Carbon dioxide reductive dimerization

Carbon dioxide reduction is thought to proceed via metallocarboxylate intermediate (s) formed by coordination of CO2 to the electron-rich Re center, although discrete steps in the process cannot be unambiguously assigned. The timing of Cl displacement from and CO2 adduction to the Re(bpy) (CO)3 unit are important mechanistic parameters. Most interpretations are based on a one-electron pathway, involving the interaction of CO2 with the product of Eq. (5) a two-electron pathway, involving interaction of CO2 with the product of Eq. (6) or a combination of these steps. Additional mechanistic considerations are the role dimeric rhenium intermediates and likely proton sources. 

The characterization and crystal structure of the dimer [Pt2( -dppm)3] (dppm = bis(diphenyl-phosphino)methane), first reported as a deep red complex in 1978, was described by Manojlovic-Muir et al. in 1986.11 The structure, the first of its type, is made up of two parallel and almost eclipsed trigonal-planar platinum moieties bridged by three diphosphine ligands. The Pf Pt separation is 3.0225(3) A, too long to be considered a bond.11 [Pt2(//-dppm)3] catalyzes the hydrogenation/reduction of carbon dioxide with dimethylamine to give dimethylformamide12 (Equation (1)) and the reverse reaction.13... 

In one mechanism, Pd° generated by reduction of Pd2+ with formic acid forms hydridoformatopalladium complex 102, which reacts with isoprene to form formato(methylbutenyl)palladium complex 103. Then, insertion of the second molecule of isoprene takes place. Finally, reductive elimination and evolution of carbon dioxide give the dimers ... 

Wagenknecht [17] found that the reduction of 1-bromobutane at graphite in carbon dioxide-saturated DMF containing TEABr is a two-electron process, and that the generated butyl carbanions lead to the formation of butane, 1-butene, octane, butyl valerate, butyl-A,A-dimethyloxamate, and dibutyl-2-methylmalonate. Earlier work by Kaabak and CO workers [26], dealing with the reduction of iodoethane and 1-bromobutane at graphite in DMF, revealed that alkanes, olefins, dimers, and solvent-derived species are produced. 

Amino acids may also be prepared from azomethine compounds by reducing them in the presence of carbon dioxide [34—38]. Thus reduction of benzylidene aniline (IV) in molten tetraethylammonium -toluenesulfonate saturated with carbon dioxide at 140°C gave V-phenyl-phenylglycine (60% yield) [34], [Eq. (9)]. In DMF a similar yield was obtained together with small amounts of a dimer and some phenylbenzylamine [38]. 

We here report the first example of an electrochemical polymerization process which leads to formation of a modified electrode having the generic formula [Ru (bpy)(CO)2Cl]n, and which displays outstanding electrochemical activity towards reduction of carbon dioxide to either carbon monoxide or formate. A crucial stereochemical effect of the leaving groups on the feasibility of polymerization is demonstrated. Formation of the polymer occurs stepwise, through the formation of a dimeric or a tetrameric intermediate. 

Scheme 9.100. A representation of the Kolbe electrolysis of the sodium salt of propanoic acid. The voltage for the electrolysis must be greater than that required for the reduction of water to hydroxide anion (OH") and hydrogen (H2) which occurs at the cathode. It is generally accepted that one electron is lost by the carboxylate anion at the anode to generate a carboxyl radical. The arrows shown on the carboxyl radical (with half-heads) are drawn to account for the apparent movement of one electron (in contrast to the usual cartoons showing two electron arrows) at a time to produce carbon dioxide (CO2) and the ethyl radical (CH3CH2 ).The latter dimerizes to butane or loses a hydrogen to a second radical, producing ethane and ethene.

Yan Y, Gu J, Bocarsly AB (2014) Hydrogen bonded pyridine dimer a possible intermediate in the electrocatalytic reduction of carbon dioxide to methanol. Aerosol Air Qual Res 14 515-521... 

Photochemistry. Photodecomposition of [Pt(PPh3)a(ox)] produces carbon dioxide and results in reduction of the platinum(n) to platinum(o). In ethanol the platinum(o) product is the dimer [Pt2(PPh8)4], in the presence of an excess of triphenylphosphine the product is [Pt(PPhs)4], and in the presence of an alkyne it is [Pt(PPh3)a(alkyne)]. In all cases the first step is photochemical generation of radical ions ... 

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