Bronsted acid-catalyzed skeletal rearrangements in polycyclic conjugated boracycles: a thermal route to a ladder diborole

Abstract

The dibenzodiboracyclobutylidene 1 was recently shown to undergo photochemical rearrangement to the more thermodynamically stable dibenzodiborapentalene 2. In order to discover a more efficient thermal route for this isomerization, the redox chemistry of both 1 and 2 was explored. Radical anions can be formed and characterized by EPR spectroscopy by one electron reduction using potassium graphite. The [Cp^*₂Co]⁺ salt of the radical anion of 2 was characterized by X-ray crystallography. The dipotassium salts 1K₂ and 2K₂ can be prepared by using two equivalents of potassium naphthalenide; both were fully characterized. It was found that smooth rearrangement of 1K₂ to 2K₂ is mediated by weak Bronsted acids such as tert-butanol or 2,6-dimethylphenol; a mechanism based on similar rearrangements in the isoelectronic neutral all-carbon framework is proposed.