Remarkably Robust Monomeric Alkylperoxyzinc Compounds from Tris(oxazolinyl)boratozinc Alkyls and O-2

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-C3H7, i-C3H7, t-Bu) that react with O-2 at 25 degrees 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)-O-center dot and To(M)ZnR. In contrast, To(M)ZnH and To(M)ZnMe are unchanged even to 120 degrees C under 100 psi of O-2 and in the presence of active radical chains (e.g., (OOEt)-O-center dot). This class of zinc alkylperoxides is unusually thermally robust, in that the compounds are unchanged after heating at 120 degrees 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 ROOSiR3'.