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

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.