Sodium borohydride demercurations

Eq. 34). The resultant p-mercurioalkyl peroxides can often be demercurated with sodium borohydride (Eq. 35), or by brominolysis (Eq. 36) without substantial cleavage of the 0-0 bond. Both peroxymercuration and demercurations occur rapidly under mild conditions 48). 

Analogously, mixtures of A-alkoxycarbonyl- and iV-tosyl-9-azabicyclo 3.3.11- and [4.2.1]nonanes were obtained by reaction of 3 with carbamates or p-toluenesufonamide in the presence of mercury(II) nitrate followed by in situ demercuration with sodium borohydride (equation 168)171,172. 

The technique of methoxymercuration-demercuration was utilized to determine the position of double bonds in the side chains. Since this method is not successful with the free alkaloids (272), the secondary amino groups must be protected as the A -heptafluorobutyramide. These amides are treated with mercuric acetate and methanol followed by reduction with sodium borohydride to yield the methoxylated compounds (273). The mass spectra of these compounds show a fragment ion (274) at m/z 59 indicating terminal double bonds in every case (Scheme 22) 16,25,410,411). 

Silver(I) catalyzed cyclizations of allenic alcohols (202) lead to 2,5-dihydrofurans (203) (79S743), whilst another mild method for the synthesis of tetrahydrofurans is the intramolecular oxymercuration-demercuration process. Geraniol, when treated with mer-cury(II) acetate and subsequently with sodium borohydride, gave a tetrahydrofuran. 

The reaction may be subject to limitations in the alkene structure similar to those of the nitrile process. Besides simple amides, one can also utilize urea and urethanes in this reaction. Demercuration is best effected by using alkaline sodium borohydride in the presence of a primary amine such as Bu"NH2. 

Alkaline sodium borohydride is the preferred demercuration reagent, but sodium-mercury amalgam in D2O is best for stereospecific reduction with retention.268... 

While many different procedures have been reported for the hydroxymercuration—demercuration of alkenes, the most useful procedure uses mercury(II) acetate in aqueous THF, followed by in situ alkaline sodium borohydride reduction (equation 205).312-313 Virtually all substitution patterns about the C—C double bond are accommodated. 

While successful demercuration using alkaline sodium borohydride has been reported, this process can also give rise to epoxides (equation 240).383 Tri-n-butyltin hydride often gives improved results.384... 

The demercuration of (3-alkoxymercurials is usually best effected using alkaline sodium borohydride. Few rearrangements during this free radical reduction process have been observed 429 When the mercury moiety is positioned a to a carbonyl group, alkaline H2S430,431 or 1,3-propanedithiol316 provide alternatives that afford complementary stereochemical results (equation 262). The use of sodium-mercury amalgam is also useful for stereospecific reduction.432 433... 

The demercuration of these cyclic mercurials is fraught with more problems than analogous mercurials formed by intermolecular processes. Alkaline sodium borohydride is once again the most common reducing agent, but elimination to the starting unsaturated alcohol is not unusual. The extent of elimination varies with the mercury ligand, the pH and the solvent used.434 Phase transfer approaches offer advant-... 

While the demercuration of (3-acyloxymercurials has not received much attention, sodium borohydride or alkaline sodium borohydride have been most widely employed for this task. Hydrolysis to the corresponding alcohol or reversion to alkene are significant side reactions, however, and they are sensitive to the solvent used.4991500 Sodium amalgam and tri-n-butyltin hydride501 have also been utilized as reducing agents. 

It appears that all but one502 (eleven-membered ring) of the examples of this reaction in the literature involve the synthesis of monocyclic or tricyclic five-membered ring lactones.503-507 Sodium borohydride,508,509 sodium trimethoxyborohydride510 and sodium amalgam483 have been employed for demercuration of these substrates. 

The acyloxymercuration of alkynes has been reported to produce a wide variety of products. Terminal alkynes afford either dialkynylmercurials516,517 or polymercurated products whose structures have not been well established (equation 290). Internal alkynes usually afford vinylic mercurials in which the mercury(II) salt has added in an anti fashion (equation 291 ).518-520 Only sodium borohydride has been used to demercurate a few of these mercurials.520... 

Oxy-Cope rearrangement. Tertiary 1,5-hexadiene-3-ols undergo oxy-Cope rearrangement at room temperature on treatment with 1 equiv. of mercury(II) trifluoroacetate and subsequent demercuration with sodium borohydride.2 The corresponding secondary alcohols undergo polymerization in the presence of this salt. 

The removal of a mercury species from a molecule. Demercuration of the products of oxymer-curation and alkoxymercuration is usually accomplished using sodium borohydride. (p. 340) (hydroxylation) The addition of two hydroxyl groups, one at each carbon of the double bond formally, an oxidation, (p. 364)... 

Addition of mercury and an alkoxy group to a double bond, usually by a solution of mercuric acetate in an alcohol. Alkoxymercuration is usually followed by sodium borohydride reduction (demercuration) to give an ether, (p. 636)... 

Conversion of synthetic picrotoxinin (1) to picrotin (2) was achieved by Corey and Pearce in four steps and 30% overall yield 124). To prevent intramolecular oxymercuration, the tertiary alcohol of picrotoxinin (1) was protected as trifluor-oacetate 346 prior to addition of mercury trifluoroacetate in a benzene/THF mixture as solvent. Demercuration with sodium borohydride failed, thus the covalent C-Hg bond was cleaved by tributylstannane in ethanol. Mild hydrolysis of the bistrifluor-oacetate 347 afforded picrotin (2) in 30% overall yield (75% corrected for recovered 1) from picrotoxinin (1). 

Phenyl-4-pentenoic acid (3) is converted to the corresponding mercury derivative 7 by treatment with mercury(II) acetate in methanol26. After demercuration with sodium borohydride in sodium hydroxide, dihydro-5-methyl-4-phenyl-2(3//)-furanone (8) is obtained as an 85 15 mixture of trans/cis-isomers in 45 % yield. 

Demercuration of metallacycle 42 with an alkaline solution of sodium borohydride provided a (7-methyl-substituted five-membered heterocycle 43 (Equation 3) <2001JGU1874>. 

The action of mercury(ll) acetate on 165 in a THF solution followed by demercuration with an alkaline solution of sodium borohydride, gave a substituted 3-silapyrrolidine 166 in 58% yield. Analogously, under the same conditions of aminomercuration-demercuration, allyl(phenylaminomethyl)dimethylsilane 167 easily formed C-methyl-substituted five-membered heterocycle 43 (Equation 29). 

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.)... 

The adducts may be demercurated in situ, frequently with sodium borohydride or with a halogen ... 

Treatment of azido(2-azidocyclopropyl)mercury(II) derivatives with sodium borohydride under basic aqueous conditions led to concomitant C —Hg cleavage and C —H bond formation and formation of the corresponding azidocyclopropanes. Reductive demercuration also took place when (cw-2-azidocyclopropyl)bromomercury(II) derivatives were reacted with sodium amalgam and deuterium oxide mercury was replaced stereospecifically with deuterium leading to c -l-azido-2-deuteriocyclopropanes. The yields are generally very low (< 25%). 

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). 

Azasilacyclopentanes are most conveniently prepared by the amidomercuration-demercuration of dimethyl(chloroalkyl)alkenylsilanes of the form (131) <87JOM(326)i59,85ZOB706). Thus, treatment of dimethyl(chloromethyl)vinylsilane (Ola, n = 0) or dimethyl(chloromethyl)allylsilane (131b, n = 1) with aniline in THF in the presence of mercury acetate, followed by reduction with sodium borohydride, afforded the azasilacyclopentanes (Ola), (132b). 

---