Radicals, formation intermediates

Detailed studies on the decomposition of organic peroxides are of fundamental interest and of high importance in polymerization reactions. The time-scales of intermediate radical formation and of their subsequent decomposition determine process parameters such as the initiator efficiency in radical polymerizations. An improved understanding of the mechanism and dynamics of photo-induced fragmentation is achieved by systematic investigations in which quantum-chemical calculations are carried out in conjunction with highly time-resolved experiments. 

The mechanisms of dehalogenations have been reviewed by Miller and in a series of papers , the stereoselectivity of the dehalogenation of the stilbene dibromides with a wide variety of reagents has been discussed. The meso-stilbene dibromide always eliminates to give the thermodynamically more stable alkene, namely tra 5-stilbene which is product of apparent a t/-elimina-tion. However, the J/-stilbene dibromide gives both cis- and rm i-stilbenes, and the ratio of these products can provide useful mechanistic information. One-electron reductants, such as chromous ion, give rise to intermediate radical formation in which rotation about the Ca-Cg bond allows thermodynamic control of the reaction. Two-electron reductants, such as iodide ion in dimethyl formamide, induce highly stereoselective a i-elimination. In protic solvents, carbonium ion intermediates were proposed to explain the trend towards thermodynamic control. Miller has proposed a reaction mechanism which embraces elimination, substitution, and electrophilic addition to alkenes. 

The rapid reaction of LiMe2Cu with trityl halides in ether produces trityl radicals, as shown by ESR. The reaction with a cyclizable alkyl halide, 6-iodo-l-heptene, produced the cyclic coupled product. Both results are strongly indicative of the intermediate radical formation by a singleelectron transfer pathway. 

Homogeneous Sonochemistry Bond Breaking and Radical Formation. The chemical effect of ultrasound on aqueous solutions have been studied for many years. The primary products are H2O2 there is strong evidence for various high-energy intermediates, including HO2,... 

The oxidation of hydrocarbons involves the sequential formation of a number of similar reactions in which various intermediate radicals which are combinations of carbon, hydrogen and oxygen are formed. In the simplest case, the oxidation of medrane, the methyl radical CH3 plays an important part both in direct oxidation to CO(g) and in indirect oxidation duough the formation of higher hydrocarbons such as CaHe before CO is formed. The chain reactions include... 

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

It is important to emphasize that direet studies sueh as those earned out on the eyelopropylmethyl radieal ean be done with low steady-state eoneentrations of the radical. In the case of the study of the eyelopropylmethyl radical, removal of the source of irradiation leads to rapid disappearance of the EPR spectrum, because the radicals react rapidly and are not replaced by continuing radical formation. Under many conditions, the steady-state concentration of a radical intermediate may be too low to permit direct detection. Failure to observe an EPR signal, therefore, cannot be taken as conclusive evidence against a radical intermediate. 

Grignard reagents are a very important class of organometallic compounds. For their preparation an alkyl halide or aryl halide 5 is reacted with magnesium metal. The formation of the organometallic species takes place at the metal surface by transfer of an electron from magnesium to a halide molecule, an alkyl or aryl radical species 6 respectively is formed. Whether the intermediate radical species stays adsorbed at the metal surface (the A-modelf, or desorbs into solution (the D-model), still is in debate ... 

Mechanistically the rearrangement is formulated to proceed via an intermediate radical-pair or ion-pair. In either case the initial step is the formation of a nitrogen-ylide 2 by deprotonation of the ammonium species with a strong base. The abstraction of a proton from the a-carbon is facilitated by an electron-withdrawing group Z—e.g. an ester, keto or phenyl group ... 

The rate-determining step in the formation of the x-lithio ethers is the formation of a carbon radical as a precursor to the anion. The intermediate radical in the tetrahydropyranyl system is expected to be nonplanar, to be capable of rapid equilibration between the quasiequatorial and quasiaxial epimers, and to exist largely or entirely in the axial configuration at — 78 °C. However, treatment of the a-phenylthio ether 4 with LDMAN at higher temperature in the presence of A, A, lV, ./V -tetramethylethylenediamine leads to the more stable equatorial epimer of the lithio ether 5 and, after addition to benzaldehyde, the axial- and equatorial-substituted products were obtained in a ratio of 13 87. 

Sulfate radical anion may be converted to the hydroxyl radical in aqueous solution. Evidence for this pathway under polymerization conditions is the formation of a proportion of hydroxy end groups in some polymerizations. However, the hydrolysis of sulfate radical anion at neutral pi I is slow (k— 107 M"1 s 1) compared with the rale of reaction with most monomers (Ar=l08-109 M 1 s 1, Table 3.7)440 under typical reaction conditions. Thus, hydrolysis should only be competitive with addition when the monomer concentration is very low. The formation of hydroxy end groups in polymerizations initiated by sulfate radical anion can also be accounted for by the hydration of an intermediate radical cation or by the hydrolysis of an initially formed sulfate adduct either during the polymerization or subsequently. 

Following three phase transformations [951] (>298 K), NH C decomposition begins [915] in the solid phase at 423 K but only becomes extensive well above the melting point ( 440 K). Decomposition with the evolution of N20 and H20 from the melt is first order [952,953] (E = 153—163 kJ mole-1), the mechanism suggested involving intermediate nitramide formation. Other proposed schemes have identified NOj [954] or the radical NH2NO [955] (<473 K) as possible participants. Studies [956,957] have been made of the influence of additives on NH C decomposition. 

Trifluoropropene is a compound of special interest in this series. Some transformations of the intermediate telomer radicals had been observed in this case, which prompted to study this reaction in more details. In addition to telomers C6H5CH2(CH2CHCp3)nBr (T Br, n = 1,2), the authors have found compound PhCH=CHCH(CF3)CH2CH2Cp3 and explained its formation by rearrangement of the intermediate radical with two monomer units followed by easy... 

Other preparative snags also occur in the addition of HHal to alkenes. Thus in solution in H20, or in other hydroxylic solvents, acid-catalysed hydration (p. 187) or solvation may constitute a competing reaction while in less polar solvents radical formation may be encouraged, resulting in anti-Markownikov addition to give 1-bromopropane (MeCH2CH2Br), via the preferentially formed radical intermediate, MeCHCH2Br. This is discussed in detail below (p. 316). 

The driving force of the reaction is the formation of a more stable radical, i.e. the unpaired electron is delocalised more effectively by Q in (1276) than by H in (127a). Migration of fluorine does not occur as its d orbitals are not accessible, and migration of Br only rarely as the intermediate radicals undergo elimination (to alkene) more readily than rearrangement. 

Treatment of iV-(a-aminobenzyl)benzotriazoles 648 with samarium diiodide generates radicals 649 that undergo coupling to form vicinal diamines 650 (Scheme 101) <1992TL4763>. Formation of intermediate radicals 649 at low temperature is confirmed by EPR <19990L1755>. Short-living radicals 649 are readily converted to more stable radicals 651 by treatment with 2-methyl-2-nitrosopropane. 

The first term on the right-hand side denotes the rate of dioxygen reaction with styrene (see Chapter 4) and the second term is the rate of catalytic free radical generation via the reaction of styrene with dioxygen catalyzed by cobaltous stearate or cobaltous acetylacetonate. The rate constants were found to be ki = 7.45 x 10-6 L mol-1 s-1, k2 = 6.30 x 10 2 L2 mol 2 s 1 (cobaltous acetylacetonate), and k2 = 0.31L2 mol-2 s 2 (cobaltous stearate) (T = 388 K, solvent = PhCl [169]). The mechanism with intermediate complex formation was proposed. 

Some evidence to suggest that peroxo complexes can be intermediates in the oxidation of Pt(II) by 02 has been presented. As shown in Scheme 41, a Pt(IV) peroxo complex was obtained by reacting cis-PtCl2(DMSO)2 and 1,4,7-triazacyclononane (tacn) in ethanol in the presence of air (200). An alkylperoxoplatinum(IV) complex is obtained in the reaction of (phen)PtMe2 (phen = 1,10-phenanthroline) with dioxygen and isopropyl-iodide. Under conditions that favor radical formation (light or radical initiators), an isopropylperoxoplatinum(IV) compound was obtained (201,202), depicted in Scheme 42. 

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