Radical fragmentations, hydrides

Fig. 1.14. NaBH4, reduction of (/Thydroxyalkyl)mercury(II) acetates to alcohols and radical fragmentation of (/f hydroxy-alkyl)mercury (II) hydrides. According to the terminology used in Figure 1.2 it is a "substitution by fragmentation."...

In the first experiments in which chemistry of metal clusters was demonstrated, the reactant was present in the carrier gas. The problem with this approach is that the reactant is also decomposed during the high-temperature vaporization process, and thus reactive radical fragments are present which may participate in the cluster growth process. Although not exploited, this may be a good method to synthesize metal cluster carbides, oxides, nitrides, sulfides, and hydrides, to name a few. It is not a viable mode of operation for the measurement of reactivity toward molecular species, but has been used to examine the stoichiometry of metal cluster compounds. ... 

Since the discovery of Binkley and Koholic, nitrate esters have been used as sources of alkoxyl radicals by treatment with either tributyltin hydride/AIBN or photolysis [16], These authors described the conversion of the nitrate ester derivative of d-allose into the inverted aleohol by a radical -fragmentation-recyclization reaction as shown (Eq. 27, Scheme 8) [16e,f[. 

The intermediacy of ion-neutral complexes is neither restricted to even-electron fragmentations nor to complexes that consist of a neutral molecule and an ion. hi addition, radical-ion complexes and radical ion-neutral complexes occur that may dissociate to yield the respective fragments or can even reversibly interconvert by hydride, proton or hydrogen radical shifts. Many examples are known from aliphatic alcohols, [180-183] alkylphenylethers, [184-187] and thioethers. [188]... 

The compounds obtained by the replacement of ring sulfur by carbon, as in the case of penicillins, show somewhat improved antibiotic properties. A free radical— based route has been described for the conversion of fermentation derived cephalosporins to their carbocyclic derivatives. The first step in this sequence consists of the condensation of the cephalosporin sulfone (36-1) with formaldehyde and dimethyla-mine the initial product from the Mannich-like reaction consists of the exomethylene derivative at the position adjacent to the activating sulfone. The product is treated in situ with phenylselenol to give the Michael adduct (36-2). This fragments with an extmsion of sulfur dioxide when heated with the free radical initiator AIBN in the presence of tributyltin hydride the reaction can be envisaged as leading to the... 

Because no tin hydride is present, intermediate radicals are only slowly intercepted by hydrogen atom abstraction. Thus, the fragmentation method is a clever alternative that avoids this course, and low concentrations are not required. As a result of this, even relatively unreactive precursors, such as glycosyl chlorides and phenylsulfides, can be used. Therefore, the method is compatible with the same molecular complexity and an extended spectrum of functionality as found in the tin hydride method. 

Conducting radical cyclizations by fragmentation reactions offers a powerful alternative to the tin hydride method. Instead of. obtaining reduced products, one obtains products of substitution an alkene is regenerated in the fragmentation step. 

In cases where the fragmentation reactions are not prohibitively rapid, it may be possible to conduct either cyclization or fragmentation by selection of appropriate reagents and conditions. Scheme 47 provides an example where the same two intermediate radicals are involved.200-201 The cyclized product is formed in the presence of tin hydride, but the fragmented product is produced under the atom transfer conditions. 

A Iky I-1.7>-hutadienes.311 A radical reaction of tri-n-butyltin hydride with a-(llydmxymclhyl)allyl lolyl sullbncs (I) results in stereoselective formation of allyltin intermediates, which fragment to 2-substilutcd 1. .Vbutadicnes when heated. 

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