A Versatile NHC-Parent Silyliumylidene Cation for Catalytic Chemo- and Regioselective Hydroboration

Abstract

This study describes the first use of a silicon­(II) complex, NHC-parent silyliumylidene cation complex [(IMe)2SiH]­I (1, IMe = :C­{N­(Me)­C­(Me)}2) as a versatile catalyst in organic synthesis. Complex 1 (loading: 10 mol %) was shown to act as an efficient catalyst (reaction time: 0.08 h, yield: 94%, TOF = 113.2 h–1; reaction time: 0.17 h, yield: 98%, TOF = 58.7 h–1) for the selective reduction of CO2 with pinacolborane (HBpin) to form the primarily reduced formoxyborane [pinBOC­(O)­H]. The activity is better than the currently available base-metal catalysts used for this reaction. It also catalyzed the chemo- and regioselective hydroboration of carbonyl compounds and pyridine derivatives to form borate esters and N-boryl-1,4-dihydropyridine derivatives with quantitative conversions, respectively. Mechanistic studies show that the silicon­(II) center in complex 1 activated the substrates and then mediated the catalytic hydroboration. In addition, complex 1 was slightly converted into the NHC-borylsilyliumylidene complex [(IMe)2SiBpin]I (3) in the catalysis, which was also able to mediate the catalytic hydroboration.

Catalytic hydroboration of carbonyl compounds and pyridine derivatives with the NHC-parent silyliumylidene cation

“…This indicates that the Si-H bond is chemically active and could possibly be suitable for the were investigated and will be reported in the following section. 60 Results and discussion…”

“…This is consistent with the experimental data, where the NHC-parent silyliumylidene-mediated catalytic hydroboration of aldehyde derivatives could be carried out at room temperature. 60 The isolation of catalytic intermediates is currently under investigation in order to support the catalytic mechanism.…”

“…activation of small molecules similar to other main group hydride catalysts reported by research groups of Jones and Kinjo. As such, the reactivity of the NHC-parent silyliumylidene cation (Cat G) was investigated via the reduction of carbon dioxide with 4,4,5,5-tetramethyl-1,3,2dioxaboralane (HBpin) as the hydride source to form HCO2Bpin (Scheme 5) 60,[61][62][63]. SchemeCatalytic hydroborationOrganoboranes are versatile intermediates in organic synthesis due to their stability, easyhandling, high functional group compatibility, and wide application in the development of new strategies for transition-metal-catalyzed carbon-carbon, carbon-oxygen, carbon-nitrogen, and carbon-halogen bond-forming reactions.…”

“…These signals did not correspond to any of the reactants and products and hence, was suspected to be a new boron intermediate formed in the hydroboration. As such, stoichiometric reactions among the NHC-parent silyliumylidene cation, benzaldehyde, HBpin were carried out to determine this intermediate compound 60. The stoichiometric reaction of the NHC-parent silyliumylidene cation and benzaldehyde under any conditions did not produce any new boron compounds.…”

“…As such, the NHC-borylsilyliumylidene complex should be a key intermediate in the catalysis. With this, the catalytic cycle wasproposed as below 60.…”