Borole

Boroles represent a class of molecules known as metalloles, which are heterocyclic 5-membered rings. As such, they can be viewed as structural analogs of cyclopentadiene, pyrrole or furan, with boron replacing a carbon, nitrogen and oxygen atom respectively. They are isoelectronic with the cyclopentadienyl cation or abbreviated as and comprise four π electrons. Although Hückel's rule cannot be strictly applied to borole, it is considered to be antiaromatic due to having 4 π electrons. As a result, boroles exhibit unique electronic properties not found in other metalloles.
The parent unsubstituted compound with the chemical formula has yet to be isolated outside a coordination sphere of transition metals. Substituted derivatives, which have been synthesized, can have various substituents at the 4 carbons and boron. 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.
Once reduced to the dianion, the borolediide complex gains aromaticity and can then participate in similar reactions as the anion, including forming sandwich complexes.

Electronic properties

Hückel analysis

According to Hückel's rule which states that a cyclic molecule is aromatic if it has π electrons and antiaromatic if there are electrons, boroles represent antiaromatic molecules. In agreement with chemical intuition, ab initio calculations on the parent borole predict it to have an antiaromatic singlet ground state. Its backbone structure features strongly alternating bond lengths, consistent with localised electrons in the π system. This characteristic is preserved in almost all structurally characterised borole derivatives except those derived from. This discrepancy was attributed to intermolecular phenyl→boron π donation within dimeric subunits.
In addition, theoretical studies also suggest that borole is significantly destabilised by the delocalisation of its four π electrons. UV-Vis spectroscopy and reactivity studies have been conducted to assess the consequences of antiaromaticity in boroles. 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.
A simple Hückel model can be used to compare the spectroscopic properties and observed reactivity of boroles against the isoelectronic cyclopentadienyl cation. Unlike which has a doubly degenerate pair of HOMOs, the introduction of a boron center lifts their degeneracy by increasing the energy of the antisymmetric molecular orbital somewhat and the symmetric molecular orbital significantly. As a result, the HOMO in boroles is doubly occupied and no biradical character is observed, in line with a singlet ground state and the diamagnetic character of boroles. Furthermore, boroles exhibit a small HOMO–LUMO gap and the lowest-energy electronic absorption of boroles is considerably red shifted in the UV-Vis spectra. Accordingly, boroles exhibit a characteristic blue color. By contrast, introducing two electrons into the vacant LUMO either by reduction or adduct formation with Lewis bases significantly increases the HOMO–LUMO gap. Consequently, a dramatic blue shift of the lowest-energy excitation is observed and the resulting species are usually yellow to red in color. A qualitative drawing is presented to the left. The small HOMO-LUMO gap also makes boroles excellent participants in Diels–Alder type reactions either with themselves or with a variety of alkenes and alkynes.

Natural bond orbitals

analysis of has been performed in order to understand the bonding of borole in the familiar Lewis picture. According to the computational results, the occupancy of the two π orbitals is about 1.9, with a tiny amount of electronic charge delocalised on the out-of-plane boron orbital, illustrated below. The standard Lewis structure of borole captures more than 50% of the overall electronic structure according to Natural Resoanance Theory analysis. As delocalisation of the 4π electrons is prevented by antiaromaticity, the unsaturated boron atom has low occupancy of its vacant orbital and is highly Lewis acidic. Along with the low energy LUMO, boroles show an inherent propensity to form Lewis acid–base adducts even with substrates of low donor strengths.
As the p orbital of boron is virtually vacant and nonbonding, borole is regarded as a good Lewis acid or electron acceptor. The figure to the right shows the lack of involvement of boron's p orbital in the HOMO and the substantial Lewis acidic character at boron in the LUMO. Chemically, borole is reactive and unstable in ambient conditions. The pentaphenylborole analog is a highly reactive green solid; it readily undergoes oxidation, partial protolysis, and Diels−Alder reaction with dienophiles. Borole, even in perarylated form, is still very labile. Due to its reactive nature, the structural parameters and thermochemical data of borole are not known.
In the optimized structure of borole shown to the left, the,, and bond lengths are approximately 1.58, 1.338, and 1.518 Å respectively, as shown to the left. The longer bond in agree with NBO analysis that the π-electron delocalizations are mainly confined on the methine carbons, supporting the antiaromatic nature of the neutral borole.

Borole dianion (borolediides)

As the boron has an empty p orbital, boroles tend to accept 2 electrons in order to have 6 π electrons and gain aromaticity. This reduction to borolediides was demonstrated in the early 1980s by Herberich et al. with the isolation of K₂. Because the atomic orbitals of boron are different from those from carbon in terms of energy, not all atoms contribute equally to the π system in the borole dianion. A Natural Resonance Theory calculation shows that there are 3 dominant resonance structures for the isolated ²⁻ dianion as illustrated below.
File:Resonance in borole 2-.png|center|thumb|464x464px|Resonance forms in the Borole dianion. Contributions to overall structure are 30.30%, 30.71% and 13.04% respectively going from left to right as determed using NRT. Analyses were performed using NBO7 on a ^2- structure optimised using BP86-D3BJ and def2- TZVPP basis set.|alt=
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. This is exactly what was found for several characterised structures, namely K₂, K₂ and K₂. Thus, the observed B–C bonds are rather short and all C–C bond lengths lie within a narrow array.

Synthesis

The first borole derivative to be isolated was pentaphenylborole , synthesised by Eisch et al. in 1969 as a deep blue solid. Referring to the figure below, the practical synthesis of was initially accomplished in two different ways: by direct reaction of 1,4-dilithio-1,2,3,4-tetraphenylbutadiene with PhBBr₂ which gives a Lewis base adduct of pentaphenylborole in diethylether, and subsequent removal of the solvent. By boron–tin exchange between 2,3,4,5-tetraphenylstannole derivatives with PhBCl₂. Eisch et al. have demonstrated that the latter method can be expanded to other borole derivatives, even though these species have only been generated in situ.
Accounting for the drawbacks of each method, the boron–tin exchange is the current method of choice and has been widely accepted for the synthesis of numerous differently substituted borole derivatives. The other approach has yet to play a significant role in the further development of borole chemistry.
Besides the development of a synthetic pathway to the perfluorinated version of, the substituents on the BC₄ backbone is largely still limited to phenyl substituents. However, substituents beside H has been attached to the boron atom, such as halide, aryl and amino functionalities. Ferrocenyl, cymantrenyl and platinum complex fragments have also been successfully attached.
Depending on the boron-bound substituent, the electron density at boron can be altered. Hence, substituents can exert strong influences on the spectroscopic properties of the whole borole system. For instance, significant π-back bonding interactions from nitrogen in raises the borole LUMO energy and a resulting blue shift of the lowest energy absorption in UV-Vis spectra compared to .
In order to synthesis less sterically congested boroles, a zirconacycle transfer strategy was adopted by Fagan et al. Reaction of with PhBCl₂ was expected to result in the formation of . However, the product was too reactive and only its Diels–Alder dimer has been isolated. Evidence for the intermediate before dimerisation was shown through trapping experiments with 2-butyne and reactivity studies using a variety of unactivated alkenes.
In 2018, Lee et al. successfully transformed a borapyramidane into a stable halogen substituted planar borole dianion which was stabilized by Li⁺ ions positioned above and below the plane of the borole ring, revealing a direct synthetic path to borolediides from borapyramidane.

Reactions

Pentaphenylborole is known to show a wide range of reactivity, owing to its antiaromatic and highly Lewis acidic nature.

Lewis acid-base adducts

As suggested previously, the high Lewis acidity of boroles allows the ready formation of Lewis acid–base adducts with a variety of different donor molecules. This simplest case of reactivity has already been realised in the early days and has frequently been used to highlight the antiaromatic nature of boroles. Pyridines, ethers, phosphines, and different carbene species have been successfully attached to the unsaturated boron center.
In general, such reactions are facile and proceed quantitatively, facilitating their isolation in high yield. 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. However, strong bond length alternation in the BC₄ backbone is still observed and remain almost unaffected by these fundamental electronic changes. In contrast, spectroscopic measurements are much more sensitive to adduct formation. Unlike the respective borole precursors which are intensely coloured, the adducts are pale yellow solids with characteristic UV-Vis excitations at λ_max = 350–380 nm which agrees with an increase in the HOMO-LUMO gap.