Mechanistic Studies on the Rearrangement of a Boron Cation: From a nido-Carborane to a Planar Boracycle

Abstract

The reaction mechanism of the [Et3BH]−-induced conversion of a dicationic boron compound ([Cp*B-IMes]2+, [1]2+) to a planar neutral carbene-coordinated borabenzene (2) is investigated experimentally and theoretically. Owing to the steric congestion around the boron center, bulky [Et3BH]− attacks the less encumbered carbon atom of Cp*, leading to a metastable 5-borabicyclo[2.1.1]­hex-2-ene borenium cation ([3-CH]+) at low temperature. Upon raising the temperature, the bicyclic-borenium ion undergoes sequential pericyclic reactions, including suprafacial [1,3]-sigmatropic shift, two-electron electrocyclic ring opening, and then [1,2]-hydrogen shift to a planar cyclic borenium ion, [4-CH]+, which can then be deprotonated to yield the neutral aromatic borabenzene. The attack of hydride at the boron center of [1]2+ was excluded through the preparation and isolation of 6-borabicyclo[3.1.0]­hexenylium, [3-BH]+, which could not be transformed into [3-CH]+ or [4-CH]+. The borenium ion rearrangement follows first-order kinetics with activation parameters of Δ H ⧧ = 19.6 (±0.31) kcal/mol and Δ S ⧧ = −2.03 (±1.15) cal/mol·K. The observed kinetic isotope effect of 0.87 at 265 K is consistent with the DFT-calculated reaction mechanism, which predicts an overall KIE value of 0.90. These results show that [1]2+ reacts with [Et3BH]− as a carbon-based electrophile and the formation of an electron-deficient borenium center is responsible for the skeletal rearrangement of the nido-cluster.

Isolation of an N‐Heterocyclic Carbene Complex of a Borasilene

“…2019, 25,1 1036 -11041 www.chemeurj.org p-electron-donating NHI. [19,22] With structurale lucidations of 4 and 5 + + in hand, we highlight three resonance structures to sufficiently describe the bondings ituation for the borasilene entity in 4 (Scheme 1): (i)the zwitterionic formulation 4 A with the negative chargea t the silicon centera nd the positive charge at the boron atom, (ii)the zwitterionic formulation 4 B with the negative charge at the silicon atom and the positive charged elocalized to the NHI, and (iii)the borasilene formulation 4 C with ac ovalentB =Si double bond. [8c, 21] Clearly,this is not asuitable indicator for chargea llocation into the ring because the average bond lengths do not substantially change for cationic three-coordinate boron complexes of I Mes .…”

“…[8c, 21] Clearly,this is not asuitable indicator for chargea llocation into the ring because the average bond lengths do not substantially change for cationic three-coordinate boron complexes of I Mes . [19,22] With structurale lucidations of 4 and 5 + + in hand, we highlight three resonance structures to sufficiently describe the bondings ituation for the borasilene entity in 4 (Scheme 1): (i)the zwitterionic formulation 4 A with the negative chargea t the silicon centera nd the positive charge at the boron atom, (ii)the zwitterionic formulation 4 B with the negative charge at the silicon atom and the positive charged elocalized to the NHI, and (iii)the borasilene formulation 4 C with ac ovalentB =Si double bond. Distinct structural parameters of 4 support particular resonance structures, that is (a)the pyramidalized geometry at the silicon site and the trigonal-planar geometry aroundt he boron atom (in favor of 4 A , 4 B ), (b) the BÀSi distance being only slightly shorter than that in Aldridge'sb orylsilylene (in favor of 4 A , 4 B ), and( c) the BÀNa nd the CÀN imino bond lengths being longera nd shorter,r espectively,t han those in the ligand-centered cation 5 + + (in favor of 4 A ).…”

Cleavage of Unstrained C−C Bonds in Acenes by Boron and Light: Transformation of Naphthalene into Benzoborepin

“…[2] Recently,m ain-group compounds have been demonstrated to be very effective in activating chemical bonds,such as H À Hbonds and C À X(X= H, F) bonds,a nd some of these main-group-based systems were successfully used in catalytic processes. [7][8][9][10] Nevertheless,none of the previously reported boron-based systems are capable of cleaving C À Cbonds in stable arenes.Aspart of our continuing efforts in the investigation and understanding of the photoreactivity of organoboron compounds,t he reactivity of the peri-substituted naphthalene compound 1a (Scheme 1) was examined. [4][5][6][7][8] Several recent reports demonstrated that reversible or irreversible CÀCb ond formation/cleavage in ab oron-supported framework can be readily achieved with light or heat.…”

“…[4][5][6][7][8] Several recent reports demonstrated that reversible or irreversible CÀCb ond formation/cleavage in ab oron-supported framework can be readily achieved with light or heat. [7][8][9][10] Nevertheless,none of the previously reported boron-based systems are capable of cleaving C À Cbonds in stable arenes.Aspart of our continuing efforts in the investigation and understanding of the photoreactivity of organoboron compounds,t he reactivity of the peri-substituted naphthalene compound 1a (Scheme 1) was examined. Surprisingly,c ompound 1a was found to undergo facile photoisomerization to produce benzoborepin 2a in good yield.…”

Cationic Complexes of Boron and Aluminum: An Early 21st Century Viewpoint

“…The conversion of 64 a [AlCl 4 ] 2 produced the uncharged borabenzene 65 which forms by hydride‐transfer‐induced insertion of the metalloid atom into the five‐membered ring of the Cp* ligand followed by proton desorption from an intermediate monocation (Figure ). The reduced species 65 was isolated in poor yield only (16 %) which might correlate to the necessity of a suitable proton acceptor to promote the reaction (note: in a later study t Bu 3 P was used as a base, however, no yield was reported for this base‐assisted conversion) –. Though theoretical investigations suggested 65 to form by initial hydride transfer to the boron center in 64 a 2+ the low‐temperature NMR study diagnosed the formation of intermediate 66 + as a consequence of hydride transfer to the Cp* group (and subsequent rearrangement, Figure ) .…”