Ethereal C–O Bond Cleavage Mediated by Ni(0)-Ate Complex: A DFT Study

Abstract

Density functional theory calculations were performed to explore the mechanism of Ni-catalyzed crosscoupling reactions involving organo-lithium and -zinc reagents through ethereal C-O bond cleavage. Based on this work, together with our previous mechanistic study on etheric Kumada-Tamao reaction, we identify and characterize a novel catalytic cycle for cross-coupling mediated by Ni(0)-ate complex.Key words cross-coupling; ether; nickel catalyst; organolithium; organozinc; density functional theory (DFT) calculation Efficient and selective cleavage and transformation of C-O bonds, particularly by means of cross-coupling methods, has attracted great interest, since compounds containing C-O moieties occur widely in nature and are also extensively utilized in industry.1-8) Among C-O compounds, ethers are particularly attractive, [1][2][3][4][5][6][7][8] as they offer the advantages of 1) high atom economy/conversion efficiency (the use of ethers as simple as MeO as substrates affords fewer by-products compared with tosylate, mesylate, triflate, phosphate, etc.), 2) environmental compatibility (cleavage of the C-O moiety in ether releases non-halogen-containing waste), and 3) excellent stability, easy accessibility and wide diversity. However, ethereal C-O bonds ( E C-O) are very unreactive, and most of the well-established Pd-catalyzed C-O bond-cleaving protocols are ineffective for ethers.On the other hand, Ni-catalysts have proved effective for many types of C-O bond cleavage, including ethers. As early as in 1979, Wenkert et al. 9,10) reported the first Ni-catalyzed Kumada-Tamao type (Mg) 9-23) reaction, in which aryl methyl ether (ArOMe) acted as the electrophilic partner, and this is now recognized as the first example of Ni-catalyzed activation of an inert C-O bond. However, this breakthrough was overlooked for decades, until quite recently. After the development of improved conditions, ArOR can now be used as a coupling partner in several types of transition metal (TM)-catalyzed cross-couplings and related transformations, such as Suzuki-Miyaura-type (B), [24][25][26][27][28][29][30][31] Negishi-type (Zn or Al), [32][33][34][35][36] Murahashi-type (Li), [37][38][39][40] and other reactions. 41-50) As a continuation of our work in this area, we reported in 2012 the first ethereal Negishi-type coupling of aryl alkyl ether 36) (Chart 1(1)) and in 2016 we reported a systematic examination of ethereal Murahashi-type reaction 37) (Chart 1(2)). More recently, we also reported an in-depth study of Ni-catalyzed cross-coupling between organoaluminums and various types of C-O electrophiles, including aryl alkyl ether. 32) Results and DiscussionThe conventional catalytic cycle for cross-coupling reaction consists of three elemental steps: oxidative addition (OA), transmetalation, and reductive elimination. [51][52][53][54] Martin and colleagues reported that direct OA of a E C-O bond to Ni(0) requires high temperature, based on experimental and theoretical findings.46) Computational studies by Nakamura and colleagues, ...

The Anionic Pathway in the Nickel‐Catalysed Cross‐Coupling of Aryl Ethers

“…[7] In addition, nickel catalysts can activate strong s-bonds outside the scope of Pd catalysis,t hus allowing unconventional, but readily available electrophiles,s uch as phenol derivatives,t o be functionalised. [8][9][10] In 1979, Wenkert et al [11] reported that aryl ethers could serve as electrophiles in Kumada-Corriu type cross-coupling reactions with Grignard reagents under Ni catalysis.U nlike aryl esters and other activated phenol-derived electrophiles [8,10] in which the C(sp 2 )ÀOb ond can be cleaved by Ni 0 species,t he direct oxidative addition of aryl ethers is thermodynamically and kinetically unfavourable, [12][13][14][15][16] which suggests that non-conventional mechanisms are in operation, particularly for transformations occurring under mild conditions.W ang, Uchiyama, and co-workers have provided theoretical support for an alternative anionic pathway (Scheme 1) through DFT calculations,w hich identified that anionic Ni 0 -ate complexes are key intermediates that enable C À Ob ond cleavage. [14,16] Nickelate intermediates have been proposed in several Ni-promoted functionalisation reactions, [17][18][19][20] but catalytically relevant examples have rarely been isolated [21] and direct experimental evidence for their role in the anionic pathway has yet to be established.…”

Cross-coupling polycondensation via C–O or C–N bond cleavage

“…Further investigations of the reaction scope (e.g., organoboron, organozinc, and/or other C–O/C–N electrophiles) and synthetic applications of the present method to photoelectrical materials, are in progress. It should be noted that previous mechanistic studies on transition metal-catalyzed C–O/C–N bond cleavage reactions from our group 27 , 35 , 69 and others 35 , 43 , 46 , 47 , 49 have found that the oxidative addition of ethereal C–O bond to Ni generally suffers from high activation barriers. However, it can be greatly facilitated either by formation of the Ni(0)-ate complex with Grignard reagent or by NHC ligand.…”

“…We next turned our attention to other organometallic species. Recently, Rueping, 40 Feringa, 41 and our group 42 – 43 have reported protocols for ethereal Murahashi-type coupling using organolithiums, which provide high efficiency under mild conditions. When di-lithium reagent 4a prepared by the bromine-lithium exchange reaction of 0a was employed, the polycondensation with 2a took place smoothly at room temperature with Ni(cod)2 catalyst (cod: 1,5-cyclooctadiene) and SIMes ligand (SIMes: 1,3-bis(2,4,6-trimethylphenyl) imidazolidin-2-ylidene), affording 3aa in 80% yield with high molecular weight ( M n = 15.3 kDa, PDI = 1.92) (Table 1 , Entry 8).…”

“…However, it was then largely neglected for almost 30 years until Dankwardt significantly extended the substrate scope 21 . Since then, several groups, including ours, have developed various types of cross-coupling reactions of aryl ethers, including Kumada–Tamao–Corriu type (Mg) 22 – 28 , Suzuki–Miyaura type (B) 29 – 35 , Negishi type (Zn, Al) 36 – 39 , Murahashi type (Li) 40 – 43 , and related C–H 44 – 50 or C–X (X = B, N, Si, etc.) 51 – 55 bond formation reactions.…”

Cross‐Coupling Reaction of Alkenyl Sulfoximines and Alkenyl Aminosulfoxonium Salts with Organozincs by Dual Nickel Catalysis and Lewis Acid Promotion

“…An interesting feature of the CCR of alkenyl sulfoximines is the general necessity of ap romotion by MgBr 2 ,w hich is effective in diethylether but not in THF.T he promotion could be due to ar ate enhancement of the oxidative addition [51] through coordination to the nucleofugal sulfonimidoyl group under formation of complex I-MgBr 2 .R ate acceleration of the Ni 0 -catalyzed CCR by Lewis acids has been previously observed and generally ascribed to an acceleration of the oxidative addition through nucleofugal complexation. [13] We had previously observed a complex formation between allylic (N-methyl)sulfoximines and BF 3 .…”