Alkyl radicals preferential formation

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

The efficiency of product formation in solution is also controlled by the stabilities of the radicals. Stable radicals such as tertiary alkyl radicals or benzyl radicals lead to efficient decarbonylation in solution. Because of steric factors involving bulky groups, tertiary radicals tend to preferentially undergo disproportionation rather than radical combination and so the quantum yield of the products formed by disproportionation exceeds that of the radical combination product. 

It has also been shown that preferential formation of primary alkyl radicals occurs in the ring-opening of ring-substituted cyclopropyl(stannyloxy)methyl and cyclopropyl(hydroxy) methyl radicals . ... 

The subsequent transformations of alkoxy radicals containing strong C-0 bonds at the surface of V-Mo- catalyst with weakly bonded oxygen atoms yields preferentially formation of full CB products with some hydrogen evolution, while alkyl radicals stabilized on acid sites at the surface of RLH catalysts as a result of C-H bonds polarization in the strong field of metal-... 

With alkyl-substituted terminal alkenes various degrees of porphyrin N-alkylation accompany epoxidation. The N-alkylation take place exclusively at the unsubstituted terminus of the double bond and could be a concerted or non-concerted reaction. Formation of an intermediate carbocation on the path to N-alkylation can be excluded because it would require the preferential formation of a primary carbocation. Radicals, on the other hand, show much lower preference for substituted unsubstituted carbon, suggesting that an initial formation of a carbon radical, followed by its collapse to the N-alkyl porphyrin is possible. While carbon radicals cannot be discrete intermediates in the epoxidation reaction vide supra), they can be intermediates in N-alkylation, if N-alkylation is a side reaction to epoxidation. 

With some substrates, HCo(CO)4 is thought to transfer H- to the alkene. This tends to happen when the substrate radical is specially stabilized (eg, PhCH. —CH3 from PhCH=CH2). The radical may then recombine with the Co to give an alkyl. This may help account for the preferential formation of the iso aldehyde from styrene. 

Benzocyclopropene reacts with a variety of radical reagents (for example A -bromosuccinimide carbon tetrachloride bromotrichloromethane bromoform/benzoyl peroxide alkyl sulfide and ethane-1,2-dithiol with photolysis) to afford products derived from cleavage of the cyclopropane ring. The preferential mode of reaction consists of a chain reaction initiated by radical addition at Cl a followed by opening of the cyclopropyl radical to afford a benzyl radical. Yields are generally low except for the addition of the alkylsulfanyl radical, e.g. formation of 1, and no products derived from addition to the central tt-bond are formed. Cyclopropa[A]naphthalene reacts similarly with radicals and gives 2-methylnapthalene derivatives, while no addition to the central 7i-bond is observed. ... 

Figure 9 drastically simplifies the major reaction paths of alkyl-naphthalene components. Via H-abstraction and successive decomposition reactions, they can easily form, either naphthalenes with unsaturated side chains (vinyl, allyl or alkenyl side chains) or RSR and smaller decomposition products. The preferential radical attack on the alkyl side chain is in the benzyl position due to the weak hydrogen bond. This makes it easy to justify either the formation of RSR or the successive / -decomposition reaction to form vinylnaphthalene. The net result of the successive recombination and condensation reactions of these aromatic species is the formation of PAH of increasing molecular weight with a progressively lower hydrogen to carbon ratio. 

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