The oxymercuration of allenes

Abstract

Addition of mercuric acetate to allene and the five possible methyl-substituted allenes was carried out in methanol. The reaction is quite rapid and even quantitative if an equivalent amount of sodium carbonate is added soon after initial mixing of the reagents. Allene yields exclusively 1,3-bis-(acetoxymercuri)-2propanone. Methylallene produces an analogous ketone plus one monoadduct. All other allenes form either one or two monoadducts. The original allenes can be regenerated from the monoadducts by shaking a benzene solution with aqueous hydrochloric acid. The orientation of mercuric acetate addition in at least the cases of methylallene and l,l-dimethylallene differs from that reported for hydrogen halide addition to the same compounds. Methylallene adds hydrogen chloride to the less substituted double bond, putting hydrogen on the terminal carbon atom. Oxymercuration of this compound gives a similar product (although not stopping at the monoadduct stage) and a substantial amount of a second product in which Markovnikov addition has occured across the more highly substituted olefinic bond. In the corresponding reactions of l,l-dimethylallene, the Lewis acid attacks the center carbon of the allenic system in both cases. However, the halide ion in hydrogen halide addition adds to give the thermodynamically more stable primary halide, while the attack of methanol in oxymercuration appears to be kinetically controlled, i.e., a tertiary ether is produced. The significance of these differences must reflect either the dissimilarities of the two reaction intermediates involved, i.e., classical carbonium ion in the case of hydrohalogenation and a bridged mercurinium ion in the case of oxymercuration, or some rather unlikely type of rearrangement before or after the addition of the electrophilic reagent to these allenes. The structural assignments of most of the products were based on detailed NMR analyses. Unambiguous results were obtained when measured Hg199-H1 spin-spin coupling constants were compared with literature values for similar mercury-proton systems. Unexplained mercury satellites which consistently appeared in the NMR spectra of some of these allene adducts prompted an investigation of possible long-range Hg199_Hl spin-spin coupling. The spectra of oxymercuration products from some twenty olefins revealed that coupling through four sigma bonds does exist and can vary in size from 0 to 50 c.p.s., depending on substituents. In addition to non-bonded steric repulsions, intramolecular electronic attractions between mercury and aromatic or halo substituents appear to be important in-determining the magnitude of this coupling.