Borohydrides from Organic Hydrides: Reactions of Hantzsch's Esters with B(C6F5)3

We report herein that the reaction between a series of Hantzsch’s ester analogues 1 a–d with the Lewis acidic species B(C6F5)3 results in facile transfer of hydride to boron. The main products of this reaction are pyridinium borohydride salts 2 a–d, which are obtained in high to moderate yields. The N‐substituted substrates (N‐Me, N‐Ph) reacted in high yield 90–98 % and the connectivity of the products were confirmed by an X‐ray crystallographic analysis of the N‐Me borohydride salt 2 a. Unsubstituted Hanztsch’s ester 1 a reacted less effectively generating only 60 % of the corresponding borohydride salt, with the balance of the material sequestered as the ester‐bound Lewis acid–base adduct 3 a. Formation of the Lewis acid–base adduct could be minimized by increasing the steric bulk about the ester groups as in 1 d. The connectivity of the carbonyl‐bound adduct was confirmed by an X‐ray crystallographic analysis of 3 e the product of the reaction of methyl ketone 1 e with B(C6F5)3. We also explored the generation of these pyridinium salts by employing frustrated Lewis pair methodology. However, the reaction of mixtures of the corresponding pyridine and B(C6F5)3 with hydrogen gas only resulted in formation of trace amounts of the pyridinium borohydride, along with the Lewis acid–base adduct of the starting material and B(C6F5)3. The 1,2‐dihydropyridine adduct was the final product of this reaction. This was ascribed to the low basicity of the pyridine nitrogen and the complicating formation of an ester bound Lewis acid–base adduct. Borohydrides from organic hydrides: Hantzsch's esters have been shown to act as hydride donors for B(C6F5)3, generating a pyridinium borohydride salt (see scheme, R′ = OEt, R = Me). Hydride transfer occurred quantitatively at temperatures as low as −50 °C. Substituents on the carbonyl group and the nitrogen atom proved to be important in terms of the stability of the resulting salt, which decomposed upon standing at room temperature by re‐addition of the borohydride to the pyridinium.