Three-Component Reaction to 1,4,2-Diazaborole-Type Heteroarene Systems

Aromaticity transfer in an annulated 1,4,2-diazaborole: facile access to Cs symmetric 1,4,2,5-diazadiborinines

A tricyclic annulated 1,4,2-diazaborole is readily accessed via reaction of a bidentate pyridyl-carbene ligand with MesBBr2 followed by reduction. Dearomatization of the flanking rings is shown to increase reactivity of this heterocycle in the form of a B-centred alkylation with MeI. Its reaction with hydrido-, fluoro-, and chloro-boranes reveal an unprecedented ring expansion reaction to form a diverse family of B2C2N2 heterocycles, reduction of which allows facile access to the first examples of Cs symmetric 1,4,2,5-diazadiborinines. DFT calculations have shed light on the electronic structures of the reduced species and provide insight into mechanistic aspects of the observed ring-expansion.

Reactivity and Stability of a Ring-Expanded N-Heterocyclic Carbene Copper(I) Boryl Imidinate

Frustrated Lewis pairs (FLPs) have evolved from a revolutionary concept to widely applied catalysts. We recently reported the ring-expanded N-heterocyclic carbene supported copper(I) boryliminomethanide, (6-Dipp)CuC(=NtBu)Bpin and noted it reacted with heterocumulenes in a fashion reminiscent of FLPs. We thus set out to explore its reactivity with a range of other substrates known to react with FLPs. This was undertaken by a series of synthetic studies using NMR spectroscopy, mass spectrometry, IR spectroscopy, and single crystal X-ray crystallography. (6-Dipp)CuC(=NtBu)Bpin was investigated for its reactivity towards water, hydrogen, and phenylacetylene. Its solution stability was also explored. Upon heating, (6-Dipp)CuC(=NtBu)Bpin decomposed to (6-Dipp)CuCN, which was characterised by SC-XRD and NMR spectroscopy, and pinBtBu. Although no reaction was observed with hydrogen, (6-Dipp)CuC(=NtBu)Bpin reacted with water to form (6-Dipp)CuC(=N(H)tBu)B(OH)pin, which was structurally characterised. In contrast to its FLP-reminiscent heterolytic cleavage reactivity towards water, (6-Dipp)CuC(=NtBu)Bpin acted as a Brønsted base towards phenyl acetylene generating (6-Dipp)CuCCPh, which was characterised by SC-XRD, IR, and NMR spectroscopy, and HC(=NtBu)Bpin