Hydrogen from hydrides + acids

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

Ethyl sulfate Flammable liquids Fluorine Formamide Freon 113 Glycerol Oxidizing materials, water Ammonium nitrate, chromic acid, the halogens, hydrogen peroxide, nitric acid Isolate from everything only lead and nickel resist prolonged attack Iodine, pyridine, sulfur trioxide Aluminum, barium, lithium, samarium, NaK alloy, titanium Acetic anhydride, hypochlorites, chromium(VI) oxide, perchlorates, alkali peroxides, sodium hydride... 

Scheme 3.7 Generation of the active hydride catalyst by hydrogen transfer from formic acid or iso-propanol via /5-hydride elimination from formate or alkoxide intermediates. The square represents a vacant site on ruthenium.

Hydrogen transfer reactions are highly selective and usually no side products are formed. However, a major problem is that such reactions are in redox equilibrium and high TOFs can often only be reached when the equilibria involved are shifted towards the product side. As stated above, this can be achieved by adding an excess of the hydrogen donor. (For a comparison, see Table 20.2, entry 8 and Table 20.7, entry 3, in which a 10-fold increase in TOF, from 6 to 60, can be observed for the reaction catalyzed by neodymium isopropoxide upon changing the amount of hydrogen donor from an equimolar amount to a solvent. Removal of the oxidation product by distillation also increases the reaction rate. When formic acid (49) is employed, the reduction is a truly irreversible reaction [82]. This acid is mainly used for the reduction of C-C double bonds. As the proton and the hydride are removed from the acid, carbon dioxide is formed, which leaves the reaction mixture. Typically, the reaction is performed in an azeotropic mixture of formic acid and triethylamine in the molar ratio 5 2 [83],... 

Aside from the multifaceted chemical conversions, there are sources to develop into industrially viable microbial conversions. 1,2,4-Butanetriol, for example, used as an intermediate chemical for alkyd resins and rocket fuels, is currently prepared commercially from malic acid by high-pressure hydrogenation or hydride reduction of its methyl ester. In a novel environmentally benign approach to this chemical, wood-derived D-xylose is microbially oxidized to D-xylonic acid, followed by a multistep conversion to the product effected by a biocatalyst specially engineered by inserting Pseudomonas putida plasmids into E. coli ... 

Experimentally, this pathway has been well established from IR spectra of the [CpRuH(C0)(PCy3)]/(CF3)30H system in CH2CI2, where large variations in hydride/alcohol ratios did not affect slow transformation of the H H complexes to hydrogen-bonded ion pairs with k values between 1.4 X 10 and 1.6 X 10 s [25]. Activation parameters for this step (Table 10.3) have been determined in hexane [6]. It is probable that a similar mechanism operates for protonation of the hydrides [ReH2(NO)(CO)(PR3)2] with CF3COOH (Table 10.3) in CD2CI2, where the reaction corresponds to first-order kinetics on the acid at hydride/acid ratios > 1 [7]. 

Thiophenes are best converted to the tetrahydro derivatives by the so-called ionic hydrogenation. This depends on the successive addition of a proton (from trifluoroacetic acid) and a hydride ion (from triethylsilane) (75T311). A subsequent improvement involved the use of HC1/A1C13 to form the thiophenium ion and then reaction with triethylsilane (78T1703) best results are obtained with the substrate/Et3SiH/AlCl3 ratio of 1 3 0.3. The mechanism of the reaction is shown in Scheme 43. Evidence for this has been provided by the use of Et3SiD, when D enters positions 3 and 5 in the product. 

An alternative application of cobalt Mb has been reported by Willner and coworkers (92). They immobilized the reconstituted cobalt Mb on the functionalized electrodes and generated cobalt(I)-Mb by the electrochemical reduction of cobalt(II)-Mb. The hydrogenation of acetylenedicarboxylic acid smoothly occurred on the functionalized electrode, and the electrocatalytic reaction in H20 and D20 reveals a clear isotope effect, kH/kD = 2.7, indicating that the hydride transfer from cobalt(III)-hydride in Mb is the key reactive process. 

Already familiar is the convenient laboratory preparation of elementary hydrogen by reduction of acids. Generally those metals lying between magnesium and tin in oxidation potential are appropriate. Less convenient but more spectacular is the production of hydrogen from action of the alkali metals on water. For small quantities of hydrogen, reaction of metal hydrides with water has been used such hydrides will be considered later in the chapter. Commercial preparations of H2 by reduction of steam with iron or coke and, finally, by the electrolysis of water should be recalled. 

FeFe-enzyme - proton or hydrogen substrate binding and also the hydride-proton reaction exclusively occurs at the iron distal to the [4Fe-4S] cluster, suggesting that mononuclear iron complexes might also be viable catalysts. Consequently, Ott and coworkers have synthesized and characterized some stable pentacoordinated Fe(II) complexes with five ligands that nicely mimic the native ones and exhibit an open coordination site [163, 164]. This approach avoids the formation of the less reactive bridging hydrides that are found in the dinuclear complexes [153]. Catalytic H2 formation from weak acids at low overpotentials with promising TOF and catalyst stability could be demonstrated [164]. 

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