Attempts toward a Silyl-Stabilized Dicoordinate Borylene: Insertion of Carbon Dioxide into the B–Si Bond

One-electron reduction
of the carbene-stabilized borane (Me2-cAAC)­B­(Cl)2Si­(SiMe3)3, 1, with potassium
naphthalenide gave the radical (Me2-cAAC)­B­(Cl)­Si­(SiMe3)3, 2. A subsequent
one-electron reduction of 2 yielded the dicoordinate
borylene (Me2-cAAC)­BSi­(SiMe3)3, 3, which rapidly underwent intramolecular C–H activation
to give cyclo-(Me2-cAAC)­B­(H)­Si­(SiMe3)3, 4, irrespective of the employed reaction conditions.
Compound 3 could be stabilized as the carbonyl complex
(Me2-cAAC)­B­(CO)­Si­(SiMe3)3, 5, that gave 4 upon irradiation with a UV light
under a CO2 atmosphere. In contrast, the two-electron reduction
of 1 under an atmosphere of CO2 yielded a
mixture of products of which (Me2-cAAC)­B­(Cl)­(H)­C­(O)­OSi­(SiMe3)3, 6, could be separated and structurally
characterized. Compound 6 is a rare example of CO2 insertion into a B–E (E = heavier main group element)
bond in which boron functions as a nucleophile, thereby mimicking
transition metal-mediated carboxylation. The mechanism for the formation
of 6 from the purported boryl anion intermediate [(Me2-cAAC)­B­(Cl)­Si­(SiMe3)3]−, 2–, was analyzed computationally.