Hydrogen atom abstraction route

In a more complex elaboration of the hydrogen atom abstraction/nucleo-philic cyclization route to tetrahydrofurans (Scheme 20), a carbohydrate-based N-alkoxy phthalimide was converted to a spirocyclic acetal in excellent yield and diastereoselectivity (Scheme 33) [136]. In this cyclization, nucleophilic attack takes place from the endo face of the trioxabicyclo [3.3.0] octane... 

The table shows the effect on product ratio of ultrasonic irradiation (Kerry Pulsatron cleaning bath 35 kHz 50 W) during electrolysis. Here there is only 8% of the bicyclohexyl dimeric one-electron product, with approximately 41 % of the two-electron product from nucleophilic capture of the intermediate carbocation. The preponderance of cyclohexene (32 %) over cyclohexane (> 3 %) shows its formation is by proton loss from the carbocation intermediate, since free-radical routes to cyclohexene (i. e. hydrogen atom abstraction) also produce cyclohexane in equal if not greater amounts... 

Some conclusions on the reaction mechanism may be drawn from the rate constants obtained. It was shown for hydroxyl reactions with saturated compounds (propane, for example) that the main reaction of OH was the hydrogen atom abstraction in the formation of water. This is an accepted point of view. However, another route is possible for reactions with unsaturated hydrocarbons, i.e., addition at the double bond. This is the case for the H atom with saturated compounds H reacts by abstraction, and with unsaturated ones by addition. 

An alternative possible route for the formation of carbamoyl radicals may involve a hydrogen atom abstraction from formamide by the excited acetone molecule in its triplet, state (45, 57). Ketonic compounds at this state of excitation mainly if the excitation is of an n - -7r transition, are known to be hydrogen atom abstraction agents, due to the electron deficient oxygen in the excited state. Thus, the formation of carbamoyl radicals through the second route may be summarized by the following scheme ... 

Radical cations are probably involved in the photooxidation of olefins [1-7, 24, 50-54, 56, 61, 66] (Eqs. 2 and 5). Many oxidizable arenes and dienes are, similarly, oxidatively cleaved or rearranged by routes consistent with initial formation of a radical cation. As we saw above, the observed oxidative cleavage product probably derives from the combination of a surface-bound radical cation with superoxide or adsorbed oxygen. With alkanes or simply substituted alkenes, however, the capture of a photogenerated hole is often thermodynamically forbidden. Thus, instead of radical cations, radicals formed by hydrogen atom abstraction by an activated oxygen species dominate the observed chemistry. With alkanes, oxygenation at sites... 

A tandem radical sequence was observed when the indole amide 120 was treated with tributylstannyl radical [81]. The initially formed aryl radical is proposed to undergo a 1,5-hydrogen atom abstraction to form an a-amido radical en route to the cyano spiroindolines 121a and 121b. Treatment with potassium tert-butoxide and oxygen in THF gave the C-3 spirooxindole in modest yield. 

It is not always easy to establish that an authentic insertion reaction has occurred. An alternative route to the same product is available it involves hydrogen-atom abstraction by the triplet carbene, followed by recombination of the fragments ... 

Two potential routes exist for the homopolymerization of 18. First, the standard vinyl addition mechanism could occur. However, an internal hydrogen atom abstraction from the W-CH3 group followed by termination or further polymerization by LnW-CH2 addition to another vinyl group can be envisioned (see Scheme 4). Thus, two limiting polymer structures, and 316, are possible. The NMR spectrum of the homopolymer is only consistent with The polymer... 

Studies involving 14a and its derivatives were scarce for the next 40 years. In an attempt to prepare [Sjannulene derivative 15a, Youngs and coworkers discovered a Pd-catalyzed route to dibenzopentalenes (Scheme 2.8) [37]. Under standard Sonogashira conditions, 14b was obtained in 67% yield with the desired cyclodimer 15a in only 10% yield. The unexpected dibenzopentalene 14b was presumed to arise from an intramolecular cyclization and subsequent hydrogen atom abstraction from the solvent. The lack of trimer and tetramer in the reaction mixture was most Hkely due to the steric bulk of the flanking ferf-butyldimethylsilyl groups. 

In the case of hydrohalomethanes, two alternative routes are possible for the transfer reaction, either halogen- or hydrogen-atom abstraction. The hydrogen transfer is favoured for chloroform, because the C-H bond is weaker than the C-Cl bonds. 

An additional effect of the use of an organic medium in the catalyst preparation is creation of mote defects in the crystalline lattice when compared to a catalyst made by the aqueous route (123). These defects persist in the active phase and are thought to result in creation of strong Lewis acid sites on the surface of the catalysts (123,127). These sites ate viewed as being responsible for the activation of butane on the catalyst surface by means of abstraction of a hydrogen atom. 

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