Oxygen atoms, abstraction reactions

The Schiff bases (23), (43), and (44) formed Mo complexes of the type cw,wgr-[Mo02(ligand)] (which was associated in the solid state) and cw,mer-[Mo02(ligand)L] (L = neutral donor). The structures, electrochemical and chemical properties of these complexes, and their oxygen-atom transfer capabilities were extensive studies. These species were generally more active catalysts than their N-O-O-donor atom Schiff base counterparts, but oxygen-atom abstraction reactions with tertiary phosphines led to dinuclear Mo species rather than monomeric Mo complexes. 

Even if atomic oxygen-carbon cluster reactions are rapid, as is assumed in the new neutral-neutral model, the synthetic power of this model can be recovered if exothermic hydrogen atom abstraction reactions of the sort,... 

Since the Barton reaction and the Hofmann-Lofifler-Freytag reaction generate very reactive oxygen-centered and nitrogen-centered radicals respectively, the next 1,5- and 1,6-hydrogen atom abstraction reaction readily happens. However, 1,5-H shift does not proceed effectively by carbon-centered radicals, because there is not so much energy difference between the C-H bond before and after 1,5-H shift. So the reactions are quite limited. Eq. 6.21 shows iodine transfer from reactive 1-iodoheptyl phenyl sulfone (40) to a mixture of 5-iodoheptyl phenyl sulfone (41a) and 6-iodoheptyl phenyl sulfone (41b) initiated by benzoyl peroxide, through 1,5-H shift by an sp3 carbon-centered radical [56-58]. 

Among these, a variety of oxygen atom transfer reactions have been described [la,b] and highly stereoselective reactions have been reported [2]. Although the formation of aziridines by the reaction of nitrenes with olefins is well known, the efficiency is moderate, because of the competition between hydrogen abstraction and insertion processes [3]. A typical example is shown (Eq. 2) [3d]. 

While electron-transfer processes are common in inorganic photochemistry, excited-state atom transfer is limited to a small class of inorganic complexes. For U022 , the diradical excited state ( U-OO is active in alcohol oxidation (2). The primary photoprocess is hydrogen atom abstraction by the oxygen-centered radical. Photoaddition to a metal center via atom transfer has been observed for binuclear metal complexes such as Re2(CO)io (3-5). The primary photoprocess is metal-metal bond homolysis. The photogenerated metal radical undergoes thermal atom-abstraction reactions. Until recently, atom transfer to a metal-localized excited state had not been observed. 

Hydroxyl radicals ( OH) are powerful oxidants and participate in a number of reactions such as addition to the double bonds forming radical adducts, electron transfer reactions, and H-atom abstraction reaction. The rate constants for the reaction of OH radicals with organic substrates are mostly diffusion controlled (10 -10 ° M" s" ). When OH radical reacts with cellular organic molecules (RH) either by hydrogen abstraction [Eq. (4)] or by addition reaction, it leaves a radical site on the molecule (R ) and sometimes these radicals can add to the oxygen present in the cells, to be converted to peroxyl radicals [ROO, Eqs. (4) and (5)]. Rate constants for these reactions vary between 10 to diffusion-controlled limits depending on the nature and substitution on RH. °... 

Besides direct reaction of atomic oxygen with a stable hydrocarbon, another path to an oxide radical on the surface was thought to be reaction of O atoms, from the gas-phase or on the surface, with hydrocarbon radicals formed from the initial H-atom abstraction reaction ... 

Atom abstraction reactions also occur with // -cyclopentadienyl substituted metal carbonyl complexes. Laser photolysis (460-490 nm) of [ / -cpM(CO)3]2 in the presence of organic halides (RX) leads to the formation of the halo compounds / -cpM(CO)3X (M = Mo, W). The reactivity trends RI > RBr > RCl, and benzyl > allyl > 3 > 2 > 1 > CH3, follow those expected for a free radical pathway. The reaction involves formation of the 17-electron intermediate / -cpM(CO)3, which then abstracts a halogen atom from the substrate (Scheme 6.9). The cpM(CO)3 radical is trapped by oxygen at a rate that is close to diffusion controlled. 

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