Esters oxidation potentials

Ortho esters, in synthesis of symmetrical trimethine thiazolocyanines, 54 Oxazolone, for neutrocyanines, 27 Oxidation potentials, of dyes, 75 of mesosubstituted dyes, in relation with absorption, 77 of polymethine dyes, 72 Oxidoreduction, relation between sensitizers, and silver halides, 78 4-Oxo-disubstituted 2-aminoselenazoles, table of products, 262 Oxonols, nomenclature of, 26 in synthesis of dimethine neutrocyanines, 62... 

A literature method for preparation of chromyl acetate by interaction of chromium trioxide and acetic anhydride was modified by omission of cooling and agitation. The warm mixture exploded violently when moved [1], A later publication emphasised the need for cooling, and summarised several such previous occurrences [2], An earlier reference attributes the cause of chromium trioxide-acetic anhydride oxidation mixtures going out of control to presence of nitric acid or nitrates in the chromium trioxide, and a simple test to check this point is given [3], Mixtures used as a reagent for the remote oxidation of carboxylic esters are potentially explosive, and must be made up and used at below 25 °C under controlled conditions [4], An attempt to purify the anhydride by warming with 2% w/v of trioxide led to an explosion at 30°C [5],... 

Further complicating factors in the choice of an enhancer include degradation of HRP by enhancer radicals [23], pH effects [24] on reduction and oxidation potentials for enhancer and acridan ester, inactivation of enhancer radicals because of dimerization or other reactions, etc. All these, and other, effects of the structures (and because of the kinetics also the concentrations) of enhancer and acridan ester may cause erratic results when optimization studies are conducted. When... 

Using esters instead of acids reduces the rate of formation of lactones and gives rise to trapping by solvent as well as the formation of overall diene substitution products. Oxidation of amidomalonic ester 57, for example, yields as major products the acetic acid trapping product 58 and the diene substitution product 59, but only 5% of lactone 60 (equation 26). The oxidation of the initially formed amidomalonic ester radical, of increased importance in this case due to the amide substituent, could be largely reduced through addition of sodium acetate or trifluoroacetic acid, which are known to reduce the oxidation potential of the Mn(III) acetate. 

In the DCA-sensitized reaction of silyl amino esters 46 (equation 16) the formation of pyrrolidines 48 must be obtained through a desilylmethylation. This process can be prevented by attaching an electron-withdrawing group to the amine that obviously reduces its oxidation potential (equation 17)48. 

Again, the exclusive formation of six-membered rings indicates that the cyclization takes place by the electrophilic attack of a cationic center, generated from the enol ester moiety to the olefinic double bond. The eventually conceivable oxidation of the terminal double bond seems to be negligible under the reaction conditions since the halve-wave oxidation potentials E1/2 of enol acetates are + 1.44 to - - 2.09 V vs. SCE in acetonitrile while those of 1-alkenes are + 2.70 to -1- 2.90 V vs. Ag/0.01 N AgC104 in acetonitrile and the cyclization reactions are carried out at anodic potentials of mainly 1.8 to 2.0 V vs. SCE. 

The advent of sulfamate ester oxidative cyclization makes available a myriad of oxathia-zinane derivatives from simple alcohol precursors. These unusual heterocycles have enjoyed only sparing use despite their potential as electrophiHc azetidine equivalents. By contrast, five-membered ring sutfamidates and cycHc sulfates are broadly recog-... 

A similar pattern of reactivity has been observed by Burrows and coworkers for the reaction between A -acetyllysine methyl ester (Lys) and dG. This reaction was studied in order to gain an understanding of structural aspects of DNA-protein cross-links (DPCs). These cross-links are regarded as a common lesion of oxidative damage to cells, but remain, from a chemical point, a poorly understood DNA lesion. As pointed out by Burrows, oxidation of protein-DNA complexes should occur preferentially at the primary amines since these sites have a lower oxidation potential (1.1 V vs. NHE, pH 10) than G. While protonation of the primary amine inhibits the oxidative process, transient deprotonation of a lysine residue would give rise to a lysine aminyl radical (or aminium radical cation). Using... 

Conjugated dienes, styrenes and electron-rich alkenes are cyclopropanated with ethyl diazoacetate using a triarylamminium salt of appropriate oxidation potential as a cata-lyst/initiator (equation 96)185. These reactions are initiated by electron transfer from the unsaturated substrate to the amminium ion and the double additions of the diazo esters to the conjugated dienes are effectively suppressed. Cyclopropanes geminally bearing two... 

The electrochemical oxidation of acyl silanes has been investigated, giving rise to esters and amides when carried out in the presence of alcohols and amines. The oxidation potentials of acyl silanes proved to be much lower than those of the corresponding ketones213. 

Perhaps the most useful type of alkene substrates for these reactions are enol ethers, enol esters and vinyl sulfides. Silyl enol ethers have excellent electron-donor properties, with an ionization potential of about 8 eV and an oxidation potential in various solvents of approximately 1.0-1.5 V vs SCE161. These compounds are easily synthesized by reaction of an enolate with a chlorosilane. (A very recent report synthesized a variety of silyl enol ethers with extremely high stereochemical yield, using the electrogenerated amidate of 2-pyrolidinone as the base.)162 An interesting point is that the use of oxidative or reductive cyclization reactions allows carbonyl functionalities to be ambivalent, either oxidizable or reducible (Scheme 65)163. 

Benzoquinones are not aromatics, but electron-deficient compounds. Photolytic treatment of O-acyl esters (2) with benzoquinones generates the corresponding 1,2-adducts. Practically, 1,2-disubstituted benzoquinones (46) bearing both alkyl and 2-thiopyridyl groups can be obtained. Since the initially formed product is 1,2-disubstituted dihydrobenzoquinone, it is smoothly oxidized to 1,2-disubstituted benzoquinone by excess benzoquinone, due to the difference in their oxidation potential, as shown in eq. 8.20 [63, 64]. 

DMSO, the oxidation potential with salts based on BF4 was higher by only 0.8 V than with solutions based on C104 . Hence, solvents from the alkyl carbonate, nitrile, and ester families may have a 6-8 V wide electrochemical window provided that appropriate salts of sufficiently high anodic stability are chosen as the electrolytes. 

SAFETY PROFILE Moderately toxic by ingestion, inhalation, and intraperitoneal routes. Mutation data reported. Flammable when exposed to heat or flame or by spontaneous chemical reaction. An oxidizer. Potentially explosive. To fight fire, use water, spray, foam, dry chemical. When heated to decomposition it emits toxic fumes of NOx. See also n-BUTYL NITRITE, NITRITES, and ESTERS. 

The carbon-carbon bond formation via photoinduced electron transfer has recently attracted considerable attention from both synthetic and mechanistic viewpoints [240-243]. In order to achieve efficient C-C bond formation via photoinduced electron transfer, the choice of an appropriate electron donor is essential. Most importantly, the donor should be sufficiently strong to attain efficient photoinduced electron transfer. Furthermore, the bond cleavage in the donor radical cation produced in the photoinduced electron transfer should occur rapidly in competition with the fast back electron transfer. Organosilanes that have been frequently used as key reagents for many synthetically important transformations [244-247] have been reported to act as good electron donors in photoinduced electron-transfer reactions [248, 249]. The one-electron oxidation potentials of ketene silyl acetals (e.g., E°o relative to the SCE = 0.90 V for Me2C=C(OMe)OSiMe3) [248] are sufficiently low to render the efficient photoinduced electron transfer to Ceo [22], which, after the addition of ketene silyl acetals, yields the fullerene with an ester functionality (Eq. 15) [250, 251]. 

A third possibility of chemical modification is conversion into an acylsilane which reduces the oxidation potential of the corresponding ketone by approximately 1 V. A peak potential of 1.45 V (relative to Ag/AgCl) for the oxidation of undecanoyltrimethylsilane has been reported. Preparative electrochemical oxidations of acylsilanes proceed in methanol to give the corresponding methyl esters. A two-step oxidation process must be assumed because of the reaction stoichiometry —oxidation of the acylsilane results in the carbonyl radical cation which is meso-lytically cleaved to give the silyl cation and the acyl radical, which is subsequently oxidized to give the acyl cation as ultimate electrophile which reacts with the solvent. A variety of other nucleophiles have been used and a series of carboxylic acid derivatives are available via this pathway (Scheme 49) [198]. 

With 1 as catalyst, alkene bonds which have oxidation potentials less than 1.6 V (vs standard calomel electrode) are considered potentially susceptible to this transformation. With the stronger oxidant 2, the scope of the reaction can be extended to include, for example, tetraalkyl-substituted double bonds, but obviously not disubstituted alkenes such as cyclohexene. On the other hand, electron-rich alkenes such as enol ethers and vinyl sulfides cannot be cyclo-propanated by this method. In order to suppress cyclodimer formation from the alkene and its radical cation, the diazo ester is sometimes applied in a four- to fivefold amount with respect to the alkene. 

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