Formic acid as a reductant

An alternative means of limiting hydrogen availability, and thus controlling undesired ammonium formation, is through the use of formic acid as a reductant which can be added in stoichiometric amounts to avoid over-hydrogenation. However, this serves an additional purpose since the... 

A similar example, using formic acid as a reductant, has also been reported... 

Carbonyl)chlorohydridotris(triphenylphosphine)ruthenium(II) was used as a catalyst in the transfer hydrogenation of benzaldehyde with formic acid as a hydrogen source. Under these conditions, the reduction ofbenzaldehyde to benzyl alcohol is accompanied by esterification of the alcohol with the excess of formic acid to provide benzyl formate (Scheme 4.16). In this microwave-assisted reaction, the catalyst displayed improved turnover rates compared to the thermal reaction (280 vs. 6700 turnovers/h), thus leading to shorter reaction times36. 

Formic acid as a stable organic hydrogen donor is an alternative to the flammable molecular hydrogen. Although Ru(II)-complexes are known to catalyze the reversible process C, the reduction of the imine is a result of transfer hydrogenation by formic acid. Interference of molecular hydrogen does not take place. 

It is a very difficult task to reduce CO2 to a useful fuel such as methanol or methane by electrochemical methods because six electrons per molecule are required for the production of methanol and as many as eight electrons for methane. Another difficulty is that high energy intermediates are involved in most steps, imposing high kinetic barriers. Hence, most electrochemical reduction experiments have yielded only formic acid as a product, according to the reaction... 

Reduction. Japanese chemists have used this reagent in place of formic acid as a reducing agent because of its high boiling point and weak acidity. The reagent is effective for selective reduction of <, /3-unsaturated ketones of the type (l)-2... 

This useful way of adding two methyl groups to a primary amine by reductive amination is sometimes called the Eschweiler-Clarke reaction. For more on formic acid as a reducing agent, see Chapter 41,... 

The CO2 reduction catalyzed by non-porphyrinic tetraaza macrocyclic complexes of Co(II) and Ni(II) yields CO and formic acid as main reduction products together with H2 as a by-product. The commonly accepted mechanism involves the reduction of the metal center M(II)L to form the one-electron-reduced species M(I)L followed by the direct reaction with CO2 to form an intermediate M(III)C02L, which is further pro-tonated and reduced to form CO and water and recover the starting form of the catalyst (Fig. 1). 

Mercuric chloride test. Add mercuric chloride solution to formic acid or a solution of formate and w arm. A white precipitate of mercurous chloride, insoluble in dil. HCl, is produced. Sometimes the reduction proceeds as far as metallic mercury, which appears as a grey precipitate. 

The conventional electrochemical reduction of carbon dioxide tends to give formic acid as the major product, which can be obtained with a 90% current efficiency using, for example, indium, tin, or mercury cathodes. Being able to convert CO2 initially to formates or formaldehyde is in itself significant. In our direct oxidation liquid feed fuel cell, varied oxygenates such as formaldehyde, formic acid and methyl formate, dimethoxymethane, trimethoxymethane, trioxane, and dimethyl carbonate are all useful fuels. At the same time, they can also be readily reduced further to methyl alcohol by varied chemical or enzymatic processes. 

Formic acid behaves differently. The expected octadienyl formate is not formed. The reaction of butadiene carried out in formic acid and triethylamine affords 1,7-octadiene (41) as the major product and 1,6-octadiene as a minor product[41-43], Formic acid is a hydride source. It is known that the Pd hydride formed from palladium formate attacks the substituted side of tt-allylpalladium to form the terminal alkene[44] (see Section 2.8). The reductive dimerization of isoprene in formic acid in the presence of Et3N using tri(i)-tolyl)phosphine at room temperature afforded a mixture of dimers in 87% yield, which contained 71% of the head-to-tail dimers 42a and 42b. The mixture was treated with concentrated HCl to give an easily separable chloro derivative 43. By this means, a- and d-citronellol (44 and 45) were pre-pared[45]. 

Formic acid is a good reducing agent in the presence of Pd on carbon as a catalyst. Aromatic nitro compounds are reduced to aniline with formic acid[100]. Selective reduction of one nitro group in 2,4-dinitrotoluene (112) with triethylammonium formate is possible[101]. o-Nitroacetophenone (113) is first reduced to o-aminoacetophenone, then to o-ethylaniline when an excess of formate is used[102]. Ammonium and potassium formate are also used for the reduction of aliphatic and aromatic nitro compounds. Pd on carbon is a good catalyst[103,104]. NaBH4 is also used for the Pd-catalyzed reduction of nitro compounds 105]. However, the ,/)-unsaturated nitroalkene 114 is partially reduced to the oxime 115 with ammonium formate[106]... 

Similarly, the m/z = 60 ion current signal was converted into the partial current for methanol oxidation to formic acid in a four-electron reaction (dash-dotted line in Fig. 13.3c for calibration, see Section 13.2). The resulting partial current of methanol oxidation to formic acid does not exceed about 10% of the methanol oxidation current. Obviously, the sum of both partial currents of methanol oxidation to CO2 and formic acid also does not reach the measured faradaic current. Their difference is plotted in Fig. 13.3c as a dotted line, after the PtO formation/reduction currents and pseudoca-pacitive contributions, as evident in the base CV of a Pt/Vulcan electrode (dotted line in Fig. 13.1a), were subtracted as well. Apparently, a signihcant fraction of the faradaic current is used for the formation of another methanol oxidation product, other than CO2 and formic acid. Since formaldehyde formation has been shown in methanol oxidation at ambient temperatures as well, parallel to CO2 and formic acid formation [Ota et al., 1984 Iwasita and Vielstich, 1986 Korzeniewski and ChUders, 1998 ChUders et al., 1999], we attribute this current difference to the partial current of methanol oxidation to formaldehyde. (Note that direct detection of formaldehyde by DBMS is not possible under these conditions, owing to its low volatility and interference with methanol-related mass peaks, as discussed previously [Jusys et al., 2003]). Assuming that formaldehyde is the only other methanol oxidation product in addition to CO2 and formic acid, we can quantitatively determine the partial currents of all three major products during methanol oxidation, which are otherwise not accessible. Similarly, subtraction of the partial current for formaldehyde oxidation to CO2 from the measured faradaic current for formaldehyde oxidation yields an additional current, which corresponds to the partial oxidation of formaldehyde to formic acid. The characteristics of the different Ci oxidation reactions are presented in more detail in the following sections. 

As with hydrogenation, hydrogen transfer of imines is a poorly developed field.126-130 However, recent arene-Ru11 systems bearing chiral 1,2-diamine co-ligands have been found to be excellent catalysts for asymmetric reduction of imines with formic acid as donor.31,131-134... 

A different dimerization takes place in formic acid (33. 34). Unlike other carboxylic acids which behave as nucleophiles, formic acid behaves as a reductant and the dimerization proceeds with partial reduction. By carrying out the reaction in a mixture of formic acid and triethylamine at 59°C using Pd(OAc)2, 1,6-octadiene (24) was obtained. 

Few studies have been carried out on the telomerization of carboxylic acids other than acetic acid. Carboxylic acids are expected to react similarly with butadiene. The exception is formic acid No telomerization takes place, as described before (33, 34), and it behaves as a reductant rather than a nucleophile, forming 1,6- and 1,7-octadienes and octatriene. 

The pyrrole ring can also be constructed starting from an 7V-vinyl-2-halobenzoic amide. The /V-(2-iodobcnzoyl)-1,4-dihydropyndine derivative shown in 3.19. underwent palladium catalysed ring closure to give a condensed isoindolone derivative. The use of formic acid as co-solvent led to the reduction of the intermediate palladium complex formed in the insertion step, instead of / -hydride elimination. The transfer of the stereochemical information from the starting material to the product was poor.25... 

Two parallel routes for the elimination of glycol formate are suggested, involving either reaction with H2 or with cocatalyst water. The detection of formic acid in the reaction products suggests another mechanism, with initial production of formic acid from H2 and C02, followed by reaction with the oxirane. This mechanism is not favored however since the yields of glycol formates varied substantially when various substituted oxiranes were reacted. This would not have been expected in a mechanism with formic acid as an intermediate. A third mechanism, not considered by the authors, could proceed through initial production of propylene carbonate, followed by reduction to the mono- or di-formate. 

The results obtained with formic and acetic acids have been corrected as described before. There is evidence that in these two reactions, reduction takes place besides H atom formation. In formic acid (50), a limiting G(H2) = 2.2 is obtained at concentrations of 5Af and up. 

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