Xanthates intermolecular reaction

Finally, studies of the intermolecular reactions between xanthates and alkenes have recently been published [510]. First, a new soluble polymer analogous to the Wang resin was prepared using the appropriate xanthate (678), which was reacted with 20 equiv. of styrene in the presence of lauroyl peroxide in toluene at 90 °C. 

Other silanes have been used as mediators in the intermolecular C—C bond formation. They can be used alone, as in the following example of the disilane (Ph2SiH)2 shown in Reaction (7.6), for the reductive addition of a bromide or a xanthate to phenyl vinyl sulfone [11,12]. 

The initiation step (i) producing the tributyltin radical can be performed by thermolysis of radical initiators such as AIBN. Complementarily, the use of a Et3B/02 system or of photochemical irradiation allows the reactions to proceed at low temperature. The reaction (ii) with the substrate forming the carbon radical applies the same substrates as those used in tin hydride chemistry, such as iodides, bromides, selenides xanthates and even thiocarbonates. Furthermore, the possible competitive addition of tributyltin radicals to the allyltin reagent is of no consequence due to the rapid /3-fragmentation of the resulting carbon-centred radical, which renders this reaction a degenerate one. It is noteworthy that less reactive radical precursors such as chlorides or phenyl thioethers can be used efficiently. The evolution of the radical (iii) via several intra- or intermolecular elementary... 

In a related study, Lusinchi [11] reported the synthesis of a new soluble polymer containing a p-alkoxybenzyl alcohol linker, analogous to Wang resin. The polymer was utilized in intermolecular radical addition reactions. The polymer was synthesized by subjecting xanthate 65, formed in three steps from alcohol 63, to styrene polymerization in the presence of lauroyl peroxide... 

The use of soluble polymers in intermolecular radical additions with xanthates gave far better results than the analogous reactions using Wang resin [llj. hi addition, the radical reactions could be monitored using... 

The most important synthetic asset of the xanthate transfer methodology lies in its ability to induce carbon-carbon bond formation by intermolecular addition to unactivated olefins. Again, this is possible because the initial radical has a comparatively long lifetime in the medium. Unhindered, terminal olefins are the best substrates, but other types of olefins (especially strained or lacking allylic hydrogens) may be made to react in some cases. Three examples of additions are collected in Scheme 18. The first involves formation and capture of a trifluoroacetonyl radical, a species hitherto only studied by mass spectrometry but never employed in synthesis [34a]. This reaction represents a convenient route to various, otherwise inaccessible, trifluoromethyl ketones. In the second example a tetrazolylmethyl radical, also a previously unused intermediate, is intercepted by a latent allyl glycine [34b]. The amino acid moiety may be part of the xanthate partner as highlighted by the last example [34c]. 

The intermolecular addition of carbon-centered radicals to unactivated alkenes followed by azidation (a formal carboazidation of alkenes) has been reported. A one-pot procedure similar to the one used for intramolecular reactions gives good results (Scheme 8.28). Slow addition of benzenesulfonyl azide is not necessary because this electrophilic reagent does not react with the initial electrophilic or ambiphilic radicals. Excellent results are obtained with a-iodo and -xanthate esters. a-Bromoacetates give also satisfactory results. The carboazidation process allows to prepare pyrroUdinone derivatives in a straightforward manner (Scheme 8.28, bottom example). A tin-free version of this reaction using triethylborane instead of hexabutylditin has also been reported. ... 

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