Free radicals fragmentation reactions

Free radical fragmentation reactions provide a convenient method for the construction of carbon-carbon bonds. A wide variety of reagents are available for carrying out allylation, vinylation and alkynylation reactions of numerous types of... 

The thione function of the Barton ester 1 can be replaced by other elements such as Se (13) without significant alteration of the reactivity (Scheme 6). Although the starting esters are more air sensitive than the analogous N-hydroxypyridine 2-thione derivatives (1), the method has some advantages for example, the adducts of the free-radical fragmentation reactions of N-hydroxypyridine-2-selenone derivatives 13 (i.e., alkylpyridylselenides) can usually be more easily modeled and transformed to other chemical functions than thioethers. 

The first step in cracking is the thermal decomposition of hydrocarbon molecules to two free radical fragments. This initiation step can occur by a homolytic carbon-carbon bond scission at any position along the hydrocarbon chain. The following represents the initiation reaction ... 

Conversion of coal to benzene or hexane soluble form has been shown to consist of a series of very fast reactions followed by slower reactions (2 3). The fast initial reactions have been proposed to involve only the thermal disruption of the coal structure to produce free radical fragments. Solvents which are present interact with these fragments to stabilize them through hydrogen donation. In fact, Wiser showed that there exists a strong similarity between coal pyrolysis and liquefaction (5). Recent studies by Petrakis have shown that suspensions of coals in various solvents when heated to 450°C produce large quantities of free radicals (. 1 molar solutions ) even when subsequently measured at room temperature. The radical concentration was significantly lower in H-donor solvents (Tetralin) then in non-donor solvents (naphthalene) (6). 

The need to better control surface-initiated polymerization recently led to the development of controlled radical polymerization techniques. The trick is to keep the concentration of free radicals low in order to decrease the number of side reactions. This is achieved by introducing a dormant species in equilibrium with the active free radical. Important reactions are the living radical polymerization with 2,2,4,4-methylpiperidine N-oxide (TEMPO) [439], reversible addition fragment chain transfer (RAFT) which utilizes so-called iniferters (a word formed from initiator, chain transfer and terminator) [440], and atom transfer radical polymerization (ATRP) [441-443]. The latter forms radicals by added metal complexes as copper halogenides which exhibit reversible reduction-oxidation processes. 

Most important, the existence of an induction or inhibition period suggests a free-radical step in the decomposition of the thiophene ring. Further evidence for the free-radical nature of the reaction was obtained from experiments conducted under less severe conditions in order to isolate the initial ring-opened intermediate before subsequent loss of the one-carbon fragment. Efforts to isolate the initial decomposition product were unsuccessful. Apparently, the loss of the one-carbon fragment occurs rapidly, consistent with a free-radical chain reaction of some type. 

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

T7ery little has appeared in the literature concerning the radiation chemistry of covalent inorganic compounds in condensed phase. In the search for new, high energy oxidizers, it appears plausible that ion fragmentation, electron capture, ion-molecule reactions, and free radical combination reactions at low temperatures may be utilized. 

Reversible addition-fragmentation chain transfer (RAFT) polymerization using 2,2 -azobisisobutyronitrile and either A, A-dimethyl-5-thiobenzoylthiopropionamide or A-dimethyl-5-thiobenzoylthioacetamide as chain transfer agents has been used to prepare low polydispersity poly(A, A-dimethylacrylamide). The chain transfer agents were unusually effective in suppressing free radical termination reaction, thereby mimicking a living polymerization reaction. 

This chapter discusses free-radical substitution reactions. Free-radical additions to unsaturated compounds and rearrangements are discussed in Chapters 15 and 18, respectively. Fragmentation reactions are covered, in part, in Chapter 17. In addition, many of the oxidation-reduction reactions considered in Chapter 19 involve free-radical mechanisms. Several important types of free-radical reactions do not usually lead to reasonable yields of pure products and are not generally treated in this book. Among these are polymerizations and high-temperature pyrolyses. 

A free radical can add to CO or an isocyanide (RNC) in the course of a free-radical cyclization reaction, too, to give an acyl radical (RC=0) or an iminyl radical (RC=NR), either of which can undergo further reactions. In the following example, an alkyl radical adds to the terminal C of f-BuNC to give an iminyl radical. The iminyl radical then fragments to give t-Bu- and an alkyl cyanide N=CR. In a different substrate, the iminyl radical may undergo an addition or an abstraction reaction instead. 

Free Radicals - An atom or group of atoms with an odd or unpaired electron. Free radicals are highly reactive and participate in free radical chain reactions such as combustion and pol5mier oxidation reactions. Scission of a covalent bond by thermal degradation or radiation in air can produce a molecular fragment named a free radical. Most free radicals are highly reactive because of their unpaired electrons, and have short half lives. 

Allyltributyltin (5) is the most commonly used reagent for carrying out allylation reactions via a free radical fragmentation process [5]. Keck reported the first practical use of allyltributyltin for free radical allylation reactions in 1982 in the context of a synthesis of perhydrohistrionicotoxin [6]. Heating bromide 4 with allyltributyltin in the presence of AIBN as a radical initiator gave the allylated derivative 6 (Scheme 3) in high yield with complete control of stereochemistry. Similar transformations had proven to be very difficult by standard ionic reactions. 

Another approach to carrying out tin-free radical fragmentation processes, developed by Fuchs, utilizes trifluoromethyl sulfone, or triflone, derivatives. Fuchs first reported examples of free radical alkynylation reactions using acetylenic triflone 102 [62]. What is most remarkable about these reactions is that the radicals being alkynylated are formed from the cleavage of C-H bonds standard radical precursors are not required. For example, when tetrahydrofuran is mixed with triflone 102 at room temperature, alkynylation occurs a to the ether oxygen in 92% yield (Scheme 21). In this case, the radical chain process is most likely initiated by traces of peroxides in the THF. Similarly, unactivated alkanes such as cyclohexane will react with triflone 102 in good yield (83% for cyclohexane) when heated with a catalytic amount of AIBN. 

For a recent review of fragmentation processes in free radical polymerization reactions see D. Colombani, P. Chaumont, Prog. Polym. Set 1996, 21, 439-503. 

Figure 6.36. Proposed mechanism for the C17-C20 lyase reaction catalyzed by CYPI7A. The key steps involve addition of a P450 ferric peroxide species to the C20 carbonyl and subsequent free radical fragmentation of the peroxyhemiacetal,...

Addition polymers are most often produced by a free-radical polymerization mechanism. A free radical is a molecular fragment that contains an unpaired electron. It can result from the decomposition of a molecule, or the reaction of a molecule with another free radical. Free radicals are energetic species that are able to attack the relatively reactive C=C bond, resulting in incorporation of the monomer in the growing chain and generation of a larger free radical. These reactions produce very long chains very quickly. 

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