Diboramacrocycles: reversible borole dimerisation-dissociation systems.

Sonja Fuchs,Maximilian Fest,Holger Braunschweig,Marta Baštovanović,Laura Haley,Arumugam Jayaraman,Krzysztof Radacki,Holger Helten,Ivo Krummenacher

doi:10.1039/d1sc06908j

Abstract

We report that the outcome of the tin–boron exchange reaction of a mixed thiophene-benzo-fused stannole with aryldibromoboranes is associated with the steric bulk of the aryl substituent of the borane reagent, leading to either boroles or large diboracycles as products. NMR spectroscopic studies indicate that the two products can reversibly interconvert in solution, and mechanistic density functional theory (DFT) calculations reveal boroles to be intermediates in the formation of the diboracyclic products. The addition of Lewis bases to the diboracycles leads to the corresponding borole adducts, demonstrating that they react as “masked” boroles. Additionally, the reaction of the title compounds with a series of organic azides affords complex heteropropellanes, formally 2 : 1 borole-azide adducts, that deviate from the usual BN aromatic compounds formed via nitrogen atom insertion into the boroles.

Highlights

The class of ve-membered boroles continues to be of great interest beyond their antiaromatic p-electron conjugation.[1]
While the reaction of doubly benzo-fused and mixed thiophene-benzo-fused stannole precursors with dichloro(phenyl)borane led to the desired boroles (I and II), the tin–boron exchange with the doubly thiophene-fused precursor led instead to a diboracyclic structure (III), where the aInstitute for Inorganic Chemistry, Julius-Maximilians-Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
Selective formation of the borole 2c was observed only with the sterically most hindered dibromo(mesityl)borane, whereas employing the less-hindered phenyl- and 2,4-xylyldibromoboranes led to the diboracycles 3a and 3b in 79% and 37% isolated yield, respectively

Summary

Introduction

The class of ve-membered boroles continues to be of great interest beyond their antiaromatic p-electron conjugation.[1]. Edge Article unfavourable cyclic four-electron delocalisation is avoided.[6] Such macrocyclic ring structures, in which the boron atoms are doubly biaryl-bridged, have been observed by Piers[7] and postulated by Wagner as intermediates in dimerisation pathways of 9-H-9-bora uorene.[8] boroles have not been previously thought to be playing a role in tin–boron exchange reactions where large diboracycles are formed,[6] they might represent viable intermediates.

Results

Conclusion