Methylenic hydrogen abstraction

The studied catalyst exhibits a significant primary isotope effect for both propane disappearance (kn/kD = 1.7) with theoretical maximum being 1.95) and propylene disappearance (ku/ko = 1.9) and a theoretical maximum of 2.05). These large, observed kinetic isotope effects are consistent with a homolytic C-H bond breaking as the rate determining step, and a relatively symmetric transition state hence, methylene hydrogen abstraction for propane activation and a-hydrogen abstraction for propylene activation. 

If every collision of a chlorine atom with a butane molecule resulted in hydrogen abstraction, the n-butyl/5ec-butyl radical ratio and, therefore, the 1-chloro/2-chlorobutane ratio, would be given by the relative numbers of hydrogens in the two equivalent methyl groups of CH3CH2CH2CH3 (six) compared with those in the two equivalent methylene groups (four). The product distribution expected on a statistical basis would be 60% 1-chloro-butane and 40% 2-chlorobutane. The experimentally observed product distribution, however, is 28% 1-chlorobutane and 72% 2-chlorobutane. 5ec-Butyl radical is therefore formed in greater anounts, and n-butyl radical in lesser anounts, than expected statistically. 

Irradiation of 4-(3-benzoylpropionyl)-1,4-morpholine (267) yielded an epimeric mixture of 9-hydroxy-9-phenylperhydropyrido[2,l-c][l,4]oxazin-6-ones 268 and 269 via hydrogen abstraction from the position 3 of the morpholine moiety of 267 (98T2529). It was assumed that the steric hinderance between the phenyl group and the hydrogen atoms of 5-methylene group of 267 in the biradicals contributed to the observed selectivity. 

The methylene intermediate abstracts a hydrogen and is converted to an adsorbed methyl. Reaction of the methyl with the methylene produces an ethyl-metal species. Successive reactions of the methylene with the formed ethyl produces a long chain adsorbed alkyl. 

Photo-induced Diels Alder reaction occurs either by direct photo activation of a diene or dienophile or by irradiation of a photosensitizer (Rose Bengal, Methylene Blue, hematoporphyrin, tetraphenylporphyrin) that interacts with diene or dienophile. These processes produce an electronically excited reagent (energy transfer) or a radical cation (electron transfer) or a radical (hydrogen abstraction) that is subsequently trapped by the other reagent. 

The reaction with active methylene compounds results in the hydrogen abstraction to give the alcohol and the organogold derivatives.178... 

Several mechanisms have been postulated in order to account for ketone-sensitized photodehydrochlorination. Benzophenone and acetophenone have been suggested to act as singlet sensitizers via a collisional deactivation process (13). An alternative mechanism proposed for benzophenone involves abstraction of a methylene hydrogen from PVC by the triplet ketone (Equation 2), followed by 3 scission of a... 

The photochemical behavior of methylene cyclopropenes is a subject of current investigation170 Previous results with some 4,4-diacyl and 4,4-dicyano triafulvenes indicate that mainly dimerization, but sometimes additional solvent incorporation and hydrogen abstraction occurs. In the case of the photodimer of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the structure 460 can be assigned from spectral evidence ... 

To explore the mechanism of allylic hydroxylation, three probe substrates, 3,3,6,6-tetradeuterocyclohexene, methylene cyclohexane, and /l-pinenc, were studied (113). Each substrate yielded a mixture of two allylic alcohols formed as a consequence of either retention or rearrangement of the double bond. The observation of a significant deuterium isotope effect (4-5) in the oxidation of 3,3,6,6-tetradeuterocyclohexene together with the formation of a mixture of un-rearranged and rearranged allylic alcohols from all three substrates is most consistent with a hydrogen abstraction-oxygen rebound mechanism (Fig. 4.48). 

Oxidation to CO of biodiesel results in the formation of hydroperoxides. The formation of a hydroperoxide follows a well-known peroxidation chain mechanism. Oxidative lipid modifications occur through lipid peroxidation mechanisms in which free radicals and reactive oxygen species abstract a methylene hydrogen atom from polyunsaturated fatty acids, producing a carbon-centered lipid radical. Spontaneous rearrangement of the 1,4-pentadiene yields a conjugated diene, which reacts with molecular oxygen to form a lipid peroxyl radical. 

For example, radical allylic bromination of pent-2-ene must produce a mixture of three products. There are two allylic positions in the substrate, and either can suffer hydrogen abstraction. If hydrogen is abstracted from the methylene, then the two contributing resonance structures for the allylic radical are equivalent, and one product results when this captures a bromine atom. Abstraction... 

A regioselective deprotonation with amide base by preferential abstraction of the a-methylene hydrogen syn to the phenylaziridyl moiety in 116 and subsequent decomposition of the resulting monoanion furnishes, with extrusion of styrene and nitrogen, the alkyllithium 118. After abstraction of the amine proton, the c -alkene 117 is formed with regeneration of the lithium amide base for further use in the catalytic cycle. 

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