Well-Defined Cationic Cobalt(I) Precatalyst for Olefin-Alkyne [2 + 2] Cycloaddition and Olefin-Diene Hydrovinylation Reactions: Experimental Evidence for Metallacycle Intermediates

Abstract

The synthesis and characterization of the cationic cobalt(I) arene complex, [(dppf)Co(η 6 -C 7 H 8 )][BAr F 4 ] (dppf = 1,1′-bis(diphenylphosphino)ferrocene; BAr F 4 = B[(3,5-(CF 3 ) 2 )-C 6 H 3 ] 4 ) from an air-stable cobalt precursor is described. Dissolution in benzene-d 6 or tetrahydrofuran (THF) resulted in rapid arene substitution and generated [(dppf)Co(η 6 -C 6 H 6 )]-[BAr F 4 ] or [(dppf)Co(THF) 2 ][BAr F 4]. The latter compound was characterized by a combination of X-ray diffraction and magnetometry and established an S = 1 cobalt(I) derivative. The isolated bis(phosphine)cobalt complexes were evaluated as precatalysts for carbon−carbon bond-forming reactions. The [2 + 2] cycloaddition of internal alkynes and olefins was observed with cobalt precatalyst loadings of 0.25 mol % with high chemoselectivity. The catalytic method was compatible with Lewis basic functional groups, an advantage over in situ-generated catalysts that rely on excess trialkyl aluminum activators. The cationic bis(phosphine) cobalt arene complex was also an effective catalyst precursor for the hydrovinylation of isoprene with ethylene. In both C−C bondforming reactions, the corresponding cobalt(0) complex, [(dppf)Co(COD)] (COD = 1,5-cyclooctadiene), was inactive, providing strong evidence of the role of cobalt(I) during catalysis. In both catalytic reactions, deuterium crossover experiments provide experimental evidence of the role of metallacyclic intermediates during turnover.

Three-Component Coupling of Arenes, Ethylene, and Alkynes Catalyzed by a Cationic Bis(phosphine) Cobalt Complex: Intercepting Metallacyclopentenes for C–H Functionalization

“…A series of cationic cobalt­(I) complexes were synthesized bearing dppf, dppe, dppbz (1,2-bis­(diphenylphosphino)­benzene), and dcype as representative bis­(phosphines), and a labile η 6 -toluene ligand. Whereas the dppf-ligated complex [(dppf)­Co­(η 6 -C 7 H 8 )]­[BAr F 4 ] was previously reported, the dppe, dppbz, and dcype-ligated variants were synthesized by analogy with our recently reported protocol using oxidatively induced reductive elimination from the corresponding cobalt­(II) dialkyl derivatives…”

“…The balance of the alkyne had predominantly been converted to cyclobutene 6 (73%), with hydrovinylation also being observed as a minor side product ( 5 , 12%). While the observation of 6 as the major product is unsurprising, as cobalt complexes bearing dppf have previously been reported as efficient catalysts for [2+2]-cycloaddition, ,, the observation of ortho -functionalized product 3aa motivated the exploration of other cationic bis­(phosphine) cobalt­(I) precatalysts.…”

“…Metallacyclopentenes , most commonly react by uni­molecular pathways, including β-hydride elimination followed by C–H reductive elimination to form hydrovinylation products ,, or direct C–C bond reductive elimination to form cyclobutenes (Scheme C) . The selectivity of these product-forming processes is known to be highly dependent on the identity of the ligand bound to the cationic cobalt center. , For example, cyclobutene formation is favored over β-hydride elimination when a wide bite angle bis­(phosphine) such as dppf (1,1′-bis­(diphenylphosphino)­ferrocene) is used. ,, Conversely, hydrovinylation has been reported with dppp- and dppe-ligated cobalt catalysts (dppp = 1,3-bis­(diphenylphosphino)­propane; dppe = 1,2-bis­(diphenylphosphino)­ethane) . Significantly, hydroarylation of the metallacyclopentene requires the intermediate to be sufficiently long-lived to promote a bimolecular C–H activation event.…”

“…Our group has reported the synthesis of a series of well-defined cobalt(0) and cationic cobalt­(I) complexes bearing chiral bis­(phosphine) ligands and demonstrated that both oxidation states are active precatalysts for asymmetric alkene hydrogenation . Similarly, (dppf)­Co­(COD) and [(dppf)­Co­(η 6 -C 7 H 8 )]­[BAr F 4 ] (COD = 1,5-cyclooctadiene) were prepared, and the cationic cobalt­(I) variant was shown to catalyze olefin-alkyne [2+2]-cycloaddition . Here, we describe the synthesis and evaluation of a series of well-defined cationic bis­(phosphine) cobalt­(I) complexes as precatalysts for the intermolecular, three-component arene-alkene-alkyne coupling (Scheme D).…”

Activator-free single-component Co(i)-catalysts for regio- and enantioselective heterodimerization and hydroacylation reactions of 1,3-dienes. New reduction procedures for synthesis of [L]Co(i)-complexes and comparison to in situ generated catalysts

“…Representative examples include a broad range of carbon–carbon bond-forming reactions such as cross-coupling reactions, 2 heterodimerizations of alkenes 3 cycloaddition reactions, 4 hydroacylation reactions, 5 functionalizations via C–H activation, 6 and reductive coupling of alkenes and alkynes. 7 Until recently, with few exceptions, 3 d ,4 c ,8 the oxidation state of cobalt and the nature of the intermediates (radicals or low-valent organometallic C–Co species) in many of these reactions have remained open to speculation. For most C–C bond-forming reactions that involve low-valent cobalt species, the reactive catalysts were often generated in situ by reduction of the corresponding Co II species with reducing agents such as Zn, Mn, Na/Hg, NaEt 3 BH, alkyl aluminum, alkyl lithium or Grignard reagents (Fig.…”