H sources

Reduction of unsaturated organic substrates such as alkenes, alkynes, ketones, and aldehydes by molecular dihydrogen or other H-sources is an important process in chemistry. In hydrogenation processes some iron complexes have been demonstrated to possess catalytic activity. Although catalytic intermediates have rarely been defined, the Fe-H bond has been thought to be involved in key intermediates. 

An alternative approach to hydroboration has utilized a chiral B-H source with either achiral or chiral rhodium complexes.58 The enantiomerically pure reagent (21) is derived from ephedrine. Notably in the reactions with BINAP, a higher enantiomeric excess is produced from (R)-BINAP (6) compared to the Y-form (Scheme 13). 

FIGURE 1—1 Formation and disappearance of methemoglobin from blood of rats exposed at 100 ppm for 8 or 12 h. Source Modified from Kim and Carlson 1986. 

FIGURE 1-2 Measured and projected methemoglobin levels in rats exposed to aniline for 8 h. Source Data from Kim and Carlson 1986. 

European Dioxin Inventory, 1999, Report for European Commission, DG XI. Hutzinger O., Fiedler, H., Sources and Emissions of PCDD/F, Chemosphere 18,1989,23-32. 

Electrochemical and Electrocatalytic Reduction. The one-electron reduction of C02 yields the radical anion C02-, which reacts with an H source to give the formate ion. The reaction, however, is not selective because various other reactions may take place. An alternative and more promising approach is the two-electron reduction of C02 in the presence of a proton source to afford formic acid. The latter process requires a considerably lower potential (—0.61 V) than does the one-electron reduction (—1.9 V) consequently, the electrolysis in the presence of catalysts may be performed at lower voltages. The control of selectivity, however, is still a problem, since other two-electron reductions, most importantly reduction to form CO and H2, may also occur.101127 The reduction of C02 to CO, in fact, is the subject of numerous studies. Electrochemical and electrocatalytic reductions of C02 in aqueous solutions have been studied and reviewed.11,128-130... 

Stodola, F. H., "Source Book on Gibberellin 1828-1957, Agricultural Research Service, U. S. Dept, of Agriculture, Peoria, IH., 1958. 

Figure 3 Hydrolysis of different film coatings after application or storage, indicating the stability of these polymers. These data were obtained under storage at 20°C/100% r.h. (Source Ref. 24 and Ref. 72).

In many reactions molecular hydrogen may be replaced by other H-sources such as methanol, isopropanol, or formic acid even cyclic ethers such as dioxane or THF can be used in homogeneous transfer hydrogenation. The hydrogen donor coordinates to the metal and undergoes /3-hydrogen transfer ... 

The general form of the material balance is the familiar one, Inputs -H Sources = Outputs + Sinks -H Accumulations... 

Source Location Notes nmolL h nmolL h nmolL h Source... 

Tetraalkylborates are mild and selective alkylation reagents [186, 187], and they are commonly considered as sources of nucleophilic alkyl groups (R ) just as borohy-drides are depicted as hydride (H ) sources. However, since organoborates represent excellent electron donors (see Table 5, Section 2.2.1), the question arises as to what role electron donor-acceptor interactions play in the nucleophilic alkyl transfer. Phenyl- and alkyl-substituted borate ions form highly colored charge-transfer salts with a variety of cationic pyridinium acceptors [65], which represent ideal substrates to probe the methyl-transfer mechanisms. Most pyridinium borate salts are quite stable in crystalline form (see for example Figure 5C), but decompose rapidly when dissolved in tetrahydrofuran to yield methylated hydropyridines (Eq. 65). 

Figure 5 Time courses for CH4 production (left) and CH4 oxidation (right) in selected 5 cm core sections (site LB2 in Lemeta Bog) that were alternately made anoxic and oxic in an attempt to mimic response to changing water table levels. Methane production hy anoxic core sections was measured for 54 h. Core sections were drained for 20 h in an oxic environment and then amended with 20 pJ L CH4, after which the time course for CH4 oxidation (right) was determined. No CH4 was produced over 48 h hy core sections rewetted and made anoxic at 120 h (source Whalen...

On the other hand, when the solvent or other components in the system have H sources, competitive abstraction from nonlipid sites occurs and the net result is to quench the radical and interrupt the chain rather than propagate it. Abstraction from multiple H sources in a system is common, and subsequent oxidation at nonlipid sites may account for oxygen consumption that exceeds LOOH formation in many systems. 

In aqueous and protic solvents where H sources are plentiful, hydrogen abstractions by LO are faster kinetically, but less effective in chain propagation (Table 7). Production of LOH can be detected in protic solvents (308), but the yields of hydro-xylated products remain low because selectivity of H abstraction decreases and H... 

The rate of H abstraction by RO increases with temperature in all solvents. This leads to marked acceleration of oxidation in neat lipids and in nonpolar solvents where the only H sources are fatty acids, and it also favors LOOK formation over cyclization. This is evident in the marked increase of mono-, di-, and trihydroperoxides over epoxides as oxidation temperature increases from room temperature to about 80°C (228, 276, 311, 312). However, heat has less effect in polar and aqueous solvents (310). The activation energy for H abstraction is lower than for 3-scission, so there is less thermal enhancement of abstraction rate and also less selectivity of abstraction sites in polar solvents. More importantly, higher temperatures enhance scission more than abstractions so, particularly at T > 100°C, the relative importance of H abstraction by LO and LOO in propagation is diminished (278, 313) and secondary processes begin to dominate. 

Apparently the H-source is elusive to NMR because it is strongly chemisorbed to the solid surface. This implies that secondary carbenium ions do not persist because they are more acidic [12] than the zeolite acid sites. 

In an attempt to characterize an H-source (of acid-catalyzed oligomerization), a homogeneous system was studied to facilitate NMR detection. 

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