Structural studies on dioxan-2-ylium ions; intramolecular attack of an oxygen atom on a carbenium ion centre
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abstract
As models for the ionization of orthoesters, a series of 1,3-dioxan-2-ylium salts have been prepared and characterized. The properties of 1,3-dioxan-2-ylium hexachloroantimonate 1, 5-ethoxymethyl-1,3-dioxan-2-ylium hexachloroantimonate 2, 2-methyl-5-ethoxymethyl-1,3-dioxan-2-ylium hexachloroantimonate 3, and 2-phenyl-5-ethoxymethyl-1,3-dioxan-2-ylium hexachloroantimonate 4, have been studied by 1H and 13C NMR spectroscopy. The structures of 3 and 4 have been determined using X-ray crystallography. Salts 3 and 4 both crystallize in monoclinic crystal systems. For 3, the space group is C2/c, with a = 30.192(4) Å, b = 9.613(2) Å, c = 12.742(2) Å, β = 109.71(1)°, V = 3482(1) Å3, and Z = 8. Complex 4 crystallizes in the space group P21/n, with a = 10.638(2) Å, b = 11.849(2) Å, c = 16.970(3) Å, β = 106.84(1)°, V = 2047(1) Å3, and Z = 4. The geometries of 3 and 4 are similar with the ether oxygen disposed over the dioxan-2-ylium ring. However, the interaction between the ether oxygen and the cationic centre is not significant due to the large internuclear distance observed. The cation 5, in which the carbon chain length between the ether oxygen and C(5) was increased, was prepared. This ion was found to undergo a reversible, intramolecular rearrangement resulting in the formation of 1-methyl-4-acetoxymethyl-tetrahydrofuranyl oxonium hexachloroantimonate 11.
