Intramolecular Oxymercuration of 4-Hexen-1-ols: Kinetic vs. Thermodynamic Products Regulated by Mercuric Salts

The rate and mode of intramolecular oxymercuration of 4-hexen-1-ol derivatives was greatly affected by mercuric salts. Mercuric acetate induced a slow reaction to give substituted tetrahydrofurans via 5-exo-trans cyclization. In contrast, 6-endo-trans cyclization proceeded exclusively by mercuric triflate with or without N,N,N’,N’-tetramethylurea. Mercuric salts mediated hydration of alkenes, so-called oxymercuration, was discovered a century ago. The intramolecular reaction of alkenyl alcohol was also investigated since the beginning of the researches and is now considered as one of the most convenient and promising methods to construct cyclic ethers. We have developed mercuric triflate and demonstrated that it is useful not only for olefin cyclization but also oxymercuration. According to the Baldwin’s rule, the exo-mode cyclization is usually preferred over the endo-mode one, and therefore, cyclic ethers with smaller ring size have been produced predominantly. Now we would like to describe that the nature of mercuric salts greatly affected the rate and cyclization mode in the competition between 5-exo-mode and 6-endo-mode oxymercuration of 4-hexen-1-ol derivatives. When the reaction of (E)-1,1-dibenzyl-4-hexen-1-ol (1) with mercuric triflate was carried out, quick reaction took place to give two diastereomers (2) and (3), while the reaction with mercuric acetate afforded 2 as a sole product (Scheme 1). Since oxymercuration usually produces the trans-cyclization product such as 2 and the cis-adduct is usually obtained only when strained alkenes such as norbornene † This paper is dedicated to Professor Sho Ito on the occasion of his 77 birthday (“Ki-Ju”).