Cobalt‐Catalyzed Diastereo‐ and Enantioselective Hydroalkylation of Cyclopropenes with Cobalt Homoenolates

Abstract

Catalylic diastereo‐ and enantioselective hydroalkylation of 3,3‐disubstituted cyclopropenes with Co‐homoenolate generated in situ from ring‐opening of easily accessible cyclopropanols promoted by a chiral phosphine–cobalt complex is presented. Such a process represents the unprecedented and direct introduction of a wide range of functionalized alkyl groups without the need of pre‐formation of stoichiometric amounts of organometallic reagents onto the cyclopropane motif, affording multi‐substituted cyclopropanes in up to 99 % yield with >95:5 dr and 98:2 er. Functionalization of the products delivered enantioenriched cyclopropanes that are otherwise difficult to access.

Zinc-Catalyzed β-Allylation of Cyclopropanols via Enolized Homoenolate

“…Et2Zn and L1 would afford ethylzinc aminoalkoxide A, which would exist in equilibrium with alkoxide-bridged dimer (not shown). Coordination of cyclopropanol 1 to A would give the intermediate B, followed by deprotonation of the cyclopropyl OH with the internal aminoalkoxide base to generate the cyclopropoxide species C. Homoenolate D, formed by ring-opening of C, would be deprotonated by another molecule of A to generate the "enolized homoenolate" E. 10 Interception of E with MBH carbonate 2 would afford a-allylated homoenolate F along with ethylzinc tert-butoxide J, which would regenerate A by releasing tBuOH. Reconstruction of the cyclopropane ring from F would be followed by intramolecular transesterification of the cyclopropoxide G to afford 3 and ethylzinc methoxide H, which would release MeOH and regenerate A.…”

Zinc-Mediated Hydroxyallylation of Aldehydes with Cyclopropanols: Direct Access to Vicinal anti-sec,tert-Diols via Enolized Homoenolate

“…The reaction is promoted using the combination of Et2Zn and 2,2'-bipyridine (bpy) at room temperature, engaging a variety of 1-substituted cyclopropanols and bicyclic cyclopropanols as well as aromatic and aliphatic aldehydes. The present reaction features the action of enolized homoenolate, [8][9][10][11] formed through Et2Zn-mediated ring-opening of cyclopropanol and subsequent enolization of the resulting homoenolate, as a stereodefined α-oxyallylzinc nucleophile toward the aldehyde. The complementary antiselectivity of the reaction was ascribed to a bicyclic chairlike transition state of allylation, where the aldehyde substituent prefers to occupy the pseudoaxial position.…”