The Bis(η6 -benzene)lithium Cation: A Fundamental Main-Group Organometallic Species

The synthesis and characterization of the bis(η⁶ -benzene)lithium cation, the benzene metallocene of the lightest metal, is reported. The boron compound FmesBCl₂ [Fmes: 2,4,6-tris(trifluoromethyl)phenyl] reacted with three molar equivalents of the lithio-acetylene reagent Li-C≡C-Fmxyl [Fmxyl: 3,5-bis(trifluoromethyl)phenyl]. Subsequent crystallization from benzene gave the [bis(η⁶ -benzene)Li]⁺ cation with the [{FmesB(-C≡C-Fmxyl)₃ }₂ Li]^- anion. This parent [(arene)₂ Li]⁺ cation shows an eclipsed arrangement of the pair of benzene ligands at the central lithium cation with uniform carbon-lithium bond lengths. The corresponding [(η⁶ -toluene)₂ Li]⁺ and [(η⁶ -durene)₂ Li]⁺ containing salts were similarly prepared. The bis(arene)lithium cations were characterized by X-ray diffraction, by solid-state ⁷ Li MAS NMR spectroscopy and their bonding features were analyzed by DFT calculations.

Tri-insertion with dearomatization of terminal arylalkynes using a carborane based frustrated Lewis pair template

Intramolecular vicinal Frustrated Lewis Pairs (FLPs) have played a significant role in the activation of small molecules, and their stabilities and reactivities are found to strongly depend on the nature of the bridging units. This work reports a new carborane based FLP, 1-PPh2-2-BPh2-1,2-C2B10H10 (2), which reacts with an equimolar amount of p-R2NC6H4C[triple bond, length as m-dash]CH (R = Me, Et, Ph) at room temperature to give C[triple bond, length as m-dash]C triple bond addition products 1,2-[PPh2C(R2NC6H4)[double bond, length as m-dash]CHBPh2]-1,2-C2B10H10 (3) in high yields. Compounds 3 react further with two equiv. of p-R2NC6H4C[triple bond, length as m-dash]CH (R = Me, Et) at 60-70 °C to give unprecedented stereoselective tri-insertion products, 3,3a,6,6a-tetrahydronaphtho[1,8a-b]borole tricycles (4), in which one of the aryl rings from arylacetylene moieties has been dearomatized with the formation of four stereocenters including one quaternary carbon center. It is noted that the phosphine unit functions as a catalyst during the reactions. After trapping and structural characterization of a key intermediate, a reaction mechanism is proposed, involving sequential alkyne insertion and 1,2-boryl migration.