Abstract
Boranes have long been known as the archetypal Lewis acids owing to an empty p-orbital on the boron centre. Meanwhile, Lewis basic tricoordinate boranes have been developed in recent years. Here we report the synthesis of an annulated 1,4,2,5-diazadiborinine derivative featuring boron atoms that exhibit both Lewis acidic and basic properties. Experimental and computational studies confirmed that two boron atoms in this molecule are spectroscopically equivalent. Nevertheless, this molecule cleaves C–O, B–H, Si–H and P–H bonds heterolytically, and readily undergoes [4+2] cycloaddition reaction with non-activated unsaturated bonds such as C=O, C=C, C≡C and C≡N bonds. The result, thus, indicates that the indistinguishable boron atoms in 1,4,2,5-diazadiborinine act as both nucleophilic and electrophilic centres, demonstrating ambiphilic nature.
Highlights
Boranes have long been known as the archetypal Lewis acids owing to an empty p-orbital on the boron centre
Representative examples include phosphine-borane-containing species, pioneered by Stephan et al.[29,30,31], in which the phosphorus centre acts as a Lewis base, whereas the boron centre acts as a Lewis acid
We have synthesized a 1,3,2,5-diazadiborinine bearing two boron atoms that are spectroscopically inequivalent. We showed that these two boron centres in an aromatic ring cooperatively activate small molecules in which one of them behaves as a Lewis acid centre, whereas the other serves as a Lewis base centre[35,36]
Summary
Boranes have long been known as the archetypal Lewis acids owing to an empty p-orbital on the boron centre. As classical trivalent boranes inherently possess an unoccupied p-orbital on the boron centre, they have been widely utilized as electron-pair acceptors or Lewis acids in synthetic chemistry[1] Apart from this line of research, isolable nucleophilic and low-valent boron species have attracted great attention in recent years since the seminal work by Nozaki, Yamashita and co-workers[2,3,4,5,6,7,8,9,10,11]. We have synthesized a 1,3,2,5-diazadiborinine bearing two boron atoms that are spectroscopically inequivalent We showed that these two boron centres in an aromatic ring cooperatively activate small molecules in which one of them behaves as a Lewis acid centre, whereas the other serves as a Lewis base centre[35,36]. We report the synthesis of 1,4,2,5-diazadiborinine in which two equivalent boron atoms act as both nucleophilic and electrophilic centres; the compound features ambiphilic nature
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