[3] The high electron deficiency leads to various reactivities such as metal free hydrogen activation and rearrangements upon cycloaddition which are unobserved in other structural analogues like pyrrole or furan.

In agreement with chemical intuition, ab initio calculations on the parent borole C4H4BH predict it to have an antiaromatic singlet ground state.

In addition, theoretical studies also suggest that borole is significantly destabilised by the delocalisation of its four π electrons (NICS 17.2; ASE 19.3 kcal mol−1).

Their antiaromatic character entails strong electrophilicity of the boron center resulting in even weak donors such as ethers or nitriles being capable of forming stable Lewis acid–base adducts.

Moreover, boroles' highly activated carbon backbone readily participates in Diels–Alder reactions and is prone to two-electron reductions affording borolediides.

Consequently, a dramatic blue shift of the lowest-energy excitation is observed (e.g. [PhBC4Ph4]·pyridine: λmax = 340 nm) and the resulting species are usually yellow to red in color.

As delocalisation of the 4π electrons is prevented by antiaromaticity, the unsaturated boron atom has low occupancy of its vacant pz orbital and is highly Lewis acidic.

As the p orbital of boron is virtually vacant and nonbonding (as indicated by its NBO energy level), borole is regarded as a good Lewis acid or electron acceptor.

The longer 2C2C bond in C4H4BH agree with NBO analysis that the π-electron delocalizations are mainly confined on the methine carbons, supporting the antiaromatic nature of the neutral borole.

Since borole dianions are isoelectronic to the ubiquitous cyclopentadienyl anion, aromatic delocalisation of the 6π electrons should cause bond lengths assimilation within the BC4 backbone.

[10] Referring to the figure below, the practical synthesis of [PhBC4Ph4] (1) was initially accomplished in two different ways:[10][7][11] (a) by direct reaction of 1,4-dilithio-1,2,3,4-tetraphenylbutadiene with PhBBr2 which gives a Lewis base adduct of pentaphenylborole (1·OEt2) in diethylether, and subsequent removal of the solvent.

In order to synthesis less sterically congested boroles, a zirconacycle transfer strategy was adopted by Fagan et al.[15][16] Reaction of [Cp2ZrC4Me4] with PhBCl2 was expected to result in the formation of [PhBC4Me4].

Upon Lewis base coordination, the former vacant p orbital at boron becomes occupied and cyclic delocalization of the π electron system is no longer feasible, corresponding to the loss of antiaromaticity.

Unlike the respective borole precursors which are intensely coloured, the adducts are pale yellow solids with characteristic UV-Vis excitations at λmax = 350–380 nm which agrees with an increase in the HOMO-LUMO gap.

As a result of only having 4 electrons in the planar π system, boroles experience a large destabilizing effect and thus exhibits high reactivity, such as in dimerisations[24] and cycloadditions.

[26] Recently, the interest in this reaction pathway was revived by Piers et al.., who studied the reactivity of perfluorinated [PhBC4Ph4] (2 in the figure above) towards alkynes in great detail.

However, the mild reaction conditions (spontaneous at room temperature) enabled the isolation of the direct Diels–Alder cycloaddition product 1,2-Et2-4, which is considered the thermodynamically favored isomer of the two possible 7-borabicyclo[2.2.1]hepta-dienes.

The potent Lewis acidity of 12 revealed a novel reaction pathway whereby the alkyne first adds to the borole nucleophilically, followed by subsequent aryl migration and ring expansion to afford the boracyclohexadiene 7 as the predominant species (75%).

Results show that the partial charges of the pyridine and dimethylamino nitrogen atoms are -0.232 and -0.446 respectively, suggesting a larger accumulation of electron density on the latter group which should make it the more reactive nucleophile.

Given the many studies on frustrated Lewis pairs (FLP) that point them towards high potential small molecule activators, Piers and coworkers set out to investigate whether the strong electrophilicity of antiaromatic boroles might entail a comparable reactivity.

[30] Their studies initially focused on perfluorinated [PhBC4Ph4] due to its exceptionally high Lewis acid strength, which readily reacted with H2 both in solution and in the solid state to form two possible isomers as shown above.

A plausible reaction mechanism involving a borole H2 adduct was proposed on the basis of the observed isomers ratios and theoretical studies.

Sindlinger[6] reported that examining the orbitals of the simpler hypothetical model complex (C4BH5)Al(C5H5) revealed similar features to the fully substituted compound.

In line with a strong localisation of electron density on Cα, bond critical points are only found for the between Al and Cα (delocalization index, DI=0.25) but not between Al and Cβ (DI=0.11) as shown below.In contrast, the gallium analogue forms a Lewis-base adduct with a dative Ga−B bond rather than the neutral heteroleptic case for Aluminium, suggesting that Ga still retains the +1 oxidation state.

Using the model compound, its frontier orbitals were calculated and they reveal covalent bonding interactions between the apical germanium atom and the borole base.

Further theoretical studies have also been conducted at the M06-2X/Def2-TZVP level of theory to investigate the stability of half sandwich complexes between C4H4BNH2 with other group 14 elements (C, Si, Sn, Pb),[17] where the borole ring binds to the divalent metal cation in η5 mode.