Initiator radical reaction

Sonoelectrochemistry has been employed in a number of fields such as in electroplating for the achievement of deposits and films of higher density and superior quality, in the deposition of conducting polymers, in the generation of highly active metal particles and in electroanalysis. Furtlienuore, the sonolysis of water to produce hydroxyl radicals can be exploited to initiate radical reactions in aqueous solutions coupled to electrode reactions. 

Optically active iV-unprotected-2-pyrrolidinones 194 were obtained from selenocarboxylate or allylamine via radical cyclization and subsequent one-step cleavage of the C-O and C-N bond of the inseparable mixture of the two bicyclic oxyoxazolidinones 192 and 193 with -Bu4NF. The initial radical reaction is highly stereoselective. Products were obtained with ee up to 90%. The mandelic acid 195, which served as the chiral auxiliary in this method, was recovered with no loss of optical activity (Equation 33) <2003T6291>. 

Keywords Atom transfer reactions Boron C C bond formation Conjugate addition Radical initiators Radical reaction Tin-free... 

Figure 4.11. Examples of redox-initiated radical reactions. Samarium diiodide reduction of the bromide gives a radical that cyclizes faster than the second reduction reaction. Manganese triacetate oxidation of the P-keto ester gives an enol radical that is not further oxidized by the manganese reagent. The IBX oxidizes anilides to the corresponding radicals. Hexamethylphosphoramide = HMPA and Tetrahydrofuran = THE.

This reagent is useful for initiating radical reactions. Since it is a radical initiator, only small amounts are needed. For reactions at higher temperature, ACN (1,1 -azobis-1 -cyclohexanenitrile [2094-98-6]) has been suggested (see Example 2).1... 

Recently, carbon black was reported to catalyze coal liquefaction (41, 42) this may initiate radical reactions of bond breakage. 

Contrary to the noncatalytic Kolbe synthesis or other anodically initiated radical reactions at Pt anodes, the anodic functionalization and anodic coupling of vinyl compounds by direct anodic oxidation of olefins in alcohols as... 

Non-activated double bonds, e.g. in the allylic disulfide 1 (Fig. 10.2) in which there are no substituents in conjugation with the double bond, require high initiator concentrations in order to achieve reasonable polymerisation rates. This indicates that competition between addition of initiator radicals (R = 2-cyanoisopropyl from AIBN) to the double bond of 1 and bimolecular side reactions (e.g. bimolecular initiator radical-initiator radical reactions outside the solvent cage with rate = 2A t[R ]2 where k, is the second-order rate constant) cannot be neglected. To quantify this effect, [R ] was evaluated using the quadratic Equation 10.5 describing the steady-state approximation for R (i.e. the balance between the radical production and reaction). In Equation 10.5, [M]0 is the initial monomer concentration, k is as in Equation 10.4 (and approximately equal to the value for the addition of the cyanoisopropyl radical to 1-butene) [3] and k, = 109 dm3 mol 1 s l / is assumed to be 0.5, which is typical for azo-initiators (Section 10.2). The value of 11, for the cyanoisopropyl radicals and 1 was estimated to be less than Rpr (Equation 10.3) by factors of 0.59, 0.79 and 0.96 at 50, 60 and 70°C, respectively, at the monomer and initiator concentrations used in benzene [5] ... 

Stereochemistry in radical reactions for organic synthesis has not been studied very extensively, because mild or low temperature-promoted radical reaction methods are extremely limited and the stereoselectivity in radical reactions is generally rather poor. Recently, however, stereoselective organic synthesis with radical reactions has become popular, since mild radical reaction methods such as the Barton decarboxylation, Et3B-initiated Bu3SnH reaction, etc. have been developed. Normally, low temperature-initiated radical reactions induce high stereoselectivity. 

Since Et3B-initiated radical reactions with Bu3SnH under aerobic conditions, can be carried out at 0 °C or lower temperature, stereoselective reduction of reactive a-haloesters or a-haloketones can be performed. Eq. 10.1 shows Et3B-mediated Bu3SnH reduction of p-methoxy-a-bromo ester in the presence and absence of MgBr2 (Lewis acid) at 0 °C. Anti/syn diastereoselectivity depends on the absence or presence of a Lewis... 

Nickel catalysis is a very active field in organometallic and organic chemistry (selected reviews [3-7]). Complexes of all oxidation states are active in two-electron transfer processes, such as oxidative addition or reductive elimination as well as in single electron transfer initiating radical reactions. Through these processes, oxidation states from Ni(0) to Ni(III) can be easily accessed under mild conditions. Occasionally, Ni(IV) intermediates were also proposed. Apart from the vast number of Ni(II) complexes, a number of organonickel(I) complexes were characterized by X-ray crystallography and their potency as active species in catalytic cycles tested [8-10]. Either radical or two-electron reactivity was observed. Recently, the structure of some alkylnickel(III) complexes was also structurally elucidated [11]. 

The features of initiation of free radical reactions in polymers by dimers of nitrogen dioxide are considered. The conversion of planar dimers into nitrosyl nitrate in the presence of amide groups of macromolecules has been revealed. Nitrosyl nitrate initiates radical reactions in oxidative primary process of electron transfer with formation of intermediate radical cations and nitric oxide. As a result of subsequent reactions, nitrogen-containing radicals are produced. The dimer conversion has been exhibited by estimation of the oxyaminoxyl radical yield in characteristic reaction of p-benzoquinone with nitrogen dioxide on addition of aromatic polyamide and polyvinylpyrrolidone to reacting system. The isomerisation of planar dimers is efficient in their complexes with amide groups, as confirmed by ab initio calculations. 

Light, heat, or added peroxide is necessary for the reaction. Light or heat provides the energy needed for hemolytic bond cleavage to form radicals. Breaking the weak 0—0 bond of peroxides initiates radical reactions as well. 

Hydrosilylation of ethylene (equation 27) is the prototype for the addition of hydrosilanes across carbon-carbon double bonds, and is achieved by means of such metal complex catalysts as H2PtCl6, IRhCl(PPh3)3], [ RhCl(CO)2l2] (equation 28), 5 [C02(CO)8], [HCo(CO)4], [NiCl2(dmpf)], 0 [Pd(PPh3)4] and also with the aid of y-rays, which initiate radical reactions. ... 

The combination of Et3B and 02 is widely used to initiate radical reactions. The mechanism involves two unusual radical reactions. Addition of 02 to the empty orbital of BEt3 provides a one-electron bond between B and O, and a fragmentation follows to provide Et- and Et2BOO-. 

The function of MAD is not only to reduce the LUMO of the /l-carbon and to fix the conformation of the a,j9-unsaturated ester, but also to initiate radical reaction [208]. This was demonstrated when a similar reaction was performed without 138 (Scheme 6.162). Reaction of 163 occurred with equal effectiveness, although with a slight dechne in diastereoselectivity. 

Dihydrotagetone (95) has been synthesized by dibenzoyl peroxide-initiated radical reaction of 3-methylbutanal (80) and the diethyl acetal (96) of methacrolein the product 97 (obtained in 42% yield) could be converted conventionally to dihydrotagetone (95). The same author has described the direct radical addition of 82 to 3-methyl-3-butenyl acetate. The product, 98, in this case was deacetylated by pyrolysis over ceramic beads to dihydrotagetone (95). ... 

Molecular, free radical and ionic mechanisms were considered in detail by Grassie and Roche [227]. The absence of any secondary volatile products, which would result from an end initiated radical reaction, are strong arguments against a free radical mechanism. Furthermore, inhibitors of radical reactions have no effect on the depolymerization. In the presence of basic catalysts the reaction is accelerated and undoubtedly... 

Inorganic compounds can also act as radical initiators. Zinc chloride (ZnCl2) has been used to initiate radical reactions at —78 °C. In following case (Scheme 19), zinc chloride acts as a radical initiator as well as a chelating agent. 

The chemistry of radical reactions is rapidly advancing as a consequence of the discovery of the new initiators. Radical reactions at low temperatures using highly active initiators facilitate the generation of radicals at specific positions in the mole-... 

Togo and Yokoyama demonstrated the Et3B-initiated radical reaction using water-soluble organosilanes 50 52 in ethanol or aqueous media [36]. Although aryl... 

Recently, it has been reported that EtiZn/air and 9-BBN are also effective to initiate radical reactions. See, I. Ryu, F. Araki, S. Minakata, M. Komatsu, Tetrahedron Lett. 1998, 39, 6335 ... 

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