Liquid-phase 1,4-diacetoxylation of conjugated dienes with tellurium(IV) oxide and alkali metal halides

Abstract

Oxidation of buta-1,3-diene, isoprene, and 2,3-dimethylbuta-1,3-diene with tellurium(IV) oxide and lithium bromide in acetic acid affords an isomeric mixture of the corresponding diacetoxyalkenes (1,2- and 1,4-addition products). The product yield and selectivity for 1,4-isomers are high [up to 61%(based on TeO₂) and 1,4-/l,2-= 90/l0 respectively in the case of buta-1,3-diene] when an excess of LiBr is employed (LiBr/TeO₂= 5–10). The reaction also proceeds in the presence of NaBr, KBr, LiCl, HBr, or I₂, in the place of LiBr, but both the selectivity for 1,4-isomers and the product yield are lower. The reaction hardly occurs using LiF, Lil, NaCl, Br₂ and NH₄Br as a halogen source. The reaction proceeds catalytically with respect to TeO₂ to some extent when a re-oxidant such as H₂O₂ or t-BuOOH is used. In the cases of 2,5-dimethylhexa-2,4-diene, cyclopenta-1,3-diene, cyclohexa-1,3-diene, and cyclo-octa-1,3-diene the results are unsatisfactory in either the product yield or the selectivity for 1,4-isomers. Halogeno- and/or acetoxy-telluriation of a diene followed by acetolysis of the produced C–X (X = halogen) and C–Te bonds are proposed as one of the possible reaction pathways.