Mechanisms of multielectron redox catalysis with low-valent cobalt: defining the roles of coordination number, stereoelectronics and field effects in selectivity for two-electron chemistry

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 (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. The role of the N(SiMe3)2 ligand in effecting two-electron chemistry with Co(I) was further probed by a structural and spectroscopic investigation of a series of (P^P)CoN(SiMe3)2 complexes. X-ray diffraction demonstrates that a distortion from planarity is observed in these complexes, consistent with weak Jahn-Teller distortion. Variable temperature nOe studies identify that a structural distortion qualitatively consistent with the bending observed in the X-ray structures occurs at the temperatures at which catalysis occurs. Initial rate kinetics further suggest that catalyst stability is a key factor in achieving effective catalysis, suggesting that the ligand geometry’s ability to support a pyramidalized structure is a defining feature of (P^P)CoN(SiMe3)2 catalysis. A C–C coupling reaction between organomagnesium halides and alkyl halides using cobalt tris(acetylacetonate) was also developed. This method is capable of forming sterically congested alkylarenes under remarkably mild conditions. The substrate scope of the reaction and functional group tolerance was explored via a screening technique for rapidly assaying reaction conditions. The catalyst is a rare example of a complex capable of neopentylation in doubly ortho-encumbered organomagnesium reagents, forming axially chiral hydrocarbon products. Finally, the synthesis of bi- and tridentate aryl carbene ligands is described. These ligands are strongly donating and the Co(III) complex synthesized is diamagnetic as a result. These ligands are potentially useful for further studies into the two-electron chemistry of first-row transition metals