Boosting Ring Strain and Lewis Acidity of Borirane: Synthesis, Reactivity and Density Functional Theory Studies of an Uncoordinated Arylborirane Fused to o‐Carborane

Abstract

Among the parent borirane, benzoborirene and ortho‐dicarbadodecaborane‐fused borirane, the latter possesses the highest ring strain and the highest Lewis acidity according to our density functional theory (DFT) studies. The synthesis of this class of compounds is thus considerably challenging. The existing examples require either a strong π‐donating group or an extra ligand for B‐coordination, which nevertheless suppresses or completely turns off the Lewis acidity. The title compound, which possesses both features, not only allows the 1,2‐insertion of P=O, C=O or C≡N to proceed under milder conditions, but also enables the heretofore unknown dearomative 1,4‐insertion of Ar−(C=O)− into a B−C bond. The fusion of strained molecular systems to an o‐carborane cage shows great promise for boosting both the ring strain and acidity.

Accessing unusual heterocycles: ring expansion of benzoborirenes by formal cycloaddition reactions

“…28 We thus employed the smaller Me 3 PO instead of the standard Gutmann-Beckett reagent Et 3 PO to afford qualitative information about the electrophilicity of benzoborirene 1. However, when 1 was treated with one equivalent of Me 3 PO in toluene we did not observe the expected 31 The phosphorus atom is coordinated in a slightly distorted trigonal bipyramidal fashion with bond angles between the equatorial carbon atoms of 118.12(4)°, 118.57(4)°and 120.64 (5)°and a bond angle between the apical carbon and oxygen atoms of 175.15(4)°. The central ring of the terphenyl moiety is tilted by 85.5°relative to the benzo[c] [1,2,5]oxaphosphaborole plane.…”

“…26,27 The related carborane-fused boriranes undergo cycloaddition reactions with various substrates resulting in an expansion of the three-membered ring. 29–31…”

“…26,27 The related carborane-fused boriranes undergo cycloaddition reactions with various substrates resulting in an expansion of the three-membered ring. [29][30][31] Clearly, the readily accessible benzoborirene 1 offers the potential for unique reactivity as it combines an electrophilic boron centre 31 and a strained three-membered as well as a distorted benzene ring. We here investigate this "ménage à trois" in its behaviour towards multiple bonds as provided by phosphine oxides R 3 PO, carbonyl compounds (R 2 CO and RCHO), and isonitriles RNC.…”

The Effect of Carborane Substituents on the Lewis Acidity of Boranes

“…An LSA is a species that has a theoretically calculated fluoride ion affinity exceeding that of SbF 5 . , Ye and co-workers have prepared analogues of anthracene, in which the two phenyl groups are replaced with ortho -carboranes ( 8 ). , Both the calculated hydride and fluoride ion affinities (HIA and FIA, respectively) follow the Lewis acidity trend of Br > Cl > Ph > N 3 > CH 3 , with all five species exceeding the HIA of B­(C 6 F 5 ) 3 and the FIA of SbF 5 . The boracyclic analogues of the cyclopropenium cation were also prepared, featuring the two carbon atoms of ortho -carborane ( 9 ). , Amino substituents as π-donors or bulky mesityl groups were required to isolate these species due to the reactivity of the strained three-membered ring.…”

“…The boracyclic analogues of the cyclopropenium cation were also prepared, featuring the two carbon atoms of orthocarborane (9). 95,96 Amino substituents as π-donors or bulky mesityl groups were required to isolate these species due to the reactivity of the strained three-membered ring.…”