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

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 (PPh₃)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 (PPh₃)₂CoN(SiMe₃)₂ (2) is likely generated within the catalytic cycle and could be independently synthesized from the reaction of (PPh₃)₃CoCl with LiN(SiMe₃)₂. Catalytic reactivity featuring the Co–amide complex, (PPh₃)₂CoN(SiMe₃)₂, showed that it is a competent catalyst, implying that the (PPh₃)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.