Initiation of radical chain reactions

Curran2 has reviewed recent applications of the tin hydride method for initiation of radical chain reactions in organic synthesis (191 references). The review covers intermolecular additions of radicals to alkenes (Giese reaction) as well as intramolecular radical cyclizations, including use of vinyl radical cyclization. 

Results of a chemical activation induced by ultrasound have been reported by Nakamura et al. in the initiation of radical chain reactions with tin radicals [59]. When an aerated solution of R3SnH and an olefin is sonicated at low temperatures (0 to 10 °C), hydroxystannation of the double bond occurs and not the conventional hydrostannation achieved under silent conditions (Scheme 3.10). This point evidences the differences between radical sonochemistry and the classical free radical chemistry. The result was interpreted on the basis of the generation of tin and peroxy radicals in the region of hot cavities, which then undergo synthetic reactions in the bulk liquid phase. These findings also enable the sonochemical synthesis of alkyl hydroperoxides by aerobic reductive oxygenation of alkyl halides [60], and the aerobic catalytic conversion of alkyl halides into alcohols by trialkyltin halides [61]. 

Ozone also reacts with ethane in the gas phase at room temperature. Rather than a direct molecular reaction, however, evidence points to the initiation of radical-chain reactions by the very small O-atom concentrations present in ozone at room temperature. Added oxygen scavenges the radicals and slows the build-up, leading to induction periods which may be in excess of 3 h. Recent advances in mechanistic investigations of gas-phase ozonolysis of alkanes have been reviewed. Oligomeric peroxides dominate the products of oxidation of nitrotoluenes with ozone in acetic acid. °... 

Simple fissions are responsible for the initiation of radical chain reactions, and some examples are... 

Symmetric azo compounds are useful initiators of radical chain reactions. However, in many instances, the radicals are released from the initial... 

Bateman, L., Hughes, H., Morris, A.L. 1953. Hydroperoxide decomposition in relation to the initiation of radical chain reactions. Disc. Faraday Soc. 14, 190-199. 

Autoxidation of saturated hydrocarbons or fragments occurs as a free radical-chain process. Azobis(isobutyronitrile) (AIBN, In-N=N-In) is frequently used as an initiator of radical-chain reactions [3]. For example, heptane (RH) is oxidized according to the following scheme [3c] ... 

Somewhat surprisingly, even M-L heterolyses can lead to initiation of radical chain reactions. In these reactions, the key feature is again the formation of CCI3 radicals (Equation (36)). 

Addition of HI to propene gave only 2-iodopropane, whether antioxidants or peroxides were added. Addition of HI to allyl bromide produced only l-bromo-2-iodopropane, and addition of HI to allyl chloride gave only l-chloro-2-iodopropane. The reason for the Markovnikov orientation may be that HI reacted with peroxides to give iodine, thus stopping initiation of radical chain reaction. Kharasch, M. S. Norton, J. A. Mayo, F. R. /. Am. Chem. Soc. 1940, 62, 81. 

Free-radical initiation of the chain reaction of oxidation can arise from the action of heat, light, metal ions and, sometimes, ozone on a variety of chemical moieties such as those indicated in Table 7.4. 

Initiator (Section 5.3) A substance with an easily broken bond that is used to initiate a radical chain reaction. For example, radical chlorination of alkanes is initiated when light energy breaks the weak Cl-Cl bond to form Cl-radicals. 

Lipid peroxidation (see Fig. 17.2) is a chain reaction that can be attacked in many ways. The chain reaction can be inhibited by use of radical scavengers (chain termination). Initiation of the chain reaction can be blocked by either inhibiting synthesis. of reactive oxygen species (ROS) or by use of antioxidant enzymes like superoxide dismutase (SOD), complexes of SOD and catalase. Finally, agents that chelate iron can remove free iron and thus reduce Flaber-Weiss-mediated iron/oxygen injury. 

Numerous autoxidation reactions of aliphatic and araliphatic hydrocarbons, ketones, and esters have been found to be accompanied by chemiluminescence (for reviews see D, p. 19 14>) generally of low intensity and quantum yield. This weak chemiluminescence can be measured by means of modern equipment, especially when fluorescers are used to transform the electronic excitation energy of the triplet carbonyl compounds formed as primary reaction products. It is therefore possible to use it for analytical purposes 35>, e.g. to measure the efficiency of inhibitors as well as initiators in autoxidation of polymer hydrocarbons 14), and in mechanistic studies of radical chain reactions. 

What is the difference between the systems initiator-RH and initiator- RH Oz that is responsible for the initiation of the chain reaction It is apparent that, in contrast to the former system, the radical R in the latter system is regenerated via a cyclic sequence of chemical conversions. The analysis of this system and other similar systems suggests that a chain reaction can be sustained only in a system where radicals are involved in a cyclic sequence of conversions with the conservation of free valence. It is the multiple repetition of conversion cycles that leads to the chain reaction [2,3]. 

Initiation of a chain reaction involves the formation of free radicals from an initiator. Benzoyl peroxide is a commonly used initiator. 

Triethylborane in the presence of very small amounts of oxygen is an excellent initiator for radical chain reactions. For a long time it has been known that trialkylboranes R3B react spontaneously with molecular oxygen to give alkyl radicals (Reaction 4.7), but only recently has this approach successfully been applied as the initiation [22]. The reactions can be run at temperatures as low as — 78 °C, which allow for a better control of stereoselectivity (see below). 

Furthermore this chapter deals chiefly with polymerizations which are catalyzed by acid-acting catalysts. A comprehensive discussion of not only the thermal but even the photochemical and free radical-initiated polymerizations is outside its scope. The free radical-initiated reactions include those which are induced by metal alkylies, peroxides, oxygen and certain other substances. They depend on free radical initiation of a chain reaction whether or not these free radicals should be considered to be catalysts has been questioned because the radicals enter into the reaction chain and are part of the reaction product. 

Effect of S/V Ratio. The reactor wall plays two important roles in this oxidation. First, the reactor surface promotes the initiation of radical chain (12), but if it is exceedingly large, it adversely affects the total rate of the oxidation reaction (6, 12). Second, it accelerates heterogene-... 

Initiation normally requires molecules with weak bonds to undergo homolytic cleavage to produce free radicals. Since bond homolysis even of weak bonds is endothermic, energy in the form of heat (A) or light (hv) is usually required in die initiation phase. However, some type of initiation is required to get any free-radical reaction to proceed. That is, you must first produce free radicals from closed-shell molecules in order to get free-radical reactions to occur. Benzoyl peroxide contains a weak 0-0 bond that undergoes thermal cleavage and decarboxylation (probably a concerted process) to produce phenyl radicals which can initiate free-radical chain reactions. 

A catalyst is a substance that participates in a chemical reaction and thereby increases the reaction rate but without a net change in the amount of that substance in the system. The catalyst is used and regenerated during each set of microscopic events leading from reactants to products, and the overall free energy of the reaction is unaltered by the presence of the catalyst which must, therefore, be in small amount relative to reactants. In radical chemistry, a catalyst can be the initiator of a chain reaction or a carrier (see Chapter 10). 

Liochev, S. and I. Fridovich. 1986. The vanadate-stimulated oxidation of NAD(P)H by biomembranes is a superoxide initiated free radical chain reaction. Biochim. Biophys. Acta 250 139-145. 

Radical cyclization reactions can be carried out by treating an unsaturated alkyl iodide or bromide with Et2Zn in the presence of palladium(II) or nickel(II) complexes.28 Under these conditions, an intermediate Pd(0) or Ni(0) complex is formed which initiates a radical chain reaction providing a new cyclized zinc cyclopentylmethyl derivative which can be trapped with various electrophiles in the presence of a copper catalyst. A related cyclization can be... 

Soluble Co compounds are generally employed in the autoxidation of hydrocarbons, i.e., the oxidation with O2 as the oxidant. In neat hydrocarbons, low concentrations of Co compounds accelerate the autoxidation since the Co2+/Co3+ couple is excellent for decomposing alkyl hydroperoxides and thus initiates free radical chain reactions. However, at high conversions, the Co may be deactivated by formation of insoluble clusters with side products of the hydrocarbon autoxidation. Moreover, high concentrations of a Co compound may actually inhibit the reaction because Co also terminates radical chains by reaction with ROO radicals ... 

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