Revisiting the Synthesis and Nucleophilic Reactivity of an Anionic Copper Superoxide Complex

Abstract

The addition of 1 equiv of KO 2 and Kryptofix222 (Krypt) in CH 3 CN to a solution of LCu(CH 3 CN) [L = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinecarboxamide] in tetrahydrofuran at −80 °C yielded [K(Krypt)][LCuO 2 ], the enhanced stability of which enabled reexamination of its reactivity with 2-phenylpropionaldehyde (2-PPA). Mechanistic and product analysis studies revealed that [K(Krypt)][LCuO 2 ] reacts with wet 2-PPA to form [LCuOH] − , which then, deprotonates 2-PPA to yield the copper(II) enolate complex [LCu(OC=C(Me)Ph)] − . Acetophenone was observed upon workup of this complex or mixtures of KO 2 and 2-PPA alone, in support of an alternative mechanism(s) to the one proposed previously involving an initial nucleophilic attack at the carbonyl group of 2-PPA.Elucidating the structure and function of putative monocopper−oxygen intermediates is imperative for understanding many oxygenase enzymes 1 and abiological oxidation catalysts. 2 Common to all mechanistic schemes for the activation of O 2 in such systems is the initial formation of a 1:1 Cu/O 2 species, typically formulated as a copper(II) superoxide comprising a [CuO 2 ] + core. 3 Significant appreciation of the structures and spectroscopic properties of [CuO 2 ] + cores has been achieved through studies of synthetic complexes. 4 Some features of the reactivity of such complexes also have been examined, but many questions concerning detailed mechanisms, supporting ligand effects, 5 and other aspects remain unanswered because of their thermal instability, challenges associated with isolating the complexes in pure form, and complicated side reactions.

Immobilization of a copper complex based on the tripodal ligand (2‐aminoethyl)bis(2‐pyridylmethyl)amine (uns‐penp)

“…Efforts to obtain the proposed superoxido species by addition of KO 2 (in combination with cryptand[2.2.2]) to an immobilized copper(II) complex, according to a procedure described by Bailey et al. failed [54] …”

“…Efforts to obtain the proposed superoxido species by addition of KO 2 (in combination with cryptand[2.2.2]) to an immobilized copper(II) complex, according to a procedure described by Bailey et al failed. [54] The monolith was treated in the same way as the functionalized silica powder and showed the same reactivity towards dioxygen at low temperatures.…”

Nucleophilic reactivity of a copper(II)-hydroperoxo complex

“…It should be noted that the reactivity of 1 (k 2 = 3.4 M -1 s -1 at -80°C) with CCA is also higher than that of a Cu(II)-superoxo complex, [Cu(βDK)(O 2 )] -(k 2 = 1.4 M -1 s -1 at -80°C), which was a highly reactive oxidant in aldehyde deformylation reported so far (Supplementary Table 8) 41 . Very recently, it has been reported that an anionic copper(II)-superoxo complex acts as a base than a nucleophile in the aldehyde deformylation of wet 2-PPA 42 . Furthermore, nucleophilic reactivity has been observed for an Fe(III) porphyrin peroxo complex in the epoxidation of electron-deficient olefins 43,44 .…”

Copper Catalyzed sp3 C-H α-Acetylation

“…We observed a low yield (28%), however, in the coupling of cyclohexylbenzene (5c) with acetophenone, perhaps due to competing side reactions that involve the cyclohexyl C-H bonds. 27 Nonetheless, addition of excess acetophenone to [Cl 2 NN]Cu-O t Bu results in second order decay of the otherwise stable copper(II) t-butoxide (Fgures S1-S2). GC/MS analysis of the resulting solution reveals the homocoupled diketone product PhC(O)CH 2 CH 2 C(O)Ph in 82% yield.…”