Peroxymercuration

In a process known as peroxymercuration, hydrogen peroxide and alkyl hydroperoxides can be alkylated by alkenes in the presence of a suitable mercury(II) salt... 

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). 

Thus the range of bicyclic peroxides available via peroxymercuration may be quite limited. Nevertheless where the method works best, namely with 1,5- and 1,4-cyclo-octadiene, it makes a valuable contribution in that each peroxymercuration is regiospecific and leads to a dioxabicyclodecane that is isomeric with the [4.2.2] compound 23 available via photooxygenation (Eq. 18). Furthermore, the [3.3.2] compounds derived from 1,5-cyclooctadiene are, to the best of our knowledge, the only bicyclic peroxides obtained to date that do not contain either a 5- or a 6-membered dioxacycloalkane ring. 

To add to the silver salt (Eq. 19) and peroxymercuration (Eq. 34-36) methods of preparing dialkyl peroxides, a third mild alkylation procedure has been developed that involves the use of alkyl trifluoromethane sulphonates (triflates)55). The peroxide transfer reaction between bistributyltin peroxide and the bistriflate 58 of cis-1,3-cyclopentanediol provided one of the first syntheses of 2,3-dioxabicyclo[2.2.1]heptane 9 (Eq. 44, H for D Tf = 02SCF3)56). Because of the sensitivity of 9, it was necessary to carry out the reaction in vacuo with rapid transfer of the volatile products to a cold trap to avoid decomposition a yield of 22% was achieved. 

Use of mild conditions was crucial and the development of diimide reduction of singlet oxygenates, silver-salt-assisted displacement of halide by peroxide nucleophiles, peroxymercuration and demercuration, peroxide transfer from organotin to alkyl triflates, and oxygen trapping of azoalkane-derived diradicals have all played a part in providing the rich harvest of new bicyclic peroxides described herein. 

Not only has the prostanoid [2.2.1] skeleton yielded to the onslaught, but seven other bicyclic peroxide systems have also been obtained. Particularly versatile have been the singlet oxygen-diimide and silver salt routes which have provided higher [n.2.2] systems (n = 2-4), and [n.2.1] systems (n = 3-5) respectively the [3.3.2] skeleton, unique in its lack of both 5- and 6-membered peroxide rings, was afforded via peroxymercuration. 

With mercury(II) nitrate, the five-membered ring peroxide was obtained as an approximately equimolar mixture of isomers, while the 1,2-dioxacyclohexane contained about three times as much trans- as ds-isomer. Peroxymercuration of alkyl-substituted 1,4-penta- and 1,5-hexadienes, followed by demercuration, afforded mixtures of isomeric cyclic alkyl peroxides in yields strongly dependent on the number and position of the substituents175. 

In addition to these extensive studies on electrophile-mediated intramolecular peroxydation of electron-rich C=C bonds, some examples of intramolecular hydroperoxide addition to electron-poor C=C bonds have been described. For example, several racemic analogues 371 of the naturally occurring plakinic acid were readily obtained by peroxymercuration followed by hydridodemercuration of the dienic acids 370 (Scheme 95 f °. Intramolecular Michael addition of hydroperoxide function to the double... 

Cyclopropanes 82 can be converted to the corresponding y-bromoalkyl ferf-butyl peroxides 83 employing a method published in 1982 by Bloodworth and Courtneidge, which uses peroxymercuration and subsequent bromodemercuration (Scheme 36). Perox-ymercuration is also useful to prepare a-iodo-/3-terf-butylperoxyethanes 85 from olefins as shown by Bloodworth and coworkers in 1987 (Scheme 37). The fraw -addition of... 

Hg(OAc)2 and Hg(OOCCFj)2 are the most frequently used reagents. Water and alcohols are the most common nucleophiles, and these processes are termed oxy-mercuration. Other additions are aminomercuration and peroxymercuration. 

The peroxymercuration-demercuration of alkenes provides a more versatile approach to the corresponding hydroperoxides or dialkyl peroxides than direct acid-catalyzed addition (equation 239).372 374... 

A wide variety of alkenes undergo Maikovnikov peroxymercuration. Even a,(3-unsaturated carbonyl compounds afford peroxymercurials whose regiochemistry is dependent on the substitution pattern about the C—C double bond.37 "381 Selective anti addition to the double bond of simple alkenes is the general rule.373 382... 

Alkyl hydroperoxides. Peroxymercuration of alkenes proceeds cleanly and in good yield. These products previously were reduced to the desired alkyl hydroperoxides with alkaline sodium borohydride. This reduction proceeds in reasonable yield in the case of terminal alkenes, but scission to form epoxides of the original alkene predominates among products from nonterminal alkenes. This difficulty is now overcome by use of tri-n-butyltin hydride for reduction.18... 

Epoxides. A new synthesis of epoxides involves peroxymercuration of an ally lie alcohol with this combination followed by reduction of the peroxymercurial with aqueous NaBH4. 

Intramolecular peroxymercuration (Eq. 18) followed by brominolysis afforded the dioxetane (10). Since the allylic hydroperoxides are readily available via ene-reaction of a suitably alkyl-substituted olefin (Eq. 16), this method has synthetic potential, especially if electrophiles other than bromine can be employed to provide dioxetanes with other substituents. 

These reactions are solvomercurations when the participating nucleophile is a part of the solvent. The terms oxymercuration for addition of oxygen nucleophiles, aminomer-curation for addition of amines, peroxymercuration for addition of OjH or OjR, etc., are used. 

It is also advantageous to use Hg(OCOCF3)2 rather than Hg(OAc)2 for peroxymercuration . This is not only to circumvent acyloxymercuration, but to enable the reactions to be carried out with moderate amounts of peroxide. 

The most popular hydroperoxide in peroxymercuration is t-BuOjH, the system used frequently with Hg(OCOCF3)2 in CH2CI2 ... 

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