Reactions of lead tetra-acetate. Part XI. The oxidation of styrene

Abstract

Styrene reacts with lead tetra-acetate to give a t least four products, PhCH (0Ac)Et. PhCH (OAc).CH,OAc, PhCH,.CH(OAc),, and PhCHO. The relative yields of the first three are strongly dependent on the conditions of the reaction. A systematic study of these variations, together with kinetic evidence, has shown that the first product results from a radical-chain reaction and the third from a heterolytic process; the 1.2-diacetate is probably formed by t w o concurrent reactions of, respectively, homolytic and heterolytic type.OLEFINS react with lead tetra-acetate to give one or more of a variety of products, depending on the structure of the olefin and the condition~.l-~ These include 1,2-diacetates, enolic and allylic acetates, derivatives of glycollic and glyoxylic acid, and 1 ,l-diacetates whose formation is accompanied by skeletal re-arrangement. Despite this complexity, however, the courses of the reactions are, in almost all cases, satisfactorily accounted for by variants of a basic heterolytic rnechanism.lL3

Sonochemical Reactions of Lead Tetracarboxylates with Styrene

“…This was indeed observed,2a and this result constitutes an example of what has become known as sonochemical switching . All the data also agree with a chain mechanism, easily inhibited by oxygen or tert -butylcatechol. 2a,− …”

“…Before a general scheme is proposed, a few important elements should be considered. (i) The existence of a chain mechanism was demonstrated previously by several authors, − using inhibition [with benzoquinone and 4- tert -butyl catechol (4-BC)] and initiation experiments (with diisopropyl peroxydicarbonate). Some of us observed the complete quenching of the formation of 4 when benzoquinone or 4-BC (20% of the stoichiometric amounts) were added to the reaction mixture 2a.…”

“…The presence of styrene is then necessary to induce the homolytic cleavage of the R−COO bonds, probably via the formation of a transitory species containing the lead reagent and the olefinic partner. Indeed, authors have observed previously that the UV-vis spectrum of a styrene-LTA mixture differs from the sum of those of the constituents with the presence of a low-intensity peak at 410 nm 5 …”

“…Notwithstanding this observation, many aspects of these reactions, both conventional and sonochemical, remain unclear. Thus, in the literature there are several references to this type of reaction, some run simply by heating the styrene-LTA mixture, 2a, whereas others report experiments performed in the presence of a base, the role of which is not clearly explained. 2b-d, The present study was undertaken to provide more mechanistic elements, particularly with respect to the sonochemical effect. A specific aim was to determine the experimental conditions, sonochemical or conventional, that might confer useful selectivity to the reaction.…”