Abstract
An enantioselective and non-oxidative methodology was developed to obtain enantioenriched cyclopropyl boronates using a diethanolamine-promoted selective decomplexation of dioxaborolane. The non-oxidative decomplexation of the dioxaborolane ligand from the cyclopropylmethoxide species formed in the dioxaborolane-mediated Simmons–Smith cyclopropanation reaction provided the enantioenriched CIDA-based (CIDA = N -cyclohexyliminodiacetic acid) borocyclopropane in 92% yield and 95.6:4.4 er. A robustness screen has shown diethanolamine to be compatible with esters, carbamates and N-heterocycles, providing a tool to access enantioenriched cyclopropanes carrying not only base-sensitive but oxidizable functional groups as well. Diethanolamine was found to be compatible with the modified zinco-cyclopropanation reaction of allyl alcohol to remove residual dioxaborolane from the corresponding cis -N-heterocycle cyclopropylmethanol, thereby leading to improved yields.
Key words borocyclopropanes - heterocycles - Simmons–Smith reaction - zinco-cyclopropanation - cross-coupling