The Cover Feature shows a carbonyl trialkyl borate, R₃B-CO, where three consecutive B-to-C alkyl group migrations are indicated by coloured (greenish, blue and violet) arrows, the last two being accompanied by the corresponding (blue and violet) arrows signaling the subsequent inverse (C-to-B) oxygen shift. This represents the consecutive formation of the formal acyl borane, boraepoxide and alkyl boron oxide intermediates resulting from the treatment of trialkyl boranes with carbon monoxide. The art work was designed by Pablo Espinosa Sánchez-Campillo. More information can be found in the Research Article by A. Espinosa Ferao.

Despite long time assumed as direct intramolecular migratory insertions, the two last of the three alkyl group migrations after alkylborane carbonylation with carbon monoxide are shown to proceed either through activated dimeric species or catalyzed by the final product, as the only low barrier possible pathways, the latter leading to scarcely aromatic boroxines.

Abstract

The classical simple picture of stepwise B-to-C migratory insertion of all three alkyl groups in the carbonylation reaction of trialkyl boranes with CO was shown not to be correct, except for the first alkyl group shift affording an acyl borane. The second and third direct alkyl shifts turned out to be kinetically hampered due to the non-activated character of the B−C bond in electron-poor B atoms. The latter can only be achieved by either the autocatalytic action of the final alkyl boron oxide or by formation of dimeric species with weakened B-alkyl bonds at borate centres. Both thermodynamic and several NICS-related parameters pointed to scarce, even “negative”, aromatic character for boroxines, the final cyclotrimerization products of alkyl-boron oxides.