Self terminating radical reaction

Since the computational studies support the general perception of good or poor leaving groups in self-terminating radical reactions, but clearly contradict the experimental findings, radical cyclization cascades initiated by N-centered radical addition to alkynes are not terminated by homolytic p-fragmentation. 

Finally, cerium(IV) ammonium nitrate can serve as the radical source by itself, generating NO3 radicals If by photolysis. The addition of such radicals to cyclo-alkynes 20 initiates an interesting tandem reaction [12]. Transannular hydrogen atom abstraction by the vinyl radical 21 affords the intermediate 22, which undergoes a 5-exo cyclization to the radical 23. In the last step, the ketone 24 is formed by elimination of NO2 in moderate yield thus, the overall sequence can be described as a self-terminating radical reaction (Scheme 7). 

On addition of S04 to the triple bond in the lO-member cycloalkyne 24 and cyclo-aUcynone 27, a nonchain, and anionic, self-terminating radical cyclization cascade is induced. In the former reaction (equation 22) the bicyclic ketones 25 and 26 are formed, and in the latter reaction (equation 23) the a,/3-epoxy ketones 28 and 29 are formed in good yields. Because of the difficulty of oxidizing isolated triple bonds, 804 does not react as an electron-transfer reagent in these reactions but acts as a donor of atomic oxygen. 

Cascade Reactions Initiated by Addition of O-Centered Radicals to Alkynes (Self-Terminating Radical Oxygenations)... 

In this sequence, NO3 can formally be regarded as a synthon for O atoms in solution. Because the released NOa is a comparatively stable radical that does not initiate a radical chain process under the applied reaction conditions, this sequence is termed a self-terminating oxidative radical cyclization —the cascade-initiating radical contains the unreactive leaving group that is released in the final step of the reaction. In contrast to reactions performed under classical radical chain conditions and that are nearly always associated with a loss in overall functionality of the molecule, in self-terminating radical cyclizations the net amount of functional groups is retained in the system. 

Self-terminating radical oxygenations are not restricted to cyclic alkynes. Electro-and photochemically generated NO3 can be used for the oxidative cyclization of cycloalkyl-clamped alkynes 67 (Scheme 2.12). This reaction leads to formation of anelated tetrahydrofurans (68 with X = or pyrrolidines (68 with X = NTs,... 

Self-terminating radical cyclizations have been explored with both aminium and amidyl radicals using the reaction with cyclodecyne (52) as a model system. However, instead of N-containing compounds, the bicyclic ketones 53 and 54 were exclusively obtained. Their formation could be explained by the mechanism that is exemplary shown for reaction of the A-benzyl acetamidyl radical, AcBnN" (Scheme 2.19). ... 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

A characteristic of free radicals is the bimolecular radical-radical reaction which in many cases proceeds at the diffusion-controlled limit. These radical-radical reactions can occur either between two identical radicals or between unlike radicals, the two processes being known as self-termination and cross-termination reactions, respectively. 

A characteristic reaction of free radicals is the bimolecular self-reaction which, in many cases, proceeds at the diffusion-controlled limit or close to it, although the reversible coupling of free radicals in solution to yield diamagnetic dimers has been found to be a common feature of several classes of relatively stable organic radicals. Unfortunatly, only the rate constants for self-termination of (CH3)jCSO (6 x 10 M s at 173 K) and (CH3CH2)2NS0 (1.1 X 10 M s at 163K) have been measured up to date by kinetic ESR spectroscopy and consequently not many mechanistic conclusions can be reached. 

The extent to which chain oxidation is inhibited depends on the activity and concentration of the antioxidant. A specific activity of an antioxidant as a retarding agent should be expressed per unit concentration of the inhibitor. If the antioxidant terminates chains, chain self-termination by the reaction of peroxyl radical disproportionation should be taken into account. As a result, one obtains the following expression for estimation of the activity F of the introduced amount of the antioxidant [18] ... 

T Depends on rate constants for radical self-termination reactions. g Rate constants for reaction of Bu3SnD. 

Rate constants for the self-reactions of a number of tertiary and secondary peroxy radicals have been determined by electron spin resonance spectroscopy. The pre-exponential factors for these reactions are in the normal range for bi-molecular radical-radical reactions (109 to 1011 M"1 sec 1). Differences in the rate constants for different peroxy radicals arise primarily from differences in the activation energies of their self reactions. These activation energies can be large for some tertiary peroxy radicals (—10 kcal. per mole). The significance of these results as they relate to the mechanism of the self reactions of tertiary and secondary peroxy radicals is discussed. Rate constants for chain termination in oxidizing hydrocarbons are summarized. 

The mechanisms behind lipid oxidation of foods has been the subject of many research projects. One reaction in particular, autoxida-tion, is consistently believed to be the major source of lipid oxidation in foods (Fennema, 1993). Autoxidation involves self-catalytic reactions with molecular oxygen in which free radicals are formed from unsaturated fatty acids (initiation), followed by reaction with oxygen to form peroxy radicals (propagation), and terminated by reactions with other unsaturated molecules to form hydroperoxides (termination O Connor and O Brien, 1994). Additionally, enzymes inherent in the food system can contribute to lipid oxidization. 

Laroff GP, Fischer H (1973) The enol of acetone during photochemical reaction of 3-hydroxy-4-methyl-2-butanone and of acetone. Helv Chim Acta 56 2011 -2020 Lehni M, Fischer H (1983) Effects of diffusion on the self-termination kinetics of isopropylol radicals in solution. Int J Chem Kinet 15 733-757... 

Apparently, the phenoxyl radical reacts also at carbon [reaction (187) the typical site for self-termination Jin et al. 1993,1995]. The resulting product undergoes an H-shift and eliminates water [reactions (188) and (189)]. The first step is certainly very fast and is expected to occur on the sub-ms time scale (cf. Capponi... 

Since it is likely that the chain may be propagated by the reaction of hydroxyl or methoxy radicals with acetone, then reaction (41) followed by (31) constitutes a branching reaction which would be favored by a rise in temperature, but would be self-terminating due to the reactions that are second order with respect to chain centers, i.e., (21) and (27). 

Offhand, one may be surprised. Intuitively, one could expect equal concentrations of R and Y", because the radicals are formed with equal rates, and therefore, the products R—Y and R—R should be formed in the statistical ratio of 2 1. However, this is not the case, except during a very short initial period. The simple cause is that the transient radicals R disappear by their self-termination (4) and by the cross-reaction, whereas by definition the persistent radicals Y do not self-terminate but disappear only by the cross-reaction.1 Hence, every self-termination event of R (3) causes a buildup of excess Y, and this buildup continues as the time goes on. The permanently increasing concentration of Y accelerates the cross-reaction at the expense of the self-termination... 

However, one may reason now that one will not find a marked lifetime prolongation if the equilibrium constant is above a critical limit. Actually, a too small rate constant of the back-reaction must lead to a fast and complete conversion of R—Y to the final products R—R and Y Also, the self-termination of the transient radicals never ceases completely. Therefore, a true stationary state does not exist, and for the given reaction mechanism, the radical concentrations will always depend on time. Moreover, the apparent equilibrium and the lifetime prolongation of R—Y are only of transient nature, because at sufficiently long times one must always find only self-termination products and persistent radicals. 

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