Hemiortho esters and hydrotrioxides as the primary products in the low-temperature ozonation of cyclic acetals: an experimental and theoretical investigation

Low-temperature ozonation (-78°C) of 1,3-dioxolanes 1a-1f and 1,3-dioxanes 1g and h in acetone-d₆, methyl acetate, and fert-butyl methyl ether produced both the corresponding hemiortho esters (2a-h, ROH) and acetal hydrotrioxides (3a-h, ROOOH) in molar ratios ROH/ROOOH ranging from 0.5 to 23. Both types of intermediates were fully characterized by ¹H, ¹³C, and ¹⁷O NMR spectroscopy. DFT calculations suggest that ozone abstracts a hydride ion from 1 to form an ion pair, R⁺ -OOOH, which subsequently collapses to either the corresponding hemiortho ester (ROH) or the acetal hydrotrioxide (ROOOH). Hemiortho esters decomposed quantitatively into the corresponding hydroxy esters. Experi-mentally obtained activation parameters for the decomposition of 2a (E_a = 13.5 ± 1.0 kcal/mol, log A = 8.3 ± 1.0) are in accord with a highly oriented transition state involving, according to B3LYP calculations (ΔH _a(298) = 13.2 kcal/mol), two molecules of water as a bifunctional catalyst. This mechanism is also supported by the magnitude of the solvent isotope effect for the decomposition of 2e, i.e., kH₂O/kD ₂O = 4.6 ± 1.2. Besides the hydroxy esters and oxygen ( ³O₂/¹O₂), dihydrogen trioxide (HOOOH) was formed in the decomposition of most of the acetal hydrotrioxides (ROOOH) investigated. The activation parameters for the decomposition of the hydrotrioxides 3a-e in various solvents were Ea = 20 ± 2 kcal/mol, log A = 13.5 ± 1.5. Several mechanistic possibilities for the decomposition of ROOOH were tested by experiment and theory. The formation of the hydroxy esters and oxygen could be explained by the intramolecular transfer of the proton to form the hydroxy ester. The assistance of water in the decomposition of ROOOH to form the hydroxy esters, either directly or via hemiortho esters, was also investigated. According to DFT calculations, the formation of a hydroxy ester via hemiortho ester is energetically more favorable (ΔH_a(298) = 14.5 kcal/mol), again due to the catalytic effect of two water molecules. HOOOH generation requires the involvement of water in the decomposition of ROOOH where the direct formation out of ROOOH is energetically preferred. The energy for a reaction between two molecules of water and singlet oxygen (Δ¹O₂) is too high to occur in solution.