Side-chain electrophilic aromatic substitution in 2-alkyloxa- and 2-alkylthiazolines initiated via σ-complex formation with super-electrophilic DNBF: a model proinsecticide

Abstract

A ¹H and ¹³C NMR study of the reaction of a series of 2-alkyloxa- and 2-alkylthiazolines 3-Xa–c with 4,6-dinitrobenzofuroxan (DNBF) in DMSO revealed the formation of the C-bonded σ-adducts 5-Xa–c. Product isolation following addition of Et₃N or KOAc, afforded the Et₃NH⁺ or K⁺ salts of the DNBF adducts, respectively. The NMR results showed conclusively that coupling of the DNBF moiety with the oxazoline or thiazoline heterocycle occurred at the α position of the 2-alkyl side-chain. A most reasonable mechanism for this coupling involves initial formation of a transient N-bonded adduct between the N-heterocycle and DNBF, which subsequently rearranges through the action of base to the C-bonded adduct as the product of thermodynamic control. The relevance of this work to oxa- and thiazoline-based proinsecticide is emphasized.