Hydrogen transfer from hydrocarbons

One reaction model that explains these results has been proposed [165]. The hydrogen atom is transferred to the ethylene molecule that is weakly adsorbed on top of the ethylidyne and in the second layer perhaps by forming an ethyl idene intermediate. This model of hydrogen transfer from hydrocarbons to ethylene was first proposed by Thomson and Webb [175]. This mechanism is of the Eley-Rideal type and is characterized by low activation energy and structure insensitivity. 

CgQ with this Zr complex, a red solution is formed, unlike the green solution ofr transition metal complexes of Cjq. The structure of the air-sensitive Cp2ZrClC5oH was confirmed by NMR spectroscopy. The hydrogen transferred from the Zr to CgQ resonates at 5 = 6.09, a typical value for fullerenyl protons [83]. Hydrolysis of Cp2ZrClC5oH with aqueous HCl provides access to the simplest hydrocarbon C5QH2 (30, Scheme 7.14). Spectroscopic characterization of CggH2 showed that the compound is the isomerically pure 1,2-addition product. 

Hydrogen Atom Transfer from Hydrocarbons to Peroxy Radicals. The ready conversion of one chain carrier to another in hydrocarbon oxidations by the addition of a hydroperoxide is illustrated in Table VII. 

Hydrogen Transfer Radical reacts with a hydrocarbon H-transfers from hydrocarbon to radical forming a lower-molecular-weight compound and a new radical... 

Copper(I) can also be regenerated by electron transfer oxidation of radicals produced by hydrogen transfer from solvent by alkoxy radicals.101 Thus, reactions carried out in hydrocarbon solvents will produce alkyl radicals that are oxidized by Cu(II) at rates approaching diffusion control.95-100... 

The products from these hydrogenations were separated into gases (analyzed by G.C.), water (analysed by azeotropic distillation), insolubles (CH2CI2 insolubles), asphaltene (CH2CI2 soluble/X4 insoluble) (Shell X4 40-60 C b.p. light petroleum), oils (CH2CI2 soluble/X4 soluble). Hydrogen transferred from the donor solvent was determined by G.L.C. analysis of the ratio of tetralin to naphthalene in the total hydrocarbon liquid product. 

The reaction is sensitive to steric hinderance. Aromatic ketones are reduced to hydrocarbons. Unsaturated ketones are fully reduced and with no selectivity. Complexes of the type Ir(Chel)(CH2=CH2)2Cl, with Chel = 2,2 -bipyridine or phenantholine derivatives, behave as catalyst precursors for hydrogen transfer from isopropanol to ketones and Schiff bases. Potassium hydroxide is required as cocatalyst to convert the isopropanol coordinated to the Ir(I) ion, in the neutral isopropoxy derivative. Enolates that are present would act as inhibitors when coordinated to the cationic derivative. Ethylene complexes are better precursors than the corresponding cyclooctadiene derivatives, because they are activated more easily and more completely, and they show high catalytic activity. The most active complexes is the 3,4,7,8-Me4 phen derivative, which, at 83°C, gives turnovers of up to 2850 cycles/min. Reduction of 4-r-butylcyclohexanone affords 97% of the tra/u-alcohol. 

A single empirical equation (I) was developed [21], which allows the prediction of rate constants k for hydrogen transfer between the hydrogen donor XH and any radical Y in any solvent S based on the following parameters (i) the rate constant k° of hydrogen transfer from XH in a reference solvent (a saturated hydrocarbon with no hydrogen bond acceptor properties), (ii) a parameter 2, which... 

The question of carbonium ion formation from saturated hydrocarbons was considered in (,1) by the writer when the possibility of participation by olefins from thermal cracking was mentioned. However, it was only somewhat later that this suggestion was seriously adopted ( ). Then it was postulated that even traces of unsaturated hydrocarbons can activate saturated hydrocarbons by first forming a carbonium ion by proton addition. This ion can then extract a hydride ion by hydrogen transfer from the paraffin or cycloparaffin. This initiates a sort of chain reaction in which new carbonium ions are formed by hydrogen transfer with a steady-state population of ions on the catalyst surface. 

Scheme 3.38 Hydrogenation of benzene or toluene to saturated hydrocarbons (i) and hydrogen transfer from isopropanol to benzaldehyde (ii).

It has also been demonstrated that trialkylsilanes, which are poor radical-based reducing agents due to their low hydrogen donation abilities [79], can reduce alkyl halides and xanthates (equation (36)) to the corresponding hydrocarbons in the presence of a catalytic amount of an alkanethiol [80]. The reaction consists of a chain process in which the key propagation step is the hydrogen transfer from the silane to the alkanethiyl radical. This approach has also been used for the hydrosilylation of chiral alkenes by Ph2SiH2 [81]. 

Products from catalytic cracking units are also more stable due to a lower olefin content in the liquid products. This reflects a higher hydrogen transfer activity, which leads to more saturated hydrocarbons than in thermally cracked products from delayed coking units, for example. 

Aquilante and Volpi indicate (2) that propanium ions formed by proton transfer from H3 + are not collisionally stabilized at propane pressures as great as 0.3 mm. and that they decompose by elimination of hydrogen or a smaller saturated hydrocarbon to form an alkyl carbonium ion. Others (16, 19) have proposed one or the other of these fates for unstabilized propanium ions. Our observations can be rationalized within this framework by the following mechanisms ... 

In addition to hydrogen abstraction by peroxyl radical from another molecule, the reaction of intramolecular hydrogen transfer occurs in peroxyl radicals in oxidized hydrocarbons (see Chapter 2). 

FIGURE 6 Speculative mechanism of Crl hydrocarbon biosynthesis from fatty acid hydroperoxides in algae. Homolytic cleavage of the hydroperoxide is assumed to give an allyl radical, which cyclizes to the thermolabile (1S,2R)-cyclopropane. The sequence is terminated by transfer of a hydrogen radical from C(16) to the -X-0 function. The cyclopropane rearranges to (6S)-ectocarpene as shown in Figure 4. 

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