Aromatic Methoxylation and Hydroxylation by Organometallic High-Valent Nickel Complexes

Abstract

Herein we report the synthesis and reactivity of several organometallic Ni(III) complexes stabilized by a modified tetradentate pyridinophane ligand containing one phenyl group. A room temperature stable dicationic Ni(III)-disolvento complex was also isolated, and the presence of two available cis coordination sites in this complex offers an opportunity to probe the C-heteroatom bond formation reactivity of high-valent Ni centers. Interestingly, the Ni(III)-dihydroxide and Ni(III)-dimethoxide species can be synthesized, and they undergo aryl methoxylation and hydroxylation that is favored by addition of oxidant, which also limits the β-hydride elimination side reaction. Overall, these results provide strong evidence for the involvement of high-valent organometallic Ni species, possibly both Ni(III) and Ni(IV) species, in oxidatively induced C-heteroatom bond formation reactions.

Switching on elusive organometallic mechanisms with photoredox catalysis

“…In a series of seminal studies, Hillhouse and coworkers demonstrated that Ni(II) alkoxide complexes do not readily undergo reductive elimination at ambient or elevated temperature and that stoichiometric conversion to a less stable Ni(III) system is required for productive C–O bond formation 17 – 19 . Similarly, Sanford and Mirica have recently demonstrated C–heteroatom reductive elimination when stoichiometrically accessing Ni(IV) intermediates 20 , 21 . In fact, computational studies have revealed that Ni(II) alkoxide reductive elimination is endothermic, in contrast to Pd, Pt, or alkyl Ni(II) variants which are exothermic ( Fig.…”

Mechanistic insights into the SN2-type reactivity of aryl-Co(iii) masked-carbenes for C–C bond forming transformations

“… 53 – 56 To achieve this goal, a variety of macrocyclic model substrates can be utilized, as they proved to be key for the synthesis and characterization of several organometallic intermediates. 57 – 59 Following this line, we have recently published the synthesis and characterization of a family of aryl-Co( iii ) complexes through C–H activation. 60 These complexes were competent intermediates during alkyne annulation reactions, as well as diazoacetate couplings.…”

Model Macrocyclic Ligands for Proof‐of‐Concept Mechanistic Studies in Transition‐Metal Catalysis

“…It is worth mentioning that at ertiary amine N3C ligand analoguew as used in 2015 by Mirica to stabilize organometallic high valentn ickel complexes. [31] In fact, this showedthe different mechanisms for the reactivity of aryl-Co III with terminal and internal phenylacetylenes. The latter showed the classical migratory insertionm echanism at aryl-Co III ,w hereast he former undergoes the formation of an aryl-Co III -acetylide.T his is an unprecedented mechanism that challenges previousm igratory insertion proposalsi nt he case of terminal acetylenes.…”

“…The use of this model arene substrate allowed us i) to determine that the C−H activation step occurs at Co III , ii) to obtain the spectroscopic signature of a well‐defined octahedral aryl–Co III and iii) to study its reactivity with phenylacetylenes. It is worth mentioning that a tertiary amine N3C ligand analogue was used in 2015 by Mirica to stabilize organometallic high valent nickel complexes …”