Hydrogen abstraction, ketones competing reactions

In summary, ketones react preferentially with y-CH bonds. Reducing their reactivity can allow more remote hydrogen abstractions to compete, but the only way to eliminate y-hydrogen abstraction is to ehminate y-hydrogens. This constraint does not apply to similar hydrogen abstraction reactions of thioketones and N-alkyHmides. ... 

Competing reactions may introduce mechanistic ambiguities Ketone or quinone triplet states abstract hydrogen atoms, forming neutral radicals. Also, many radical cations are proton donors and radical anions are comparably strong bases. Thus, geminate radical ion pairs may generate neutral radicals by proton transfer. 

Since intramolecular hydrogen abstraction could not compete with decay of the two ketones having it,it lowest triplets, the reaction must have a rate constant of < 103 sec-1, the rate of their decay in solution.155,337 Consequently a factor of at least 10 exists between the reactivities of pure n,ir and -it,it ketone triplets in this particular reaction. Since the phenyl ketone triplets are about one-tenth as reactive as the triplets of aliphatic ketones, we have suggested that... 

Due to the low rate constants for y-hydrogen abstraction ( 1% of those for analogous phenyl ketone triplets), more remote hydrogen abstractions and cycloadditions to remote double bonds commonly compete and sometimes are the only reactions of pyruvates. 

Two types of competing reaction pose potential drawbacks to the PET method. First, the principal types of electron acceptor are ketones and quinones, the triplet states of which are known to abstract hydrogen atoms with formation of neutral radicals. Second, many of the radical cations generated by PET are potential proton... 

Stereoselective radical addition on the exo-face of levoglucosenone 250 leads to the C-linked dimer 251 in a modest 26% yield. The expected competing reaction is the direct hydrogen abstraction by the initial radical species. Stereoselective hemiketal and ketone reductions afford the 1,6-anhydro derivative 252, which is deprotected and opened to give the final C-ana-log of a /3-(1 4) disaccharide 253. 

Like (5-alkoxy ketones, (5-amido ketones also undergo photoinduced 8-hydrogen abstraction to give proline derivatives (in <50% chemical yield) (Scheme 6.126).967 The reaction stereoselectivity depends on the biradical cyclization rate, which competes with conformational changes. Whereas singlet biradicals couple without reaching conformational equilibrium, the triplet biradicals allow bond rotation before the ring forms. 

In the hypochlorite reactions, intramolecular hydrogen abstraction competes with p-cleavage and other reactions. For example, tertiary hypochlorite 15 on photolysis gives ketones and alkyl chlorides from P-cleavage and radical coupling reactions [10]. 

With the exception of implications regarding solubility, a feature not yet apparent is any recognized trend in the emissions from sulphur cures with variations in the base polymer. This is not the case with peroxide cures, where the reactivity of the polymer can influence both the quantity and type of emissions. A well-studied example is that of NR which carries an abundance of abstractable allylic hydrogens to favour alcohol formations (eqn (29)). Thus when DTOP (R = Me) is the peroxide, fert-butanol (BP 82°C) is obtained, whilst cumyl alcohol (2-phenyl-2-propanol BP 202°C) is obtained from Dicup (R = Ph). Ketone formation (eqn (30)) competes with hydrogen abstraction and can predominate in the presence of a different polymer emissions from formulations based on EPDM, silicone and a fluoroelastomer have been characterized. Other by-products include alkenes from alcohol dehydration, although numerous other reactions can occur. 

Two side reactions can limit the utility of the Paterno- Biichi reaction. The presence of active hydrogen [allylic hydrogen atom] on the olefin makes hydrogen abstraction by excited ketone which complete with Paterno-Biichi reaction. If the triplet energy of ketone is comparable to the triplet excitation energy of olefin, energy transfer will compete with the reaction. 

A wide variety of carbonyl compounds undergo photoinduced intramolecular hydrogen atom abstraction to form 1-hydroxy-IvJc-biradicals, which then undergo two common competing reactions (1) coupling to produce cycKc alcohols and (2) disproportionation back to ketone or to various enols. The overall process closely parallels the well-known bimolecular photoreduction of ketones, the most common products of which are formed by radical coupling. These intramolecular hydrogen abstraction processes have been widely studied, and several reviews are available. ... 

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