Boron-Mediated Carbon–Carbon Bond Cleavage and Rearrangement of Benzene Forming the Borepinyl Radical and Borole Derivatives

Abstract

The reaction of atomic boron with benzene in solid neon has been investigated by matrix isolation infrared spectroscopy with isotopic substitutions as well as quantum chemical calculations. The reaction is initiated by boron atom addition to benzene in forming an η 2 -(1, 4) π adduct (A). A borepinyl radical (B) formed by C−C bond insertion is also observed on annealing. The η 2 -(1,4) π adduct photoisomerizes to an unprecedented borole substituted vinyl radical intermediate (C) via ringopening and rearrangement reactions involving an antiaromatic borole subunit. A previously unconsidered 1-ethynyl-2-dihydro-1H-borole radical (D) is generated as the final product under UV light irradiation. The results presented herein give new insight into the benzene carbon−carbon bond cleavage and rearrangement reactions mediated by a nonmetal and provide a possible route for the construction of heterocyclic borepinyl and borole species via benzene ring opening and rearrangement reactions.

Formation of 1-ethynyl-1H-silole from the reaction of silicon atoms with benzene: matrix infrared spectroscopy and quantum chemical calculations

“…28,48,49 It should be pointed out that the planar C 2v structure is about 8 kJ mol À1 less stable than the C 2 structure in the seven-membered ring borepinyl radical due to the unpaired electron occupying a twisted bonding p orbital located mainly at the CBC moiety. 30 The 1-ethynyl-1H-silole (C) is predicted to have a 1 A 0 ground state with C s symmetry involving a planar five-membered silole ring with the ethynyl subunit bonded to the silicon center. The C-C bond length of the ethynyl subunit is predicted to be 1.205 Å, which is typical for the CRC triple bond.…”

“…It is necessary to mention that another possible exothermic reaction channel undergoes Z 1 complex intermediate and the C-C bond insertion reaction proceeds to form the borepinyl radical (B 0 ) and requires negligible activation energy. 30,54,55 These are the reasons that the borepinyl radical (B 0 ) could be observed upon annealing, and 1-silacycloheptatrienylidene species B is only observed under UV light excitation in a solid neon matrix. The isomerization reaction from species B to the 1-ethynyl-1H-silole (C) is predicted to be exothermic by 20 kJ mol À1 , while the species B 0 conversion to 1-ethynyl-2-dihydro-1H-role radical (C 0 ) is predicted to be exothermic by 45 kJ mol À1 .…”

“…Although both the final products are the ethynyl substituted five-membered ring species in the reaction with benzene, the 1-vinyl-1H-borole radical intermediate was trapped and characterized in the isomerization reaction in which species A 0 and B 0 convert to the C 0 compound. 30…”

“…28 Boron atoms can be spontaneously and selectively inserted into the C-C bond forming the borepine radical and further photo-isomerization to five-membered borole derivatives, during the reaction of boron atoms with a benzene molecule under matrix isolated conditions. 30 Due to its small size and similar charge/mass ratio, boron is different to other group 13 members, but it resembles silicon, the second element of group 14 and they exhibit a diagonal relationship. [31][32][33] The research of cationic dihydride boryl and dihydride silyl osmium(IV) NHC complexes showed this remarkable boron-silicon diagonal relationship.…”

Spectroscopic characterization of two boron heterocyclic radicals in the solid neon matrix

“…Recent studies show that boron atoms can spontaneously and selectively insert into the carbon-carbon bonds of benzene, forming a seven-membered borepine radical and that is further isomerized to a five-membered borole derivative under UV light irradiation in matrix-isolation conditions. 30 This result not only provides new insight into the activation of benzene molecules but also can guide the future design and synthesis of the corresponding boron heterocyclic compounds. However, benzene is highly toxic and carcinogenic, which raises an interesting question: can other reactants react with boron atoms to form borole derivatives or boron heterocycles in the inert matrix?…”