A synthetic and mechanistic investigation into the cobalt(i) catalyzed amination of aryl halides

Abstract

The Co( i) catalyzed amination of aryl halides with lithium hexamethyldisilazide is reported.

Abstract

Employing first-row transition metals in catalytic two-electron transformations remains a synthetic challenge. In order to overcome the common and often deleterious single-electron reactivity, an electron rich ligand was targeted on cobalt. Herein, we report the Co( i) catalyzed amination of aryl halides with lithium hexamethyldisilazide. This transformation features (PPh ₃) ₃CoCl ( 1) as the catalyst and affords structurally diverse and electronically varied primary arylamines in good chemical yields, with the scope of the reaction featuring arylamines that cannot be synthesized via traditional metal-catalyzed amination routes, including 4-aminophenylboronic acid pinacol ester. Stoichiometric reactivity revealed that (PPh ₃) ₂CoN(SiMe ₃) ₂ ( 2) is likely generated within the catalytic cycle and could be independently synthesized from the reaction of (PPh ₃) ₃CoCl with LiN(SiMe ₃) ₂. Catalytic reactivity featuring the Co–amide complex, (PPh ₃) ₂CoN(SiMe ₃) ₂, showed that it is a competent catalyst, implying that the (PPh ₃) ₃CoCl may be serving as a pre-catalyst in the reaction. Both stoichiometric and kinetic studies support the catalytic cycle involving a Co( i) complex. Catalytic reactions featuring Co( ii) complexes resulted in undesired biaryl formation, a product that is not observed under standard catalytic conditions and any productive catalytic reactivity likely arises from an in situ reduction of Co( ii) to Co( i). A Hammett study was carried out to differentiate between a closed-shell or radical mechanism, the results of which are consistent with the proposed closed-shell mechanism. Initial studies indicate that this reactivity may be expanded to other bulky nucleophiles.