Deoxymercuration

One of the pieces of evidence in favor of this mechanism is the fact that the for deoxymercuration of h an5-2-meflioxycyclohexylmercuric iodide is about 8 kcal/mol less than for the cis isomer. Only the trans isomer can undergo elimination by an anti process... 

The reductive demercuration was marred by the loss of about half of the peroxide due to competing deoxymercuration which afforded 4-cycloocten-l-ol. An additional complication was the formation of a small amount of trans-1,2-epoxy-cw-cyclooct-5-ene. The bicyclic peroxide 50 was readily separated from the unsaturated alcohol by silica chromatography, but complete removal of the epoxide was more difficult. Preservation of the peroxide linkage was markedly higher in the bromodemercuration. The diastereoisomeric dibromoperoxides 51 were separated by HPLC, although only one isomer was fully characterised. 

The other point concerns the persistent controversy about the detailed mechanisms of general reactions of this type—i.e., the deoxymercuration reactions, not only of methoxycarbonyl compounds but also of the corresponding olefin adducts. These are clearly concerted processes that are aided in some way by coordination of X- on the mercury and coordination of H+ on the oxygen. There has been controversy, in which Wright and others have participated, as to whether the detailed mechanism involves a cyclic intermediate, in other words whether undissociated HX participates in the reaction or whether the assistance at the metal and oxygen centers are independent processes. Have you any comments on this ... 

Dr. Dessy First, I think Maurice Kreevoy feels that the deoxymercuration reaction... 

Hydrolysis of the dithiane groups with mercury(II) chloride/cadmium carbonate followed by deoxymercuration of the alkene with potassium iodide yielded the diketone... 

Price and Hammett s rule has found confirmation in the reaction of benzaldehyde with acetone and ethyl methyl ketone (Gettler and Hammett, 1943), in the acid-catalyzed hydration of olefins (Taft, 1956a), in the hydrolysis of esters catalyzed by ion-exchange resins (Samelson and Hammett, 1956), in acid-catalyzed deoxymercuration (Kreevoy et al., 1962), and in the esterification of carboxylic acids in methanol (Smith, 1939). Taft (1956b) has noted that the rule seems to require the following modifications. The entropy-bearing substituent... 

The products obtained from the oxymercuration-demercuration of alkenylacetates under standard conditions (NaBH4/NaOH 3 M) are diols. However, the yields are significantly lower than with methoxy-and hydroxy-alkenes because of competitive deoxymercuration.Increasing the amount of base results in major increases in the yield of hydrated products. A less basic procedure has been developed, which allows for the survival of the acetate group. 

Phase transfer reagents sometimes avoid deoxymercuration during the reduction step, as with the products (18) to (20) from hydroxymercuration, amidomercuration and from spiroacetals. ... 

In the reaction between peroxymercurials and sodium borohydride, epoxide formation and deoxymercuration compete with hydrogenodemercuration (Scheme 30). Except for the synthesis of t-butyl... 

Apparently this procedure gives higher yields of carbonyl products than obtained with Pd(II) and the olefin alone. This procedure overcomes the low solubility of the olefins in aqueous solution. There is the possibility that the reaction proceeds via deoxymercuration, followed by direct reaction of Pd(II) with the olefin. However this seems unlikely, and the transmetallation reaction for oxymercurials have been demonstrated previously and the yields using mercurials are much higher than in the case of direct reaction with olefins. 

Evidence that deoxymercuration does not occur to give free olefin, followed by oxidation by Pd(II) to give product, is provided by the fact that undec-l-ene was not oxidized by PdCl2 under the reaction conditions in 2 hours, but a 63% yield was obtained in 30 minutes when the mercurial was employed. A very significant deuterium-labeling experiment was also carried out. 2-d -3,3-Dimethylbut-l-ene was prepared. 

Similarly to the mechanism of the Claisen rearrangement, the Overman rearrangement is a suprafacial, concerted, nonsynchronous / 3,37-sigmatropic rearrangement. The reaction is irreversible, which is the result of the significant driving force associated with the formation of the amide functionality. The mechanism of the metal catalyzed reaction is believed to proceed via an iminomercuration-deoxymercuration sequence and it is only formally a [3,3]-sigmatropic shift. 

Solvolytic deoxymercuration catalysed by iodide has been reported and the reaction shows both a first- and second-order component in iodide - It seems unlikely that other halides will be as useful as iodide in dehalogenation as they are oxidised less easily. Rearrangement rather than gamma elimination is observed when 1,3-dihalosubstrates are treated with iodide ion . ... 

Nucleophilic catalysis by halide ion would be most reasonable, and such possibilities have been realized in studies H8g) on deoxymercuration, and suggested by the late Professor Hughes for apparent S l processes involving scission of the c—hg bond in some -COCH2HgX compounds 107). 

The problem of the oxymercuration reaction, and the deoxymercuration process, alluded to above, has only begun to resolve itself in the last few years. The problem has been fundamentally one of assigning stereochemistry and mechanistic pattern to the processes... 

Kreevoy studied in depth the deoxymercuration of CH3CH(OR)CH2HgI with perchloric and acetic acids in methanol, and found that the process was pseudo-first order in mercurial, with ka. [HA]. Specific hydronium ion catalysis was involved, and solvent HOH/DOD isotope effects were those predicted by the Butler equations for a pre-rate-determining proton transfer. Further studies on what had been termed - and /9-2-methoxy-cyclohexylmercuric iodides under similar conditions, led to similar findings concerning the solvent isotope effects, and correlation of log k with — Hq. This suggested that the transition state differs from substrate only by a proton. 

This was substantiated by later work on secondary deuterium isotope effects in deoxymercuration. Coplanarity of the two carbons, and the RO and HgX functions is implied. However, thermodynamic parameters for... 

Deoxymercuration, and hence oxymercuration, still presents many pitfalls to broad generalizations. Kreevoy and Turner 118g) have shown that by judicious choice of substrate deoxymercuration can be solvolytic in nature, or involve acid catalysis with conucleophilic catalysis (or assistance)... 

The similarities to the deoxymercuration processes reported earlier are obvious. Related is the rearrangement reported by Brook (246). 

Lambert, J. B., Emblidge, R. W. (1993). Nucleophilic catalysis in deoxymercuration the beta effect of mercury. Journal of Physical Organic Chemistry, 6,555—560. 

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