Letter pubs.acs.org/OrgLett
Cu-Catalyzed Stereoselective Borylation of gem-Difluoroalkenes with B2pin2 Juan Zhang, Wenpeng Dai, Qingyun Liu, and Song Cao* Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University of Science and Technology (ECUST), Shanghai 200237, China S Supporting Information *
ABSTRACT: A novel and efficient method for the synthesis of (Z)-fluorinated alkenylboronic acid pinacol esters via Cu-catalyzed stereoselective borylation of gem-difluoroalkenes using bis(pinacolato)diboron (B2pin2) as the boron source with the assistance of NaOtBu and Xantphos at room temperature was developed.
O
obtained (Scheme 1a).11 In 2015, the group of Martin described the Ni-catalyzed borylation of monofluoroarenes with B2nep2 via
rganoboronic acid and their derivatives, especially alkenylboronic esters, are important and versatile reagents in organic synthesis because of their high stability, low toxicity, and great synthetic utility in the Suzuki−Miyaura coupling reaction.1 Consequently, considerable attention has been devoted to the development of novel and efficient methods for the synthesis of vinylboronate esters (VBEs).2 Traditional methods for the synthesis of alkenylboronic esters involve the transmetalation of 1-alkenyllithium or Grignard reagents with trialkylborates.3 Transition-metal-catalyzed hydroboration of alkynes is one of the most efficient and straightforward methods to access a variety of alkenylboron compounds.4 Alternatively, rhodium- or iridium-catalyzed direct C−H dehydrogenative borylation of alkenes represents an attractive approach due to its atom and step economy.5 In addition, a palladium-catalyzed cross-coupling reaction of bis(pinacolato)diboron (B2pin2, pin = Me4C2O2) with alkenyl halides or triflates provides a convenient and alternative method for the preparation of alkenylboronic esters.6 However, attempts by Miyaura and Ishiyama to borylate dihaloalkenes such as 2,2-dibromo-1-phenylethene with B2pin2 in the presence of PdCl2(PPh3)2 and Ph3P were unsuccessful.6a Interestingly, when 1,1-dibromoalkenes reacted with B2pin2 with the assistance of n-butyllithium at −110 °C, 1,1-diborylalkenes were obtained.7 It should be noted that when an alkenyl halide containing both an alkene C−F and C−Br bond such as (Z)-(2bromo-2-fluorovinyl)benzene was used as a substrate, the borylation reaction took place selectively at the C−Br bond and the C−F bond remained intact.8 The cleavage, activation, and functionalization of the carbon− fluorine bond have received considerable attention in recent years because they offer unique methods for synthesis of structurally diverse molecules from fluoroorganic compounds.9 The transition-metal-catalyzed borylation of the inert C−F bond in polyfluorinated alkenes and arenes is of particular interest to the synthetic community due to the ability of borylated products to be readily converted into the fluorinated amine, alcohol, alkyl, and aryl functionalities.10 In 2015, Zhang et al. disclosed the first example of an ortho-selective C−F bond borylation of polyfluoroarenes with commercially available [Rh(cod)2]BF4 as a catalyst, and a wide range of borylated fluoroarenes were © 2017 American Chemical Society
Scheme 1. C−F Bond Borylation of Fluoroarenes and Fluoroalkenes
C−F activation (Scheme 1b).12 Almost at the same time, the group of Niwa and Hosoya independently developed an efficient method for the synthesis of various borylarenes by the Ni/Cucatalyzed borylation of monofluoroarenes with B2pin2 (Scheme 1c).13 In 2016, Marder and Radius et al. reported an alternative method for the C−F borylation of polyfluoroarenes using Ni(IMes)2 as a catalyst and B2pin2 as the boron source (Scheme 1d).14 Unfortunately, to date, examples regarding the transformation of alkenyl fluorides into alkenylboronic acid pinacol esters are very scarce. The first and only example of C−F bond borylation of fluorinated olefin was described by Braun and coworkers in 2009. They found that hexafluoropropene can be coupled with HBpin in the presence of 0.4 mol % RhH(PEt3)3 and a mixture of 2-fluoroalkyl-1,3,2-dioxaborolanes was obtained in quantitative yields (Scheme 1e).15 Received: May 11, 2017 Published: June 5, 2017 3283
DOI: 10.1021/acs.orglett.7b01430 Org. Lett. 2017, 19, 3283−3286
Letter
Organic Letters It is well-known that gem-difluoroalkenes exhibit excellent reactivity toward nucleophiles due to their highly electrondeficient nature of carbon−carbon double bond and the high leaving-group ability of the fluoride ion.16 Although gemdifluoroalkenes could react readily with strong nucleophiles such as nitrogen, oxygen, and sulfur nucleophiles through an addition−elimination process (SNV reaction),17 the crosscoupling reaction of gem-difluoroalkenes with organometallic reagents such as Grignard reagents,18 organozinc reagents,19 organosilicon reagents,20 and arylboronic acids21 requires the use of a catalytic amount of transition-metal catalyst to activate the carbon−fluorine bond. Even though the intermolecular activation of bis(pinacolato)diboron can generate a reactive nucleophilic boryl B(sp 2) synthon,22 the borylation of fluoroalkenes with B2pin2 remains an extremely attractive but challenging subject for organic chemists due to the attenuated nucleophilicity of the boryl moiety. In continuation of our research on the functionalization of the C−F bond of gemdifluoroalkenes,23 in this letter, we reported a novel and efficient method for the synthesis of (Z)-fluorinated alkenylboronic acid pinacol esters via Cu-catalyzed stereoselective borylation of gemdifluoroalkenes with B2pin2 in the presence of NaOtBu and Xantphos at room temperature (Scheme 1f). We began our investigation using the borylation of 1-(2,2difluorovinyl)-4-methoxybenzene 1a with bis(pinacolato)-diboron B2pin2 2 as the model reaction to optimize the reaction conditions (Table 1). Initially, the borylation reaction was performed in the presence of various cuprous compounds. Among the copper(I) salts examined, CuCN and Cu(OAc) turned out to be the most effective, providing 3a in 81% yields (entries 2−6). Expectedly, no detectable product was observed
in the absence of a copper catalyst (entry 1). When cupric salts such as Cu(OAc)2 were used as catalyst, the reaction did not proceed at all. Xantphos is a preferred ligand compared to other common ligands such as dppe, dppf, dppb, PPh3, PCy3, and DPEphos (entries 6 and 8−13). Notably, the reaction has proven to be ineffective without addition of ligand, as only 43% of the borylated product was obtained (entry 7). Further screening of different bases revealed that only NaOtBu could afford the desired product 3a in optimal yield (entries 6 and 14−17). Solvent played an important role in this transformation. DMF was the most suitable solvent for the reaction (entry 6). Other solvents such as THF, MeOH, CH3CN, toluene, and DMAc (N, N-dimethylacetamide) all resulted in no reaction or low efficiency (entries 18−22). To our delight, this borylation reaction occurred with high (Z)-selectivity and no (E)-isomer was detected. With the optimal reaction conditions in hand (Table 1, entry 6), the scope of this Cu(OAc)-catalyzed borylation of various gem-difluoroalkenes was investigated (Scheme 2). In most cases, Scheme 2. Reactions of gem-Difluoroalkenes 1a−t with B2pin2a
Table 1. Optimization of the Reaction Conditionsa
entry
Cu salt
ligand
base
solvent
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
none CuI CuBr CuCl CuCN Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc) Cu(OAc)
Xantphos Xantphos Xantphos Xantphos Xantphos Xantphos none dppe dppf dppb PPh3 PCy3 DPEphos Xantphos Xantphos Xantphos Xantphos Xantphos Xantphos Xantphos Xantphos Xantphos
NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu CsF NaOMe KOtBu LiOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu
DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF THF MeOH toluene CH3CN DMAc
0 trace trace 60 81 81 43 16 20 23 18 25 40 trace trace 53 65 trace 0 0 0 60
a Reaction conditions: 1a−t (1.0 mmol), 2 (1.2 mmol), Cu(OAc) (0.1 mmol), Xantphos (0.1 mmol), NaOtBu (2.0 mmol), DMF (6 mL), 25 °C, 6−12 h, Ar. Isolated yields. The configuration of the (Z)-isomer 3a−t was determined by its 3JH−F coupling constant in the 1H NMR spectra (ca. 46.0 Hz for (Z)-isomer and 16.0 Hz for (E)-isomer).
this novel borylation reaction proceeded efficiently to give the corresponding (Z)-fluorinated alkenyl boronates in moderate to good yields and no (E)-isomers were observed. gem-Difluoroalkenes having a strong electron-donating group on the benzene ring proceeded smoothly, whereas a substrate bearing a strong electron-withdrawing group such as cyano failed to give the desired product. Substituents, such as methylthio (1f), acetoxy (1l), chloro (1m−o), bromo (1p and 1t), and iodo (1q) groups, were tolerated under the optimized reaction conditions. The tolerance of chloride, bromide, and iodide substituents was particularly useful, which offers the opportunity for further modifications. When the aryl ring of the substrates was changed to naphthalene or thiophene, the reactions could also provide the
a
Reaction conditions: 1a (0.2 mmol), 2 (0.24 mmol), solvent (3 mL), 6 h. bYields determined by GC analysis and based on 1a. 3284
DOI: 10.1021/acs.orglett.7b01430 Org. Lett. 2017, 19, 3283−3286
Letter
Organic Letters borylation products in modest yields (3r and 3t). In addition, (E)-(4,4-difluorobuta-1,3-dien-1-yl)benzene was a suitable substrate for this reaction, the corresponding product (3s) being obtained in moderate yield. Unfortunately, when 1,1-diaryl-2,2difluoroethenes, gem-dibromoalkenes, and gem-dichloroalkenes were used as substrates under the optimal reaction conditions, only a trace amount of borylated products was detected. Furthermore, when (4,4-difluorobut-3-en-1-yl)benzene was subjected to this borylation reaction, a small amount of a mixture of E/Z-borylated product was observed (GC-MS). The activation and functionalization of the C−F bond of monofluoroalkenes remain a largely unexplored area. Generally, monofluorinated alkenes are much less reactive in the addition− elimination reaction than gem-difluoroalkenes. Encouraged by the above results obtained with the gem-difluoroalkenes, we next applied the Cu-catalyzed borylation process to examine some monofluoroalkenes (Scheme 3). Much to our surprise, the
Scheme 4. Derivatization of (Z)-3a
Scheme 3. Reactions of Vinyl Monofluorides 1u−x with B2pin2a
gem-difluoroalkenes (for detailed experimental procedures and characterization of products, see the Supporting Information). Based on the above observations and previous reports,24 the mechanism for the Cu-Catalyzed stereoselective borylation of gem-difluoroalkenes with B2pin2 is proposed (Scheme 5). Scheme 5. Proposed Reaction Mechanism
a
Reaction conditions: 1u−x (1.0 mmol), 2 (1.2 mmol), Cu(OAc) (0.1 mmol), Xantphos (0.1 mmol), NaOtBu (2.0 mmol), DMF (6 mL), 25 °C, 3−6 h, Ar. Isolated yields. The configuration of the (E)-isomer 3u−x was determined by its 3JH−H coupling constant in the 1H NMR spectra (ca. 16.0 Hz for (Z)-isomer and 18.0 Hz for (E)-isomer).
borylation of a mixture of E/Z-monofluoroalkenes (1u−x) under the same conditions afforded the (E)-isomers (3u−x) exclusively. No unreacted (Z)-monofluoroalkene starting materials and (Z)-borylated products were observed. Notably, the borylation of pure (Z)-1-(2-fluorovinyl)-4-methoxybenzene 1u alone also gave (E)-borylated product 3u, though a longer reaction time was required to complete the reaction (12 h). These results therefore suggested that carbon−carbon double bond cleavage might be involved in the reaction. The novel pure (Z)-fluorinated pinacol alkenylboronates we reported here are expected to be highly useful and versatile building blocks in organic synthesis due to the unique reactivity of the boryl moiety. To demonstrate the synthetic utility of (Z)β-fluorinated vinylboronates, we converted pure (Z)-3a into other valuable β-fluorostyrene derivatives using established chemistry (Scheme 4). We were delighted to find that these derivatization reactions proceeded smoothly and afforded the corresponding (Z)- or (E)-fluorinated olefins in moderate to good yields with complete retention of configuration of the double bond. No obvious isomerization was observed. One of the remarkable advantages of this method is that it provides direct and facile access to expensive and not readily available isomerically pure mixed-halogen alkenes ((E)-5−7). More importantly, the C−F bond of (E)-7 and (Z)-11 could also be transformed into a C−N bond ((E)-12) and C−C bond ((E)13) through an addition−elimination process and coupling reaction, respectively, despite the lower reactivity of the C−F bond of monofluoroalkenes compared with the C−F bond in
Initially, transmetalation of the phosphine-ligated copper catalyst LCuOtBu, which is generated in situ from LCu(OAc) and NaOtBu, with the diboron reagent B2pin2 provides nucleophilic phosphine-coordinated Cu(I)-boryl complexes LCu−Bpin. Subsequently, insertion of a difluoroalkene into the Cu−B bond of borylcopper complex would lead to the generation of βborylalkyl copper species I. Rotation of the key intermediate I by ±60° results in the formation of two conformations, II and III. The intermediate III was relatively unstable due to steric repulsion between the bulky boryl group and the aryl group. βFluoride elimination of borylalkyl copper intermediate II affords the desired pure (Z)-fluorinated pinacol alkenylboronate along with LCuF. Finally, LCuF further reacts with B2pin2 and NaOtBu to regenerate the active catalyst LCu−Bpin and finish the catalytic cycle. In summary, we have developed an efficient and practical method for the synthesis of (Z)-fluorinated alkenylboronic acid pinacol esters via Cu-catalyzed stereoselective borylation of gemdifluoroalkenes with B2pin2 in the presence of NaOtBu and Xantphos at room temperature. The borylation reaction exhibits a wide substrate scope and good functional group compatibility and affords a variety of (Z)-fluorinated borylation products in moderate to good yields. Specifically, the borylation of 3285
DOI: 10.1021/acs.orglett.7b01430 Org. Lett. 2017, 19, 3283−3286
Letter
Organic Letters
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monofluoroalkenes also proceeds smoothly and furnishes the geometrically pure (E)-borylated alkenes only, irrespective of the configuration (E or Z) of the starting monofluoroalkenes. To demonstrate the synthetic utility of the valuable fluorinated and borylated building blocks obtained by the present method, derivatizations of the product pure (Z)-3a using the boryl group, such as protodeboronation, halogenation, and Suzuki−Miyaura coupling, were conducted. Delightfully, (Z)-configuration of a borylation product such as 3a can be retained completely in these transformations and a range of versatile geometrically pure multisubstituted alkenes were obtained. This method has allowed convenient access to the novel (Z)-fluorinated alkenylboronic acid pinacol esters, which are anticipated to be a powerful synthetic module for the construction of isomerically pure β-fluorostyrene derivatives.
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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.7b01430. Experimental details and spectral data for all products (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Song Cao: 0000-0002-3231-6136 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (Grant Nos. 21472043 and 21272070).
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REFERENCES
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DOI: 10.1021/acs.orglett.7b01430 Org. Lett. 2017, 19, 3283−3286
