Oxymercuration of 2,6-Dimethylhepten-5-ol-2

Abstract

Positional isomers are usually not obtained when an alkene is oxymercurated, although several exceptions have been noted (1, 2, 3). However these exceptions do not occur unexpectedly. On the other hand Sand and Singer (4,5) reported two pairs of isomers which were formed by oxymercuration of 2,6-dimethylhepten-5-ol-2. They implied by analogy with the oxymercuration products from -terpineol that these were diastereomers, but this is obviously impossible since only one asymmetric carbon atom would be present in the products from dimethylheptenol.The oxymercuration of 2,6-dimethylhepten-5-ol-2 was carried out by Sand and Singer with mercuric acetate in water, and potassium iodide was added subsequently. The pair of isomers with the composition CgHigChHgl (II, X = iodide) was alkali-soluble and wras represented as 2,6-dimethyl-3-iodomercuriheptanediol-2,6. The other pair, containing one molecule less of the elements of wrater, was designated by the structure 2,2,6,6-tetramethyl-3-iodomercuritetrahydropyrans. These structure designations wrere assumed solely by analogy with the three oxymercurials which Sand and Singer obtained from a-terpineol.Each of the pairs from dimethylheptenol comprised an oil and a crystalline solid. Furthermore the oil could be easily, or wras spontaneously converted to the solid. In general, oxyalkanemercurials are reasonably stable except in acidic media (3, 6); furthermore an alkene of specific configuration has always been

The Kinetics of Alkene Oxymercuration

“…The nonionic mechanism involving coordination of the alkene with alkoxymercuric acetate has been criticized extensively by Chatt (9), but chiefly because of lack of evidence for the existence of the basic salt (II). In the present report we shall present evidence supporting the reality of this species, II. It has been shown recently (10) that good agreement with observed kinetics is obtained by taking into consideration the acetic acid which is formed during the reaction. Applying this consideration to the reaction wherein the alkene (cyclohexene), mercuric acetate, and the alcohol are separate compounds and assuming the establishment of a steady state for the formation and consumption of alkoxymercuric acetate, one may write for the reactions governed by ki, k2, and k3 (Formulation 1) the following expressions: when ki [R'OH] = k. Wlien the molar quantities of cyclohexene and mercuric acetate are initially equal, the concentration of each may be expressed as "a" at the beginning of the reaction and as "ax" after a time period t. The concentration of acetic acid which is added deliberately is denoted by "b", so that "b + x" is the acetic acid concentration at time t. The specific rate constant, k, may then be expressed…”

“…The analytical method has been described previously (10). The ratio k2/k3 has been determined by trial and error to be 2,0 for the methoxymercuration of cyclohexene.…”

Deoxymercuration

“…A definite aliquot is then withdrawn by pipette from the cup. The subsequent colorimetric procedure has been described previously (3,12). Further detail is available in Mr. Berg's M. A. thesis.…”

“…Among these, equations [I], [2], [3], [7], and [B] represent reactions in anhydrous media, while [4], [5], and [6] represent reactions in which water is present. Since the addition of water decreases the rates of deoxymercuration by hydrogen chloride that are described in the report, it is doubtful t h a t equation [6] is significant to these reactions.…”

“…Reaction according t o the equations [7] and [B] is thus plausible. However, the experimental results may also be esplained in terms of equations [I], [2], and [3], assuming that the concentratio~l of methoxonium ion is reduced by addition of water according t o equation [5]. .Actually the paucity of information respecting the true composition of the methanol -hydrogen chloride system makes a choice between the two nlechanisms relatively meaningless except insofar a s the results may be compared with those in a solvent of lower dielectric constant.…”