Abstraction process

At low energies the abstraction process dominates and at higher energies the exchange mechanism becomes more important. The cross-sections for the two processes crossing at 10 eV. The END calculations yield absolute cross-sections that show the same trend as the experimentally determined relative cross-sections for the two processes. The theory predicts that a substantial fraction of the abstraction product NHjD, which are excited above the dissociation threshold for an N—H bond actually dissociates to NH2D" + H or NH3 during the almost 50-ps travel from the collision chamber to the detector, and thus affects the measured relative cross-sections of the two processes. 

With other ring sizes, the photochemistry of cyclic enones may take different courses. For cyclopentenones, the principal products result from hydrogen abstraction processes. Irradiation of cyclopentenone in eyclohexane gives a mixture of 2- and 3-cyclohexyleyclopentanone. These products can be formed by intermolecular hydrogen abstraction, followed by recombination of the resulting radicals ... 

In benzonitrile both types of processes occur in parallel with similar efficiencies. In hydrogen-donating solvents (toluene, ether, dioxane, ethanol), hydrogen abstraction processes prevail [toluene (114) -> (123)]. 

Irradiation of the polymer in air prior to grafting introduces hydroperoxide groups in the backbone polymer by the intermolecular, intramolecular, or hydrogen abstraction process. 

The S-S linkage of disulfides and the C-S linkage of certain sulfides can undergo photoinduced homolysis. The low reactivity of the sulfur-centered radicals in addition or abstraction processes means that primary radical termination can be a complication. The disulfides may also be extremely susceptible to transfer to initiator (Ci for 88 is ca 0.5, Sections 6.2.2.2 and 9.3.2). However, these features are used to advantage when the disulfides are used as initiators in the synthesis of tel ec he lies295 or in living radical polymerizations. 96 The most common initiators in this context are the dithiuram disulfides (88) which are both thermal and photochemical initiators. The corresponding monosulfides [e.g. (89)J are thermally stable but can be used as photoinitiators. The chemistry of these initiators is discussed in more detail in Section 9.3.2. 

That this mechanism can take place under suitable conditions has been demonstrated by isotopic labeling and by other means. However, the formation of disproportionation and dimerization products does not always mean that the free-radical abstraction process takes place. In some cases these products arise in a different manner.We have seen that the product of the reaction between a carbene and a molecule may have excess energy (p. 247). Therefore it is possible for the substrate and the carbene to react by mechanism 1 (the direct-insertion process) and for the excess energy to cause the compound thus formed to cleave to free radicals. When this pathway is in operation, the free radicals are formed after the actual insertion reaction. 

Hydroxy Benzophenone 4-Hydroxy Benzophenone Hydrogen Abstraction Processes Photocrosslinking ... 

Hydroxy Benzophenone O- CH2CH2OCH2CH2OCH3 Methoxyethoxyethoxy Hydrogen Abstraction Processes Photocrossslinking" ... 

Stratification of operational tasks, shown in Fig. 2 (Stephanopoulos, 1990). At the lowest level of abstraction, process data at a scale of seconds or minute are used to carry out a variety of numerical and logical tasks. 

The oxidation of di-2-chloroethyl ether is first-order with respect to ether, but is autocatalytic and chloride ion is liberated. A hydrogen atom abstraction process, similar to that above, probably takes place, viz. 

The final example to be discussed in this chapter is a domino 3-fragmentation/ hydrogen abstraction process developed by Suarez and coworkers [24]. Besides other related examples reported by the same group [25], the formation of 5-91 from 5-84 is probably the most striking due to its similarity to the natural product li-monin (5-92) [26]. 

The addition of the diphenylcarbene, generated by the photolysis of diazo-diphenylcarbene, to cis- or trans-butene gives mostly the abstraction product, and a small amount of the addition product. However, the abstraction processes are... 

Nucleophilic substitution occurs at C-2, and to a lesser extent C-4, as might be predicted from similar reactions with pyridine. Chichibabin amination occurs rather more readily than with pyridine, giving 2-aminoquinoline. A typical hydride abstraction process occurs when qninoline is heated with sodinm... 

In most cases, either R1 or R2 in amide 1 is a hydrogen. For configurational characterization the terms syn and anti for enolates having the non-hydrogen substituent in the position cis or trans to the oxygen, respectively, will be employed (see also Section A.I.). The ratio of syn- to anh-enolale formed in the proton abstraction process sets the upper limit for the stereoselectivity that can be achieved in a subsequent alkylation reaction, since it is generally assumed that attack of the electrophile occurs from the same side on both the syn- and anti-tnolate if the other side is blocked by a bulky substituent. 

The LFP techniques make it possible to observe directly the hydrogen atom abstraction process by triplet carbenes from appropriate H donors. The techniques also give useful kinetic information. 

The point here is that the entire symmetry classification of reaction (69) is based on the presumption that a plane of symmetry is conserved thoughout the hydrogen abstraction process (the plane incorporating the C—H bond and the carbonyl group). If one or more atoms are displaced slightly so as to destroy the symmetry plane the crossing becomes avoided. A comparison of the in-plane and slightly out-of-plane cases is illustrated in Fig. 16a and b (Salem etal., 1975). 

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