A New Perspective on Borane Chemistry: The Nucleophilicity of the B−H Bonding Pair Electrons

Abstract

A number of recently discovered nucleophilic boron compounds, such as boryl anions and borylenes, are breaking the rules regarding boron and boron‐containing compounds and their reputation as Lewis acids/electrophiles. In a similar fashion, the B−H bonding pair electrons in boranes also show nucleophilicity which is ascribed to the lower electronegativity of boron relative to that of hydrogen. However, this nucleophilicity of the B−H bond has received far less attention. Explorations of the nucleophilicity of the B−H bonding pair electrons have led to the formation of B−H−B bonded units and B−H⋅⋅⋅H−Y dihydrogen bonds, based on which new chemistry has been uncovered, including the elucidation of the mechanism of formation of aminodiborane (ADB), the diammoniate of diborane (DADB), and lithium or sodium salts of octahydrotriborates (B3H8−), as well as the development of more convenient and straightforward synthetic routes to these reagents. Moreover, the recognition of the nucleophilic properties of the B−H bonding pair electrons will also help to more deeply understand the different mechanisms operating in hydroboration reactions.

Synthesis of Acylboron Compounds

“…[61] Current nucleophiles either do not possess the necessary reactivity or are difficult to synthesize on al arger scale. [28,29] Recent prospects on photoredox chemistry via boron radical species could present another promising approach to be yet explored. [62] Access toward more a-amino KATs could provide more sustainable approaches to peptide synthesis.…”

Synthesis of Acylboron Compounds

“…A direct route from the α‐amino acid would be more desirable and we hypothesize that this can be achieved with further research in nucleophilic anionic boron species or development of radical‐based borylation methods . Current nucleophiles either do not possess the necessary reactivity or are difficult to synthesize on a larger scale . Recent prospects on photoredox chemistry via boron radical species could present another promising approach to be yet explored .…”

“…The most attractive approach to acylborons would be coupling of a nucleophilic boron species to a carboxylic acid derivative. The development of an anionic nucleophilic boron species is challenging as the bases that would be needed to deprotonate borane species preferentially coordinate to the boron rather than abstract the proton of a B−H bond. Current boron nucleophiles, which were employed for acylboron synthesis, use bulky stabilizing ligands to circumvent this issue .…”

B−B Bond Nucleophilicity in a Tetraaryl μ‐Hydridodiborane(4) Anion

“…[10,11] If we consider [FluBÀBFlu(R)] À as containing a [:BFlu] À moiety stabilized via formation of an adduct through its lone pair of electrons with a Lewis acidic 9-R-9-borafluorene molecule, it would be justified to regard the entire electron density between the two boron atoms as the actual nucleophilic entity (cf. the related idea of nucleophilic B À H bonds [12] ). Other species possessing nucleophilic BÀB s bonds include the three-membered ring species B, [13] the doubly guanidinate-bridged diborane(4) C, [14,15] and compound D with a B À B-supporting BHB twoelectron, three-center bond (Figure 1).…”