Cross-Coupling Reaction of Allylic Ethers with Aryl Grignard Reagents Catalyzed by a Nickel Pincer Complex
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molecules Article Cross-Coupling Reaction of Allylic Ethers with Aryl Grignard Reagents Catalyzed by a Nickel Pincer Complex Toru Hashimoto * , Kei Funatsu, Atsufumi Ohtani, Erika Asano and Yoshitaka Yamaguchi * Department of Advanced Materials Chemistry, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan; [email protected] (K.F.); [email protected] (A.O.); [email protected] (E.A.) * Correspondence: [email protected] (T.H.); [email protected] (Y.Y.); Tel.: +81-45-339-3940 (T.H.); +81-45-339-3932 (Y.Y.) Academic Editor: Kouki Matsubara Received: 30 May 2019; Accepted: 18 June 2019; Published: 21 June 2019 Abstract: A cross-coupling reaction of allylic aryl ethers with arylmagnesium reagents was investigated using β-aminoketonato- and β-diketiminato-based pincer-type nickel(II) complexes as catalysts. An β-aminoketonato nickel(II) complex bearing a diphenylphosphino group as a third donor effectively catalyzed the reaction to afford the target cross-coupled products, allylbenzene derivatives, in high yield. The regioselective reaction of a variety of substituted cinnamyl ethers proceeded to give the corresponding linear products. In contrast, α- and γ-alkyl substituted allylic ethers afforded a mixture of the linear and branched products. These results indicated that the coupling reaction proceeded via a π-allyl nickel intermediate. Keywords: pincer-type nickel(II) complex; cross-coupling; allylic ether 1. Introduction Transition metal-catalyzed cross-coupling reactions are efficient and widely utilized synthetic protocols for constructing carbon–carbon bonds [1]. The reactions of allylic electrophiles with aryl metal reagents provide promising methodologies for the formation of C(sp3)–C(sp2) bonds. Allylic electrophiles with efficient leaving groups, such as allylic halides, acetates, and phosphates, are commonly used in these reactions [2–6]. In contrast to these allylic electrophiles, allylic ethers have been considered less-reactive electrophiles in cross-coupling reactions, whereas these electrophiles are easily accessible, cheap, and easy to handle. Therefore, the development of effective metal catalysts for coupling reactions utilizing allylic ethers as electrophiles in a highly regioselective manner has attracted considerable attention. The choice of the metal catalyst is critical for linear and branched selectivity. Although precious metals such as Pd and Ir have been extensively explored in selective allylic substitution reactions [2–4], recent efforts have focused on first-row late transition-metals such as Cu, Co, Fe, and Ni, whose complexes act as highly active catalysts. For example, Feringa reported the asymmetric allylic alkylation of acyclic allylic ethers with organolithium reagents using a Cu/phosphoramidite system [7]. Oshima and Yorimitsu reported a cobalt-catalyzed cross-coupling reaction of allylic ethers with Grignard reagents in a linear manner [8]. Iron catalysts also enable the allylic arylation reaction to be performed [9–12]. Recently, Ni pincer complexes have been reported as effective catalysts for the coupling reaction of allylic ethers with aryl zinc reagents [13]. In recent years, tridentate pincer-type complexes have generated significant interest because the pincer-type ligand stabilizes the metal complex and its properties can be tuned to achieve optimal reactivity [14–25]. We have recently reported the synthesis of a range of pincer-type nickel(II) complexes Molecules 2019, 24, 2296; doi:10.3390/molecules24122296 www.mdpi.com/journal/molecules Molecules 2019, 24, 2296 2 of 12 Molecules 2019, 24 FOR PEER REVIEW 2 (nickel(II)1a–d) utilizing complexes a combination (1a–d) ofutilizingβ-aminoketonato a combination or β-diketiminato of β-aminoketonato frameworks or with β- adiketiminato third donor suchframeworks as an amino with a or third phosphino donor such group as [an26 ,amino27]. Our or systematicphosphino studygroup on[26,27 these]. Our nickel(II) systematic complexes study revealedon these thatnickel(II) modifying complex the ligandes reveal frameworked that hasmodif a significantying the influenceligand framework on the catalytic has a performance significant ininfluence the cross-coupling on the catalytic reaction performance of aryl halides in withthe arylmagnesiumcross-coupling reagents.reaction Inof particular,aryl halides nickel(II) with complexesarylmagnesium1b and reagents.1d bearing In a diphenylphosphinoparticular, nickel(II) group compl as theexes third donor1b and enable 1d the bearing utilization a ofdiphenylphosphino aryl fluorides as group electrophiles as the third in the donor reaction enable [27 the]. Inutilization order to of elucidate aryl fluorides the reactivity as electrophiles of the nickel(II)in the reaction complex [27 toward]. In order the relatively to elucidate robust the C-O reactivity bond in anof allylicthe nickel(II) aryl ether, complex we herein toward describe the therelatively cross-coupling robust C- reactionO bond ofin allylican allylic ethers aryl with ether arylmagnesium, we herein describe reagents the usingcross- nickel(II)coupling complexesreaction of (allylic1a–d) ethers (Scheme with1). arylmagnesium reagents using nickel(II) complexes (1a–d) (Scheme 1). Scheme 1. The coupling r reactioneaction of allylic ethers with aryl Grignard reagents using Ni(II)Ni(II) pincer complexes (1a1a–d).). 2. Results and Discussion 2. Results and Discussion 2.1. Optimization of the Reaction Conditions 2.1. Optimization of the Reaction Conditions 2.1.1. Initial Optimization of the Nickel(II) Complexes 2.1.1. Initial Optimization of the Nickel(II) Complexes To evaluate the catalytic activity of nickel(II) complexes 1a–d, we carried out the reaction between cinnamylTo evaluate phenyl etherthe catalytic2a and 3 equivalentsactivity of ofnickelp-tolylmagnesium(II) complexes bromide 1a–d, we3a usingcarried nickel(II) out the complexes reaction 1abetween–d (2.5 mol%)cinnamyl in THF phenyl at room ether temperature 2a and 3 (Tableequivalents1), referring of p to-tolylmagnesium our previously studybromide on the3a biarylusing couplingnickel(II) reactioncomplex ofes aryl1a–d chlorides (2.5 mol%) with in arylmagnesium THF at room reagentstemperature [26]. (Table Complex 1),1a referringgave the to linear our productpreviously4a studyin only on 7%the yield.biaryl coupling Complex reaction1b showed of aryl high chloride catalytics with activity arylmagnesium to give 4a reagentsin 86% yield. [26]. MoleculesMoleculesMoleculesMolecules 2019 20192019 2019, ,,24 24,24 24 FOR FORFOR FOR PEER PEERPEER PEER REVIEW REVIEWREVIEW REVIEW 333 3 Nickel(II)Complex 1a complexes gave the 1clineandar product1d bearing 4a Nin, Nonly,N- and7% yield.N,N,P C-typeomplex ligands 1b showed were not high catalytic activity to give 4aTableTableTableTable in 86 1. 1.1. 1. The% TheThe The yield. cross crosscross cross-- coupling-couplingN-couplingcouplingickel(II) reaction reactionreaction reaction complexes of ofof of cinnamyl cinnamylcinnamyl cinnamyl 1c and phenyl phenylphenyl phenyl 1d ether ether ether bearingether 2a 2a2a 2a with withwith with N p, pNp- -ptolylmagnesium-tolylmagnesium-tolylmagnesium,tolylmagnesiumN- and N,N, Pbromide bromidebromide -bromidetype ligands 3 33a a3a a were not effectiveusingusingusingusingTable Ni NiinNi Ni(II)(II) (II)t(II)he1. pincer pincerpincer pincerThe reaction cross-couplingcomplexes complexescomplexes complexes and ( (1 ( 1(1afforded))1.). ). a a. a areaction the of 2atargetwith product3a using Ni(II)in 2% pincer and complexes4% yield, (1respectively.). a In these cases, a large amount of 2a remained unreacted (see 2.3 Reaction mechanism section). In all cases, the branched product 5a was not detected and 4,4′-bitolyl (6a) derived from 3a was formed as a by-product (9–30%). In the absence of complex 1a–d, the reaction did not give any coupled products. Ni cat 1 Yield of 4a (%) b Recovery of 2a (%) b NNNNii i cati catcat cat 1 11 1 11a11a1aa a 111b1b1bb b 111c1c1cc c 111d1d1dd d 7 86 2 4 YYYieldYieldieldield of ofof of 4a 4a4a 4a (%) (%)(%) (%) b b b b 43777 7 88806866 6 28322 2 44904 4 RRaRecoveryRecoveryTheecoveryecovery reaction of ofof of 2a 2a2a was2a (%) (%)(%) (%) carried b b b b out using4443433 2a3 (0.5 mmol) and 3a000 (1.50 mmol) in the presence8883833 3 of 1 (0.0125 mmol,9990900 0 2.5 mol%) b 1 in THFaa a The aTheThe The (5 reaction mL)reaction reactionreaction for 24was was waswas h atcarried carried carriedcarried room temperature.outout outout using using usingusing 2a 2a2a 2a ( (0.5( 0.5(0.5The0.5 mmol) mmol)mmol) mmol) yield and ofand andand4a 3a 3a3a and3a ( (1.5( 1.5(1.51.5 recovery mmol) mmol)mmol) mmol) in ofin inin 2athe the thethewere presence presence presencepresence determined of of ofof 1 1 1 1( (0.0125( 0.01250.0125( by0.0125H NMR analysis using pyrazine as the internal standard. mmolmmolmmolmmol,, , 2.5 ,2.5 2.52.5 mol% mol% mol%mol%)) ) in) in inin THF THFTHF THF ( (5( 55( 5mL) mL)mL) mL) for for forfor 24 2424 24 h hh h at atat at room roomroom room temperature. temperature.temperature. temperature. b b b Theb TheThe The yield yieldyield yield of of ofof 4a 4a 4a4a and and andand recovery recovery recoveryrecovery of of ofof 2a 2a 2a2a werewerewerewere determined determineddetermined determined by byby by 1 1H1 HH1H NMR NMRNMR NMR analysis analysisanalysis analysis using usingusing using pyrazine pyrazinepyrazine pyrazine as asas as the thethe the internal internalinternal internal standard. standard.standard. standard. 2.1.2.2.1.2.2.1.2.2.1.2.