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, <b>1</b>, with potassium naphthalenide gave the radical (Me₂-cAAC)B(Cl)Si(SiMe₃)₃, <b>2</b>. A subsequent one-electron reduction of <b>2</b> yielded the dicoordinate borylene (Me₂-cAAC)BSi(SiMe₃)₃, <b>3</b>, which rapidly underwent intramolecular C-H activation to give cyclo-(Me₂-cAAC)B(H)Si(SiMe₃)₃, <b>4</b>, irrespective of the employed reaction conditions. Compound <b>3</b> could be stabilized as the carbonyl complex (Me₂-cAAC)B(CO)Si(SiMe₃)₃, <b>5</b>, that gave <b>4</b> upon irradiation with a UV light under a CO₂ atmosphere. In contrast, the two-electron reduction of <b>1</b> under an atmosphere of CO₂ yielded a mixture of products of which (Me₂-cAAC)B(Cl)(H)C(O)OSi(SiMe₃)₃, <b>6</b>, could be separated and structurally characterized. Compound <b>6</b> 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 <b>6</b> from the purported boryl anion intermediate [(Me₂-cAAC)B(Cl)Si(SiMe₃)₃]^-, <b>2</b> ^-, was analyzed computationally.