Hydrides organomercurials

Perlmutter used an oxymercuration/demercuration of a y-hydroxy alkene as the key transformation in an enantioselective synthesis of the C(8 ) epimeric smaller fragment of lb (and many more pamamycin homologs cf. Fig. 1) [36]. Preparation of substrate 164 for the crucial cyclization event commenced with silylation and reduction of hydroxy ester 158 (85-89% ee) [37] to give aldehyde 159, which was converted to alkenal 162 by (Z)-selective olefination with ylide 160 (dr=89 l 1) and another diisobutylaluminum hydride reduction (Scheme 22). An Oppolzer aldol reaction with boron enolate 163 then provided 164 as the major product. Upon successive treatment of 164 with mercury(II) acetate and sodium chloride, organomercurial compound 165 and a second minor diastereomer (dr=6 l) were formed, which could be easily separated. Reductive demercuration, hydrolytic cleavage of the chiral auxiliary, methyl ester formation, and desilylation eventually led to 166, the C(8 ) epimer of the... 

The reductive decomposition of alkylmercury compounds is also a useful source of radicals.205 206 207 The organomercury compounds are available by oxymercuration (Section 4.3) or from an organometallic compound as a result of metal-metal exchange (Section 7.3.3). The mercuric hydride formed by reduction undergoes chain decomposition to generate alkyl radicals. 

However, recent findings of Singh and Khanna on the LAH reduction of organomercury(II) halides suggested an electron transfer mechanism of the SrnI type, involving attack of R- on the metal hydride (Scheme 24).In other respects, LAH has been relatively little used as a reducing reagent for demercuration in the last decade. 

Stereochemical studies have brought an important contribution to the radical mechanism of the boro-hydride reduction of organomercurials. A recent H NMR study of the reduction of norbomyl-type mercurials with NaBD4 confirmed the previous finding of Gray and Jackson about the composition of the product mixture. The major components were clearly nortricyclanol and anti-7-norbomeol. H NMR data provided accurate measures of the preferred directions of deuterium abstraction by the radical intermediate. ... 

In the laboratory, alkenes are often hydrated by the oxymercura-tion procedure. When an alkene is treated with mercurydl) acetate [Hg(02CCH3)2, usually abbreviated HgOAclal in aqueous tetrahydrofuran (THF) solvent, electrophilic addition to the double bond rapidly occurs. The intermediate organomercury compound is then treated with sodium boro-hydride, NaBH4, and an alcohol is produced. For example ... 

Alkene oxymercuration is closely analogous to halohydrin formation. The reaction is initiated by electrophilic addition of (mercuric) ion to the alkene to give an intermediate mercurinium ion, whose structure resembles that of a bromonium ion (Figure 7.5). Nucleophilic attack of water, followed by loss of a proton, then yields a stable organomercury addition product. The final step, reaction of the organomercury compound with sodium boro-hydride, is not fully understood but appears to involve radicals. Note that... 

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... 

Scheme 4.1 Radical chain reduction using organomercury hydride.

As described in Scheme 4.3, the influence of a labile organomercury hydride which gives rise to a carbon radical appears to be a logical explanation. Aliphatic organomercury hydrides are so labile that there is usually no need to add any initiator traces of oxygen (or even some metallic impurities) are sufficient to trigger the radical chain. 

The organomercury radical extrudes mercury to produce a carbon radical which propagates the chain by abstracting a hydrogen atom from another molecule of hydride (path A). If the reaction is conducted in the deliberate presence of oxygen, the carbon radical can be captured to give ultimately an alcohol by reduction of the intermediate hydroperoxide (path B). 

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