Other Hydrogen Donors

One of the most common reactions of photoexcited carbonyl groups is hydrogen-atom abstraction from solvent or some other hydrogen donor. A second common reaction is cleavage of the carbon-carbon bond adjacent to the carbonyl group ... 

The reaction between the photoexcited carbonyl compound and an amine occurs with substantially greater facility than that with most other hydrogen donors. The rate constants for triplet quenching by amines show little dependence on the amine a-C-H bond strength. However, the ability of the amine to release an electron is important.- - This is in keeping with a mechanism of radical generation which involves initial electron (or charge) transfer from the amine to the photoexcited carbonyl compound. Loss of a proton from the resultant complex (exciplex) results in an a-aminoalkyl radical which initiates polymerization. The... 

These composite data strongly suggest that the presence of adventitious water or other hydrogen donors can markedly affect the observed rate of epoxidation. For example, Murray and Gu have reported AH = 5.0 kcalmol" for the DMDO epoxidation of cyclohexene in CDCI3 and 7.4 kcalmol" in acetone as solvent . Curci and coworkers also reported an a value of 9.3 kcalmol" for the DMDO epoxidation of isobutylene in acetone . These barriers are significantly lower than the 13-18 kcalmoD gas-phase barriers reported " at the B3LYP level of theory (Tables 3 and 4). Activation barriers of 12.6,... 

At the end of reaction the initial hydrocarbon concentration is still important, but there are also many other hydrogen donors. Thus, in the absence of oxygen, reactions of alkoxy radicals will lead to aldehydes by pyrolysis and to alcohols by abstraction. Let us examine these two possible reactions for each alkoxy radical. 

In plants, catalase appears to have two functions. First is the ability to dispose of the excess H202 produced in oxidative metabolism, and second is the ability to use H202 in the oxidation of phenols, alcohols, and other hydrogen donors. The difference in heat stability of catalase and peroxidase was demonstrated by Lopez et al. (1959). They found that blanching of southern peas for one minute in boiling water destroys 70 to 90 percent of the peroxidase activity and 80 to 100 percent of the catalase activity. 

Transition Metal Catalyzed Reductions with Group 14 Metal Hydrides 3.5.65 Transition Metal Catalyzed Reductions with other Hydrogen Donors... 

Other hydrogen donors which have been applied to the reduction of diazonium fluoroborates are formamide and A -benzyl-l,4-dihydronicotinamide (equation 97). ... 

Reductive dimerization of alkenes and alkynes can be achieved with various catalysts in the presence of dihydrogen or other hydrogen donors, such as tetrahydroborates or / -H-eliminat-ing aluminum alkyls. 

Both in structure and in functions, catalase is akin to peroxidase. Catalase is the most effective among all known catalysts for the decomposition of hydrogen peroxide (k = 10 s ) and it displays, moreover, a peroxidase activity. The catalase molecule with a molecular mass of 225,000 is comprised of four subunits, each of which contains a hematin group. Hydrogen peroxide forms three compounds with catalase. Decomposition of H2O2 or ROOH occurs as a result of interaction between compound I of the catalase and the second substrate molecule. Not only hydroperoxides but other hydrogen donors, for instance, ethanol, are capable of reacting with compound I. 

Other monooxygenase mimics are obtained if ascorbic acid in Udenfriend s system is replaced by other hydrogen donors, such as enediols [37], reduced nicotinamide coenzymes [41], thiosalicylic acid (Ullrich s system) [52], cysteine [53], pyrimidine [40], or tetrahydropteridine [54,55]. Low-valent transition metal compounds (e.g. FeCl ) with dioxygen and reducing agents (hydrazobenzene,... 

The hydrogen atom abstraction can be either intramolecular or intermolecular. Many aromatic ketones undergo hydrogen abstraction from solvent or some other hydrogen donor, followed by coupling of the resulting a-hydroxy radicals. Such reactions go best in solvents that have easily abstractable hydrogens ... 

This section is divided into three different parts Sect.B, catalytic hydrogenation over palladium catalysts Sect.C, Pd-catalyzed reductions with group 14 metal hydrides and Sect.D, Pd-catalyzed reductions with other hydrogen donors. 

D. PALLADIUM-CATALYZED REDUCTIONS WITH OTHER HYDROGEN DONORS... 

Finally, the use of other hydrogen donors, including various formate salts, are convenient reducing reagents for the Pd-catalyzed reduction of a,/3-unsaturated carbonyl compounds. 

Some of these enzyme-substrate complexes, in a second reaction sequence, can oxidize a hydrogen donor molecule. If this donor happens to be HiOs we speak of catalatic activity, and if other hydrogen donors are oxidized, of peroxidatic activity. Catalase can exhibit both activities, whereas peroxidase can act only peroxidatically. 

Between 1972 and 1990, Maruyama and Takuwa reinvestigated the mechanistic scenario of the photoacylation for several quinone/aldehyde pairs. - For 9,10-phenanthrenequinone (1), Maruyama was able to show, via CIDNP investigations and in comparison with other hydrogen donors (e.g., ethers, alkyl benzenes, xanthene), that the photoacylation precedes via the unstable vibrationally excited 1,2-photoadduct 74 (Scheme 21) Subsequent rearrangement through dissociation into singlet radical pairs leads... 

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