Esters reduction mechanism

Not a great deal of work has been done on the acid or ester reduction mechanisms. There is no fundamental reason why a carboxylic acid should not be reduced electrolytically just as easily as the ester or amide. However, one... 

Scheme 3 Ester reduction mechanisms on lithium J butyrolactone and methyl formate [189,4],...

The spectroscopic identification of the y3-keto ester and the reduction mechanism leading to it have been previously described by Aurbach and co-workers when studying the surface chemistry of yBL-based electrolytes on various electrodes polarized to low potentials. 123,208,209... 

The way in which the dominant reduction mechanism for chromate changes with the reaction conditions and how this is related to the toxicity of chromate is not as yet clear. As outlined above, the products of the reaction may depend on the mechanism of reduction and these, as yet, unidentified chromium complexes are probably the agents responsible for the mutagenicity of chromate. The substantial stability of the chromium(V) complexes and thiolate esters generated in the reaction of GSH with chromate suggests that if similar complexes were formed in vivo they would have time to reach many intracellular compartments and could hence be the crucial active intermediates in the toxicity of chromate. 

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 have studied the electrolysis of y-butyrolactone (BL) and methyl formate (MF) in TBAP solutions. A typical voltammogram of y-BL/TBAP with a gold electrode is also shown in Figure 1. Butyrate (CH3CH2CH2COO ) and a cyclic (3-keto ester were identified as the major electrolysis products. The latter is a product of a nucleophilic attack of y-BL anion (in the a position) on the carbonyl center of another molecule [3], The FTIR spectra of this product, as well as its lithiated derivative, are shown in Figure 2. The basic reduction mechanisms of y-BL, based on the above product analysis, as well as on other arguments [3], are presented in Scheme 2. 

Alternatively, the electron-withdrawing properties of the a-heteroatom and its ability to coordinate Sml2 may facilitate reduction of the ester carbonyl group and formation of ketyl radicals 37 (Scheme 4.22). Once the activating heteroatom group has been removed, no further ester reduction is possible. A similar mechanism is possible for the reduction of a-heteroatom-substituted amides. 

Problem-Solving Strategy Proposing Reaction Mechanisms 1007 Mechanism 21-8 Transesterification 1008 21-7 Hydrolysis of Carboxylic Acid Derivatives 1009 Mechanism 21-9 Saponification of an Ester 1010 Mechanism 21-10 Basic Hydrolysis of an Amide 1012 Mechanism 21-11 Acidic Hydrolysis of an Amide 1012 Mechanism 21-12 Base-Catalyzed Hydrolysis of a Nitrile 1014 21-8 Reduction of Acid Derivatives 1014... 

Mechanism 21-13 Hydride Reduction of an Ester 1015 Mechanism 21-14 Reduction of an Amide to an Amine 1016 21-9 Reactions of Acid Derivatives with Organometallic Reagents 1017 Mechanism 21-15 Reaction of an Ester with Two Moles of a Grignard Reagent 1018... 

Mechanism The mechanism of acids and esters reduction with LiAlH4 is shown in Schemes 6.9 and 6.10, respectively. The acidic hydrogen in acid reacts first. Then the reduction of carbonyl group proceeds via the usual alkoxyaluminate intermediate. 

Mechanism Mechanism of esters reduction involves the electron transfer from metal, followed by protonation by EtOH as shown in Scheme 6.29. 

The mechanism for ester reduction here has rather more detail than the simplified one we presented to you in Chapter 10. 

Enantiosdective allyic substitution processes have been developed over the course of 30 years. Initial observations of the reactions of nucleophiles with paUadium-allyl complexes led to the observation of catalytic substitutions of aUylic ethers and esters, and then catalytic enantioselective aUylic substitutions. The use of catalysts based on ottier metals has led to reactions that occur with complementary regiochemistry. Moreover, flie scope of the reactions has expanded to include heteroatom and unstabilized carbon nucleophiles. Suitable electrophiles for these reactions indude allyhc esters of various types, allyhc ethers, aUylic alcohols, and aUylic halides. Enantioselective reactions can be conducted with monoesters or by selection for deavage of one of two equivalent esters. The mechanism of these reactions occurs by initial oxidative addition to form a metal-aUyl complex. The second step involves nudeophilic attadc on ttie aUyl ligand for reaction of "soft" nudeophiles or inner-sphere reductive eUmination for reactions of "hard" nudeophiles. The external nudeophilic attack typicaUy occurs by reaction of the nudeophile with a cationic aUyl complex at the face opposite to that to which Uie metal is bound. Exceptions indude reactions of certain molybdenum-aUyl complexes. Dissociation of product then regenerates the starting catalyst. Because of the diversity of the classes of these reactions, aUylic substitution—in particular asymmetric aUylic substitution—has been used to prepare a wide variety of natural products. 

The mechanism of the reaction is as shown in equations (13.139) and (13.140). This reaction is also catalyzed by compounds of other metals of groups 8 and 9 such as ruthenium and iridium. Higher alcohols EtOH, Pr"OH, Pr OH also undergo carbonylation to give corresponding carboxylic acids.However, the rate of the reaction is lower. It is assumed that in this case, the oxidative addition of alkyl iodide to the rhodium(I) complex proceeds according to a radical mechanism. Hydrocarboalkoxylation, carbonylation of esters, reductive carbonylation of... 

Taking advantage of the LFER treatment of the electrochemical data, we have found an irregularity in reduction of all compounds where the phenyl ring is substituted in p-position by methyl ester function [23] (I e, II e, V e, VI e). While all other substituents follow the linear Hammett relationship, the derivatives substituted by the p-carbonyl group are always reduced more easily and the observed values of red are always shifted by 150 mV toward less negative potentials (cf., anomalous values in Fig. 48.4). This observation points to a different reduction mechanism, caused by a different, more-delocalized system where the carbonyl is involved. 

Scheme 3 describes reduction mechanisms of two selected esters — methyl formate and y-butyrolactone on lithium, lithiated carbon or noble metals polarized to low potentials (Li salt solutions). FTIR spectra of Li electrodes in contact with ester solutions clearly show absorption bands of surface species which contain Li carboxylate groups (-COOLi). This is demonstrated in Figure 19, which shows FTIR spectra of a Li surface covered by a thin layer of... 

The enantioselective reduction of pyrilium cations has been achieved. For example, the calcone (196) is treated with catalytic chiral acid (BINOL-derived phos-phonic acid or triflimide) and subjected to photolysis in the presence of the Hantzsch ester. A mechanism is proposed involving formation of the pyrilium cation (197). The chiral counterion provides an asymmetric environment around the ion (197) and the hydride is delivered stereoselectively from the Hantzsch ester. Reduction of the pyrilium ion gives product (198). 

Carbonylation of Alcohols and Esters. The mechanism of ttie Rh/I" catalysed alcohol carbonylation has been studied in detail. Rates decrease sharply from methanol to n-propanol. Formation of isobutyric acid as a by-product points to a p-H elimination-reinsertion sequence.This sequence has also been demonstrated for ethanol carbonylation by selective C labelling (eqn.l8). The reductive carbonylation of methanol in the presence of Col2 and PPhg generates acetaldehyde, ethanol and methyl acetate. Only diphenylether and alkanes as solvents did not decompose under the reaction conditions (17CPC,... 

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