Hydrides of s-block metals are an important class of compound due to their utility in synthetic transformations in versatile roles such as bases, catalysts, or reducing agents, thereby serving as a sustainable substitute to transition metals. It is also well known that their reactivity generally increases on descending the group on account of increasing ionic character. However, access to usable hydrocarbon soluble heavy alkali metal hydrides (AM—H) of enhanced, but controllable reactivity is challenging due to the absence of suitable supporting organic ligands. 2-t-Butyldihydropyridyl (tBuDHP) compounds of lithium, sodium and potassium provide a convenient alternative by mimicking a hydride storage/release vessel for applications in catalytic hydroboration and dehydrogenative cyclisation as shown previously by our group. Encouraged by these initial successes, and aware that alkali metal mediation (AMM) can be dependent on the specific alkali metal employed, we sought to synthesise and isolate the heavier analogues [Rb(tBuDHP) and Cs(tBuDHP)] and investigate their activity in the transfer hydrogenation catalysis of imines with 1,4-cyclohexadiene as the hydride source (left of Figure). To further probe the nature of the tBuDHP anion, an amide carrying a surrogate hydride, we transported the AM-tBuDHP chemistry to the area of heterobimetallic aluminates by co-complexing with di-iso-butylaluminium-2,2,6,6-tetramethylpiperidide [(iBu)2Al(TMP)], a reagent employed previously in ‘ate’ chemistry. From a structural perspective this opened up a new avenue to isolate rarer aluminates of heavier alkali metals (right of Figure). Calculations provided evidence of unprecedented ionic and dispersive stability provided by the rigid TMP anion towards the heavy alkali-metal cations.

Alkali metal dihydropyridines: Hydride surrogates for synthesis and catalysis

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