Metal alkylperoxides are remarkable, highly effective, yet often thermally unstable, oxidants that may react through a number of possible pathways including O-O homolytic cleavage, M-O homolytic cleavage, nucleophilic O-atom transfer, and electrophilic O-atom transfer. Here we describe a series of zinc alkyl compounds of the type To(M)ZnR (To(M) = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; R = Et, n-C(3)H(7), i-C(3)H(7), t-Bu) that react with O(2) at 25 °C to form isolable monomeric alkylperoxides To(M)ZnOOR in quantitative yield. The series of zinc alkylperoxides is crystallographically characterized, and the structures show systematic variations in the Zn-O-O angle and O-O distances. The observed rate law for the reaction of To(M)ZnEt (2) and O(2) is consistent with a radical chain mechanism, where the rate-limiting S(H)2 step involves the interaction of (•)OOR and To(M)ZnR. In contrast, To(M)ZnH and To(M)ZnMe are unchanged even to 120 °C under 100 psi of O(2) and in the presence of active radical chains (e.g., (•)OOEt). This class of zinc alkylperoxides is unusually thermally robust, in that the compounds are unchanged after heating at 120 °C in solution for several days. Yet, these compounds are reactive as oxidants with phosphines. Additionally, an unusual alkylperoxy group transfer to organosilanes affords To(M)ZnH and ROOSiR(3)'.
[Show abstract][Hide abstract] ABSTRACT: Synthesis and reactivity of monomeric magnesium and zinc silyl compounds To(M)M-Si(SiHMe(2))(3) and To(M)M-Si(SiMe(3))(3) are described (To(M) = tris(4,4-dimethyl-2-oxazolinyl)phenylborate). The magnesium compounds react slowly with water and air, while the zinc compounds are inert. With CO(2), To(M)Mg-Si(SiHMe(2))(3) provides To(M)MgO(2)CSi(SiHMe(2))(3) through CO(2) insertion, whereas To(M)Zn-Si(SiHMe(2))(3) affords To(M)ZnOCHO.
Chemical Communications 11/2012; 49(39). DOI:10.1039/c2cc36953b · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The formation of alkylidenezinc carbenoids by 1,4-addition/carbozincation of dialkylzincs or alkyl iodides based on zinc atom radical transfer, in the presence of dimethylzinc with β-(propargyloxy)enoates having pendant iodo- and bromoalkynes, is disclosed. Formation of the carbenoid intermediate is fully stereoselective at -30 °C and arises from a formal anti-selective carbozincation reaction. Upon warming, the zinc carbenoid is stereochemically labile and isomerizes to its more stable form.
[Show abstract][Hide abstract] ABSTRACT: The epoxidation of enones by zinc alkylperoxides is a challenging task receiving considerable attention in contemporary research; however, until now no well-defined zinc alkylperoxide based systems have been described. Here, a new catalytic method of epoxidation of enones in the presence of zinc alkylperoxides supported by N,N-bidentate ligands and tert-butyl hydroperoxide is reported. A new dimeric zinc alkylperoxide complex supported by an aminotroponiminate ligand is also presented. The studied catalytic systems show high activity in the epoxidation of trans-chalcone, and in the case of a chiral catalyst with the (S,S)-N,N′-bis(1-phenylethyl)aminotroponiminate ligand a moderate enantioselectivity was achieved.
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