DIBOR · Unsymmetric diborane(4) compounds for small molecule activation and B-B coupling

Unsymmetric diborane(4) compounds for small molecule activation and B-B coupling

Organoboron complexes are ubiquitous reagents in synthesis due to their utility in functional group transformations including the Nobel Prize winning Suzuki-Miyaura reaction. Many routes to organoboranes use diboron(4) compounds (e.g. (RO)2B-B(OR)2, thus these compounds have received considerable interest, as have diboron(5) derivatives that possess two distinct boron moieties. Most unsymmetrical diboron(5) species are accessed by adding a neutral or anionic Lewis base to a (RO)2B-B(OR)2 precursor, quaternizing one boron center. This leads to the polarization of the B-B σ bond and these mixed sp2/sp3 species can then react as a source of nucleophilic boron that reacts with an array of carbon electrophiles, thus providing a powerful route for forming C-B bonds. Whilst the use of diborane(5) as a source of nucleophilic boron is now well established reports on the development of unsymmetrical diboron(4) and diboron(5) compounds that have significant electrophilicity are much less common. However, the limited reports to date have demonstrated that electrophilic diboron compounds exhibit unusual reactivity towards small molecules and C-H bonds for example, indicating that more need to be synthesised and further exploration of their reactivity performed. Thus our initial objective was to synthesise a family of new unsymmetrical diboron(5) precursors that are readily amenable to further transformations, specifically: (i) to readily access cationic diboron(4) electrophiles thereby enabling us to explore their use in small molecule and sigma-bond activation; (ii) to undergo reductive coupling to form extended chains of homocatenated boron linked by electron precise B-B and B=B bonds. The latter objective is of import as it seeks to develop the fundamental chemistry of electron precise boron compounds, particularly targeting systems containing both B-B single bonds and B=B double bonds. While the chemistry of boron’s near neighbour carbon exhibits a rich diversity of conjugated C-C, C=C and C≡C containing structures, before this work no boron compound containing a conjugated B-B=B-B structure had been reported. Thus this objective will develop our understanding of the fundamental properties of boron analogues of conjugated hydrocarbons and thus enable future developments in the highly topical area of electron precise boron compounds.

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