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Synthesis of Functionalized Organoboron/Silicon Compounds by Copper-Catalyzed Coupling of Alkylsilyl Peroxides and Diboron/ Silylborane Reagents Takumi Seihara,† Shunya Sakurai,† Terumasa Kato,† Ryu Sakamoto,*,† and Keiji Maruoka*,†,‡ †

Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China

‡

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ABSTRACT: The facile synthesis of functionalized organoboron/silicon compounds by copper-catalyzed coupling of alkylsilyl peroxides and diboron/ silylborane reagents is reported. The reactions proceed smoothly under mild, neutral conditions in short reaction times to generate organoboron/silicon compounds bearing a ketone moiety, which are useful synthetic intermediates that are otherwise difficult to access. The results of mechanistic investigations suggest the radical-mediated formation of carbon−boron and carbon−silicon bonds via β-scission of alkoxy radicals.

O

Scheme 1. General Synthetic Routes to Alkylboronic Esters

rganoboron compounds are structurally important molecules in synthetic chemistry due to their wide applications as synthetic building blocks.1 In particular, functionalized alkylboronic esters are highly valuable intermediates for the introduction of an alkyl chain with the corresponding functional group.2 Although considerable research efforts have been devoted to the development of synthetic routes to alkylboronic esters, the limited functionalgroup tolerance observed in most cases still requires improvement. The transmetalation of organolithium/magnesium reagents and the hydroboration of olefins, which represent the most common and reliable means of synthesizing alkylboronic esters, often suffer from the incompatibility of several functional groups such as ketones or aldehydes (Scheme 1a,b).3−5 Although the transition metal-catalyzed borylation of alkyl halides, the so-called Miyaura-type borylation, has recently emerged as a promising synthetic approach to alkylboronic esters, it usually requires strong bases, which often limits the substrate scope (Scheme 1c).6 Therefore, the development of novel synthetic routes to functionalized alkylboronic esters is highly desirable. In this report, we provide an alternative synthetic approach via a radical mechanism for alkylboronic esters bearing a ketone group, which are otherwise difficult to access, by the coppercatalyzed coupling of alkylsilyl peroxides and diboron reagents under mild, neutral conditions (Scheme 1d).7 Moreover, we report that this approach can also be applied to silylation reactions that generate organosilicon compounds bearing a ketone moiety by simply switching the reaction partner from a diboron to a silylborane reagent. The selective cleavage of inert carbon−carbon bonds for the generation of alkyl radicals is an important research topic in modern organic synthesis because it can offer a new synthetic approach to organic molecules.8,9 In this field, β-scission of alkoxy radicals generated from cycloalkanols has emerged as a © XXXX American Chemical Society

promising approach for the synthesis of functionalized ketones through the cleavage of C(sp3)−C(sp3) bonds. Although various types of radical reactions have been applied to the βscission of alkoxy radicals,10,11 a radical-mediated formation of carbon−boron bonds through β-scission of alkoxy radicals has not yet been developed. This is probably due to the incompatibility of organoboron compounds/agents with the strong oxidative conditions usually required for the generation Received: March 11, 2019

A

DOI: 10.1021/acs.orglett.9b00874 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Table 1. Optimization of Reaction Conditionsa

of alkoxy radicals from alcohols.12 On the other hand, our group has recently reported a novel reductive β-scission strategy using alkylsilyl peroxides 1, which are readily prepared from alcohols or alkenes.13 As alkylsilyl peroxides 1 can easily generate alkoxy radicals under mild and reductive conditions with a copper catalyst, we envisioned that our reductive βscission strategy using alkylsilyl peroxides would be suitable for the radical-mediated formation of carbon−boron bonds via the β-scission of alkoxy radicals, thus providing an alternative synthetic strategy for organoboron compounds bearing a ketone moiety. Scheme 2 shows the catalytic cycle that we

[M]

[B] (equiv)

c

CuI CuI CuI CuI CuOAc CuCl2 Cu(acac)2 FeCl2 CuI CuI CuI

B2pin2 (1.3 equiv) B2pin2 (1.3 equiv) B2pin2 (1.3 equiv) B2pin2 (1.3 equiv) B2pin2 (1.3 equiv) B2pin2 (1.3 equiv) B2pin2 (1.3 equiv) B2pin2 (1.3 equiv) B2pin2 (1.0 equiv) B2cat2 (1.0 equiv) B2nep2 (1.0 equiv)

1 2 3d 4e 5 6 7 8 9f 10f 11f

Scheme 2. Proposed Reaction Mechanism for the CopperCatalyzed Borylation Reaction of Alkylsilyl Peroxides with Bis(pinacolato)diboron

yield of 6 (%)b

entry

38 95 32 71 53 64 74 not 97 70 70

(6a) (6a) (6a) (6a) (6a) (6a) (6a) determined (6a) (6a′) (6a″)

a

Unless otherwise specified, the reactions were carried out in the presence of 1a (0.2 mmol), B2pin2 (1.3 equiv), a metal catalyst (5 mol %), and 1,10-Phen (5 mol %) in MeCN for 2 h under an argon atmosphere. bThe yield was determined by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane as the internal standard. cThe reaction was conducted instead of 1,10-Phen. dBenzene was used as the solvent. eTHF was used as the solvent. fThe reaction time was 0.5 h.

membered (1b) and seven-membered (1c) cyclic peroxides furnished the corresponding products (6b and 6c, respectively) in good yields. Different substituents on the aryl moiety of the peroxides (1d−f) were well tolerated, affording the respective products (6d−f) in moderate to high yields. The reaction with 1g, which bears an ether moiety, afforded 6g in 58% yield. When ethyl-substituted cyclopentyl peroxide 1h was used, the ring opening of the cyclopentyl moiety occurred selectively to furnish δ-boryl ketone 6h in an acceptable yield. Peroxide 1i derived from 1-indanone generated 6i. Acyclic alkylsilyl peroxides 1j and 1k were also used to give 6j and 6k, respectively. Furthermore, acyclic peroxide 1l, which is derived from pentoxifylline, also underwent this transformation to afford highly functionalized alkylboronic ester 6l. We next envisioned that the use of a silylborane instead of a diboron reagent for the reaction with alkylsilyl peroxides should lead to a radical-mediated formation of carbon−silicon bonds via a process analogous to that depicted in Scheme 2.17,18 Similar to the case of organoboron compounds, access to organosilicon compounds bearing a ketone moiety remains nontrivial.19,20 After extensive screening of the conditions, we discovered that the reaction of 1a and (dimethylphenylsilyl)boronic acid pinacol ester (1.3 equiv) in the presence of CuI (5 mol %) and 1,10-Phen (5 mol %) at 130 °C furnishes δ-silyl ketone 7a in 78% yield (Scheme 4).21 The reactions with various cyclic and acyclic alkylsilyl peroxides also proceeded smoothly to afford 7b−m in moderate to high yields. A series of control experiments for the borylation reaction were then carried out (Scheme 5).22 When the reactions were performed using cycloalkanol 8 or alkyl hydroperoxide 9 instead of alkylsilyl peroxide 1a under standard conditions, the formation of 6a was not observed (Scheme 5a). On the other hand, attempted reactions of 8 with B2pin2 under oxidative conditions were not successful (Scheme 5b, conditions a or b).11a,d These results demonstrate that the reductive β-scission of alkylsilyl peroxides is crucial for this transformation. The

propose for the copper-catalyzed borylation of alkylsilyl peroxide 1 with bis(pinacolato)diboron (B2pin2).9d,14 Initially, single-electron transfer (SET) from a copper catalyst to 1 should lead to the generation of an alkoxy radical (2) and a copper−silanoxide complex (3). Subsequent β-scission of 2 could generate the corresponding alkyl radical (4), while transmetalation between 3 and B2pin2 would generate a copper−boron complex (5). Finally, coupling of the alkyl radical (4) and the copper−boron complex (5) should lead to the formation of a carbon−boron bond, thus generating an organoboron product (6).15 Initially, we examined the reaction between cyclic alkylsilyl peroxide 1a and B2pin2, which should furnish δ-boryl ketone 6a (Table 1). When the reaction was conducted using 5 mol % copper iodide in acetonitrile, the desired product (6a) was obtained in 38% yield (entry 1). The addition of 1,10phenanthroline (1,10-Phen) improved the yield significantly (95%, entry 2). Solvents other than acetonitrile and copper sources other than copper iodide were less effective (entries 3−7), while the use of other metal salts such as iron(II) chloride did not promote the reaction (entry 8). After additional slight modifications of the conditions, we found that the amount of B2pin2 and the reaction time could be reduced (from 1.3 to 1.0 equiv, from 2 to 0.5 h) without affecting the product yield (entry 9). Bis(catecholato)diboron (B2cat2) or bis(neopentyl glycolato)diboron (B2nep2) also furnished the corresponding product (6a′ or 6a″, respectively), albeit with a yield slightly lower than that of B2pin2 (entries 10 and 11). With the optimized conditions in hand, we subsequently examined the scope of this reaction with respect to the alkylsilyl peroxide substrates (1) (Scheme 3).16 The use of sixB

DOI: 10.1021/acs.orglett.9b00874 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 3. Scope of Alkylsilyl Peroxides 1a,b,c

Scheme 4. Scope of the Silylation of 1a,b,c

a Unless otherwise specified, the reactions were carried out in the presence of 1 (0.2 mmol), B2pin2 (1.0 equiv), CuI (5 mol %), and 1,10-Phen (5 mol %) in MeCN for 0.5 h under an argon atmosphere. b Isolated yield. c1H NMR yield given in parentheses. dOn a 0.5 mmol scale. eThe reaction time was 1 h. fThe temperature was 80 °C.

a

Unless otherwise specified, the reactions were carried out in the presence of 1 (0.1 mmol), (dimethylphenylsilyl)boronic acid pinacol ester (1.3 equiv), CuI (5 mol %), and 1,10-Phen (5 mol %) in chlorobenzene (0.3 M) for 0.5 h under an argon atmosphere. b Isolated yield. c1H NMR yield given in parentheses. dThe reaction time was 2 h. e(Dimethylphenylsilyl)boronic acid pinacol ester (1.0 equiv) was used. f(Dimethylphenylsilyl)boronic acid pinacol ester (1.5 equiv) was used.

hypothesis that alkyl radical intermediates are formed in this reaction was supported by the results of the following experiments. The addition of the radical scavenger 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO) under standard conditions inhibited the reaction, affording radical adduct 10 (Scheme 5c). Furthermore, we tested the radical clock reaction using alkylsilyl peroxide 11 with B2pin2 under standard conditions, which resulted in the formation of ring-opened adduct 12 instead of 13 (Scheme 5d). In conclusion, a copper-catalyzed ring-opening borylation of alkylsilyl peroxides with diboron reagents via cleavage of C(sp3)−C(sp3) bonds has been developed. The reaction proceeds smoothly under mild conditions in short reaction

times to generate functionalized alkylboronic esters. Additionally, this approach allows the synthesis of organosilicon compounds bearing a ketone moiety by using silylborane instead of diboron reagents. Further investigations of the applications of such a reductive β-scission strategy for alkylsilyl peroxides for the formation of carbon−carbon or carbon− heteroatom bonds are currently in progress in our laboratory. C

DOI: 10.1021/acs.orglett.9b00874 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

Preparation and Applications in Organic Synthesis Medicine and Materials, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2011. (2) Kubota, K. Synthesis of Functionalized Organoboron Compounds through Copper(I) Catalysis; Springer, 2017. (3) (a) Brown, H. C. Organic Synthesis via Organoboranes; WileyInterscience: New York, 1975. (b) Brown, H. C.; Cole, T. E. Organoboranes. 31. A Simple Preparation of Boronic Esters from Organolithium Reagents and Selected Trialkoxyboranes. Organometallics 1983, 2, 1316−1319. (4) (a) Brown, H. C. Hydroboration; W. A. Benjamin Inc.: New York, 1962. (b) Fu, G. C. Transition Metals for Organic Synthesis: Building Blocks and Fine Chemicals; Beller, M., Bolm, C., Eds.; WileyVCH Verlag GmbH: Weinheim, Germany, 2008; p 193. (5) (a) Männig, D.; Nöth, H. Catalytic Hydroboration with Rhodium Complexes. Angew. Chem., Int. Ed. Engl. 1985, 24, 878− 879. A few examples of the chemoselective hydroboration of alkenes in the presence of ketones or aldehydes have recently been reported. However, these often require precious transition metal catalysts and/ or specific ligands. See: (b) Obligacion, J. V.; Chirik, P. J. Bis(imino)pyridine Cobalt-Catalyzed Alkene Isomerization−Hydroboration: A Strategy for Remote Hydrofunctionalization with Terminal Selectivity. J. Am. Chem. Soc. 2013, 135, 19107−19110. (c) Zhang, L.; Zuo, Z.; Leng, X.; Huang, Z. A Cobalt-Catalyzed Alkene Hydroboration with Pinacolborane. Angew. Chem., Int. Ed. 2014, 53, 2696−2700. (d) Docherty, J. H.; Peng, J.; Dominey, A. P.; Thomas, S. P. Activation and Discovery of Earth-abundant Metal Catalysts using Sodium tert-Butoxide. Nat. Chem. 2017, 9, 595−600. (e) Ibrahim, A. D.; Entsminger, S. W.; Fout, A. R. Insights into a Chemoselective Cobalt Catalyst for the Hydroboration of Alkenes and Nitriles. ACS Catal. 2017, 7, 3730−3734. (6) (a) Ito, H.; Kubota, K. Copper(I)-Catalyzed Boryl Substitution of Unactivated Alkyl Halides. Org. Lett. 2012, 14, 890−893. (b) Dudnik, A. S.; Fu, G. C. Nickel-Catalyzed Coupling Reactions of Alkyl Electrophiles, Including Unactivated Tertiary Halides, To Generate Carbon−Boron Bonds. J. Am. Chem. Soc. 2012, 134, 10693−10697. (c) Bose, S. K.; Fucke, K.; Liu, L.; Steel, P. G.; Marder, T. B. Zinc-Catalyzed Borylation of Primary, Secondary and Tertiary Alkyl Halides with Alkoxy Diboron Reagents at Room Temperature. Angew. Chem., Int. Ed. 2014, 53, 1799−1803. (d) Atack, T. C.; Lecker, R. M.; Cook, S. P. Iron-Catalyzed Borylation of Alkyl Electrophiles. J. Am. Chem. Soc. 2014, 136, 9521−9523. (e) Atack, T. C.; Cook, S. P. Manganese-Catalyzed Borylation of Unactivated Alkyl Chlorides. J. Am. Chem. Soc. 2016, 138, 6139−6142. (f) Bose, S. K.; Brand, S.; Omoregie, H. O.; Haehnel, M.; Maier, J.; Bringmann, G.; Marder, T. B. Highly Efficient Synthesis of Alkylboronate Esters via Cu(II)Catalyzed Borylation of Unactivated Alkyl Bromides and Chlorides in Air. ACS Catal. 2016, 6, 8332−8335. (7) The conjugate borylation of α,β-unsaturated carbonyl compounds represents a facile route to β-boryl carbonyls. For a review, see: Hartmann, E.; Vyas, D. J.; Oestreich, M. Enantioselective Formal Hydration of α,β-Unsaturated Acceptors: Asymmetric Conjugate Addition of Silicon and Boron Nucleophiles. Chem. Commun. 2011, 47, 7917−7932. (8) (a) Murakami, M.; Ito, Y. Activation of Unreactive Bonds and Organic Synthesis; Springer, 2001; pp 97−129. (b) Jun, C.-H. Transition Metal-Catalyzed Carbon−Carbon Bond Activation. Chem. Soc. Rev. 2004, 33, 610−618. (c) Marek, I.; Masarwa, A.; Delaye, P.-O.; Leibeling, M. Selective Carbon−Carbon Bond Cleavage for the Stereoselective Synthesis of Acyclic Systems. Angew. Chem., Int. Ed. 2015, 54, 414−429. (d) Souillart, L.; Cramer, N. Catalytic C−C Bond Activations via Oxidative Addition to Transition Metals. Chem. Rev. 2015, 115, 9410−9464. (e) Fumagalli, G.; Stanton, S.; Bower, J. F. Recent Methodologies That Exploit C−C Single-Bond Cleavage of Strained Ring Systems by Transition Metal Complexes. Chem. Rev. 2017, 117, 9404−9432. (9) Recently, the decarboxylative borylation reactions of esters via a radical mechanism emerged as a promising synthetic method for the generation of boronic esters. However, they also still suffer from some synthetic drawbacks, such as the requirement of photoirradiation and

Scheme 5. Control Experiments

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00874. Experimental procedures and characterization data for all compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shunya Sakurai: 0000-0003-1331-2794 Ryu Sakamoto: 0000-0001-8350-2636 Keiji Maruoka: 0000-0002-0044-6411 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grants JP26220803 and JP17H06450 (Hybrid Catalysis). S.S. thanks the Japan Society for the Promotion of Science for Young Scientists for a Research Fellowship.

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REFERENCES

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DOI: 10.1021/acs.orglett.9b00874 Org. Lett. XXXX, XXX, XXX−XXX