Sodium borohydride reductive demercuration

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)... 

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. 

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). 

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

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... 

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). 

The aminomercuration-demercuration reaction has provided two examples for primary to secondary amine conversion.In one, Markovnikov addition of the aminomercurial (10) to an alkene, followed by ligand exchange with sodium hydroxide and subsequent reduction with sodium borohydride, yields the secondary amine (11) in a one-pot reaction (Scheme 10). In the other,vicinal diamines (12) are the products from the one-pot reaction of alkenes with tetrafluoroboric acid and mercury(ll) oxide in the presence of excess primary amine (Scheme 11). Both reactions work equally well with secondary amines. 

Y. S. Shabarov, S. S. Mochalov, T. S. Oretskaya, and V. V. Karpova. Mercuration of some styrenes and phenylcyclopropanes. The effect of the aromatic nitro group on reductive demercuration with sodium borohydride. /. Organometallic Ghent., 1978, 150, 7. 

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