Directed Rh^I-Catalyzed Asymmetric Hydroboration of Prochiral 1-Arylcycloprop-2-Ene-1-Carboxylic Acid Derivatives

A full account on rhodium-catalyzed asymmetric, directed hydroboration of functionalized prochiral cyclopropenes affording enantiomerically enriched cyclopropylboronates is reported. The scope and limitations of two alternate directing groups, ester and carboxamide, are evaluated. It was found that hydroboration of esters appeared to be more sensitive to substitution in the aromatic ring of the substrates. Specifically, ortho-halogens were detrimental for diastereo- and enantioselectivity, possibly because of additional coordination with rhodium. In contrast, more Lewis-basic amide directing groups allowed for stronger chelation to the transition metal, leading to consistently high diastereo- and enantioselectivity in hydroboration across a broader range of substrates.