Ionic and radical pathways

Under similar profiles of raising in temperature, it was shown that the selectivity favoring 1,2 Stevens rearrangement is exemplified under the action of microwaves. A tentative explanation can be to consider that, under the action of radiation, the more polar mechanism (1,2 ionic shift) is favored when compared to less polar one (2,3 radical shift). Maybe this result is indicative of a competition between ionic and radical pathways. 

In addition to their use for the calibration of rates for radical reactions, radical clocks can be employed to distinguish between ionic and radical pathways. In the simplest embodiment of this idea, a suitable clock reaction that undergoes a known fast rearrangement with easily identifiable products is incorporated into the reaction system to be studied. This approach has been exploited in the pioneering work of Newcomb and co-workers in studies of the mechanism of cytochrome P450 oxidation reactions [13]. Newcomb has developed a range of ultrafast radical clocks able to detect radical species with lifetimes of 80-200 fs. 

Miscellaneous examples of the photoaddition of solvent and other simple molecules to nitrogen-containing systems have been described. Methoxylation, methylation, and hydroxymethylation arising via ionic and radical pathways have been observed on irradiation of dimethyl pyridine-2,4-dicarboxylate in methanol. The photomethoxylation of methyl pyridine-2-carboxylate in acidified methanol is facilitated by added 4-substituted pyridines such as 4-cyanopyridine an excited complex is thought to be involved. 

Scheme 6.20. A representation of the ionic and radical pathways for reaction of hydrogen bromide (HBr) with a generic terminal alkene [R2COCH2] showing both Markownikoff ...

The seminal work of Hassner has shown that halogen azides react with alkenes both in ionic and radical pathways leading to opposite regiochemistry (Scheme 8.13). The nature of the reagent and the polarity of the media play a fundamental role. Iodine azide reacts via an ionic pathway whereas the two mechamsms may operate for bromine and chlorine azides. Apolar solvents such as pentane and irradiation of the reaction mixture favor the radical process. 

Nature has effectively designed chemical events that produce halogenated compounds by both ionic and radical pathways. Cyclic halogenated natural products are produced by the electrophihc activation of olefins and the subsequent capture of the resultant cationic species by a nucleophilic motif of the molecules. The stereochemical... 

Lead(IV) in acidic media has been found to promote oxidative addition of Cl-, CF3CO2-, AcO-, MeS03- and CIO4- to cyclohexene, 1-hexene and styrene433. Sonochemical switching from ionic to radical pathway in the reactions of styrene and fraws-/l-methylstyrene with (AcO Pb has been observed434. 

The Bond-Forming Initiation theory (Scheme 1), originally proposed in 1983, extends the Huisgen hypothesis and proposes that these same tetramethylenes are the true initiators for the observed spontaneous polymerizations, and that this concept is valid for both ionic and radical polymerizations [9, 10]. The tetramethylenes offer a lower energy pathway for initiation than ion radicals. 

In the dibenzoylperoxide decompositions the effects are caused in pairs which are themselves part of a minor reaction pathway. The same may be true for other reactions, especially of the rearrangement type where ionic and radical intermediates may be present simultaneously. CIDNP effects are evidence for radical intermediates, though others may be present as well, and for quantitative studies CIDNP has to be combined with other techniques to elucidate the relative importance of various possible pathways. 

At higher energies, several additional reaction pathways open. These are shown in reactions (6) to (11). These reactions are endothermic in all cases, and their cross sections can be analyzed to provide thermodynamic information regarding the products. A particularly interesting aspect of reactions (6) to (8) is that both ionic and radical silicon hydrides are formed such that coupled information about the cations and neutrals can be obtained from these data. This is discussed in more detail in Section III.E. 

Reductive elimination is the product-forming step in some of the most important catalytic cycles, including hydrogenation, the Monsanto acetic acid process, and various types of cross-couplings. For this reason, detailed studies of this process have been conducted. Hrese studies have revealed examples of reductive eliminations to form H-H and C-H bonds, as well as reductive eliminations to form C-G and C-X bonds (in which X = halide, amide, alkoxide, thiolate, and phosphide). The mechanisms of these processes include the same pathways as have been deduced for oxidative addition (i.e., concerted, ionic, and radical), because reductive elimination is the same as oxidative addition, but in the reverse direction. 

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. 

We have now seen two pathways for adding HBr across a donble bond the ionic pathway (which gives Markovnikov addition) and the radical pathway (which gives anti-Markovnikov addition). Both pathways are actnally in competition with each other. However, the radical reaction is a mnch faster reaction. Therefore, we can control the regiochemistry of addition by carefully choosing the conditions. If we use a radical initiator, like ROOR, then the radical pathway will predominate, and we will see an anti-Markovnikov addition. If we do not use a radical initiator, then the ionic pathway will predominate, and we will see a Markovnikov addition ... 

Several methods and reaction pathways have been reported for the conversion of glycerol in the literature, such as etherification, esterification [1], and oxidation [2], Via ionic dehydration acetol [3] and acrolein can be produced. The radical steps result in aldehydes, allyl alcohol, etc. [4], If the dehydration is followed by a hydrogenation step, propanediols (1,2- or 1,3-) can be obtained [5-6]. 

It has been reported that (TMS)3SiCl can be used for the protection of primary and secondary alcohols [55]. Tris(trimethylsilyl)silyl ethers are stable to the usual conditions employed in organic synthesis for the deprotection of other silyl groups and can be deprotected using photolysis at 254 nm, in yields ranging from 62 to 95%. Combining this fact with the hydrosilylation of ketones and aldehydes, a radical pathway can be drawn, which is formally equivalent to the ionic reduction of carbonyl moieties to the corresponding alcohols. 

The addition of hydrogen halides to simple olefins, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule.116 When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 751).137 It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCI only rarely. In the rare cases where free-radical addition of HCI was noted, the orientation was still Markovnikov, presumably because the more stable product was formed.,3B Free-radical addition of HF, HI, and HCI is energetically unfavorable (see the discussions on pp. 683, 693). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides (4-9). Markovnikov addition can be ensured by rigorous purification of the substrate, but in practice this is not easy to achieve, and it is more common to add inhibitors, e.g., phenols or quinones, which suppress the free-radical pathway. The presence of free-radical precursors such as peroxides does not inhibit the ionic mechanism, but the radical reaction, being a chain process, is much more rapid than the electrophilic reaction. In most cases it is possible to control the mechanism (and hence the orientation) by adding peroxides... 

The addition of thiols to C—C multiple bonds may proceed via an electrophilic pathway involving ionic processes or a free radical chain pathway. The main emphasis in the literature has been on the free radical pathway, and little work exists on electrophilic processes.534-537 The normal mode of addition of the relatively weakly acidic thiols is by the electrophilic pathway in accordance with Markovnikov s rule (equation 299). However, it is established that even the smallest traces of peroxide impurities, oxygen or the presence of light will initiate the free radical mode of addition leading to anti-Markovnikov products. Fortunately, the electrophilic addition of thiols is catalyzed by protic acids, such as sulfuric acid538 and p-toluenesulfonic acid,539 and Lewis acids, such as aluminum chloride,540 boron trifluoride,536 titanium tetrachloride,540 tin(IV) chloride,536 540 zinc chloride536 and sulfur dioxide.541... 

Depending on whether the electron pair of the broken bond is shared or not by the two new entities, the reaction sequence will involve either an ionic or a free-radical pathway, as shown in Equation (4.49) and Equation (4.50). 

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