Lead tetraacetate reactions with styrene

Figure 3 - Lead tetraacetate reaction with styrene...

It would appear that a similar mechanism probably applies to the many aziiidinations of alkenes with lead tetraacetate and a wide variety of other A -aminoheterocycles. In any case, such reactions are now thought unlikely to occur via a nitrene in competition eiqieriments, A -aminqphthalimide/lead tetraacetate reacts with styrene in preference to methyl aciylate (1.5 1), whereas with the supposed genuine nitrene (27) (whose nature is also in doubt " ) prepared by pyrolysis (Scheme 32), the ratio is reversed (1 3). Other sources of supposed (27) by pyrolysis include (28) and (29). Very recently, strong... 

There have been examples of sonochemical switching in homogeneous reactions. The decomposition of lead tetraacetate in acetic acid in the presence of styrene at 50 °C generates a small quantity of diacetate via an ionic mechanism. Under otherwise identical conditions sonication of the mixture gives 1-phenylpropyl acetate predominantly through an intermediate methyl radical which adds to the double bond (Scheme 3.8) [55,56]. These results are in accord with the proposition that radical processes are favoured by sonication. 

Lead tetraacetate initiates a similar type of oxidation with terminal alkenes, in the presence of acid, to give an aldehyde hy selective oxidation of the terminal carhon. l Ajj example is the conversion of styrene to phenylacetaldehyde in 98% yield. Palladium chloride (PdCl2) reacts with terminal alkenes, in the presence of oxygen and copper salts, to give a methyl ketone (this reaction is called the Wacker process and is discussed in sec. 12.6.A). It is more useful than the LTA oxidation. Oxidation of terminal alkenes with LTA leads to the aldehyde, whereas oxidation with PdCl2 leads to the methyl ketone. The PdCl2 oxidation is illustrated hy conversion of 402 to 403 in 77% yield, in Ikegami s synthesis of coriolin. ... 

Another example is, probably, the reaction of lead tetraacetate with styrene in acetic acid (p. 77). Kinetic measurements have shown that it occurs in the liquid phase via the formation of the methyl radical. The role of the solvent and the exact sonochemical mechanism are, however, unknown. 

Lead tetraacetate (LTA) reacts with olefins via several mechanisms.23 The first one implies a methyl radical intermediate generated by the decomposition of LTA and results in the formation of a monoacetate after addition of the methyl group. Polar mechanisms give the gem- and pfc-diacetate. Sonochemical enhancement of the first process was predicted by Ando et ah, who indeed found an example of sonochemical switching when the reaction was performed on styrenes (Fig. 3). 24.25... 

The high reactivity of the Pb—O bond raises the question whether oxyplumbation is of importance in the often useful reactions of lead tetraacetate (40), as is commonly assumed. At least, oxyplumbation is postulated for the cleavage of the cyclopropane ring by Pb(OAc)4 212) as in the reaction of styrene with acetylacetone in the presence of Pb(OAc)4 119). 

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