Remarkably Robust Monomeric Alkylperoxyzinc Compounds from Tris(oxazolinyl)boratozinc Alkyls and O_2

摘要

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, nudeophilic O- atom transfer, and electrophilic O-atom transfer. Here we describe a series of zinc alkyl compounds of the type To~MZnR (To~M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; R = Et, n- C_3H_7, i-C_3H_7, t-Bu) that react with O_2 at 25 ℃ to form isolable monomeric alkylperoxides To~MZnOOR 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~MZnEt (2) and O_2 is consistent with a radical chain mechanism, where the rate-limiting SH_2 step involves the interaction of OOR and To~MZnR. In contrast, To~MZnH and To~MZnMe are unchanged even to 120 ℃ 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 ℃ in solution for several days. Yet, these compounds are reactive as oxidants with phosphines. Additionally, an unusual alkylperoxy group transfer to organosilanes affords To~MZnH and ROOSiR_3'.