Process for producing isoprene

A process for producing isoprene, which comprises decomposing a mixture of at least two compounds represented by the general formula ##STR4## wherein (i) W and Y each represent a hydrogen atom, and X and Z are identical or different and each represent the group OR, (ii) W and X together form a single bond, Y represents a hydrogen atom and Z represents the group OR, (iii) W represents a hydrogen atom, X and Y together form a single bond, and Z represents the group OR, or (iv) W represents a hydrogen atom, X represents the group OR, and Y and Z together form a single bond, in which R represents a hydrogen atom, a methyl group, a methoxymethyl group, a methylbutenyl group, a methylbutenyloxymethyl group, a 1,1-dimethyl-3-hydroxypropyl group, a 1,1-dimethyl-3-hydroxypropyloxymethyl group, a 3-methyl-3-hydroxybutyl group, a 3-methyl-3-hydroxybutyloxymethyl group, a 1,1-dimethyl-3-methoxypropyl group, a 1,1-dimethyl-3-methoxypropyloxymethyl group, a 3-methyl-3-methoxybutyl group or a 3-methyl-3-methoxybutyloxymethyl group,

• and which contains both at least one formal linkage-free compound of general formula (I) in which R is a hydrogen atom, a methyl group, a methylbutenyl group, a 1,1-dimethyl-3-hydroxypropyl group, a 3-methyl-3-hydroxybutyl group, a 1,1-dimethyl-3-methoxypropyl group or a 3-methyl-3-methoxybutyl group and at least one formal linkage-containing compound of the general formula (I) in which R is a methoxymethyl group, a methylbutenyloxymethyl group, a 1,1-dimethyl-3-hydroxypropyloxymethyl group, a 3-methyl-3-hydroxybutyloxymethyl group, a 1,1-dimethyl-3-methoxypropyloxymethyl group or a 3-methyl-3-methoxybutyloxymethyl group, in the presence of water and an oxygen-containing boron compound selected from the group consisting of boron-oxyacids and boron compounds capable of generating the boron-oxyacids in situ, in the liquid phase at a temperature of at least 150.degree. C. while adjusting the ratio of the oxygen-containing boron compound to the entire water present in the reaction system such that the weight ratio of orthoboric acid to water, calculated on the assumption that all the oxygen-containing boron compound changes in aqueous solution to orthoboric acid, is at least maintained at 15:85.