Tetrahedral intermediates. Part 1. The generation and characterisation of some hemiorthoesters

Abstract

Dimethyl hemiorthoformate, the putative intermediate in the methanolysis of methyl formate, has been detected by ¹H n.m.r. spectroscopy as an intermediate in the hydrolysis of dimethoxymethyl acetate in solution in mixtures of [²H₆]acetone (90–86 volumes) and deuterium oxide (10–14 volumes) at –35 °C. Similar experiments with 2-acetoxy-1,3-dioxolan lead to the detection of 2-hydroxy-1,3-dioxolan and with 2-acetoxy- and 2-chloroacetoxy-4,4,5,5-tetramethyl-1,3-dioxolan to 2-hydroxy-4,4,5,5-tetramethyl-1,3-dioxolan. The last compound was also detected by ¹³C n.m.r. spectroscopy. The relative rates of breakdown of dimethyl hemiorthoformate, 2-hydroxy-1,3-dioxolan, and 2-hydroxy-4,4,5,5-tetramethyl-1,3-dioxolan were approximately 1, 0.3, and 0.03. The breakdown of 2-hydroxy-4,4,5,5-tetramethyl-1,3-dioxolan was shown to be acid catalysed with a second-order rate constant 29.4 l mol^–1 s^–1 in [²H₆]acetone–deuterium oxide (9:1 v/v) at –40 °C.