Carbon dioxide reduction, scheme

More complicated reactions that combine competition between first- and second-order reactions with ECE-DISP processes are treated in detail in Section 6.2.8. The results of these theoretical treatments are used to analyze the mechanism of carbon dioxide reduction (Section 2.5.4) and the question of Fl-atom transfer vs. electron + proton transfer (Section 2.5.5). A treatment very similar to the latter case has also been used to treat the preparative-scale results in electrochemically triggered SrnI substitution reactions (Section 2.5.6). From this large range of treated reaction schemes and experimental illustrations, one may address with little adaptation any type of reaction scheme that associates electrode electron transfers and homogeneous reactions. 

Catalysis of carbon dioxide reduction thus appears as a chemical catalysis process in which the most important step is stabilization of the catalyst-substrate adduct rather than its decomposition, which closes the catalytic loop. With divalent cations, Scheme 4.8 applies. 

Scheme 144 Catalytic cycle of cathodic carbon dioxide reduction with rhenium complexes to carbon monoxide.

The relationship of the acid and methane phases to the scheme in Fig. 12.1 raises several interesting questions, such as where the acetogenic bacteria are located and whether methane is formed exclusively from acetate or from both carbon dioxide reduction and acetate in the methane-phase reactor. The experimental data useful in answering these questions include the observations that for a glucose-fed, two-phase digestion system, more than 96% of the products from the acid-phase reactor were carbon dioxide, hydrogen, acetate. 

Scheme 9.19 Expected mechanism of NADH regeneration and further carbon dioxide reduction to methanol...

Industrially, polyurethane flexible foam manufacturers combine a version of the carbamate-forming reaction and the amine—isocyanate reaction to provide both density reduction and elastic modulus increases. The overall scheme involves the reaction of one mole of water with one mole of isocyanate to produce a carbamic acid intermediate. The carbamic acid intermediate spontaneously loses carbon dioxide to yield a primary amine which reacts with a second mole of isocyanate to yield a substituted urea. 

Carbon dioxide instead of aldehydes can be involved in Ni(0)-promoted reductive coupling reactions (Equations (76) and (77) Scheme 90).434,434a 434c A stoichiometric amount of Ni(COD)2/DBU reacts with C02 and dienes, alkynes, or allenes to afford a metallacycle intermediate. This metallacycle reacts with organozinc compounds or aldehydes in one-pot to give carboxylic acid derivatives. As shown in Scheme 90, double carboxylation occurs in the presence of dimethylzinc, where the stereochemical outcome is opposite to that of the reaction with diphenylzinc. 

The reduction of carbon dioxide (Section 2.5.4) raises the question of possible competition between a radical-radical coupling and a radical-substrate coupling according to Scheme 6.3, in which the competition shown in the upper part of Scheme 2.34 is represented symbolically. 

The most probable mechanism for the reduction requires the initial nucleophilic attack by the hydridometal complex on the nitro group, followed by intramolecular cyclization and extrusion of carbon dioxide. Repetition of the cyclization and extrusion sequence, followed by proton transfer, leads to aniline (Scheme 11.6). 

Scheme 84 Cathodic reduction of carbon dioxide to carbon monoxide.

Carbon dioxide has been reduced to methanol with the Everitt s salt (ES)-mediated electrode in the presence of l,2-dihydroxybenzene-3,5-disul-phonato(iron) ferrate(III) complex (Scheme 103) [408, 409]. The reduction proceeds as follows a weak coordination bond is first formed between the central metal of ES and ethanol, then the subsequent insertion of CO2 onto... 

Scheme 7 Reduction of carbon dioxide by transfer hydrogenation...

The classical preparation of alkyllithium compounds by reductive cleavage of alkyl phenyl sulfides with lithium naphthalene (stoichiometric version) was also carried out with the same electron carrier but under catalytic conditions (1-8%). When secondary alkyl phenyl sulfides 73 were allowed to react with lithium and a catalytic amount of naphthalene (8%) in THF at —40°C, secondary alkyllithium intermediates 74 were formed, which finally reacted successively with carbon dioxide and water, giving the expected carboxylic acids 75 (Scheme 30) °. 

The reductive ring opening of six-membered nitrogen-containing heterocycles was studied with A-phenyltetrahydroisoquinoline (391). Its lithiation with lithium and a catalytic amount of DTBB (4.5%) afforded the benzylic intermediate 392, which was allowed to react with electrophiles giving, after hydrolysis, functionalized amines 393 (Scheme 110) . It is noteworthy that in the reaction with carbon dioxide, instead of the corresponding lactam, amino acid 393 with X = CO2H was exclusively isolated. 

Metallation of 3,4-dimethyl-l,2,5-thiadiazole (55) to the anion (56) was accomplished with the use of a nonnucleophilic base, lithium diisopropylamide <82JHC1247>. Nucleophilic attack at sulfur resulted in an alkyllithium reagent <70CJC2006>. The lithiomethyl derivative (56) was carboxylated to (57) with carbon dioxide and converted to the vinyl derivative (58) via an esterification, reduction, mesylation, and base elimination sequence (Scheme 12). 

Figure 15-22 Tentative scheme for reduction of carbon dioxide to methane by methanogens. After Rouviere et al.352 and Thauer et al.435...

Another route to the carbon dioxide anion radical is reduction of hydrogen peroxide with Ti3+ in the presence of formate ions (Morkovnik Okhlobystin 1979), Scheme 1-86 ... 

The carbon dioxide anion radical was used for one-electron reductions of nitrobenzene diazonium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik Okhlobystin 1979). This anion radical reduces organic complexes of Com and Rum into appropriate complexes of the metals in the valence 2 state (Morkovnik Okhlobystin 1979). In the case of the pentammino-p-nitrobenzoato-cobalt(III) complex, the electron-transfer reaction passes a stage of the formation of the Co(III) complex with the p-nitrophenyl anion radical fragment. This intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand as a result of an intramolecular electron transfer. Scheme 1-89 illustrates this sequence of transformations ... 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

Treatment of substituted phthalans 1172 with lithium metal in the presence of catalytic quantities of naphthalene leads to reductive cleavage of the arylmethyl carbon-oxygen bond to form a stable dilithium compound 1173, which upon trapping with carbon dioxide furnishes isochroman-3-ones 1174 (Scheme 289) <1996JOC4913>. 

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