page_title: Borylene
section_title: Bis-Lewis base-stabilized borylenes
hierarchical_section_title: Borylene / Bis-Lewis base-stabilized borylenes
caption_attribution_description: English: BisLB LUMO
context_page_description: A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic residue and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp² orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride and boron monohydride the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.
  The first stable terminal borylene complex [(OC)₅WBN(SiMe₃)₂] was reported by Holger Braunschweig et al. in 1998. In this compound a borylene is coordinated to a transition metal. Borylenes are also stabilized as Lewis base adducts, e.g. with a NHC carbene. Other strategies are the use of cyclic alkyl amino carbenes and other Lewis bases, and their use as bis-adducts.
context_section_description: Taking inspiration from Robinson's above diborene synthesis, Bertrand et al. swapped NHC for CAAC and successfully isolated the first bis-Lewis base-stabilized borylene in 2011. Reduction of (CAAC)BBr₃ with KC₈ in the presence of excess CAAC afforded the bis(CAAC)BH. A labeling study indicated that the H-atom was abstracted from an aryl group associated with the CAAC. Reduction of (CAAC)BBr₃ yields the same terminal borylene even in the absence of additional Lewis base via a mechanism that remains poorly understood. Exploitation of this procedure has been used to form mixed bis-Lewis base-stabilized borylenes as well. Several other routes have also been proposed. A more novel one employs methyl triflate to abstract a hydride from (CAAC)BH₃. Treatment with a Lewis base, followed by triflic acid and KC₈ afford the desired (CAAC)(Lewis base)BH. Although the reported case uses only specific Lewis bases, the approach is argued to be highly generalizable. A number of other compounds in this class have been generated using borylene-transition metal complexes as precursors. Treatment of (OC)₅M=B-Tp with carbon monoxide or acetonitrile yields the corresponding adducts: (CO)₂B-Tp and (MeNC)₂B-Tp.
  Bonding in these complexes is quite similar to that in mono-Lewis base compounds. At least one π-acceptor ligand is present in all known examples of these compounds, and the B-L bond strength tends to scale with the π-acidity of the Lewis base. Low-energy σ-donation orbitals from the base to boron are present in these compounds, and the π-interaction from boron's lone pair to the Lewis base serves as the HOMO. Calculated electronic structure for a number of borylene complexes were compared with their isoelectronic homologues: carbone complexes (CL₂) and nitrogen cation complexes ((N⁺)L₂).
caption: Bis(CAAC)BH LUMO.[23]
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