Mechanistic Investigation of Well-Defined Cobalt Catalyzed Formal E-Selective Hydrophosphination of Alkynes

Abstract

A formal E-selective hydrophophination of terminal and internal alkynes catalyzed by well-defined [Co(PMe3)4] (A) complex is achieved under mild conditions in good-toexcellent yield. The reaction does not require any additives and/or external base for an efficient hydrophosphination reaction. The reaction provided excellent scope and good functional tolerance. Detailed spectroscopic analysis (NMR, EPR, and UV-Vis) revealed that the low valent cobalt(0) complex undergoes oxidative addition with diphenylphosphine followed by hydrometallation with alkyne and subsequent reductive elimination led to the expected product. The detailed spectroscopic analyses along with the isotopic labelled experiments facilitate to intercept the active intermediates that are involved in the catalytic cycle, which are detailed. It was revealed that the suprafacial (vide infra) delivery of H and phosphorus to π-alkynes in synfashion lead to formal E-vinyl phosphine.

Associative Ligand Exchange and Substrate Activation Reactions by a Zero-Valent Cobalt Tetraisocyanide Complex

“…While it is recognized that 17-electron transition-metal carbonyl radicals (e.g., V­(CO) 6 and M­(CO) 5 ; M = Mn, Re) undergo ligand substitution reactions ,− via a second-order associative mechanism, − no direct experimental studies have been carried out on Co­(CO) 4 and its simple ligand substitution pathways. Indeed, both dissociative and associative substitution mechanisms have been proposed for the reaction of Co­(CO) 4 with incoming ligands. − For the dissociative processes in particular, it is notable that high-energy, 15e – species Co­(CO) 3 has been postulated to be a viable intermediate, although rigorous experimental data has not been disclosed to support this claim. , Nevertheless, following these reports, dissociative mechanisms have been proposed for certain other zero-valent S = 1 / 2 CoL 4 complexes, , yet detailed kinetic information supporting such proposals has also not appeared.…”

The first ring-expanded NHC–copper(i) phosphides as catalysts in the highly selective hydrophosphination of isocyanates

“…These mid-dblock systems, however, are prone to redox-switching and single-electron transfer reactions which, coupled with the stability of phosphorus centred radicals, often result in undesirable side reactions. [5][6][7] The propensity of copper(I) towards the well-defined isohypsic reaction steps of s-bond metathesis, and insertion of unsaturated substrates should allow copper(I) phosphides to overcome such issues. Despite this appeal, copper phosphides often form only as complex oligomers, with a concomitant reduction in reactivity.…”

Solvent-Free Hydrosilylation of Alkenes Catalyzed by Well-Defined Low-Valent Cobalt Catalysts

“…In 2016, Ge reported that for the alkene hydrosilylation catalyzed by Co­(acac) 2 Markovnikov products were formed with xantphos, and anti -Markovnikov silanes were produced with the dppf ligand . Recently, the catalytic applications of metal complexes containing PMe 3 ligands have been reported. ,− The studies indicate that PMe 3 ligands may have good effects in the control of reaction selectivity. , Based on this inference and as part of our ongoing work on hydrosilylation reactions, we herein report the highly regioselective hydrosilylation of alkene catalyzed by well-defined low-valent cobalt catalysts Co­(PMe 3 ) 4 and Co­(PMe 3 ) 3 Cl. The former displayed Markovnikov-type regioselectivity for aryl alkenes, while the latter had anti -Markovnikov-type regioselectivity for alkyl alkenes.…”

“…A slightly higher temperature is required for this system (entries 16, 22, and 23, Table ). The possible reason is that the higher temperature is conducive to the dissociation of the PMe 3 ligand and the formation of the active real catalyst . In addition to the reaction temperature, the solvents also have a great impact on the reaction (entries 11–16, Table ).…”