Sodium borohydride organomercurials

When an aqueous effluent stream containing organomercurials cannot be recycled, it may be treated with chlorine to convert the organomercury to inorganic mercury. The inorganic compounds thus formed are reduced to metallic mercury with sodium borohydride. The mercury metal is drained from the reactor, and the aqueous solution discarded. The process utilising sodium borohydride is known as the Ventron process (27). 

With mercuric acetate (Hg(OOCCH2)2), olefins and / fZ-butyl hydroperoxide form organomercury-containing peroxides (66,100). The organomercury compound can be treated with bromine or a mild reducing agent, such as sodium borohydride, to remove the mercury. 

Sometimes the reaction conditions used in this reaction are too harsh since heating is involved and rearrangement reactions can occur. A milder method that gives better results is to treat the alkene with mercuric acetate [Hg(OAc)2] then sodium borohydride. The reaction involves electrophilic addition of the mercury reagent to form an intermediate mercuronium ion. This reacts with water to give an organomercury intermediate. Reduction with sodium... 

Sodium borohydride, NaBHorganomercury compounds, prepared by oxymerc-uration of alkenes, to convert the C-Hg bond to C-H (Section 7.4). 

The 2-deoxyglucoside 6 can be prepared by cyclization of (2/ ,3/ ,4/ )-(Z)-1,3,4-tris(ben-zyloxy)-6-cyclohexyloxy-5-hexen-2-ol (5). The selective formation of the a- or /(-anomer depends on the configuration of the double bond. When the (Z)-enol ether is treated with mercury(II) trifluoroacctate in tetrahydrofuran at low temperature, and the resulting organomercury compound is cleaved with sodium borohydride, the 2-deoxyglucoside is obtained in 70 % yield and a 96 4 (a// ) mixture, as determined by HPLC analysis94. 

An indirect hydration reaction may be performed using mercury(TI) acetate (Scheme 2.10b). The mercury salt behaves as the electrophile, forming an organomercury intermediate. The C Hg bond is subsequently cleaved by reduction with sodium borohydride and the alcohol is generated by hydrolysis of the acetate. 

A number of metals salts can be used as the source of electrophiles in reactions with alkenes. One of the most interesting of these involves the attack of mercury(II) acetate in acetic acid. Reductive cleavage of the organomercury compound with sodium borohydride leads to the overall hydration of the alkene in a Markownikoff sense. There are a number of preparative advantages, such as a reduced tendency to rearrange, associated with this and similar relatively mild procedures when compared to the direct protonation of a double bond (Scheme 3.14)... 

The reduction of organomercurials by sodium borohydride has been widely studied and the noncage free radical chain mechanism indicated in Scheme 23 is generally accepted. 

The two-stage process of oxymercuration-demercuration is fast and convenient, takes place under mild conditions, and gives excellent yields—often over 90%. The alkene is added at room temperature to an aqueous solution of mercuric acetate diluted with the solvent tetrahydrofuran. Reaction is generally complete within minutes. The organomercurial compound is not isolated but is simply reduced in situ by sodium borohydride, NaBH4. (The mercury is recovered as a ball of elemental mercury.)... 

Of the other metals or metalloid hydrides that could be conceivably participate in radical processes, by far the most important from the synthetic point of view are organomercury hydrides. Interest in the free radical chemistry of organomercurials blossomed when it was established that the classical demer-curation reaction with sodium borohydride was in fact a radical chain reaction (Scheme 4.1).2... 

Aminomercuration leads to substituted organomercurials 5.5, which also suffer demercuration with sodium borohydride, preferentially under PTC conditions [BEl, EB4] (Figure 5.5). The mechanism proposed for this reduction in protic solvents is an ionic one, implying the intermediate formation of aziridinium salt [L3]. This method has been applied to the synthesis of cyclic amines from a,P-ethyienic precursors 5.6 [EB4] (Figure 5.5). When the reduction in run in alcohol or water, mixtures of five- and six-membered cyclic amines are obtained from each precursor 5.6 (n = 1 or 2). 

Giese and co-workers " developed a special technique for studying the effects of the substituents upon the relative reactivity of vinyl monomers toward free radicals. Briefly it is as follows. Free radicals are produced by reducing organomercury halides with sodium borohydride. The radicals undergo competitive additions to pairs of various substituted olefins. The adducts in turn are trapped by hydrogen transfers from the formed organomercury hydrides. Relative quantities of each product are then determined ... 

Reduction of the organomercury compound by sodium borohydride, NaBH, replaces HgOAc by H. 

Reduction of organomercurials formed from cyclic allenes (10—14-membered rings) with sodium borohydride yields an increasing ratio of trans. cis monosubstituted olefins as the ring size is increased. The stereochemistry and mechanism of the reduction of cyclic allenes using di-imide and sodium in liquid ammonia have been investigated. A stereospecific reduction with sodium in liquid ammonia of the intermediate cyclopropylallene... 

In the laboratory, alkenes are often hydrated by the oxymercuration-demercuration procedure. Oxymercuration involves electrophilic addition of Hg2+ to the alkene on reaction with mercury(II) acetate [(CH3C02)2Hg, often abbreviated Hg(OAc)2] in aqueous tetrahydrofuran (THF) solvent. When the intermediate organomercury compound is then treated with sodium borohydride, NaBH4, demercuration occurs to produce an alcohol. For example ... 

The organomercury compound is reduced with sodium borohydride, and the HgOAc group is replaced by a hydrogen atom. The mechanism is not well established, but is thought to involve free radicals. Thus, the reaction is not necessarily stereospecific. Only the location of the hydroxyl group can be predicted from knowledge of the formation of the mercurinium ion and the ditection of attack of water on that ion. 

---