Asymmetric ring-opening (ARO) of strained bicyclic olefins is an important platform for building-up molecular complexity. Merging ARO reactions with transition-metal-catalyzed C–H activation has become an area of recent interest (Chem. Soc. Rev. 2021, 50, 3013-3093). However, the C–H activation-induced ARO reactions of 7-oxabenzonorbornadienes via β-oxygen elimination remained elusive. Mechanistically, it was regarded that the ARO reactions proceeded via the exo-coordination of a carbon–metal species to the olefin moiety, migratory insertion and cis-β-heteroatom elimination. And the migratory insertion was speculated as the enantioselectivity-determining step in the ARO reactions (Fig. 1).
Fig. 1. Common mechanistic manifold of asymmetric ring-opening of bicyclic alkenes
In this work, high yields and excellent stereoselectivity were obtained for a wide range of cis-ring-opening products under the optimal conditions. More interestingly, the utilization of Cp*Co(CO)I2
as the catalyst led to reversed diastereoselectivities favoring the trans ring-opening products (Fig. 2).
Fig. 2. Co-cat. asymmetric ring-opening of 7-oxabenzonorbornadienes
Combined experimental and computational studies suggested the mechanistic pictures of the CpCo-catalyzed ARO reactions where cis- and trans-β-oxygen elimination work as the selectivity-determining step for enantioselective and racemic reactions, respectively (Fig. 3). Such results are different from the previous cognition (Angew. Chem. Int. Ed. 2019, 58, 322-326; Angew. Chem., Int. Ed. 2019, 58, 2514-2518).
Fig. 3. Proposed catalytic cycle and mechanism experiment
As an extension, 7-azabenzonorbornadiene could also be applied in the similar ARO reactions in a diastereodivergent manner (Fig. 4).
Fig. 4. ARO reactions of 7-azabenzonorbornadiene
More details of this work could be found here: “Cobalt(III)-catalyzed asymmetric ring-opening of 7-oxabenzonorbornadienes via indole C–H functionalization” in Nature Communications.