Z161100000116032), the Collaborative Innovation Center for Diagnosis and Treatment of Infectious Disease, and TsingPeking Centre for Life Science.

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