Radical chain substitution mechanism

Allyl)CpFe(CO)(PR3) complexes (14) have been prepared from the dicarbonyls, by photolysis in the presence of phosphine or phosphite. The substitution is often aided by a trace of (CpFe(CO)2)2, which is indicative of a radical (see Radicals) chain substitution mechanism. Phosphite ligand (as in 15) has been reported as being a particularly good replacement ligand from the standpoint of thermal stability.Nevertheless, neither C-3-substituted ()] -allyl)Fp complexes (14b) nor cyclic allyl complexes (see Allyl Complexes) may be made directly by this method the carbocyclic cases have been prepared by methoxide-induced proton abstraction of aUcene cation (16) (Section 4.3.2). " ... 

There is still another substitution mechanism to consider, although it is much less common. It is called SrnI (substitution, radical, nucleophilic, unimolecular) and involves a radical chain mechanism, unlike the SET mechanism just described. We have seen a radical chain substitution mechanism in Chapter 10 when we considered radical aromatic substitution (Section 10.22). 

It is assumed that all similar fluorination reactions proceed via an intricate radical chain-reaction mechanism. The overall reactions for the substitution of hydrogen by fluorine (RH + F2 - RF + HF, AH298 -430 kJ/mol per carbon atom) are more exothermic than the reactions for adding fluorine to the double bonds... 

In a reaction with a moderately long chain, much more of the product will be produced by abstraction (4) than by coupling (5). Cleavage steps like (2) have been called SHl (H for homolytic), and abstraction steps like (3) and (4) have been called Sh2 reactions can be classified as ShI or Sh2 on the basis of whether RX is converted to R by (2) or (3).9 Most chain substitution mechanisms follow the pattern (3), (4), (3), (4). . . . Chains are long and reactions go well where both (3) and (4) are energetically favored (no worse that slightly endothermic, see pp. 683, 693). The IUPAC designation of a chain reaction that follows the pattern (3), (4). . . is ArDR + ARDr (R stands for radical). 

Halothiophenes undergo photostimulated reaction with acetone enolate ion to form substitution products (76H(5j377). This is believed to occur by the radical-chain SrnI mechanism. The propagation steps are as follows ... 

Pyridazine and its derivatives were substituted with nucleophilic radicals. They react either with 1-formylpyrrolidine or with A/-acetylproline in the presence of radical generators to give 5-substituted pyridazines (78TL619 86MI6). Also, reactions of 3-chloro-6-methoxypyridazine with ketone enolates in liquid ammonia show typical characteristics of a radical chain (SrnI) mechanism, and ketones 105 are obtained (81JOC294). 

A novel electron transfer free radical mechanism has been elucidated for sodium naphthalenide induced demercuration. A new reductant for the cleavage of the C—Hg bond, A -benzyl-1,4-dihydro-nicotinamide (BNAH), has also been proposed it reduces alkylmercury(II) acetates via an electron transfer chain substitution mechanism. ... 

The two types of reaction follow different reaction mechanisms. Side-chain chlorination occurs by a radical chain reaction mechanism, nuclear chlorination by electrophilic substitution. 

H transfer to organic radicals is a common reaction of transition metal hydride complexes. The reaction in Equation 3.128, which is parallel to RjSnH reductions in organic chemistry, is frequently used for the preparation of stable derivatives and for quantifying the amount of hydride present (by measuring the amount of CHClj formed). The mechanism is related to that of the radical chain substitution reactions discussed in Chapter 4. 

There has been a major review of substitution by the radical-chain 5rn1 mechanism. It has been shown that reaction by the SrnI pathway of the enolate anions of 2- and 3-acetyl-l-methylpyrrole may yield a-substituted acetylpyrroles. The dichotomy of reactions of halonitrobenzenes with nucleophiles has been nicely summarized major pathways include reduction via radical pathways and. SnAt substitution of halogen. EPR spectroscopy has been used to detect radical species produced in the reactions of some aromatic nitro compounds with nucleophiles however, whether these species are on the substitution pathway is questionable. The reaction of some 4-substimted N,N-dimethylanilines with secondary anilines occurs on activation by thallium triacetate to yield diphenylamine derivatives radical cation intermediates are proposed. ... 

For unactivated aromatic and heteroaromatic substrates, where a polar substitution is not favorable, nucleophilic substitution is feasible through processes that involve electron transfer (ET) steps. In these reactions, an aromatic compound bearing an adequate leaving group is substituted at the ipso position by a nucleophile in a unimolecular radical nucleophilic substitution mechanism (or S,y.jl), which is a chain process that involves radicals and radical anions as intermediates. 

Zhang, X., Yang, D., Liu, Y, Chen, W., and Cheng, J., Kinetics and mechanism of the reactions of 0- and p-nitrohalobenzenes with the sodium salt of ethyl cyanoacetate carbanion a non-chain radical nucleophilic substitution mechanism. Res. Chem. Intermed., 11,281, 1989. 

Bromine reacts with alkanes by a free radical chain mechanism analogous to that of chlorine There is an important difference between chlorination and brommation how ever Brommation is highly selective for substitution of tertiary hydrogens The spread m reactivity among pnmary secondary and tertiary hydrogens is greater than 10 ... 

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13. 

Reactions of 3-chloro-6-methoxypyridazine with ketone enolates in liquid ammonia exhibit characteristics consistent with a radical chain mechanism for substitution (8UOC294). 

Radical substitution reactions by iodine are not practical because the abstraction of hydrogen from hydrocarbons by iodine is endothermic, even for stable radicals. The enthalpy of the overall reaction is also slightly endothermic. Thus, because of both the kinetic problem excluding a chain reaction and an unfavorable equilibrium constant for substitution, iodination cannot proceed by a radical-chain mechanism. 

Because the bromine adds to the less substituted carbon atom of the double bond, generating the more stable radical intermediate, the regioselectivity of radical-chain hydrobromination is opposite to that of ionic addition. The early work on the radical mechanism of addition of hydrogen bromide was undertaken to understand why Maikow-nikofF s rule was violated under certain circumstances. The cause was found to be conditions that initiated the radical-chain process, such as peroxide impurities or light. 

Structurally simple alJkyl halides can sometimes be prepared by reaction of an alkane with Cl2 or Br2 through a radical chain-reaction pathway (Section 5.3). Although inert to most reagents, alkanes react readily with Cl2 or Br2 in the presence of light to give alkyl halide substitution products. The reaction occurs by the radical mechanism shown in Figure 10.1 for chlorination. 

The results of kinetic studies suggest that alkane substitution reactions typically proceed by a radical chain mechanism (Section 13.9). The initiation step in the chlorination of methane is the dissociation of chlorine ... 

Alkane substitution takes place by a radical chain mechanism. 

The reductive elimination of a variety of )3-substituted sulfones for the preparation of di-and tri-substituted olefins (e.g. 75 to 76) and the use of allyl sulfones as synthetic equivalents of the allyl dianion CH=CH—CHj , has prompted considerable interest in the [1,3]rearrangements of allylic sulfones ". Kocienski has thus reported that while epoxidation of allylic sulfone 74 with MCPBA in CH2CI2 at room temperature afforded the expected product 75, epoxidation in the presence of two equivalents of NaHCOj afforded the isomeric j ,y-epoxysulfone 77. Similar results were obtained with other a-mono- or di-substituted sulfones. On the other hand, the reaction of y-substituted allylic sulfones results in the isomerization of the double bond, only. The following addition-elimination free radical chain mechanism has been suggested (equations 45, 46). In a closely related and simultaneously published investigation, Whitham and coworkers reported the 1,3-rearrangement of a number of acyclic and cyclic allylic p-tolyl sulfones on treatment with either benzoyl peroxide in CCI4 under reflux or with... 

It is now clearly demonstrated through the use of free radical traps that all organic liquids will undergo cavitation and generate bond homolysis, if the ambient temperature is sufficiently low (i.e., in order to reduce the solvent system s vapor pressure) (89,90,161,162). The sonolysis of alkanes is quite similar to very high temperature pyrolysis, yielding the products expected (H2, CH4, 1-alkenes, and acetylene) from the well-understood Rice radical chain mechanism (89). Other recent reports compare the sonolysis and pyrolysis of biacetyl (which gives primarily acetone) (163) and the sonolysis and radiolysis of menthone (164). Nonaqueous chemistry can be complex, however, as in the tarry polymerization of several substituted benzenes (165). 

Generated from diacetyl peroxide, methyl radicals attack 2-methylfuran at position 5 preferentially if both 2- and 5-positions are occupied as in 2,5-dimethylfuran there is still little or no attack at the 3(4)-position. If there is a choice of 2(5)-positions, as in 3-methylfuran, then that adjacent to the methyl substituent is selected.249 These orientation rules are very like those for electrophilic substitution, but are predicted for radical attack by calculations of superdelocalizability (Sr) by the simple HMO method. Radical bromination by IV-bromsuccinimide follows theory less closely, presumably because it does not occur through a pure radical-chain mechanism.249... 

Russell and coworkers have made an extensive study of the photolytically initiated substitution reactions of a variety of reagents with 1-alkenyl derivatives of SnBu333,34, the general reaction being as shown in reaction 26. The process is thought to involve addition-elimination in a free radical chain mechanism, illustrated in Scheme 3. 

Stannyl radicals are usually generated by homolytic substitution at hydrogen in a tin hydride, or at tin in a distannane, or, conjugatively, at the y-carbon atom in an allylstannane.453 The initiator is commonly AIBN at ca. 80 °C. In the presence of a trace of air, organoboranes are oxidized by a radical chain mechanism, and triethylborane is now commonly used as an initiator at temperatures down to —78°C,519 and it can be used in aqueous solution.520 9-Borabicyclo[3.3.1]nonane (9-BBN) has similarly been used to initiate the reaction of tin hydrides at 0 and —78°C,521 and diethylzinc works in the same way.522... 

Selective ring closure of cyclic secondary alkyl radicals onto the central carbon atom of allenes have been investigated in the course of pyrrolizidine alkaloid syntheses [69]. Thus, reduction of the phenylselenyl-substituted N-(l,2-buten-4-yl)pyrroli-done 42 with Bu3SnH via a radical chain mechanism provides 51% of target compound 44 as a 78 22 mixture of diastereomers (Scheme 11.14). The stereoselectivity... 

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