1-Alkali-metal-2-alkyl-1,2-dihydropyridines: soluble hydride surrogates for catalytic dehydrogenative coupling and hydroboration applications

Ross McLellan, Alan R Kennedy, Robert E. Mulvey, Samantha A Orr, Stuart D Robertson

Index: 10.1002/chem.201703609

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Abstract

Equipped with excellent hydrocarbon solubility, the lithium hydride surrogate 1-lithium-2-t-butyl-1,2-dihydropyridine (1tLi) functions as a precatalyst to convert Me₂NH·BH₃ to [NMe₂BH₂]₂ (89% conversion) under competitive conditions (2.5 mol%, 60h, 80°C, toluene solvent) to that of previously reported LiN(SiMe₃)₂. Sodium and potassium dihydropyridine congeners produce similar high yields of [NMe₂BH₂]₂ but require longer times. Switching the solvent to pyridine induces a remarkable change in the dehydrocoupling product ratio, with (NMe₂)₂BH favoured over [NMe₂BH₂]₂ (e.g., 94%:2% for 1tLi). Demonstrating its versatility, precatalyst 1tLi was also successful in promoting hydroboration reactions between pinacolborane and a selection of aldehydes and ketones. Most reactions gave near quantitative conversion to the hydroborated products in 15 minutes, though sterically demanding carbonyl substrates require longer times. The mechanisms of these rare examples of group 1 metal catalysed processes are discussed.