Nickel(0)-Catalyzed Inert C–O Bond Functionalization: Organo Rare

Letter Cite This: Org. Lett. 2018, 20, 624−627

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Nickel(0)-Catalyzed Inert C−O Bond Functionalization: Organo Rare‑Earth Metal Complex as the Coupling Partner Xiangqian Yan,† Fanzhi Yang,*,‡ Guilong Cai,† Qingwei Meng,† and Xiaofang Li*,† †

School of Chemistry and Chemical Engineering, Key Laboratory of Cluster Science of Ministry of Education, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, China ‡ Advanced Research Institute of Multidisciplinary Science, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, China S Supporting Information *

ABSTRACT: An organo rare-earth metal complex has been employed as a highly efficient nucleophile in Ni(0)-catalyzed C−O bond functionalization. The optimized catalytic system which consists of Ni(cod)2, PCy3, and t-BuONa could smoothly convert 1 equiv of naphthyl ethers to alkylated naphthalene analogues with 0.4 equiv of Ln(CH2SiMe3)3(THF)2, delivering good to excellent yields. The reaction system could also activate the ArCH2−O bond with mild base.

Scheme 1. Application of Organo Rare-Earth Metal Complexes in C−O Bond Activation

C−C bonds comprise the skeleton of organic molecules. During the past decades, transition-metal-catalyzed crosscouplings have been recognized as powerful tools for C−C bond formations.1 Because of the abundance of phenol derivatives in natural compounds, and because of the lower toxicity of corresponding byproducts in comparison with aryl halides, C−O bond functionalization has played an important role in the category of sustainable cross-coupling chemistry.2 Despite the high dissociation energy of the C−O bond, the activation of simple, stable, and easily available aryl ethers has drawn increasing attention from chemists during the past decade.3 Several organometallic reagents (including Grignard reagent,4 organoboronic ester,5 organoaluminum,6 organolithium,7 and organozinc8) have been employed as nucleophiles in transition-metal-catalyzed etheric cross-coupling reactions (Scheme 1, eq 1). The metal core of organometallic compound has been believed to coordinate with the oxygen atom to form an alkoxide−Lewis acid adduct, which will facilitate the subsequent C−O bond cleavage.6,9 Therefore, we assume that other organo−metal complexes with strong Lewis acidic metal cores (such as rare-earth metals) could be promising nucleophiles in transition-metal-catalyzed inert C− OR (R = Alk or Ar) bond functionalizations. On the other hand, the widely used organolithium and Grignard reagents with strong basicity could induce unwanted side reactions (such as deprotonation), while the f-block element-cored organometallic reagents might improve the chemoselectivity. As a family of elements with unique properties in chemical processes, rare-earth metals have proven efficacy in various C−C bond formations, however, mostly as catalysts or additives taking advantage of their strong Lewis acidity.10 Due to the instability of organo rare-earth metal complexes, the employment of such compounds as coupling reagents in © 2018 American Chemical Society

transition-metal catalysis has been far less explored (Scheme 1, eq 2). As early as 1984, phenylytterbium iodide was initiated into a transition-metal-catalyzed cross-coupling reaction.11 After a quarter century, in situ generated triaryllanthanum reagents were successfully utilized in palladium-catalyzed arylation reactions.12 Very recently, palladium-catalyzed alkylation and olefination were reported using trialkyl rare-earth metals and aryl/vinyl bromides,13 of which the reactive vinyl triflates were used as electrophiles in coupling reactions.13b To the best of our knowledge, unactivated electrophiles have remained unexplored in crosscoupling reactions involving organo rare-earth metal complexes. Received: December 2, 2017 Published: January 10, 2018 624

DOI: 10.1021/acs.orglett.7b03753 Org. Lett. 2018, 20, 624−627

Letter

Organic Letters

Scheme 2. Ni-Catalyzed C(sp2)−O Bond Functionalization of 2-Naphthyl Ethers and 2a

As part of our interest in enriching the toolbox of nickelcatalyzed cross-coupling reactions,3e,14 we herein report nickel(0)-catalyzed inert C−O bond functionalization with alkyl rare-earth metal complex as the nucleophile, introducing the synthetically useful CH2SiMe3 group15 into various naphthalene derivatives (Scheme 1, eq 3). Considering the high reactivity of alkyl rare-earth metal complexes, we initiated the optimization with 2-methoxynaphthalene (1a) and relatively stable Y(CH2SiMe3)3(THF)n (n = 2) (Table 1 and Table S1).16,17 Base has significant influence Table 1. Optimization of Reaction Conditionsa

entry

Ln

1c 2 3 4 5 6 7 8 9 10 11 12d 13 14 15 16

Y Y Sc Lu Sc Sc Sc Sc Sc Sc Sc Sc Sc Sc Sc Sc

17e 18f

 

catalyst (5.0 mol %) Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 Ni(cod)2 (Ph3P)2NiCl2 (Ph2PCH2)2NiCl2 Pd(Pt-Bu3)2 [Ru(p-cymene) Cl2]2 Ni(cod)2 Ni(cod)2

solvent

yieldb (%)

P(t-Bu)3 XPhos SPhos IPr·HCl PCy3 PCy3 PCy3 PCy3 PCy3 PCy3 PCy3

PhMe PhMe PhMe PhMe PhMe PhMe PhMe PhMe PhMe THF n-hexane n-hexane PhMe PhMe PhMe PhMe

31 78 95 94 72 56 80 56 58 16 95 75 0 0 0 0

PCy3 PCy3

PhMe PhMe

76 25

ligand (10 mol %) PCy3 PCy3 PCy3 PCy3

Ethyl- and isopropyl-substituted naphthols delivered lower yield than 1a. The Np−O bond was selectively cleaved when 2-phenoxynaphthalene (1d) was used as the substrate. The conversion of OPy group to a synthetically useful group has been challenging for a long period.5e,h Taking advantage of the present system, 3a was generated in moderate yield from 1e. Subsequently, we evaluated the versatility of nickel-catalyzed C(sp2)−O bond functionalization with 2a and methyl aryl ethers bearing different substituents (Scheme 3). Functional Scheme 3. Ni-Catalyzed C(sp2)−O Bond Functionalization of Methyl Aryl Ethers and 2a

a Reaction conditions: 1a (0.3 mmol), 2 (0.12 mmol), catalyst (5 mol %), ligand (10 mol %), t-BuONa (0.3 mmol), solvent (2.0 mL). bYield of isolated product. cWithout base. d At room temperature; e LiCH2SiMe3 (0.3 mmol) was used as the coupling reagent; f ClMgCH2SiMe3 (0.3 mmol) was used as the coupling reagent.

on the reaction process. Without additional base, only 31% of 3a was obtained (Table 1, entry 1), while t-BuONa delivered the best yield among all the tested bases (Table 1, entry 2, and Table S1, entries 12−15). We then examined the reactivity of another two rare-earth metalsSc and Lu from periods 4 and 6, respectively, both delivering excellent yields (Table 1, entries 3 and 4). The reaction without ligand delivered a much lower yield of 72% (Table 1, entry 5). Afterward, we tested different phosphine ligands and NHC ligand; however, no better result was obtained (Table 1, entries 6−9). Substrate 1a could deliver excellent yield in hexane at 80 °C or at room temperature (Table 1, entries 11 and 12); however, this system is inapplicable to other substrates, probably due to the poor solubility of hexane. Nickel(II) catalysts and other transition metals like palladium and ruthenium were inactive toward this reaction system (Table 1, entries 13−16). The reactions using LiCH2SiMe3 and ClMgCH2SiMe3 as the coupling reagents delivered lower yields of 76% and 25%, respectively. With the optimized reaction conditions in hand, we first studied the influence of different ether derivatives (Scheme 2).

groups including trimethylsilyl, 4-morpholino, and 1-pyrrolyl groups are well tolerated. However, the catalytic system did not work toward 1-methyl-2-methoxylnaphthalene, indicating the electron-rich α-position plays an important role in forming a stable transition state. Moreover, the reactions between Sc(CH2SiMe3)3 and nonbenzannulated anisole could give very few conversions. An intramolecular competition experiment demonstrated the priority of β-position in nickel-catalyzed C−O bond activation/alkylation (Scheme 4). Benzylalkyl ethers are generally too stable to be cleaved.18 We noticed that the ArCH2−O bond could be successfully 625

DOI: 10.1021/acs.orglett.7b03753 Org. Lett. 2018, 20, 624−627

Organic Letters

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Scheme 4. Ni-Catalyzed C(sp2)−O Bond Functionalization of 1q and 2a

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03753. Experimental procedures, characterization data, and 1H and 13C NMR spectra for new compounds (PDF)

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activated using Na2CO3 as the base, delivering alkylated products with good to excellent yields (Scheme 5).

AUTHOR INFORMATION

Corresponding Authors

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

Scheme 5. Ni-Catalyzed ArCH2−O Bond Functionalization of 4 and 2a

ORCID

Fanzhi Yang: 0000-0002-5971-2032 Xiaofang Li: 0000-0003-3788-5431 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support by the National Natural Science Foundation of China (No. 21274012, 21322401), the 111 project (No. B07012), and Beijing Institute of Technology (No. 1750012331710) is gratefully acknowledged.

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A catalytic cycle was proposed as shown in Scheme 6: First, 1 will coordinate with [Ni]0 and organo rare-earth metal

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Scheme 6. Proposed Catalytic Cycle of Ni-Catalyzed C−O Bond Functionalizationa

a

REFERENCES

L = ligand; R = CH2SiMe3; m + n = 3.

complex, forming transition state A.19 The coordination of Ln···O will facilitate the oxidative insertion of [Ni]0 into C(sp2)−O bond to generate [Ni]II species. Reductive elimination will deliver the desired product 3 and regenerate [Ni] 0. The byproduct LnRm−1(OMe)n+1 will continue participating as the nucleophile in the next catalytic cycle until all alkyl groups are transferred to 3. In summary, we have first investigated the application of organo rare-earth metal complex as a highly efficient coupling reagent in nickel-catalyzed C−O bond functionalization. The reaction system is effective toward ethers bearing various leaving groups and functional substituents, delivering good to excellent yields. Further research on the reaction mechanism is still in progress. 626

DOI: 10.1021/acs.orglett.7b03753 Org. Lett. 2018, 20, 624−627

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DOI: 10.1021/acs.orglett.7b03753 Org. Lett. 2018, 20, 624−627