Insertion of an Overcrowded Silylene into Hydro- and Haloboranes: A Novel Synthesis of Silylborane Derivatives and Their Properties

Reactions of stable silylene-isocyanide complexes Tbt(Mes)SiCNAr (3a, Ar = Mes*; 3b,
Ar = Tbt; 3c, Ar = Tip; Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes = mesityl,
Mes* = 2,4,6-tri-tert-butylphenyl, Tip = 2,4,6-triisopropylphenyl) with boron compounds were
investigated. Reactions of 3a,b with BH3·THF afforded the first stable silylborane-isocyanide
complexes, Tbt(Mes)SiHBH2·CNAr (4a,b). Silylene complex 3a reacted with BH3·PPh3 as
well to give the corresponding silylborane-phosphine complex Tbt(Mes)SiHBH2·PPh3 (12).
In addition, silylene 2 thermally generated from the corresponding disilene, Tbt(Mes)SiSi(Mes)Tbt (1), gave 12 when reacted with BH3·PPh3. These results strongly suggested that
the reactions of 3 with BH3·THF and BH3·PPh3 proceeded via insertion of a silylene, Tbt(Mes)Si:  (2), into the B−H bond rather than the nucleophilic attack of the silicon atom of
3 toward the boron atom. Thermal dissociation of 4 and 12 into a base-free silylborane,
Tbt(Mes)SiHBH2 (9), was evidenced by the base-exchange reactions. Furthermore, silylene
2 was found to insert into boron−halogen bonds as well as boron−hydrogen bonds. The novel
reactivity of silylene 2 was applied to the syntheses of a variety of silylborane derivatives
such as base-free silylboranes, silylborane-isocyanide complexes, and silylborates.