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

McLellan, Ross and Kennedy, Alan R. and Mulvey, Robert E. and Orr, Samantha A. and Robertson, Stuart D. (2017) 1-Alkali-metal-2-alkyl-1,2-dihydropyridines : soluble hydride surrogates for catalytic dehydrogenative coupling and hydroboration applications. Chemistry - A European Journal, 23 (66). pp. 16853-16861. ISSN 0947-6539

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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.