Elimination of radicals

Electron impact induced cleavages of heteroatom-carbon bonds with elimination of radicals and charge localization on the heteroatom function require considerable energy and therefore provide intense fragment ions only in exceptional cases44,45. At suitably substituted 5-membered heterocycles, for instance 4346 9 one al-... 

The following compounds o-nitroacetanilide and phenyl-(p-nitrophenyl) acetate lead to the elimination of radicals, such as (a) NOj (and also NHCOCH3 for acetanilide) directly from the initial negative molecular ion (b) NO, OH, (NO -f CH2CO) after molecular ion isomerization. Under collision conditions in the FFR, direct cleavages occur without isomerization of the molecular ion and lead to the formation of [M-NO2] , [M-O-Ph] and [M-PhCOO]" ions. 

Fig. 5.11. Left panel Schematic of the pulsed valve discharge modulation source. A 100 kHz square wave discharge 1 kV, lA) is strongly confined in the 1 mmxSOO /am X 4 cm region behind the slit expansion jaws. This yields slabs of spatially modulated jet-cooled radicals and molecular ions for detection via direct IR laser absorption and lock-in detection methods. Right panel Upper trace direct absorption spectra with conventional discharge modulation in a cw discharge source but without lock-in detection Lower trace spectra with frequency modulated discharge and lock-in detection, revealing substantial elimination of radical precursor and improves the absorption sensitivity to the near shot-noise level.

ABSTRACT. A fundamental approach is outlined for the kinetic modeling of complex processes like thermal cracking or catalytic hydrocracking of mixtures of hydrocarbons. The reaction networks are written in terms of radical mechanisms in the first case and of carbenium ion mechanisms in the second case. Since the elementary steps of the networks pertain to a relatively small number of classes, the number of rate coefficients is kept within tractable limits. The reaction networks are generated by computer through Boolean relation matrices. The number of continuity equations is limited by the elimination of radicals or carbenium ions through the pseudo-steady-state approximation. 

In the synthesis of molecules without functional groups the application of the usual polar synthetic reactions may be cumbersome, since the final elimination of hetero atoms can be difficult. Two solutions for this problem have been given in the previous sections, namely alkylation with nucleophilic carbanions and alkenylation with ylides. Another direct approach is to combine radical synthons in a non-polar reaction. Carbon radicals are. however, inherently short-lived and tend to undergo complex secondary reactions. Escheirmoser s principle (p. 34f) again provides a way out. If one connects both carbon atoms via a metal atom which (i) forms and stabilizes the carbon radicals and (ii) can be easily eliminated, the intermolecular reaction is made intramolecular, and good yields may be obtained. 

With 2-methyl- and 2,4-dimethylthiazole, the methyl thiirenium ion (m/e 72) is obtained, which can easily lose a hydrogen radical to give the ml ell ion (confirmed by the metastable peak). This latter can rearrange by ring expansion to give the thietenyl cation whose structure was confirmed in certain spectra by the presence of a metastable peak corresponding to the decomposition of the m/e 71 ion to give the thioformyl cation m/e 45, probably by elimination of acetylene. 

For 4,5-dialkylthiazoles, the molecular ion decomposes by two competitive pathways, either loss of HCN followed by elimination of the radical R in the position /3 to the double bond of the resulting substituted thiirene, or by p cleavage followed by elimination of HCN (119). 

Oligomeric Vinylphosphonate. A water-soluble oligomer, Fyrol 76 [41222-33-7] is produced by reaction of bis(2-chloroethyl) vinylphosphonate and dimethyl methylphosphonate with elimination of all the chlorine as methyl chloride (127,128). This Hquid, containing 22.5% P, is curable by free-radical initiation, on cotton or other fabrics. Nitrogen components, such as A/-methylolacrylamide or methylolmelamines, are usually included in the finish, which can be durable to multiple launderings (129,130). 

The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

Interest in the synthesis of 19-norsteroids as orally active progestins prompted efforts to remove the C19 angular methyl substituent of readily available steroid precursors. Industrial applications include the direct conversion of androsta-l,4-diene-3,17-dione [897-06-3] (92) to estrone [53-16-7] (26) by thermolysis in mineral oil at about 500°C (136), and reductive elimination of the angular methyl group of the 17-ketal of the dione [2398-63-2] (93) with lithium biphenyl radical anion to form the 17-ketal of estrone [900-83-4] (94) (137). 

Photochemistry. Vinyl chloride is subject to photodissociation. Photexcitation at 193 nm results in the elimination of HCl molecules and Cl atoms in an approximately 1.1 1 ratio (69). Both vinyUdene ( B2) [2143-69-3] and acetylene have been observed as photolysis products (70), as have H2 molecules (71) and H atoms [12385-13-6] (72). HCl and vinyUdene appear to be formed via a concerted 1,1 elimination from excited vinyl chloride (70). An adiabatic recoil mechanism seems likely for Cl atom elimination (73). As expected from the relative stabiUties of the 1- and 2-chlorovinyl radicals [50663-45-1 and 57095-76-8], H atoms are preferentially produced by detachment from the P carbon (72). Finally, a migration mechanism appears to play a significant role in H2 elimination (71). 

Action of catalytic amounts of vanadium compounds on oxaziridine (52) yields caprolactam almost quantitatively. Reductive opening of the oxaziridine ring and /3-scission yield radical (118), which recyclizes with elimination of the metal ion to form the lactam (63) (77JPR274). 

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. ... 

When the radicals have p hydrogens, alkenes are formed by a process in which carbocations are probably bypassed. Instead, the oxidation and the elimination of a proton probably occur in a single step through an alkylcopper species. The oxidation state of copper in such an intermediate is Cu(III). 

Experiments in which radical scavengers are added indicate that a chain reaction is involved, because the reaction is greatly retarded in the presence of the scavengers. The mechanism shown below indicates that one of the steps in the chain process is an electron transfer and that none of the steps involves atom abstraction. The elimination of nitrite occurs as a unimolecular decomposition of the radical anion intermediate, and the SrnI mechanistic designation would apply. 

Similarly, reaction of B6H10 with Fe2(CO)9 (p. 1 104) at room temperature results in the smooth elimination of Fe(CO)s to form [Fe(r -B6Hio)(CO)4] as a. stable, volatile yellow solid. Use of these electron-donor propcities of B(,H o towards reactive (vacant orbital) borane radicals resulted in the preparation of several new coti/uncm-boranes. e.g. BnHiy, BuHji and B15H23 (p. 162),... 

Oxaziranes derived from isobutyraldehyde react with ferrous salts to give only substituted formamides fEq. (23)], The chain propagating radical 30 thus suffers fission with elimination of the isopropyl group. An H-transfer would lead to substituted butyramides, which are not found. Here is seen a parallel to the fragmentation of alkoxyl radicals, where the elimination of an alkyl group is also favored over hydrogen. The formulation of the oxazirane fission by a radical mechanism is thus supported. 

In their thermal stability the diaziridines approximate to the oxaziranes. As with most oxaziranes, they are stable at 100° C for short periods they are decomposed by heating at 200°C 1,2-di-n-butyl-3-ri-propyldiaziridine thus eliminates butylamine. The thermal decomposition has not yet been investigated in detail. Similarly no information is available on the reaction of radical reagents on diaziridines. 

The stereospecificity of the methylene transfer provides compelling support for a concerted mechanism and this conclusion has rarely been disputed. It is instructive, however, to review the experimental evidence that allowed for the elimination of the alternative mechanistic proposals, namely, a radical addition and a carbome-tallation (Scheme 3.4). 

Michael acceptors and 1,4-addiiion of alkyl group is a normal process. The reaction mechanism is not clear, but the process via addition of alkyl radicals and subsequent elimination of NO radical is one of the possible routes. Recently, several related reactions have been reported, as shown in Eq. 4.76, Eq. 4.77, and Eq. 4.78, in which alkyl radicals are involved. The reaction of trialky Igalliiim compounds with nitrostyrene gives also a similar snbsdtiidon product fEq. 4.791. ° ... 

Because anions of nitro compounds are good electron-transfer reagents, they can serve as reducing agents in radical type eliminations of vicinal dinitro compounds. In fact, N- azolyl- sub-... 

Braun and Schurek [9] assumed that during polymerization a reaction can occur between the polymer and free radicals that leads to the elimination of hydrogen chloride and formation of a double bond. The formation of HCl during the polymerization of vinyl chloride has been observed [10],... 

An interesting free radical carbon-carbon bond formation with concomitant elimination of a /5-thio substituent was achieved during the course of Boger s impressive synthesis of CC-1065.26-27 In the event, treatment of aryl bromide 70 (see Scheme 13) with tri-n-... 

The -elimination of a thiyl radical (RS ) terminated a remarkably productive tandem radical bicyclization in Parker s formal total syntheses of ( )-codeine and ( )-morphine (see Scheme 14).29 Subjection of aryl bromide 72 to the conditions indicated generates transient aryl radical 73, an intermediate which engages the substi-... 

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