Reduction of Other Substrates

Selenoesters can be reduced with or without decarboxylation (equations 15-49 and 15-50).152 

Isothiocyanates, RN=C=S, are reduced first to the isocyanides RNC ,153 154 and then the isocyanides are reduced to the compounds RH (equation 15-51).155 

Secondary and tertiary nitroalkanes RNO2 can be reduced to RH (equation 15-52 and 15-53).156 

Pattenden has described the reduction of a triene selenyl ester, RCOSeR, by a cascade sequence to give a tetracyclic steroidal ketone,157 and of an acyclic heptaene selenyl ester, by a seven-fold cascade of ring closures to give a heptacyclic system.158 

3 Specific examples can be retrieved from the database by searching for the keywords SnH and (for example) LiAlH4. 

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Thus, the Lowe-Thomeley scheme has proved extremely useful over the past 15 years. At the time it explained all available data on N2 fixation by the enzyme, it has been predictive and allowed investigation of the reduction of other substrates, and it has been developed so that some of the steps shown in Figs. 8 and 9 have been broken down into more detailed reaction schemes that can still be encompassed within the overall mechanism. For example, the coupling of... 

Scheme 17 Asymmetric reduction of other substrates with iridium catalysts...

The brief review emphasizes the useful catalytic activities of metal oxides, i.e., Ru02, towards 02-evolution but points to their limitations as a result of surface recombination with intermediate H-atoms. Possible routes to circumvent these difficulties could involve elimination of surface H-atom through the application of homogeneous H2-evolution catalysts (see Sect. 4.3), and compartmentalization of the oxidation catalyst from the H2-evolution catalyst, i.e., liposomes. Alternatively, reduction of other substrates rather than water i.e. C02, could lead to intermediate carbonous species being insensitive to oxidation by intermediate O-species. 

On the other hand, one electron reduction of other substrates, for instance [MH(C0)2(L-L)2] (M = Cr, Mo, W L-L = dppm, dppe) [24] and [CpCoH(PR3)2]+ [25], leads to expulsion of Hj. This type of reaction is of fundamental importance in the electrocatalytic reduction of protons to dihydrogen [7] and could play a fundamental role in other electrocatalytic reduction processes. 

The best mutants were then tested as catalysts in the asymmetric reduction of other substrates 7b-7d without resorting to additional mutagenesis experiments. The results proved to be encouraging in many cases enantioselectivities better than 95% for the (R)- and (S)-products were observed [37]. However, some of the (S)-selective mutants showed low activity. In these cases additional mutagenesis experiments are necessary. 

The special salt effect is a constant feature of the activation of substrates in cages subsequent to ET from electron-reservoir complexes. In the present case, the salt effect inhibits the C-H activation process [59], but in other cases, the result of the special effect can be favorable. For instance, when the reduction of a substrate is expected, one wishes to avoid the cage reaction with the sandwich. An example is the reduction of alkynes and of aldehydes or ketones [60], These reductions follow a pathway which is comparable to the one observed in the reaction with 02. In the absence of Na + PFg, coupling of the substrate with the sandwich is observed. Thus one equiv. Na+PFg is used to avoid this cage coupling and, in the presence of ethanol as a proton donor, hydrogenation is obtained (Scheme VII). 

In the absence of other substrates which are easily reduced, halosilanes can be reduced by cathodic reaction. However, it is rather difficult to determine the reduction potentials of halosilanes, because halosilanes are readily hydrolyzed during voltammetric measurements. Although early reports stated that the reduction potential of Me3SiCl is not very negative, extensive studies by Corriu... 

Zirconium(IV) isopropoxide, 352 Reductive alkylation of aromatic rings Birch reduction, 32 (S)-Prolinol, 261 of carbonyl groups Trityl perchlorate, 339 of other substrates Lithium-Ammonia, 158 Reductive cleavage (see also Reduction of epoxides)... 

Reductive coupling of carbonyls to alkenes Titanium(IV) chloride-Zinc, 310 of carbonyls to pinacols Titanium(III) chloride, 302 Titanium(IV) chloride-Zinc, 310 of other substrates Samarium(II) iodide, 270 Reductive cyclization 2-(Phenylseleno)acrylonitrile, 244 Tributylgermane, 313 Tributyltin hydride, 316 Triphenyltin hydride, 335 Trityl perchlorate, 339 Reductive hydrolysis (see Hydrolysis) Reductive silylation Chlorotrimethylsilane-Zinc, 82... 

The reduction of lactam substrates containing proximal exo double bonds may be achieved in high e.e. as demonstrated by the reduction of 3-alkylidene-2-piperidones (Scheme 19)119. Cyclic amino acids may be prepared by, for example, asymmetric hydrogenation of 3 to 4 in up to 79% e.e.120 and the reduction of 5 to 6 in 99% e.e.121. In the latter case a number of chiral diphosphines were screened, and the best results were obtained using BINAP as a ligand with rhodium metal. Several other diphosphines, notably DuPHOS and DIOP, also performed well. The research group which produced... 

Reduction of acyl chlorides to aldehydes,13 Tri-n-butyltin hydride reduces acyl chlorides to a mixture of aldehydes and esters. In the presence of a soluble Pd catalyst usually Pd[P(C6H5)3]4, only aldehydes obtain. This reduction is very general, and yields are usually > 80%. Reduction of double bonds is a minor competing reaction with a, / -unsaturatcd substrates (< 5% reduction). Of other reducible groups, only an allylic bromide group competes with the COC1 group. 

Shein (1983) paid attention to the following facts. Dimethylsulfoxide used as a solvent may contain water and MeSNa. Water may hydrolyze the initial 4-nitro-l-chlorobenzene (spectrophotometry uses solutions extremely diluted with respect to the substrate). The presence of MeSNa may cause the formation of 4-nitrophenylmethyl sulfide and its anion radical, and this was not included in the kinetic equations. Solodovnikov (1976) considers neither the production of 4-nitroanisole nor the formation of the other products of a deeper reduction of the substrate. 

Nitrogenases are very versatile enzymes. They reduce, in addition to N2, a lot of other substrates, for example, protons, acetylene, azide, nitriles, and isonitriles. All of these substrates are reduced by multiples of [2 H+/2 e ] reductions. Both CO and NO inhibit nitrogenase activity. The apparent [2 H+/2 e ] multiplicity of substrate reductions and a couple of other findings strongly suggest diazene and hydrazine to be intermediates of the N2 —> NH3 reduction. 

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