Transformation of Ethylzinc Species to Zinc Acetate Mediated by O₂ Activation: Reactive Oxygen‐Centered Radicals Under Control

Abstract

The mechanisms of O 2 activation by main-group metal alkyl compounds and the character of reactive intermediate oxygen species, along with the origins of reaction outcomes, have been a challenge to understand since the Franklands pioneering studies. [1] For decades, these oxygenation reactions have been commonly considered as difficult to control owing to their radical-chain character. [2] Metal alkyl peroxide complexes have been proposed as intermediates in these reactions, [3,4] and the formation of the most commonly observable alkoxides as final products has traditionally been explained by s bond metathesis involving an alkyl peroxide intermediate and the starting metal alkyl complex. [2, 4a] Our recent systematic investigations have not only advanced a plausible hypothesis concerning the mechanism of O 2 activation by organometallic compounds, [4b,c,k,l, 5] but also convincingly demonstrated a high tendency of incipient zinc peroxide species to form relatively stable adducts with the parent zinc alkyl complex. [4m, 5] Moreover, we have revealed a long overlooked decomposition pathway of zinc alkyl peroxides via homolysis of the O À O bond, which is responsible for the formation of oxo complexes. [4l,m, 5] To date, the formation of alkyl peroxide (path a, Scheme 1), alkoxide (path b), and oxo species (path c) in the oxygenation of ZnÀR species has been well-documented. To our knowledge, homolytic MO À OR bond cleavage to give the corresponding metal carboxylate species (path d, scheme 1) has not been considered as a mode of decomposition of both zinc and other metal alkyl peroxides. [6] Our previous studies demonstrated that N,N-donor pyrroloimines, HL 1 , are versatile supporting ligands that can have diverse bonding modes to metal centers. [4m, 7] Moreover, the controlled oxygenation of [RZn(L 1 )] complexes provides a novel zinc alkyl peroxide or a zinc oxo-encapsulated cluster, the formation of which was mediated by the nature of the zinc-bonded alkyl substituents. [4m] Herein we present a novel extension of the latter investigations, which involves 2,5bis[(2,6-diisopropylphenyl)aldimino]pyrrole (HL 2 ) as a N,N,N-donor supporting ligand, and the oxygenation of the corresponding ethylzinc derivative [EtZn(L 2 )] leading to the zinc acetate species.The reaction of Et 2 Zn with one equivalent of HL 2 in toluene affords the alkyl zinc complex [{EtZn(L 2 )} n ] (1) in quantitative yield. Although we were not able to obtain single crystals of 1, the 1 H NMR data (see below) are fully consistent with the anticipated formula. Molecular weight measurements revealed that 1 occurs predominantly as a dimeric species in a benzene solution. In the next step, a solution of 1 in toluene at À20 8C was treated with an excess of dry dioxygen, the reaction mixture was stirred for 10 minutes, and then the excess O 2 was removed. From this oxygenation reaction, we expected new zinc alkyl peroxide or oxo aggregates. To our surprise, colorless crystals of the zinc carboxylate [{(L 2 )Zn 2 (m-O 2 CMe) 3 } 2 ] (2) deposite...

Probing the Role of π Interactions in the Reactivity of Oxygen Species: A Case of Ethylzinc Aryloxides with Different Dispositions of Aromatic Rings toward the Metal Center

“…These results are also in accord with the oxygenation processes suggested by the NMR spectroscopic investigations and the composition of the isolated crystalline material 7 ⋅2 ArOH. These findings are also in accord with our report on the long‐overlooked decomposition pathway of zinc alkylperoxides through the homolysis of the OO bond,9g–i which is likely responsible for the formation of the EtO . and ZnO .…”

“…The oxygenation of 3 to 7 demonstrates for the first time that the formation of hydroxide species occurs and substantiates the transformation of dioxygen into a peroxo anion in the O 2 activation processes mediated by alkylzinc compounds (Scheme , paths e and f). The formation of alkylperoxide,9c–f,h, 10b alkoxide,9d–f,j, 10 oxo,9g,h,j and even carboxylate species9i in the oxygenation of ZnR units has been well documented (Scheme , paths a–d). Finally, the new reaction outcomes clearly indicate that the outlined approach, which involves the suitable positioning of aromatic functionalities near the metal center, may be useful as a model system for the examination of transformations that involve either O 2 and ROS and mediation by means of organometallic compounds and various bioinorganic oxygenation systems.…”

Unveiling Complexity of the Oxygenation of Aluminum Alkyls by the Isolation of Unique Alkylperoxide and Oxoalkoxide Compounds

“… 11 , 27 , 28 Moreover, considering the chemistry of NR MOOR species, particular mention should be made of the O–O bond scissions. Although regularly overlooked, homolysis of the O–O bond appears to be the critical factor responsible for the formation of a vast array of products including metal oxide, 30 − 33 hydroxide, 27 , 34 , 35 and even carboxylate 29 , 32 clusters ( Scheme 1 , path 3).…”