Boronic ester functionalised 1,8-diboryl-naphthalene scaffolds: fluoride versus oxide chelation

Abstract

1,8-Bis(boronic ester) derivatives of naphthalene, 1,8-C₁₀H₆{B(OR)₂}₂, present an attractive target as receptors for the fluoride ion via B–F–B chelation, but are synthetically challenging to access due to the competing formation of a very stable anhydride containing a B–O–B motif. By contrast, unsymmetrical systems of the type 1,8-C₁₀H₆{B(OR)₂}(BR′₂) can be synthesized for (OR)₂ = 1,2-O₂C₆H₄ (i.e. Cat) and R′ = Mes. This system is shown to be competent for the uptake of F⁻, making use of a chelating mode of action and the formation of a bridging B–F–B motif between the two boron centres. However, both experimental and quantum chemical studies indicate that the μ₂-F adduct is the kinetic product of fluoride uptake, with an alternative structural motif featuring a terminal B–F bond and a B–O–B bridge using one of the catechol oxygens being (marginally) more favourable thermodynamically.