Radical cleavage

Methyl free radicals, generated either by thermolysis of lead tetracetate in acetic acid solution (401) or by radical cleavage of dimethylsulfoxide by H2O2 and iron (II) salts (408), afford 2- and 5-methylthiazole in the proportion of 86 and 14%, respectively, in agreement with the nucleophilic character of alkyl free radicals and the positive charge of the 2-carbon atom of the thiazole (6). 

In contrast to the extensive body of work on the preparation of these zinc carbenoids, few investigations are on record concerning the mechanism of the Furu-kawa method for carbenoid formation. Two limiting mechanisms can be envisioned - a concerted metathesis via a four-centered transition structure or a stepwise radical cleavage-recombination (Scheme 3.11). 

In free-radical substitution, Y- is usually produced by a previous free-radical cleavage, and X goes on to react further. 

The (a-sulfanylalkyl)phosphonates and their difluorinated derivatives are also, by easy radical cleavage of the C-S bond, useful precursors of phospho-nomethyl or phosphonodifluoromethyl radicals, which can be added to double bonds and so, introduced in a variety of structures. Besides, the use of phospho-nodithioformates as radical trapping agents and their use as RAFT reagent for controlled polymerization open a new interesting field of appHcation for these dithioesters. 

Besides such dissociation into long-lived radicals in solution, numerous examples are known of radical cleavage in the gas phase into unstable or reactive radicals. Two factors, the strain of hydrocarbon molecules and the stability of the radicals, are suggested as the major controlling factors for radical fission (Riichardt and Beckhaus, 1980, 1986). 

In contrast to the abundant examples of radical cleavage, only a few proposals of ionic cleavage of carbon-carbon a bonds have been put forward in the long saga of mechanistic studies on heterolytic cleavage reactions. 

Barondeau DP, Kassmann CJ, Tainer JA, Getzoff ED (2007) The case of the missing ring radical cleavage of a carbon-carbon bond and implications for GFP chromophore biosynthesis. J Am Chem Soc 129 3118-3126... 

Syntheses of the monooxaadamantane skeleton may be classified as follows (a) addition of electrophiles to bicyclononadiene (39) (b) ring closure of bicyclononadione (31) or its monomethylene analog (35) (c) dehydration of alcohols of the type 105 (d) radical cleavage of 2-ada-matanol (106) (e) miscellaneous. 

Both thermodynamic and kinetic factors are involved in the competition between concerted and stepwise mechanisms. The passage from the stepwise to the concerted situation is expected to arise when the ion radical cleavage becomes faster and faster. Under these conditions, the rate-determining step of the stepwise process tends to become the initial electron transfer. Then thermodynamics will favor one or the other mechanism according to equation (18). AG )eav is also the standard free energy of cleavage of the ion radical. 

ROLE OF SOLVENT IN ION-RADICAL CLEAVAGE AND IN STEPWISE VS. CONCERTED COMPETITIONS... 

In the stepwise case, the intermediate ion radical cleaves in a second step. Adaptation of the Morse curve model to the dynamics of ion radical cleavages, viewed as intramolecular dissociative electron transfers. Besides the prediction of the cleavage rate constants, this adaptation opens the possibility of predicting the rate constants for the reverse reaction (i.e., the reaction of radicals with nucleophiles). The latter is the key step of SrnI chemistry, in which electrons (e.g., electrons from an electrode) may be used as catalysts of a chemical reaction. A final section of the chapter deals... 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

The intron group I ribozymes feature common secondary structure and reaction pathways. Active sites capable of catalyzing consecutive phosphodi-ester reactions produce properly spliced and circular RNAs. Ribozymes fold into a globular conformation and have solvent-inaccessible cores as quantified by Fe(II)-EDTA-induced free-radical cleavage experiments. The Tetrahy-mem group I intron ribozyme catalyzes phosphoryl transfer between guanosine and a substrate RNA strand—the exon. This ribozyme also has been proposed to use metal ions to assist in proper folding, to activate the nucleophile, and to stabilize the transition state. ... 

Chemical/Physical. In the gas phase, cycloate reacts with hydroxyl and NO3 radicals but not with ozone. With hydroxy radicals, cleavage of the cyclohexyl ring was suggested leading to the formation of a compound tentatively identified as C2H5(Cff0)NC(0)SC2H5. The calculated photolysis lifetimes of cycloate in the troposphere with hydroxyl and NO3 radicals are 5.2 h and 1.4 d, respectively. The relative reaction rate constants for the reaction of cycloate with OH and nitrate radials are 3.54 x lO " and 3.29 x 10 cm /molecule-sec, respectively (Kwok et al., 1992). 

Yeh and Falvey 1991). However, the cyclobutyl dimer of a quinone, and a dithymoquinone, also cleaves on single-electron rednction but much slower than the pyrimidine dimers (Robbins and Falvey 1993). It is truly an unresolved issue as to why the anion-radical cleavage depicted in Scheme 3.72 is so facile. Water participation can probably decrease the barrier of the cycloreversion on physiological conditions (Saettel and Wiest 2001, 2006). Besides, hydration effect is known to increase the electron affinities of nucleic bases (Scheidt et al. 1998, Kim and Schaefer 2007a, 2007b). 

Hvattum, E. and Ekeberg, D., Study of the collision-induced radical cleavage of flavonoid glycosides using negative electrospray ionization tandem quadrupole mass spectrometry, J. Mass Spectrom., 38, 43, 2003. 

Compounds of type 43 and 44 tend to undergo radical cleavage readily. Carbonyl radicals formed are relatively stable and do not decarbonylate provided that an aromatic nucleus in the -position exerts a stabilizing effect. Minimal amount of by-products were observed when at least one of the substituents Rx or R2 was phenyl. The photochemical conversion 43 44 is... 

The temperature of decomposition depends also on steric factors The spiro compound 196 c with the five-membered dithiaphospha ring is much more stable than the spiro compound 196 d with the six-membered hetero ring. Only in the latter compound can a stable 1.2-dithia-cycloalkane be formed by radical cleavage Once again the preparative behaviour is in accord with the results of mass spectroscopy (p. 112). 

Homolytic (radical) cleavage. Each separating group takes one electron, e.g. 

In media of low proton availability (MeCN, dmf, hmpa)762, ylide formation was found and products derived from the radical cleavage of the phosphonium ion have been observed. The initial cation would interfere in the reaction process as an acid. A competition can exist between the one-electron pathway (dimerization, disproportionation of R ) and the two-electron pathway (ylide formation, Hofmann degradation, phosphine oxide formation) (Table 24). 

Initial electrode electron transfer Competitive radical cleavage Dimerization... 

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