Chloride abstraction from [(R,R)-(iPrDuPhos)Co(μ-Cl)]2 with NaBArF4 (BArF4=B[(3,5-(CF3)2)C6H3]4) in the presence of dienes, such as 1,5-cyclooctadiene (COD) or norbornadiene (NBD), yielded long sought-after cationic bis(phosphine) cobalt complexes, [(R,R)-(iPrDuPhos)Co(η2,η2-diene)][BArF4]. The COD complex proved substitutionally labile undergoing diene substitution with tetrahydrofuran, NBD, or arenes. The resulting 18-electron, cationic cobalt(I) arene complexes, as well as the [(R,R)-(iPrDuPhos)Co(diene)][BArF4] derivatives, proved to be highly active and enantioselective precatalysts for asymmetric alkene hydrogenation. A cobalt–substrate complex, [(R,R)-(iPrDuPhos)Co(MAA)][BArF4] (MAA=methyl 2-acetamidoacrylate) was crystallographically characterized as the opposite diastereomer to that expected for productive hydrogenation demonstrating a Curtin–Hammett kinetic regime similar to rhodium catalysis.
All Science Journal Classification (ASJC) codes
- alkene hydrogenation
- asymmetric catalysis
- catalyst–substrate complex