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

Structure-constraint induced increase in Lewis acidity of tris(ortho-carboranyl)borane and selective complexation with Bestmann ylides

The Lewis acidity of tris(ortho-carboranyl)borane has been slightly increased by mimicking the structural evolution from triarylborane to 9-aryl-9-borafluorene. The o-carborane-based analogue (C2B10H10)2B(C2B10H11), obtained via salt elimination between LiC2B10H11 and (C2B10H10)2BBr, has been fully characterized. Gutmann-Beckett and computational fluoride/hydride ion affinity (FIA/HIA) studies have confirmed the increase in Lewis acidity, which is attributable to structural constraint imposed by the CC-coupling between two carboranyl groups. Selective complexation of (C2B10H10)2B(C2B10H11) with Bestmann ylides R3PCCO (R = Ph, Cy) has been achieved, enabling further conversion into the zwitterionic phospholium salt through NHC-catalyzed proton transfer.

Formal (2+2) ring expansion prevails over (4+2) cycloaddition of a kinetically stabilized benzoborirene with reactive cycloaddends

Benzoborirene carrying a bulky Trip2C6H3 (Trip = 2,4,6-tri-iso-Pr3C6H2) group at boron reacts with the dienophile 4-phenyl-1,2,4-triazoline-3,5-dione and the diene 3,6-di(4-pyridyl)-1,2,4,5-tetrazine by opening of the borirene ring rather than undergoing the typical Diels-Alder reactions. The formal insertion results in diazaborole and azaborolo[1,5-b][1,2,4,5]tetrazine derivatives, respectively.

Reactivity study of Lewis superacidic carborane-based analogue of 9-bromo-9-borafluorene towards Lewis bases

In this contribution, we present the reactions of the o-carborane-based analogue of 9-bromo-9-borafluorene, a Lewis superacid, with diverse Lewis bases. A range of acid-base adducts, along with an intramolecular C-H activation product, were generated. All new compounds have been fully characterized.

Borirenes and Boriranes: Development and Perspectives

Strained compounds constitute a highly topical area of research in chemistry. Borirene and borirane both feature a BC2 three-membered ring. They can be viewed as the structural analogues of cyclopropane and cyclopropene, where a CH2 unit of the carbonaceous counterparts is replaced with BH, respectively. Indeed, this structural variation introduces numerous intriguing aspects. For instance, borirane and borirene are both Lewis acidic due to the presence of a tricoordinate borane center. In addition, borirene is 2π aromatic according to Hückel's rule. In addition to their ability to form adducts with Lewis bases and the capacity of borirenes to act as ligands in coordination with metals, both borirenes and boriranes exhibit ring-opening reactivity due to the considerable ring strain. Under specific conditions, coordinated boriranes can even cleave two BC bonds to serve as formal borylene sources (although the reaction mechanisms are quite complex). On the other hand, recent successful syntheses of benzoborienes and their carborane-based three-dimensional analogues (also referred to as carborane-fused boriranes) have introduced novel perspectives to this field. For instance, they display excellent ring-expanding reactivity, possibly attributed to the boosted ring strain arising from the fusion of borirenes with benzene and boriranes with o-carborane. Importantly, their applications as valuable "BC2 " synthons have become increasingly evident along with the newly disclosed reactivity. Additionally, the boosted Lewis acidity of carborane-fused boriranes, thanks to the potent electron-withdrawing effect of o-carborane, combined with their readiness for ring enlargement, makes them promising candidates as electron-accepting building blocks in the construction of chemically responsive luminescent materials. This review provides a summary of the synthesis and reactivity of borirene and borirane derivatives, with the aim of encouraging the design of new borierene- and borirane-based molecules and inspiring further exploration of their potential applications.

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

Benzoborirenes are a very rare class of strained boron heterobicyclic systems. In this study a kinetically stabilized benzoborirene 1 is shown to react with multiple bonds of trimethylphosphine oxide, acetaldehyde, and tert-butyl isonitrile. The (2 + 2) cycloaddition product with trimethylphosphine oxide, benzo[c][1,2,5]oxaphosphaborole, has a long apical PO bond (194.0 pm) that must be considered on the border line between ionic and covalent according to the natural bond orbital, quantum theory of atoms in molecules, and compliance matrix approaches to the description of chemical bonding. The coordination compound between the benzoborirene and phosphine oxide was observed by NMR spectroscopy at 213 K. The Lewis acidity of 1 is similar to that of B(OCH2CF3)3 and B(C6F5)3 based on the 31P{1H} NMR chemical shift of the Lewis acid base complexes with trimethylphosphine oxide at 213 K. Benzoboriene 1 does not react with acetone, but forms a (2 + 2) cycloaddition product, an oxaborole, with acetaldehyde. In contrast, it undergoes a double (2 + 1) reaction with tert-butyl isonitrile to yield a boro-indane derivative under mild conditions. The observed reactivity of 1 is in agreement with computational analyses of the respective potential energy surfaces.

A Journey from Benzoborirene to Benzoborole-Supported 1,2-Diimine and Antiaromatic Borolediide

The reaction of benzoborirene with one equivalent of isocyanides leads to benzene-fused boretes bearing one imine function, while the reaction with two equivalents of isocyanide affords 2,3-dihydro-2,3-diiminoboroles with perfect regioselectivity. The isocyanide double insertion products represent a novel type of 1,2-diimine with a benzoborole backbone. The reduction chemistry of the benzoborole-supported 1,2-diimine was investigated. Single- and two-electron reduction allow for the isolation and full characterization of a radical anion and a dianion, respectively. In stark contrast to the ordinary boroles, which should turn aromatic by accepting two electrons, the antiaromatic character of the benzoborole backbone is highlighted upon reduction, thus presenting a rare example of antiaromatic borole dianion. Detailed quantum chemical calculations provide a rationale for the observed regioselectivity and the electronic structure of the reduced borole diimine species.

The Effect of Carborane Substituents on the Lewis Acidity of Boranes

The Lewis acidity of primary, secondary, and tertiary boranes with phenyl, pentafluorophenyl, and all three isomers of the C-substituted icosahedral carboranes (ortho, meta, and para) was investigated by computing their fluoride, hydride, and ammonia affinities as well as their global electrophilicity indices and LUMO energies. From these calculations, it was determined that the substituent effects on the Lewis acidity of these boranes follow the trend of ortho-carborane > meta-carborane > para-carborane > C6F5 > C6H5.