Radical-Organometallic Hybrid Reaction System Enabling Couplings

Letter Cite This: ACS Catal. 2018, 8, 6791−6795

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Radical-Organometallic Hybrid Reaction System Enabling Couplings between Tertiary-Alkyl Groups and 1‑Alkenyl Groups Kimiaki Nakamura,† Reina Hara,† Yusuke Sunada,‡ and Takashi Nishikata*,† †

Graduate School of Science and Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-ku, Tokyo 153-8505, Japan

‡

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S Supporting Information *

ABSTRACT: Suzuki−Miyaura couplings of tertiary-alkyl moieties are accomplished in the presence of a copper catalyst, in which quaternary carbons possessing various functional groups can be synthesized via a radical reaction. Mechanistic studies revealed that 1-alkenylcopperI plays an important role in this coupling reaction. We expect that the radical-organometallic combined process will become one of the best options for the synthesis of quaternary carbons. KEYWORDS: radical, organometallics, copper, tertiary-alkylation, hybrid catalyst sysytem uzuki−Miyaura coupling is one of the most powerful Csp2− Csp2 bond forming methods in synthetic organic chemistry. The large number of applications are attributed to its simple catalytic cycle which is composed of the following: (1) oxidative addition of Pd(0) to Csp2−X bond leading to a Csp2−Pd−X; (2) transmetalation between Csp2−Pd−X and an organoboronic compound leading to a Csp2−Pd−C′sp2 species; and (3) reductive elimination of Csp2−Pd− C′sp2 leading to a Csp2−C′sp2 bond.1−3 Those elemental steps are suitable for the formation of Csp2−Csp2 bonds, whereas the couplings of alkyl halides including tertiary-alkyl groups are limited owing to the slow oxidative addition and rapid β-hydrogen elimination from a σ-alkyl metal intermediate.3−5 Therefore, the alkylative couplings are one of the remaining but hottest issues in this area.6−20 The impact of radical reactions has been well-recognized in organic synthesis. The radical species can be combined with ionic species because they are neutral. In this context, some chemists have been devoted to develop radical-ionic combined reactions.21,22,23 On the other hand, the radicals could be combined with organometallic intermediates.24 For example, Nevado’s group recently reported the elegant reaction, in which the combination of alkyl radicals and arylnickel species enables accurate aryl-alkylations of alkynes.25−27 They proved the reaction mechanism by using an arylnickel complex. The metalcatalyzed cross-coupling reactions involving both radials and organometallic intermediates are still challenging.28,29 In this context, we focused on using α-bromocarbonyl compounds which are very attractive tertiary-alkyl sources because further functional group transformations can be conducted after the alkyl loading. It is well-known that αbromocarbonyl compounds easily generate a tertiary-alkyl radical species via single electron transfer from a Cu(I) salt. In this reaction, both oxidative addition and β-hydrogen

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© XXXX American Chemical Society

elimination are not problematic in generating a reactive alkyl intermediate.30,31 Thus, we expected that a tertiary-alkyl radical generated by single-electron transfer of an organocopperI intermediate can overcome the inherent mechanistic limitations mentioned above for tertiary-alkylative Suzuki−Miyaura couplings (Scheme 1). The resulting compounds obtained by Scheme 1. Working Hypothesis: Radical-Organometallic Hybrid Reaction System

this method can easily transform into various derivatives possessing a quaternary carbon center, to which numerous possible manipulations can be applied at the carbonyl and alkene functionalities. Herein, we report copper-catalyzed tertiary-alkylative Suzuki−Miyaura type couplings to synthesize 1-alkenylated quaternary product possessing a carbonyl group. Optimization studies employed the combination of ethyl 2− bromoisobutyrate (1a, 1.5 equiv) and styrylpinacolborane (2a, 1.0 equiv) in the presence of a CuI catalyst (10 mol %), K2CO3 (20 mol %), amine (1.5 equiv), and alcohol (2.0 equiv) in toluene under nitrogen atmosphere at 100 °C (Table 1). The Received: April 22, 2018 Revised: May 31, 2018

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DOI: 10.1021/acscatal.8b01572 ACS Catal. 2018, 8, 6791−6795

Letter

ACS Catalysis Table 1. Optimizationa

amines, diisopropylamine was selected as the optimum. The reaction in the presence of diisopropylamine did not require K2CO3 (Entry 12 and 13). To improve the yield, we checked the effect of NHC ligand (1,3-bis(2,6-diisopropylphenyl)imidazolium chloride was used) and Pt cocatalyst (entries 14 and 15). As a result, adding Pt resulted in 92% yield. Under optimal conditions (with or without Pt catalyst), various boronic esters having electron-donating or -withdrawing groups or heteroaryl substituents reacted smoothly with bulky 2-bromoesters in good yields (Table 2). The yields Table 2. Substrate Scopea

a

The yields were determined by 1H NMR analysis. bWithout K2CO3. Isolated yield. dCu(OtBu)(NHC) was used instead of CuI. e2 mol % Pt(dba)2 was used as a cocatalyst. c

addition of the copper catalyst is crucial to obtain the desired cross-coupling product 3a (Table 1, entry 1). A nitrogen ligand is also important in this reaction. When phosphorus ligands were used, the reaction did not occur (entry 2). On the other hand, nitrogen ligands such as TMEDA (N,N,N′,N′tetramethylethylenediamine) and PMDETA (N,N,N′,N,″N″pentamethyldiethylenetriamine) were effective in this reaction (entries 3 and 4). The pyridine-based ligand TPMA (tris(2− pyridylmethyl)amine) gave 3a in 65% yield (entry 5). There are numerous reports on arylation via Suzuki−Miyaura couplings, though the reaction of tertiary-alkyl groups with 1-alkenylboron reagents is rare, probably owing to stability and reactivity of the intermediates.1,32,33 In our case, a proper catalyst system enabled smooth cross-coupling reaction. To check the pyridyl effect in ligands, various pyridine-based ligands were screened (entries 6−10). As a result, electron-rich TPMA bearing two methoxy groups resulted in the highest yields (74%, entry 10). The effect of the presence of alcohol was not clear, although the reaction smoothly proceeded with CF3CH2OH (entry 11). Amine as a base may activate the boron reagent during transmetalation with a copper catalyst. After careful optimization of amines including primary-, secondary-, and tertiary-

Conducted at 100 °C for 20 h in toluene with 10 mol % CuI, 10 mol % L5, (CF3)2CHOH (2.0 equiv), iPr2NH (1.5 equiv), 1 (1 equiv), and 2 (1.5 equiv). The yields of 3 were Isolated. b2 mol % Pt(dba)2 was used as a cocatalyst.

a

of 3b, 3c, 3g, 3h, 3k, 3m, and 3n were good to excellent, which indicates that simple styrylboronic esters or 2-bromoesters having a 4- or 6-membered ring tend to give high reactivities. Sterically bulky 2-bromoesters giving 3d and 3j also showed nice reactivities, and the yields were 70 and 64%, respectively. Although bromolactone giving 3e resulted in low yield because of the rapid decomposition of the bromide, other combinations giving 3f, 3i, and 3l resulted in moderate to good yields. Indications of good functional group tolerance were observed by the synthesis of 3o−3w. Thiophene, quinolone, and benzodioxole substituents, which are medicinally relevant heterocycles, typically suffer from catalyst poisoning by sulfur 6792

DOI: 10.1021/acscatal.8b01572 ACS Catal. 2018, 8, 6791−6795

Letter

ACS Catalysis or nitrogen atom linking to the catalyst. However, smooth reactions with functionalized acyclic and cyclic 2-bromoesters 1 were obtained. We also checked highly functionalized or congested substrates including boryl, alkynyl, amide, and amine groups. Those substrates were low-to-moderate yields, but the yields were improved in the presence of Pt(dba)2 as a cocatalyst (3x−3bb). The same effects were observed in the reaction giving 3e, 3l, 3o, and 3p. Especially, Pt was very effective in the reaction of 2a and 2-bromomalonate ester to give 3cc. Pt might play an important role in the coupling step (see Table 3). In

Scheme 3. Transformations

Scheme 4. Proposed Reaction Mechanism

Table 3. Stoichiometric Reactions of 12

entry

conditions

yield of 3a

1 2 3 4

none (CF3)2CHOH (2 equiv) iPr2NH (1.5 equiv) (CF3)2CHOH (2 equiv) iPr2NH (1.5 equiv) Pt(dba)2 (2 mol %) TEMPO (1 equiv) without L5

54 42 45 47

5 6 7

alkenylcopper intermediate A. Then, the corresponding product 3a could be obtained via a transient intermediate (B or C). To support our proposed mechanism, we carried out some control experiments. Styrylcopper(I) complex was prepared from the reaction of 11 and 2a via transmetalation. The fine Xray structure was also obtained (Scheme 5. See SI). We tried to

71 0 0

this reaction, styrene-derived boronic esters or 2-bromoesters showed excellent reactivities, although other derivatives including cyclohexenyl-substituted alkyne and 2-bromoketone giving 3dd−3ff showed poor results. Overall, over 20 coupling examples were successfully accomplished. Our reaction conditions were able to be applied to 1alkenylboronic esters, but arylboronic esters were not effectively transformed via our protocol probably because transmetalation with copper possessing multidentate nitrogen ligand is slow. Therefore, 1-alkenylboronic ester 4 possessing an aryl−boron bond can be transformed to various functionalized quaternary carbon centers via Suzuki−Miyaura coupling (Scheme 2).

Scheme 5. Preparation of Styrylcopper Complex 12

synthesize styrylcopper(I) complexes possessing a nitrogen ligand, but they were too unstable to carry out control experiments. On the other hand, styrylcopper(I) complex possessing NHC ligand (N-heterocyclic carbene) (12) was enough stable to check the catalytic cycle.34 We next checked the reactivity of 12 under various conditions shown in Table 3. After mixing of 12 and L5, 1a smoothly reacted with styrylcopper species to give 3a in 54% yield (entry 1). The additives (CF3)2CHOH and iPr2NH did not affect the coupling step (entries 2−4). Those additives might be important in the transmetalation step. When Pt(dba)2 was added to this reaction mixture, 71% yield of 2a was obtained. Pt as a cocatalyst promoted the coupling reactions but the accurate role was not clear. The reaction was inhibited by adding TEMPO (entry 6), which suggests the existence of radical species from 1. The generation of α-radicals from the reaction of α-bromocarbonyls and a copper catalyst have demonstrated in previous our results.30,31 Our new ligand L5 was also important to carry out the reaction (entry 7). We also checked the catalytic reactivity of 12 (10 mol %) and the corresponding product 3a was obtained in 61% yield (Scheme 6). We failed to have any proof of proposed intermediates (copper-stabilized alkyl radicals (B) and oxidative adduct of CuIII (C)35), but the existence of B or C cannot be excluded. This type of transient intermediate has been proposed in

Scheme 2. Suzuki−Miyaura Couplings

Borylated 1-alkenylboron ester 4 reacted with 1b to produce 5 in 74% yieldaryl while leaving the carbon−boron bond intact. The desired two benzene-substituted compound 6 was obtained in 83% yield via Suzuki−Miyaura coupling. To obtain various quaternary carbon centers, some possible manipulations were applied at the carbonyl and alkene functionalities. For example, epoxide 7 and alcohol 8 were obtained from 3a via oxidation or reduction (Scheme 3). While the exact reaction mechanism is currently unclear, one possibility involves a radical pathway (Scheme 4). The reaction could begin with the reaction of CuI and 2a to generate 16793

DOI: 10.1021/acscatal.8b01572 ACS Catal. 2018, 8, 6791−6795

Letter

ACS Catalysis

Cataryzed by Dichloro[1,1′-Bis(Diphenylphosphino) ferrocene]nickel (II). Chem. Lett. 1980, 9, 767−768. (7) Joshi-Pangu, A.; Wang, C.-Y.; Biscoe, M. R. Nickel-Catalyzed Kumada Cross-Coupling Reactions of Tertiary Alkylmagnesium Halides and Aryl Bromides/Triflates. J. Am. Chem. Soc. 2011, 133, 8478−8481. (8) Wang, X.; Wang, S.; Xue, W.; Gong, H. Nickel-Catalyzed Reductive Coupling of Aryl Bromides with Tertiary Alkyl Halides. J. Am. Chem. Soc. 2015, 137, 11562−11565. (9) Ghorai, S. K.; Jin, M.; Hatakeyama, T.; Nakamura, M. CrossCoupling of Non-activated Chloroalkanes with Aryl Grignard Reagents in the Presence of Iron/N-Heterocyclic Carbene Catalysts. Org. Lett. 2012, 14, 1066−1069. (10) Someya, H.; Ohmiya, H.; Yorimitsu, H.; Oshima, K. SilverCatalyzed Benzylation and Allylation Reactions of Tertiary and Secondary Alkyl Halides with Grignard Reagents. Org. Lett. 2008, 10, 969−971. (11) Mitamura, Y.; Asada, Y.; Murakami, K.; Someya, H.; Yorimitsu, H.; Oshima, K. Silver-Catalyzed Benzylation and Allylation of Tertiary Alkyl Bromides with Organozinc Reagents. Chem. - Asian J. 2010, 5, 1487−1493. (12) Tsuji, T.; Yorimitsu, H.; Oshima, K. Cobalt-Catalyzed Coupling Reaction of Alkyl Halides with Allylic Grignard Reagents. Angew. Chem., Int. Ed. 2002, 41, 4137−4139. (13) Dunsford, J. J.; Clark, E. R.; Ingleson, M. J. Direct C(sp2)C(sp3) Cross-Coupling of Diaryl Zinc Reagents with Benzylic, Primary, Secondary, and Tertiary Alkyl Halides. Angew. Chem., Int. Ed. 2015, 54, 5688−5692. (14) Reina, D. F.; Ruffoni, A.; Al-Faiyz, Y. S. S.; Douglas, J. J.; Sheikh, N. S.; Leonori, D. Visible-Light-Mediated Reactions of Electrophilic Radicals with Vinyl and Allyl Trifluoroborates. ACS Catal. 2017, 7, 4126−4130. (15) (a) Terao, J.; Ikumi, A.; Kuniyasu, H.; Kambe, N. Ni- or CuCatalyzed Cross-Coupling Reaction of Alkyl Fluorides with Grignard Reagents. J. Am. Chem. Soc. 2003, 125, 5646−5647. (b) Liu, C.; Liu, D.; Zhang, W.; Zhou, L.; Lei, A. Nickel-Catalyzed Aromatic C−H Alkylation with Secondary or Tertiary Alkyl−Bromine Bonds for the Construction of Indolones. Org. Lett. 2013, 15, 6166−6169. (16) (a) Terao, J.; Todo, H.; Begum, S. A.; Kuniyasu, H.; Kambe, N. Copper-Catalyzed Cross-Coupling Reaction of Grignard Reagents with Primary-Alkyl Halides: Remarkable Effect of 1-Phenylpropyne. Angew. Chem., Int. Ed. 2007, 46, 2086−2089. (b) He, C.; Guo, S.; Huang, L.; Lei, A. Copper Catalyzed Arylation/C−C Bond Activation: An Approach toward α-Aryl Ketones. J. Am. Chem. Soc. 2010, 132, 8273−8275. (17) Iwasaki, T.; Takagawa, H.; Singh, S. P.; Kuniyasu, H.; Kambe, N. Co-Catalyzed Cross-Coupling of Alkyl Halides with Tertiary Alkyl Grignard Reagents Using a 1,3-Butadiene Additive. J. Am. Chem. Soc. 2013, 135, 9604−9607. (18) Ariki, Z. T.; Maekawa, Y.; Nambo, M.; Crudden, C. M. Preparation of Quaternary Centers via Nickel-Catalyzed Suzuki− Miyaura Cross-Coupling of Tertiary Sulfones. J. Am. Chem. Soc. 2018, 140, 78−81. (19) (a) Zultanski, S. L.; Fu, G. C. Nickel-Catalyzed Carbon−Carbon Bond-Forming Reactions of Unactivated Tertiary Alkyl Halides: Suzuki Arylations. J. Am. Chem. Soc. 2013, 135, 624−627. (b) Zhou, Q.; Cobb, K. M.; Tan, T.; Watson, M. P. Stereospecific Cross Couplings To Set Benzylic, All-Carbon Quaternary Stereocenters in High Enantiopurity. J. Am. Chem. Soc. 2016, 138, 12057−12060. (20) Cu catalyzed primary- and secondary-alkylative Suzuki−Miyaura couplings: (a) Yang, C.-T.; Zhang, Z.-Q.; Liu, Y.-C.; Liu, L. CopperCatalyzed Cross-Coupling Reaction of Organoboron Compounds with Primary Alkyl Halides and Pseudohalides. Angew. Chem., Int. Ed. 2011, 50, 3904−3907. (b) Sun, Y.-Y.; Yi, J.; Lu, X.; Zhang, Z.-Q.; Xiao, B.; Fu, Y. Cu-Catalyzed Suzuki−Miyaura reactions of primary and secondary benzyl halides with arylboronates. Chem. Commun. 2014, 50, 11060−11062. (c) Zhang, Z.-Q.; Yang, C.-T.; Liang, L.-J.; Xiao, B.; Lu, X.; Liu, J.-H.; Sun, Y.-Y.; Marder, T. B.; Fu, Y. Copper-Catalyzed/ Promoted Cross-coupling of gem-Diborylalkanes with Nonactivated

Scheme 6. Catalytic Activity of 12

copper catalyzed reactions.36−38 Such a species could undergo rapid reductive elimination to form the carbon−carbon bond. In summary, the copper catalyzed tertiary-alkylative SuzukiMiyaura coupling of 1-alkenylboronic esters with functionalized tertiary-alkyl bromides under mild conditions is reported. Although the role of Pt cocatalyst was not clear, but the control experiments revealed that the 1-alkenylcopper species could be a key intermediate in the catalytic cycle. The reaction is very useful to synthesize quaternary carbons possessing C−C double bonds. Further investigations, including asymmetric reactions, are currently underway.

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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/acscatal.8b01572. Experimental details and chemical compound information (PDF) X-ray data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yusuke Sunada: 0000-0002-8954-181X Takashi Nishikata: 0000-0002-2659-4826 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We warmly thank Yamaguchi University and JSPS KAKENHI Grant Number JP18H04654 in Hybrid Catalysis for Enabling Molecular Synthesis on Demand (to T.N.), JP 18H04262 (to T.N.), and 18H04240 (to Y.S.) in Precisely Designed Catalysts with Customized Scaffolding.

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REFERENCES

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DOI: 10.1021/acscatal.8b01572 ACS Catal. 2018, 8, 6791−6795