Azobis compounds, radical initiators

Azo compounds having functional groups that stabilize the radical products are especially reactive. The stabilizing effect of the cyano substituent is responsible for the easy decomposition of azobis(isobutyronitrile) (AIBN), which is frequently used as a radical initiator. 

The radical alkylation of ketones is achieved by their conversion into the desired N-silyloxy enamines 81 (Scheme 13). The reaction of 81 with diethyl bromomalonate in the presence of EtsB (0.5 equiv) in benzene was performed in open air and stirred at room temperature for 3h. With nitro compounds it is achieved by their conversion into the desired ]V-bis(silyloxy)enamines (82) (Scheme 13). When the reaction is carried out with 82 and alkyl iodides with an electron-withdrawing substituent at the a-position, using V-70 as radical initiator (2,2 -azobis(4-methoxy-2,4-dimethylvaleronitrile)), it underwent a clean radical alkylation reaction to yield an oxime ether. Successful radical alkylation of... 

The two most important classes of radical initiators are azo-compounds and peroxides (Fig. 10.1). The most commonly used azo-initiators are 2,2/-azobis(isobutyronitrile) (AIBN) and l,l/-azobis(cyclohexane-l-carbonitrile) (ACN). The shorter half-life of AIBN ( ti/2 = 1.24 h at 80°C in benzene) has led to international shipping restrictions and ACN (ti/2 = 29.61 h under the same conditions [l])isbecominganincreasinglyvaluablereplace-ment. As explained in Section 10.4, however, this appreciable difference in decomposition rate means that ACN cannot replace AIBN as an initiator for all radical reactions. Furthermore, AIBN is soluble in a wider range of solvents, polar as well as non-polar (including alcohols, acetonitrile and benzene), compared with ACN which is restricted to use in nonpolar solvents, such as benzene, toluene and cyclohexane. 

Radical initiators are thermally labile compounds, which decompose into radicals upon moderate heating or photolysis. These radicals initiate the actual radical chain through the formation of the initiating radical. The most frequently used radical initiators are azobis-isobu-tyronitrile (AIBN) and dibenzoyl peroxide (Figure 1.11). AIBN has has a half-life of 1 h at 80 °C, and dibenzoyl peroxide a half-life of 1 h at 95 °C. 

Bicyclic eight-membered ring phosphorus compounds are usually obtained by the free radical reactions of the corresponding alkenes with phosphines. In most cases, these reactions require the presence of such free radical initiators as azobis(isobutyronitrile) (AIBN) or azobis(isovaleronitrile) (VAZO). 

A thermal initiator is typically either an azo compound or a peroxide based initiator, e.g., 2,2 -azobis-isobutyronitrile. Each initiator molecule can decompose to give one or two primary radicals (R ) and possibly inert by-products. Initiation rate constants for several common thermal initiators are listed in Table 1. Each primary radical can attack a monomer molecule (M) to initiate a polymer chain (P ). The recombination... 

Only free radical polymerisation, which requires the formation of reactive free radical species to initiate polymerisation, appears to have been used to form MIPs. Free radicals are produced by the decomposition of an initiator species by the action of heat or light. Commonly used initiators are benzoyl peroxide and azobis compounds such as azobisisobutyronitrile (AIBN) or 2,2 -azobis(2,4-dimethylvaleronitrile) (ABDV) Figure 6.20). 

Most functional monomers and cross-linkers contain one or more vinyl functionalities. Polymerization of this type of compound for the preparation of MIPs is traditionally performed as a free-radical polymerization, initiated via either ther-molytic or photolytic homolysis of an initiator. One of the most commonly used free radical initiators for this purpose is 2,2 -azobis (isobutyronitrile) (AIBN). Other examples of free-radical polymerization initiators are phenyl-azo-triphenyl-methane, tert-butyl peroxide (TBP), acetyl peroxide, benzoyl peroxide (BPO), lauroyl peroxide, tert-butyl hydroperoxide and tert-butyl perbenzoate. 

A RAFT polymerization system consists of initiator, monomer, chain transfer agent, solvent, and temperature. RAFT polymerization can be performed by simply adding a chosen quantity of an appropriate RAFT agent (thiocarbonylthio compounds) to a conventional free radical polymerization. Usually the same monomers, initiators, solvents and temperatures can be used. Because of the low concentration of the RAFT agent in the system, the concentration of the initiator is usually lower than in conventional radical polymerization. Radical initiators such as Azobisisobutyronitrile(AIBN) and 4,4 -Azobis(4-cyanovaleric acid)(ACVA) are widely used as the initiator in RAFT. RAFT polymerization is known for its compatibility with a wide range of monomers compared to other controlled radical polymerizations. These monomers include (meth)acrylates, (meth)... 

AAPH-induced Lipid Peroxidation. 2,2 -azobis(2-amidinopropane) hydrochloride (AAPH) is a water-soluble and azo compound, which works as a radical initiator and causes lipid peroxidation. The effects of THpCs on AAPH-induced lipid peroxidation were determined according to the method described before (74, 24). Briefly, 1.4 mL of 50 mM linoleic acid-50 mM SDS micelles solution was added to the reaction vessel equipped with an oxygen electrode, kept at 37°C and stirred to saturate oxygen. Various concentrations of THpCs (0.1 and 1 mM) or H2O as a control was added to the mixture, kept for 2 min. 

The aerobic oxidation of nitrobenzene catalyzed by H5[PV2Moio04o] proceeded regioselectively to form 2-nitrophenol in 5% yield (14). On the other hand, a mixture of 2-, 3-, and 4-nitrophenols with a nearly statistical distribution ( 1 0.9 0.45 ratio) was obtained for the aerobic oxidation of nitrobenzene in the presence of l,l -azobis(cyclohexanecarbonitrile) as radical initiator [158]. This study provides the first example of an aerobic regioselective hydroxylation of an arene compound. Such a unique regioselective hydroxylation resulted from an intramolecular interaction between the POM and substrate. 

Such reactions can be initiated by free radicals, derived from compounds (initiators) such as benzoyl peroxide, ammonium persulphate or azobis-isobutyronitrile or by ionic mechanisms... 

The same research group has further performed radical carbonylation reactions on the same microreactor system [36]. First, alkyl halides were initiated and effectively reacted with pressurized carbon monoxide to form carbonyl compounds. The principle was subsequently successfully extrapolated to the multicomponent coupling reactions. 1-Iodooctane, carbon monoxide and methyl vinyl ketone were reacted in the presence of 2,2 -azobis(2,4-dimethylvaleronitrile) (V-65) as an initiator and tributyltin hydride or tris(trimethylsilyl)silane (TTMSS) as catalyst (Scheme 15). 

For perhaps a century, the primary interest in azo compounds has been in dye chemistry. Recently, aliphatic azo compounds, which are thermally less stable than their aromatic counterpart, have enjoyed attention as sources of free radicals for polymerization reactions. In this regard, an initiator such as a,a -azobis(isobutyronitrile) is important for two reasons ... 

In a similar way as has been described for syntheses of type al, the majority of examples of type b involve polycondensation of a,ea bifunctional, small molecule reaction partners. Some examples are the reaction of AIBN or AIBN derivatives with 1,4-cyclohexane bismethyl diamine78), 1,2-ethylene diamine78), 1,6-hexamethylene diamine 78-80 , bisphenol A 78,81 and mono-, di- and tetraethylene glycol 55-64 . In almost all case using the AIBN derivative 4,4 -azobis(4-cyano valeryl chloride), an interfacial polymerization was employed. These polymeric azo compounds could be used as initiators for radical block copolymerizations. 

In the first experiment we will chlorinate 1-chlorobutane because it is easier to handle in the laboratory than gaseous butane and we will use sulfuryl chloride as our source of chlorine radicals because it is easier to handle than gaseous chlorine. Instead of using light to initiate the reaction we will use a chemical initiator, 2,2 -azobis-(2-methylpropionitrile). This azo compound (R—N=N—R) decomposes at moderate temperatures (80-100°C) to give two relatively stable radicals and nitrogen gas ... 

Regarding the initiation process of polymerization, it can be started by y-radiation. It is a method that has been used for the synthesis of hydrogels of PEO as well as hydrogels based on vinyl monomers " in this latter case, azo-compounds such as 2,2-azo-isobutyroni-trile (AIBN)f or 2,2 -azobis (2-amidine-propane) dihydrochloride or V-SO, and aqueous salt solutions such as aqueous ammonium peroxodisulfate are also used. Among the monomers most used in the preparation of hydrogels through free-radical polymerization are 2-hydroxyethyl methacrylate (HEMA) and A-vinyl-2-pyrrolidone (VP). ... 

Although AIBN is a popular choice, other azo compounds can give superior results due to their varying half-life. Indeed, the nature of the substitution of a newly formed carbon radical plays an important role in their half-life, as can be seen from the following for 2, 2 -azobis(4-methoxy)-3,4-dimethylvaleronitrile (AMYN), with a t /2 in toluene of 1 h at 56 °C and 10 h at 33 °C. There are also hydrophilic azo compounds, such as 2, 2 -azobis(2-methylpropionamidine) dihydrochloride (APPH), with a t j2 in water of 10 h at 56 °C. Typically, 5-10 mol% of the initiator is added, either in one portion or via slow addition. There... 

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