Radical mechanisms thallium oxidations

Intramolecular electron transfer in a stepwise manner from the amine substrate to die silver(III) center in a 1 2 complex, [Ag(OH)4] -iV,/V-dimcthylanilinc, has been observed.44 The kinetics of oxidation of some aliphatic, heterocyclic, and aromatic aldehydes towards bis(dihydrogentellurato)cuprate(III) and argentate(III) in alkaline medium have been studied.45 A negative salt effect was observed in the oxidation of aminoacetic acid by diperiodatocuprate(III) complex in alkaline medium.46 The oxidation of glutamic acid by thallium(III) perchlorate is catalysed by Ru(M), Os(III), and Nd(III) in a free radical mechanism and the rate is inversely dependent on [H+] concentration.47... 

However, in certain cases under photolytic conditions, spectra of the corresponding arylmercury radical cations 6 developed, whereas no mercuration occurred in dark [81] signifying collapse of the ArH +,Hg(TFA)2 radical ion pair 4, provides an alternative path way to Wheeland complex 2 and hence to ArHg(TFA) + 6. Arene radical cations can also be generated from arene and thallium(III) tris-(trifluoroacetate) in trifluoroacetic acid [82], but with a different mechanism proposed by Eberson et al. [83]. Oxidation of anthracene showed 9-trifluoroacetoxy and 9,10-bis(trifluoroacetoxy)anthracene [84, 85], benzo[a]pyrene, 7-methylbenzo-[a]pyrene and 12-methylbenzo[a]pyrene yielded radical cations of 7- and/or 12-trifluoroacetates [86], triptycene (9,10-dihydro-9,10-[l,2]benzanthracene) showed... 

Thallium Dorfman and Gryder, investigating the kinetics of the Ce(IV)-Tl(I) reaction, concluded that the mechanism involves two 1-equivalent oxidation steps with a Ce(rV) dimer, with T1(II) as an intermediate. They pointed out that the reaction may be complicated by the formation of OH or NOs free radicals that oxidize TI(I). Sinha and Mathur studied the Ce(IV)-Tl(I) reaction in the presence of the catalysts Ag(I) and Os(VIII). The rate was found to be first order with respect to both Ce(IV) and T1(I) but unaffected by Ce(III) or T1(III). Bisulfate and hydrogen ions acted as inhibitors. The slow step in sulfuric acid medium was considered to be... 

Lunarine (26), one of the typical neolignans, is biosynthesized by the ortho-para radical coupling between two molecules of p-hydroxycinnamic acid. In this connection, oxidative coupling reactions of 4-substituted phenols have been extensively stndied using thallium trifluoroacetate (TTFA), potassium ferricyanide (K3[Fe(CN)g]) and other reagents. p-Cresol (27) was also electrolyzed at a controlled potential (+0.25 V vi. SCE) in a basic medium to afford Pummerer s ketone 28 in 74% yield. The snggested mechanism is given in Scheme 4. 

Oxidative-addition reactions of alkyl halides also occur with metal ions such as thallium(I). In this case, the HOMO is an s orbital on the metal of a symmetry. The concerted mechanism is now impossible and only a two-step reaction can occur. The same is true for reactions of metals in the free state, such as zinc or magnesium, with alkyl halides. Note that free radicals may be formed, instead of ions, in all cases discussed. Free radical reactions are usually allowed by symmetry. 

A new acetylene synthesis involves the reaction of 3,4-disubstituted 4-haIo-2-pyrazolin-5-ones with aqueous sodium hydroxide and potassium ferricyanide. The reaction proceeds particularly well for arylacetylenes and is believed to involve the oxidation of an intermediate vinyl radical to a vinyl cation (Scheme 69). A similar reaction mechanism may be involved also in the thallium(iii) nitrate oxidation of pyrazolones prepared from -ketoesters and hydrazine. Full experimental details of this procedure have now been published with yields in the range 68—7-1 % (Scheme 70). 

Treatment of 4-alkyloxycinnamic acids with thallium(iii) trifluoroacetate and trifluoroacetic acid results in the very rapid formation of bis-lactones (70) in yields varying between 12 and 54%. There is good evidence for a mechanism involving a one-electron oxidation of the acid followed by dimerization of the radical cation so produced. 

---