A Well-Defined Isocyano Analogue of HCo(CO)₄. 3: Hydride Migration to Olefins, H-Atom Transfer and Reactivity toward Protic Sources

Abstract

The tetraisocyano cobalt hydride complex HCo­(CNArMes2)4 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3) has previously been shown to serve as a well-defined structural and spectroscopic analogue to the classical carbonyl hydride HCo­(CO)4. The latter has been established as a precatalyst for olefin hydroformylation (i.e., the oxo reaction) and, in certain instances, as an H-atom transfer reagent for radical hydrogenation reactions. In this report, a detailed account of the reactivity of HCo­(CNArMes2)4 toward olefin substrates is reported to provide additional points of comparison with HCo­(CO)4. Treatment of HCo­(CNArMes2)4 with a range of monoene substrates leads rapidly to η6-arene iminoacyl complexes of the formulation Co­(η6-(Mes)-κ1 C-C­(R)NArMes2)­(CNArMes2) and is accompanied by the loss of two isocyanide ligands. In each case, the incoming olefin is both reduced and incorporated into the α-carbon position of the newly formed iminoacyl unit. Mechanistic considerations for this transformation points to the proclivity of HCo­(CNArMes2)4 to undergo hydride migration to the CNArMes2 ligand. However, experiments with diene substrates demonstrate that α-migration is reversible and competes with β-hydride elimination from putative Co alkyl intermediates. In addition to its reactivity with olefins, the ability of HCo­(CNArMes2)4 to engage in both hydrogen-atom transfer (HAT) and protolytic dihydrogen evolution processes was also probed. Upon reaction with the nitroxide radical TEMPO, TEMPOH was generated along with the η2 O,N-nitroxide complex (η2 O,N-TEMPO)­Co­(CNArMes2)2. The latter likely arises from efficient trapping of the d9 metalloradical Co­(CNArMes2)4 by TEMPO after HAT from HCo­(CNArMes2)4. This reaction outcome places an upper limit on the bond dissociation energy (BDE) of the Co–H unit in HCo­(CNArMes2)4 of ca. 69 kcal/mol, which is in line with that measured for related cobalt hydrides, including HCo­(CO)4. Upon treatment of HCo­(CNArMes2)4 with triflic acid (HOTf; [OTf]− = [F3CSO3]−), substoichiometric quantities of H2 are evolved. However, the major product of this reaction is the aminocarbene salt [Co­(η6-(Mes)-κ1 C-C­(H)­N­(CH3)­ArMes2)­(CNArMes2)]­OTf, signifying that HCo­(CNArMes2)4 possesses dual sites of Brønsted basicity and lacks utility as a potential catalyst for the hydrogen evolution reaction (HER).

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

“…The catalytic 1,1-hydrogenation of the isocyanide CNAr Mes2 was achieved in good yields (76 ± 13%) using 91 as a catalyst, with a loading of 5 mol % (16 h, r.t.). It is worth noting that the authors carried out a full evaluation of the relative acidity of 91 , and its reactivity toward olefins and Brønsted acids, demonstrating its ability to undergo insertion chemistry and H atom transfer reactions. ,, …”

“…It is worth noting that the authors carried out a full evaluation of the relative acidity of 91, and its reactivity toward olefins and Brønsted acids, demonstrating its ability to undergo insertion chemistry and H atom transfer reactions. 135,155,158 Prolonged stirring of [PPN] [87] in n-hexane resulted in the dissociation of one isocyanide CNAr Mes2 ligand, followed by precipitation of the zwitterionic tris(isocyano) complex (η 2 -PPN)Co(CNAr Mes2 ) 3 (94, Scheme 10b), which has been proposed as a highly reactive synthon for the coordinatively unsaturated monoanionic cobaltate [Co(CNAr Mes2 ) 3 ] − . 127 The Scheme 9 (101).…”

Well-Defined Low-Valent Cobalt Complexes in Catalysis: An Overview

“…This catalyst showed reactivity toward isocyanides themselves when brought back to room temperature and was applied in the catalytic hydrogenation of MesCN (Scheme ). In 2021, the same group also disclosed some stoichiometric reactivity of Co-9 toward olefins …”

“…53b In 2021, the same group also disclosed some stoichiometric reactivity of Co-9 toward olefins. 91 The synthesis of HCo(CO) 4 (Co-2) was followed few years later by other phosphine-based cobalt(I) catalysts (Figure 4). Klein synthesized in the 1970s the trimethylphosphine-based catalysts HCo(PMe 3 ) 4 (Co-37), XCo(PMe 3 ) 3 (Co-38; with X = Cl, Br), and MeCo(PMe 3 ) 4 (Co-39), bearing respectively a hydride, 92a a halide, 92b or a methyl 92c as the X ligand.…”

Copper/Chiral Phosphoric-Acid-Catalyzed Intramolecular Reductive Isocyanide-Alkene (1 + 2) Cycloaddition: Enantioselective Construction of 2-Azabicyclo[3.1.0]hexanes

“…Overall, formimidoylcopper­(I) A is nucleophilic and inert to unactivated alkenes. It is known that some transition metals, such as Ni and Co, are prone to coordinate with isocyanide to form imidoylmetal complexes that can resonate with the corresponding aminocarbeniods after protonation on the iminonitrogen. − It is postulated that the analogous formimidoylcopper­(I) A will behave more like a carbenoid rather than a nucleophile under acidic conditions, thus leading to a (1 + 2) cycloaddition reaction with unactivated alkenes. − In this Communication, we would like to disclose a new protocol for the generation of α-aminocarbenoids from isocyanides under a Cu/silane/CPA catalytic system, and as a result, an alternative double-ring-forming strategy toward enantioenriched and N–H free 2-azabicyclo[3.1.0]­hexanes has been successfully developed. Notably, when racemic α-quaternary alkenyl isocyanide was used as a substrate, enantiodivergent cyclopropanation took place smoothly to give two 2-azabicyclo[3.1.0]­hexanes in good to excellent enantioselectivities, suggesting excellent enantiomeric control by the CPA during cyclopropanation (Scheme d).…”