Radical reaction pathways

Accepting all three of the basic postulates, the origin of CIDNP can now be described qualitatively. Figure 2 illustrates a variety of different radical reaction pathways known to produce polarized spectra. 

The same group recently disclosed a related free radical process, namely an efficient one-pot sequence comprising a homolytic aromatic substitution followed by an ionic Homer-Wadsworth-Emmons olefination, for the production of a small library of a,/3-unsaturated oxindoles (Scheme 6.164) [311]. Suitable TEMPO-derived alkoxy-amine precursors were exposed to microwave irradiation in N,N-dimethylformam-ide for 2 min to generate an oxindole intermediate via a radical reaction pathway (intramolecular homolytic aromatic substitution). After the addition of potassium tert-butoxide base (1.2 equivalents) and a suitable aromatic aldehyde (10-20 equivalents), the mixture was further exposed to microwave irradiation at 180 °C for 6 min to provide the a,jS-unsaturated oxindoles in moderate to high overall yields. A number of related oxindoles were also prepared via the same one-pot radical/ionic pathway (Scheme 6.164). 

The reaction of [H2C(SiMe3)2C]2Si 180 with cyclopropylmethyl chloride proceeds via ring opening and formation of product 192 containing a spirocyclic Si atom, whose formation can be attributed to a radical-reaction pathway... 

Radical fragmentation of 2-nitrophenyl-azo-trityl resin was studied in the presence of various radical acceptor solvents to elucidate possible radical reaction pathways. When using benzene as solvent, only 2-nitro-bi-phenyl was formed as the product of radical substitution reaction (SNR) in 67% yield. Hydrogen-radical abstraction from the polymer backbone (e.g., from the benzylic units of polystyrene) was completely suppressed. When toluene was used as solvent, a mixture of the following products was obtained nitrobenzene, 4-methyl-2 -nitrobisphenyl, 2-methyl-2 -nitro-bisphenyl, and 3-methyl-2 -nitrobisphenyl (9 9 1 1). In the case of toluene, the nitro-aryl radicals undergo H-abstraction with radical substitution as a competing reaction pathway. These results indicate that H-abstraction... 

As examples of the calculation of OH radical reaction rate constants using the method discussed above (Kwok and Atkinson, 1995), the OH radical reaction rate constants for lindane [y-hexachlorocyclohexane cyclo-(-CHCl-)6], trichloroethene (CHC1=CC12), 2,6-di-tert-butylphenol, and chloropyrofos appear below. As the section dealing with OH radical addition to aromatic rings mentions, at present the rate constant for the reaction of the OH radical with anthracene (and other PAH) cannot be estimated with the method of Kwok and Atkinson (1995). In carrying out these calculations, one first must draw the structure of the chemical (the structures are shown in the appendix to Chapter 1). Then one carries out the calculations for each of the OH radical reaction pathways which can occur for that chemical. 

Further investigations on the photochemical bissilylation of C6o with disilanes [275c] indicate that 3C o and Qo do not play an important role in these reactions due to the fact that no absorption band for the C6o radical anion is observed in the course of the reaction and the decay of 3C 6o is not accelerated by addition of the disilanes. Another hint for the involvement of the phenylsilyl radical is that no product is formed upon irradiation at >300 nm where the cleavage of the disilane does not take place. The radical reaction pathway is further confirmed by a lower yield obtained by addition of radical scavengers. 

Azidoselenenylations are known as well in these reactions, the azide ion serves as a nitrogen nucleophile.118-120 The resulting azides can then be used for further transformations to modify the nitrogen functionality. Under certain reaction conditions, however, a radical reaction pathway is possible leading to non-stereospecific addition products.121... 

Triorganotin hydrides are common reductants in a large number of organic and organo-metallic syntheses. In many cases these reductions proceed by a normal radical reaction pathway. However, ionic reductions using tin hydrides also occur, but they were only sparsely investigated. Studies of ionic reduction processes with tin hydrides by using Sn NMR are found elsewhere . 

Newcomb M, Miranda N. Kinetic resnlts impheating a polar radical reaction pathway in the rearrangement catalyzed by a-methyleneglntarate mntase. J. Am. Chem. Soc. 2003 125 4080-4086. 

A number of methyl- and ethyl-substituted cyclopropanes 7 were brominated under conditions that guaranteed a radical reaction pathway (irradiation at — 78°C in dichloromethane). Methylcyclopropane (7a) underwent exclusive cleavage of the bond next to the substituent to give a quantitative yield of 1,3-dibromobutane (8a). Analogous results were obtained with ethylcyclopropane (7b) and 1,1-dimethylcyclopropane (7c). In 1,2-dimethylcyclopropane (7d) the bond between the substituted and the nonsubstituted carbon atom was cleaved. Higher substituted cyclopropanes 7e and 7f showed a similar reaction course. 

The oxidation of cycloalkanes or alkylarenes with molecular oxygen and acetaldehyde as a co-reductant takes place efficiently in scC02 under mild conditions. No catalyst is required and high-pressure ATR-FTIR online measurements showed that a radical reaction pathway was heterogeneously initiated by the stainless steel of the reactor walls (Equation 4.34) [67]. 

The major OH radical reaction pathway is OH radical addition to the aromatic ring to yield hydroxycyclohexadienyl or alkyl-hydroxycyclo-hexadienyl radicals [reaction (63b)]. The reported rate constants for the reactions of the hydroxycyclohexadienyl-type radicals are in reasonable agreement, and the most recent data (Knispel et al., 1990 Zetzsch et al., 1990 Goumri et al., 1990, 1992) show that the hydroxycyclohexadienyl and methylhydroxycyclohexadienyl radicals both react rapidly with N02 with similar room-temperature rate constants of 3 X 10 11 cm3 molecule-1 s-1. The corresponding reactions with 02 have much lower reported room temperature rate constants, of 1.8 X 10-16 cm3 molecule-1 s-1 for the hydroxycyclohexadienyl radical and 5 X 10 16 cm3 molecule-1 s-1 for... 

Studies show that the measured composition of the product mixture at constant temperature depended on the water density (Fig. 7.7). This was taken as an indication that these products could be formed by competing ionic and free-radical reaction pathways. Usually in gas-phase kinetics the product composition changes with temperature because of the different activation energies and, to a minor extent with pressure, mainly because of the concentration effect on bimolecular elementary reaction steps. In water, the drastic dependence on pressure is likely a consequence of the competition between reactions with different polarity. Free radical reaction rates (involving large free radicals beyond the RRKM high-pressure limit, see, for example, [25]) should decrease with pressure as a result... 

The experimental and calculated conversions are in good accordance except in the temperature range in which neither the ionic nor the free-radical reaction pathways dominate. This effect can also be seen in Fig. 7.8. At around 420 C the experimental reaction-rate constant is higher than the calculated one. At around 392 °C the experimental reaction-rate constant is lower than the calculated one. 

The ab initio results suggest that it is unlikely that water-kerogen interactions occur by a hydrocarbon thermal radical reaction pathway. This conclusion is supported by experimental and natural observations. For example, at 330°C, P-scission of an alkyl radical is 300 times faster than hydrogen abstraction from water so olefin formation will greatly exceed saturates formation (Ross 1992). However, formation of large amounts of olefin in hydrous pyrolysis has not been reported (Larson 1999) and olefins are rare components of crude oils (Hunt 1996). 

In 2008, Itami reported a transition-metal-free C-H arylation of simple azines such as pyrazines, pyrimidines, pyridazines, and quinoxalines with aryl iodides under the influence of KOt-Bu. Unfortunately, this reaction showed no regios-electivity due to its radical reaction pathway (e.g., the ratio for the arylation of... 

Apart from the described radical reaction pathways, there are several important side and consecutive reactions that also proceed in the cracking furnace. The higher the product concentration in the stream (i.e., at high feedstock conversion), the higher is the probability of these side and consecutive reactions. Important side and consecutive reactions include isomerization, cyclization, aromatization, alkylation, and also condensation reactions. The aromatic compounds found in the steam cracker product stream are formed, for example, by cycloaddition reactions of alkenes and dienes followed by dehydrogenation reactions. Moreover, monoaromatic compounds transform into aromatic condensates and polyaromatics (see also Scheme 6.6.2) by the same reactions. Typically, more than 100 different products are found in the product mixture of a commercial steam cracker. 

Ferino et al. [242], exploring the reaction of2-methylfuran to 2-methylthio-phene on Me-Y zeolites (Me = Li, Na, K, Rb, Cs), proposed ionic and radical reaction pathways. The ratio of these two pathways was found to be directly correlated to the partial charge on the oxygen as calculated by the Sanderson electronegativity equaUzation principle. The selectivity to form 2-methylthiophene increased in the order Li,Na-Y < Na-Y < K,Na-Y < Rb,Na-Y = Cs,Na-Y, which is in Hne with the increasing basicity of these zeolites. As the catalytic activity exhibited a rather complex behavior due to the contribution of the two reaction pathways, more work seems necessary to allow use of this reaction on a broader basis. 

The copper-catalyzed synthesis of imidazo[l,2-a]pyridines via C—H activation using oxygen as the sole oxidant has been reported by Adimurthy et al. ° The reaction proceeds in the presence of the radical-scavenger TEMPO, thns confirming that it does not occur via a radical reaction pathway. 

After a long discussion [107 to 111, 116, 117, 121] NH2 + NO N2 +H20 (a) is accepted as the main reaction pathway, as already pointed out in an earlier mass-spectrometric investigation [43], contributing about 85 to 90%. The water obtained in this highly exothermic reaction was found to be vibrationally excited [43, 111]. The production of OH radicals (reaction pathway (b)) contributes about 10 to 15% of the total reaction. This is in agreement with earlier determinations of a quantum yield of c )(N2) = 1 for N2 molecules [58, 59, 94, 95, 125]. A more pronounced contribution of the reaction channel (b) would lead to a significantly higher quantum yield of c )(N2) = 2 due to secondary NH2 formation, since OH is an efficient chain carrier at any temperature via OH + NH3 NH2 + H20 (NH3 is available in excess in the photolysis systems). The problem that is not yet completely solved is whether channel (b) should be written... 

Radical reaction pathways govern the crosslinking and degradative reactions of UHMWPE. For these reactions to occur, macroradicals must be induced in the polymer, for example by thermal decomposition of hydroperoxides or by high-energy radiation, which leads to homolytic bond scissions with the production of alkyl macroradicals. In previous chapters, we have often referred to crosslinking... 

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