Esters reduction potentials

Electrolysis at a Hg cathode, -0.5 to -0.6 V, LiC104, CH3CN, Pyr. The trichloroethyl ester, which requires a greater reduction potential for cleavage, is retained under these conditions." ... 

Chung [34] concluded that the semiconducting properties of a metal species influence discoloration. In contrast to metals belonging to the insulator group, metals belonging to the semiconductor group promote yellowing, perhaps due to catalysis of the polymerization of vinyl esters. The formation of chromophores is enhanced when the metal has a variable valency with a reduction potential near to zero. 

Because of the highly negative reduction potentials ( —3.0 V vs. SCE) [32], the electroreduction of esters of aliphatic carboxylic acids to primary alcohols by direct electron transfer from the cathode is very difficult and the electrochemical Birch-type reduction of aliphatic esters in MeNH2 or liquid NH3 has not been reported until recently (Scheme 15) [33, 34]. This reaction is not a reduction by direct electron transfer from the cathode to the C=0 bonds of the ester but the reduction by a solvated electron. 

Esters of ethenetetracarboxylic acid, (37), and disubstituted (fluoren-9-ylidene)me-thane derivatives, (38), are reduced sequentially to radical anions and dianions [2, 68, 84, 85]. Only the dianions are sufficiently basic to be useful as EGBs [53,86]. For (37), the two reduction potentials are separated by 0.2 V [68], and even with a working potential allowing formation only of the radical anion, the dianion can be formed by disproportionation. The protonated form of the dianionic EGBs, (37H) and (38H) , will normally be stable in solution since the pK values of the dihydro products are expected to be in the range 12 to 16. 

Fig. 14. pH dependence of the heme reduction potential vs NHE in the heme protein maquette, [H10A24J2, demonstrating the 210 mV range of observed between pH 4 and 11. Data for Fe(protoporph3rrin IX, filled squares) and Fe(protoporph3rrin IX dimethyl ester, open circles) are shown. Reprinted with permission from Ref. (160) copyright 1998 American Chemical Society. 

Because 3-substituted 1,2-amino alcohols and even P-alkyl-y-hydroxy-5-amino esters are potentially precursors to pharmacologically interesting materials, further investigations have been carried out to extend the methodology in this direction. Thus, the reduction of the ketone moiety of 56 by applying L-selectride or lithium tri-t-butoxyaluminum hydride opened access to the cis-amino lactones 58 (45-54% yield de = 90-94%) and the trans-amino lactones 58 (67-76% yield de > 98%), respectively (Scheme 1.1.16). Monodebenzylation with cerium ammoni-... 

Cyclizations of carbonyl-containing a. -unsaturated esters like those illustrated in Scheme 38164>16S are conceptually very similar to ketyl cyclizations, but they may be very different mechanistically. At least for electrochemical cyclizations, Litde has proposed, based on reduction potentials, that the unsaturated ester is reduced first.164 Even though questions remain about the timing of election transfer and proton transfer steps, such cyclizations often provide good yields of products. 

Steenken S, Jagannadham V (1985) Reaction of 6-yl radicals of uracil, thymine, and cytosine and their nucleosides with nitrobenzenes via addition to give nitroxide radicals. OH catalyzed ni-troxide heterolysis. J Am Chem Soc 107 6818-6826 Steenken S, Jovanovic SV (1997) How easily oxidizableis DNA One-electron reduction potentials of adenosine and guanosine radicals in aqueous solution. J Am Chem Soc 119 617-618 Steenken S, Behrens G, Schulte-Frohlinde D (1974) Radiation chemistry of DNA model compounds, part IV. Phosphate ester cleavage in radicals derived from glycerol phosphates. Int J Radiat Biol 25 205-210... 

For instance, the reduction potential of many solvents depends on the salt used and, in particular, on the cation. The reduction potentials of alkyl carbonates and esters in the presence of tetraalkyl ammonium salts (TAA) are usually much lower than in the presence of alkaline ions (Li+, Na+, etc.). Similar effects were observed with the reduction potential of some common contaminants (e.g., H20, 02, C02). Moreover, the reduction products of many alkyl carbonates and esters are soluble in the presence of tetraalkyl ammonium salts, while in the presence of lithium ions, film formation occurs, leading to passivation of the electrode [3],... 

Tetraalkyl ammonium (TAA) salts are characterized by very low reduction potentials, along with good solubility in many organic solvents. Thus, nonaqueous solutions composed of such salts (e.g., tetrabutyl ammonium perchlorate and organic solvents such as ethers, esters, and alkyl carbonates) can be electrolyzed using noble metal electrodes. In contrast to lithium salt solutions, in TAA-based solutions there is no precipitation of insoluble products on the electrode, which leads to its passivation. Therefore, it is possible to isolate and identify the electrolysis products and thus outline precise reduction mechanisms for the various systems. 

We examined the representative esters, y-butyrolactone (BL), methyl formate (MF), and methyl acetate (MA). Figures 16 and 17 show FTIR spectra measured (ex situ) from noble metal electrodes polarized to low potentials in LiC104 solutions of BL and MF, respectively [30,39], As shown in these figures, at the onset reduction potential of around 1.3-1.2 V (Li/Li+), stable surface films precipitate on the electrode surfaces. Table 1 shows the spectral analysis for the surface films formed on noble metals at low potentials in BL. The conclusion drawn from the spectroscopic study is that the major surface compound formed is the dilithiated cyclic P-keto ester, which is similar to the electrolysis product of BL in TAA salt solutions (Scheme 2). 

Surface film formation on noble metal electrodes at reduction potentials was studied extensively with solutions of DME, THF, 2Me-THF, and DN. Basically, these solvents are much less reactive at low potentials than are alkyl carbonates and esters. However, in contrast to ethereal solutions of TBA+ whose electrochemical window is limited cathodically by the TBA+ reduction at around OV (Li/Li+), in Li+ solutions, ether reduction processes that form Li alkoxides occur at potentials below 0.5 V (Li/Li+) [4], It should be emphasized that the onset potential for surface film formation on noble metals in ethereal solutions is as high as in... 

Voltammetric data for ester reductions are available for several aromatic esters [51-54], and in particular cyclic voltammetry shows clearly that in the absence of proton donors reversible formation of anion radical occurs [51]. In dimethylfonnamide (DMF) solution the peak potential for reduction of methyl benzoate is —2.29 V (versus SCE) for comparison dimethyl terephthalate reduces at —1.68 V and phthalic anhydride at —1.25 V [4]. Half-wave potentials for reduction of aromatic carboxylate esters in an unbuffered solution of pH 7.2 are linearly correlated with cr values [51] electron-withdrawing substituents in the ring or alkoxy group shift Ei/o toward less negative potentials. Generally, esters seem to be more easily reducible than the parent carboxylic acids. Anion radicals of methyl, ethyl, and isopropyl benzoate have been detected by electron paramagnetic resonance (epr) spectroscopy upon cathodic reduction of these esters in acetonitrile-tetrapro-pylammonium perchlorate [52]. The anion radicals of several anhydrides, including phthalic anhydride, have similarly been studied [55]. 

Mixed hydrocoupling, in DMF, between simple 1,3-dienes, 63 (R, R", R = H, Me), and alkenes activated by ester or keto groups has been attempted [152]. Yields of the MHC are usually poor (0-58%), the LHD of the activated alkene predominating. Again, yields of the MHC increase as the difference between the reduction potentials of the two components decreases for example, no MHC was formed when ethyl 3,3-diphe-nylacrylate (—1.70 V) or 22b (—1.89 V) was coelectrolyzed with butadiene (63a (—2.66 V), or if cyclopentadiene (< —2.8 V) was coelectrolyzed with ethyl 3-methylcrotonate, 13b (—2.46 V). The only combination giving reasonable yield (58%) of the MHC was coelec-... 

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