Cobalt‐Catalyzed Dimerization and Homocoupling of Terminal Alkynes

Abstract

Methods toward the dimerizationa nd homocoupling of terminal alkynesh ave been established that employ Co complexesa long with tBuOKu nder different atmosphere.Avariety of Z-1,3-enynes were furnishedw ith ah igh Z:E (up to 93:7) ratio in moderate to very good yields under inert atmosphere.Alternatively,the Glaser homocoupling reaction of terminal alkynes was also accomplished with aC oc atalyst, successfully affording various 1,3-diynes in excellent yields (up to 93 %) under oxygen.Cobalt catalysis has emerged as av ery active research area over the past decades owing to the fact that cobalt is abundant, less toxic and more biocompatible.[1] Meanwhile, the cobalt complexesw ith high affinity toward p bonds enable them to catalyze the CÀCb ond formation of terminal alkynes. As the transition-metal-catalyzed CÀCb ond formation is one of the most vital transformations in organic synthetic chemistry, [2] the reactions involvinga lkynes were investigated widely. [3][4][5] In particular,d irect dimerization [6] and Glaserh omocoupling [7] of terminal alkynes are of significant interestd ue to their atom-economy assembling av ariety of useful organic compounds, such as 1,3-diynes [8] and 1,3-enynes. [9] In addition, these products are valuable building blocks for the synthesis of polysubstituted aromatic rings, drug intermediates and natural products.I np reviousr eported works, dimerizationo ft erminal alkynes often gave E-1,3-enynes [10] and Z-1,3-enynes, [11] and only af ew examples has been reportedg iving high selectivity.A lso, to our knowledge,t he efficient Glaser homocoupling of terminal alkynes catalyzed by cobalt complexes has rarely been developed.[12]Thus, we envisionedt hat cobaltc omplexes promoted reaction of terminal alkynes would provide alternative access to 1,3-diynes and Z-selective 1,3-enynes through fine tuningo f the ligand structure. In this regard,w eh erein describe cobaltcatalyzed dimerization/homocoupling of terminala lkynes in the presence of tBuOK, depending on the cobalt complexes and reaction conditions. Notably,o nly dimer 1,3-enynes were obtained in the presence of pincer-CoCl 2 complex with ap roline based tridentate nitrogen ligand under inert atmosphere (Scheme 1). The homocoupling product 1,3-diynesw erea lso furnished under oxidative atmosphere.Initially,w ec arriedo ut the dimerization reactiono fp henylacetylene with 1.0 mol %d ifferentC oc omplexes ( Figure 1) in the presence of tBuOK at 80 8CinPhCl under inert atmosphere. To our delight, the dimerization of terminal alkyne proceeded successfully and provided 1,3-enyne by cobalt complex C1 with good yields. Further,t he Co complex C2 with proline derived tridentate nitrogen ligand delivered the dimerization with ah igh Z:E ratio (Table 1, entry 2; 89:11).[13] Not surprisingly,t he use of CoCl 2 as catalyst only led to poor result (entry 4), and the ligand could not promote the dimerization of alkyne separately (entry 5). Then, different anions of Co complexes were examined. When the complexw as prepared using Co(OT...

Late 3d Metal-Catalyzed (Cross-) Dimerization of Terminal and Internal Alkynes

“…SCHEME 17 | Cobalt(I)-catalyzed Z-selective head-to-head dimerization (Weber et al, 2020). SCHEME 18 | Z-Selective homodimerization and Glaser-type coupling of terminal alkynes catalyzed by cobalt complexes (Xu et al, 2018 The substrate scope showed that many aryl alkynes underwent the cross-dimerization with alkyl-substituted alkynes in good to excellent yields and excellent chemo-and regioselectivities. All substitution patterns were tolerated and a high functional group tolerance, including electron-donating, electron-neutral, and electron-withdrawing groups, was found.…”

“…Another method to generate Z -configured enynes of type 3 was reported by Wang, Sun, and coworkers. The authors reported a cobalt(II)-pincer complex which was able to catalyze the homodimerization of terminal aryl alkynes under argon atmosphere in good to excellent yields with good stereoselectivities toward the Z -enyne ( Scheme 18 ) ( Xu et al, 2018 ).…”

“… Z -Selective homodimerization and Glaser-type coupling of terminal alkynes catalyzed by cobalt complexes ( Xu et al, 2018 ). …”

“…Also, cyclohexyl acetylene could be homocoupled to the 1,3-diyne in 67% yield. First mechanistic investigations with radical clock experiments revealed that no radical mechanism should be present, neither in the dimerization toward product 3 nor in the Glaser-type coupling ( Xu et al, 2018 ).…”

Selective Manganese-Catalyzed Dimerization and Cross-Coupling of Terminal Alkynes

“…This process is accompanied by H-H and metal-σ-C bond cleavage and is typically the final step in both stoichiometric and catalytic hydroformylations of alkenes (Scheme 1). 5,6 Scheme 1. Formation of a Mn(I) Hydride and Acetylide Species via Alkyl Migration followed by Aldehyde Release upon E-H Bond Cleavage We have recently described the hydrogenation of alkenes and nitriles utilizing Mn(I) complexes fac-[Mn(dpre)(CO)3(R)] (dpre = 1,2-bis(di-n-propylphosphino) ethane, R = CH3, CH2CH3, CH2CH2CH3) and fac-[Mn(dippe)(CO)3(CH2CH2CH3) (dippe = 1,2-bis(di-iso-propylphosphino)ethane) where we took advantage of the migratory insertion and hydrogenolysis processes to create the active 16e -Mn(I) hydride catalysts (Scheme 1).…”