PSiP-Pincer Type Palladium-Catalyzed Dehydrogenative Borylation of Alkenes and 1,3-Dienes

Dehydrogenative borylation of alkenes and 1,3-dienes was realized by carrying out the reaction in the presence of bis(pinacolato)diboron (B(2)pin(2)) and a catalytic amount of PSiP-pincer palladium complex. This protocol has the following notable features. 1) Monoanionic nature of the PSiP-pincer ligand prevents the formation of boryl(hydrido)- or dihydridopalladium species, enabling synthesis of various vinyl- or dienylboronic esters in good yield from a 1:1 mixture of B(2)pin(2) and alkenes or 1,3-dienes without forming hydroboration or hydrogenation products. 2) Due to the strong trans influence of the silicon atom, PSiP-pincer palladium complex showed high activity toward migratory insertion. 3) Suppression of these side-reactions and the high reactivity of the PSiP-pincer palladium complex enabled an efficient, successive dehydrogenative borylation to give 1,1- or 1,2-diborylated products depending on the kind of substituent on alkenes by using more than 2 equivalents of B(2)pin(2). Mechanistic study revealed that PSiP-pincer borylpalladium complex was generated from hydridopalladium complex and B(2)pin(2), and this complex underwent alkene insertion followed by beta-hydride elimination to give alkenylboronic ester with regeneration of the hydridopalladium complex.