Stereoelectronic effects on acetal cleavage. The separation of the π-donor and σ-acceptor properties of oxygen

Abstract

The spontaneous, general acid and H₃O⁺-catalysed hydrolyses of the bis-acetals (3) are unusual, semi-concerted fragmentations in which the rate of loss of the 4-nitrophenolate leaving group is controlled by the geometry at the remote acetal centre. The hydrolysis of the cis-axial compound (3ca), in which the remote oxygen atom can act as a π-donor, is up to 200 times faster than that of its trans-isomer (3ta), in which it cannot, but acts simply as a σ-acceptor to slow C–OAr cleavage. The magnitude of the stereoelectronic effect on ΔH^‡ is of the order of 7 kcal (30 kJ) mol^–1.