Lead tetraacetate, 204, Table

In Table III-33 results for the methylation of thiazoles in acetic acid are given (lead tetraacetate is used as radical source), but in this case some discrepancies appear, the acidic medium being too weak, and the heterocyclic base not fully protonated. Thiazole has also been methylated by the DMSO-H2O2 method (201), and the results are in agreement with those described previously. 

Oxidative cyclization of acylhydrazones 110a, derived from aldehydes or ketones, with the use of lead tetraacetate (LTA) has been developed into a useful route to several disubstituted and tetrasubstituted oxadiazole derivatives 122, being a convenient source of relatively stable carbenes, like N(0)C , S(0)C , 0(0)C , or S(S)C <2000J(P1)2161 >. Some representative recent examples of the syntheses are collected in Table 2. 

The equilibrium between 1,1-dimethylethylperoxyl radicals and 1,1-dimethylethyl tetroxide was first evidenced by Bartlett and Guaraldi [157] for peroxyl radicals generated by irradiation of bis( 1,1-dimethylethyl) peroxycarbonate in CH2C12 at 77 K and oxidation of 1,1-dimethylethyl hydroperoxide with lead tetraacetate at 183 K in CH2C12. A series of studies of this equilibrium were performed later using the EPR technique (see Table 2.12). It is seen that the enthalpy of tetroxide decomposition ranges from 29 to 47 kJ mol-1. 

The oxidation of both linear and cyclic ethers to the corresponding acids and lactones by aqueous H202 as catalyzed by TS-1 and TS-2 was reported by Sasidharan et al. (241) (Scheme 17 and Table XXXV). The titanosilicates exhibited significantly better activity (about 55% conversion) and selectivity (98%) than chromium silicates, although vanadium silicates totally failed to catalyze the reaction. Such conversions are usually accomplished using either stoichiometric amounts of chromium trioxide, lead tetraacetate, or ruthenium tetroxide as oxidants (242) or catalytic amounts of Ru04 in the presence of... 

Reaction of 1,2 -dicarboxylic acids has been used for the formation of a number of strained alkenes and also applied to the Diels-Alder addition products from maleic anhydride (Table 9.5). Both cis- and tr s-diacids take part in the process. Aqueous pyridine containing, triethylamine as a strong base, is considered the best solvent and higher yields are obtained at temperatures of around 80 "C [130]. Use of a divided cell avoids a possibility of electrocatalytic hydrogenation of the product at the cathode. The addition of /a/-butylhydroquinone as a radical scavenger prevents polymerization of the product [127], An alternative chemical decarboxylation process is available which uses lead tetraacetate [131] but problems can arise because of reaction between the alkene and lead tetraacetate. 

The yields with the nickel hydroxide electrode and nickel peroxide are comparable (Table 14), which again demonstrates the similarity of the two reagents. Remarkable is however, that at the nickel hydroxide electrode the conversion occurs at much lower temperatures and that the diamine is anodically oxidized in much better yield. With lead tetraacetate as oxidant the yields are lower and side products are found in major amounts... 

Oxidation of certain iV-methoxyphenylazetidinones gave tetracyclic products. The action of CAN on triazole 498 gave 499 and lead tetraacetate reacted with tetrazole 500 to yield 501 C1999T8457, 2003JCM759>. Other routes to polycyclic /3-lactams are listed in Table 5. 

The free radical substitution reactions, other than phenylation, of pyridine and its derivatives have received but scant attention. Alkylation of pyridine itself has been studied briefly, the alkyl radicals being generated either by the thermolysis of diacyl peroxides or of lead tetraacetate in acetic acid, or by electrolysis of the carboxylic acid precursor (for summary, see Norman and Radda369). Most of the available results are summarized in Table XIV. These figures on isomer ratios are not very reliable since the analyses were carried out by... 

For example, the oxyamination of (E)- or (Z)-2-butene using dimethylamine and lead tetraacetate gave different diastereomers as shown by GC and H-NMR analysis. The diastereoselectiv-ity of the reaction was almost 100%, but was lowered by using bromine as the oxidizing agent (Table 3). 

Particular advantage was taken of the solubility of lead tetraacetate, it is soluble even at — 78 °C in the presence of at least six mol equivalents of trifluoroacetic acid. This allows the aziridination reaction to be performed at low temperatures with a stoichiometric amount of the alkene, hence the diastereoselectivity was greatly improved (Table 2). 

Enolic derivatives of other ketones, e.g. enol acetates of saturated C(3> and C 20> ketones, have been converted into a -substituted ketones by some of the reagents listed in Table 19, but the reactions do not warrant special mention. Lead tetraacetate has been employed for the introduction of 16-acetoxy groups by attack on A -enol acetates of androstan-17-ones [2i8,2ig]. The products are i6 9-acetoxy 17-ketones, in contrast to the i6a-configuration of bromination products from the same enol acetates. As i6j8-derivatives are the more stable isomers, it seems likely that the reaction involves initial a-attack followed by epimerisation. The enol acetate of a i4jS-i7 ketone likewise gives the more stable iba-acetoxy-iy-ketone [2ig]. 

In the first of three comprehensive studies of the relative rates of glycol cleavage by lead tetraacetate in acetic acid, Criegee noted that with the six pairs of cis-trans isomers or near isomers listed in Table 1 the cis isomer reacts much faster than the trans. On the assumption that the relationship is general, and that the... 

Thallinm(ni) °, particnlarly as its triflnoroacetate salt , has been successfully used for the synthesis of phenols. This method can be carried out in a single step and is subject to isomer orientation control" . The aromatic compound to be hydroxylated is first thallated with thallium trifluoroacetate (TTFA)" and, by treatment with lead tetraacetate followed by triphenylphosphine and then dilute NaOH, it is converted to the corresponding phenol (equation 57). Table 1 shows some examples of these transformations . ... 

A few typical reactions of l,4-dihydro-l,2,4,5-tetrazines are listed in Scheme 55.1,4-Dihydrotetra-zines (2) are easily oxidized to 1,2,4,5-tetrazines (1) by nitrous acid, nitric acid, oxygen, halogens, iron(III) chloride, hydrogen peroxide, and lead tetraacetate <78HC(33)1077>. In contrast, 1,2,4,5-tetrazines are strong oxidants, as the half wave reduction potentials of Table 4 show. The redox system is quite mobile. 

Note that catechols (1,2-dihyroxybenzenes) are readily oxidized to o-quinones, l5 but the products are often sensitive to the electrophilic or nucleophilic species in the reaction medium. Catechol itself gives 125. Dimerization is as much a problem with catechols as with monophenols (see Table 3.4). The conversion of catechol to 125 used silver carbonate and it is noted that silver salts are the classical oxidation reagent for such transformations. Other reagent have been used to oxidize catechol derivatives, including ceric sulfate, lead tetraacetate, DDQ (2,3-dichloro-5,6-dicyano-l,4-benzoquinone), iodate, and periodate. ... 

C.i. Lead Tetraacetate. The reduction potential of lead compounds was not included in Table 3.2, but lead tetraacetate [Pb(OAc)4, LTA] is a common reagent for oxidizing organic compounds. The reduction potential for LTA in perchloric acid has been reported to be 1.6 V,502 making it one of the more powerful... 

Table 6.7 Oxidation of Alcohois With Lead Tetraacetate ...

Table 6.7 Oxidation of Alcohols With Lead Tetraacetate —cont d...

Di-tert-butyl peroxide does not react (48). Hydroperoxide can be distinguished from other types of peroxides by a positive reaction with lead tetraacetate, which is accompanied by the evolution of oxygen. It is carried out with a solution of the tested substance in glacial acetic acid by addition of a grain of lead tetraacetate. A list of reactions of peroxy compounds is given in Table 11. 

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