Selective hydroboration of unsaturated bonds by an easily accessible heterotopic cobalt catalyst

Abstract

Homogeneous earth-abundant metal catalysis based on well-defined molecular complexes has achieved great advance in synthetic methodologies. However, sophisticated ligand, hazardous activator and multistep synthesis starting from base metal salts are generally required for the generation of active molecular catalysts, which may hinder their broad application in large scale organic synthesis. Therefore, the development of metal cluster catalysts formed in situ from simple earth-abundant metal salts is of importance for the practical utilization of base metal resource, yet it is still in its infancy. Herein, a mixture of catalytic amounts of cobalt (II) iodide and potassium tert-butoxide is discovered to be highly active for selective hydroboration of vinylarenes and dihydroboration of nitriles, affording a good yield of diversified hydroboration products that without isolation can readily undergo further one pot transformations. It should be highlighted that the alkoxide-pinacolborane combination acts as an efficient activation strategy to activate cobalt (II) iodide for the generation of metastable heterotopic cobalt catalysts in situ, which is proposed to be catalytically active species.

H-BPin/KOtBu Promoted Activation of Cobalt Salt to a Heterotopic Catalyst for Highly Selective Cyclotrimerization of Alkynes

“…The use of Zn powder as a reductant together with KO t Bu only slightly influence the results (Table , entry 5 vs entry 1). It is noteworthy that, in 2017, Thomas et al reported that a boron “ate” reductive species can be formed in situ by the reaction between alkoxide salt and HBPin and activate high-oxidation-state cobalt complexes as hydride donors. , Inspired by this pioneering work and our recent studies, HBPin (1.0 mol %) was tested as an additive and the yield of 2a was increased to 96% with very high regioselectivity (95/5; see Table , entry 6). However, when HBCat was used as the additive, the reaction only gave extremely poor results (Table , entry 7).…”

“…Despite these remarkable progresses made on cyclotrimerization of alkynes mediated by homotopic or heterotopic catalysts, the reaction is still challenging, in terms of catalytic efficiency, regioselectivity, and identification of the catalytically active species, especially in the field of ligand-free metal clusters/nanoparticles catalysis. Recently, our group disclosed that a metastable heterotopic cobalt catalyst can be efficiently generated in situ via the reduction of a simple cobalt salt using an alkoxide-pinacolborane mixture, which is highly active in hydroboration of unsaturated bonds . As a continuous effort in our development of new catalytic reactions using naked multimetallic species, we disclose herein an efficient and selective cyclotrimerization of alkynes using CoI 2 as the catalyst precursor (0.5 mol % in most cases).…”

“…The presence of the selective homotopic catalyst poison dibenzo-[a,e]­cyclooctatetraene (dct) (2.0 equiv per Co) did not significantly affect the cyclotrimerization rate (Figure d). From these observations, it is most likely that heterotopic or nanoparticulate catalyst species are operative, which may be generated from the reduction of simple cobalt salts by the boron “ate” reductive species. , …”

“…Recently, our group disclosed that a metastable heterotopic cobalt catalyst can be efficiently generated in situ via the reduction of a simple cobalt salt using an alkoxide-pinacolborane mixture, which is highly active in hydroboration of unsaturated bonds. 15 As a continuous effort in our development of new catalytic reactions using naked multimetallic species, we disclose herein an efficient and selective cyclotrimerization of alkynes using CoI 2 as the catalyst precursor (0.5 mol % in most cases). In this transformation, a combination of catalytic amounts of HBpin and KO t Bu was found to serve as a reducing agent, to induce the in situ formation of a catalytically active heterotopic cobalt species.…”

Bicyclic (alkyl)(amino)carbene (BICAAC) as a metal-free catalyst for reduction of nitriles to amines

“…16 Various reports from the laboratories of Szymczak, 17 Wang, 18 Panda, 19 Hill, 20 Okuda, 21 Roesky 22 and others have contributed to the advancement of hydroboration of nitriles using main group, d-, and f-block metal-based catalysts over the last few decades. 23–25 However, such catalytic reduction under completely metal-free conditions is extremely rare in the literature. Yan and co-workers very recently demonstrated a metal-free nitrile hydroboration using a mesoionic N -heterocyclic olefin (mNHO).…”