Allyl radicals dimerization

The concentration of copper(II) has a pronounced effect on the course of the reaction. In the presence of very low copper(II) concentrations, oxidation of allyl radical 69 is slow and major amounts of allyl radical dimer are formed. In the presence of very high concentrations of copper(II), radical 68 is oxidized rapidly before addition to diene can take place. An optimum yield of product 71 can therefore only be achieved at certain copper(II) concentrations. The metal-ion-promoted addition of chloramines to butadiene appears to follow the same mechanism93. 

The trap is to form the product by dimerizing these allylic radicals. Dimerizing of radicals is known (in the acyloin reaction, for example, p. 1032), and these radicals will sometimes dimerize ut it is a rare process. 

However, a very unexpected situation is found24 for the phenyl allyl sulphone (53), for which a one-electron cleavage occurs in aprotic non-aqueous solvents. The allyl radical is apparently not electroactive at the cleavage potential, and forms the dimer. Therefore, in this one-electron bond scission no strong base is formed and the isomerization into the vinylic isomer is not observed (Figure 9). Similarly, the cleavage of phenyl propargyl... 

The regioselectivity in the dimerization of allyl radicals has been studied by a variety of methods. One of the earliest investigations into this field employed the Kolbe electrolysis... 

In summary, it appears that the trapping of allyl radicals with closed-shell trapping agents is quite selective, especially in those cases in which the allyl radical contains one substituted and one unsubstituted terminus. Trapping with radicals appears to produce mixtures of isomers, especially in the dimerization of allyl radicals. The observed regio-selectivities do, however, depend on the reaction conditions, allowing for some control of the reaction outcome for a given substrate. 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

The dimer 352 of 351 was isolated from the product mixtures of two experiments conducted to trap 351 by alkenes, one with 350 and the other with 354 as substrate. Although no cycloadduct with the alkene was observed in one case, the yield of 352 amounted to only 0.8%. Nevertheless, the structure of 352 is interesting, since it suggests that the tetramethyleneethane diradical assumed to be the intermediate undergoes ring closure preferentially between two different allyl-radical termini. 

The scheme of reactions proposed to explain the products obtained is shown, after small modifications, in Scheme 8. Primary radicals 12 formed at the anodes produce with added 30 or 36 (equation lOe) the substituted benzyl or allyl radicals 38, which can dimerize to 39 or can couple with the added olefin to form radicals 40 or 41. For allyl radical (38) a 1,1 - or l,3 -coupling is possible yielding 41 and 40, respectively. Further couplings of 40 and 41 with the primary radical 12 produce 39 and head-to-tail dimer 42, respectively. It was evident from the products obtained that the coupling of 38 in the 1-position occurs 5 to 11 times faster than in the 3-position. However, for readily polymerizable olefins, rather polymerization occurs, in particular at graphite electrodes. At Pt electrodes both dimers 39 and 42 are formed, but for Cu electrodes exclusively dimers 39 were obtained with moderate yields. Thus, an indirect electrolysis including the oxidation of copper to Cu+ ions and their further reaction with 5 yielding intermediate RCu was considered, but not proved . ... 

The first organometallic compound of the transition metals to be characterized (1827) was Zeise s salt, K[(C2H4)PtCl3]-H20 (Fig. 18.1). It forms when K2[PtCl4] in aqueous ethanol is exposed to ethylene (ethene) a dimeric Pt—C2H4 complex with Cl bridges is also formed. In both species, the ethylene is bonded sideways to the platinum(II) center so that the two carbon atoms are equidistant from the metal. This is called the dihapto-or T]2 mode. A ligand such as an allyl radical with three adjacent carbons directly bonded to a metal atom would be trihapto- or t 3, and so on. 

The oxidative dimerization has recently attracted attention, both from a fundamental viewpoint and as a means for synthesizing aromatics from lower olefins. The reaction is essentially a combination of allyl radicals, by which the oxidation is limited to the abstraction of one hydrogen atom. Typically, the catalysts applied here do not contain Mo03 or a similar component that promotes the selective incorporation of oxygen. 

Many authors assume that the initial reaction step in the dimerization is identical with that in the acrolein production, namely hydrogen abstraction and formation of an allylic intermediate. Dimerization is then supposed to occur because the ability to oxidize the allyl radical to acrolein is absent. 

An important distinction between dimerization and acrolein formation is that the selectivity of the former is evidently connected with a partially reduced state of the catalyst. It is commonly accepted, therefore, that cations like Bi3+, Sn4+, etc. play a role, presumably by adsorbing the allyl radical intermediate. Several authors assume that this is the case for allylic oxidation in general and that the role of a second oxide component is to promote dimerization byi stabilization of the allyl radical, or to direct the oxidation to aldehyde formation via a cationic allyl complex. Seiyama et al. [285] further suggest that the acidity of the promoting oxides is an important factor in this connection, and may, in part, explain why acidic oxides like Mo03 direct the oxidation to aldehydes, while basic compounds favour dimerization. 

The photochemistry of ir-allylpalladium complexes has been studied to a limited extent. Two basic reactions have been observed. Irradiation at 366 nm of ir-allylpalladium complexes produced 1,5-diene dimers, reportedly via a radical coupling mechanism.334 333 Similar irradiations in the presence of species capable of trapping the presumed allyl radical intermediate, such as BrCCb, BrCH2Ph or allyl bromide, now yield alkylated and halogenated allyls, in addition to 1,5-diene dimer. This reaction fails for simple alkyl or aryl halides due to the instability of the associated radical (equations 130 and 131 ).336... 

Anodic oxidation of cyclic enol esters with /1-hydrogens leads to allyl radicals, which then lose acyl radical to form a, /1-unsaturated ketones. When the electrolysis is performed in an undivided cell, these are converted by the cathode into enolate anion radicals, which then couple to form /1-dimers (Scheme 66)164. 

With R = benzyl and in the absence of 02, the major product (73%) is the de-carbonylation product [reaction (209) possible formed to a large extent within the solvent cage], and the dimer of the allylic radical [reaction (207)] is formed only in small amounts. Addition of a thiol increases the yield of Thd [reaction (208)]. If an evaluation of the data reported for the reduction of the allylic OH-adduct to 1,3-cylohexadiene by a thiol (Pan et al. 1988), estimated at 104 dm3 mor1 s"1, is a good guide the rate constant for reaction (208) should be similar. This would revise an assumed rate constant of 106 dm3 mol-1 s-1 and the conclusions as to the repairability of allylic Thy in DNA radicals by cellular thiols (Anderson et al. 2000). 

The mass spectrum of (7r-C3H5PdCl)2 shows loss of both chlorine and allyl radicals from the molecular ion, but no ion C3H5PdCl+ corresponding to half the dimerized molecule was observed, and metal-metal interaction is proposed to account for the abundance of fragments containing the Pd2Cl unit. The base peak of the spectrum corresponds to C3H5Pd+, which can be formed from many of the other ions (132). The mass spectrum of the methoxyallyl complex (CXXIII)... 

Fig. 15. Orbital correlation for the dimerization, allyl radical + allyl radical. The process is symmetry forbidden (Hoffmann and Woodward, 1965b).

It was thus of interest to determine whether a transannular addition could occur for the cyclo-octenyl radical. The cyclo-octenyl radical has been prepared from its parent halohydrocarbon in several matrices in the rotating cryostat. At 77°K the radical was stabilized in all of the matrices but when the samples were warmed reaction took place. The e.s.r. spectra showed that the main reaction in matrices of water, benzene or camphane was hydrogen abstraction adjacent to the double bond to give a cyclic allyl radical (9) rather than the bicyclic radical. However, in a matrix of bicyclopentadiene dimer cyclization occurs to give the bicyclic radical and in a matrix of adamantane both allylic and bicyclic radicals were formed. 

Tertiary j -nitroso allylthiols 338, heated at 40 °C in benzene or dichloromethane for 6-7 h gave a mrKture of disulfides 339 and nitroso dimers 340 (Equation 51) <2002CC2394>. First formed from 338 is the allyl thiyl radical which isomerizes reversible into the thiiranylcatbinyl radical. Dimerization, after the latter had added to NO, gave 340 and dimerization of the allylthiyl radical gave 339. This topic is also discussed in Section 1.05.6.6. 

The medium has no specific oxidative action towards the alkyl radicals, and in the presence of protonated base the major reaction is a substitution. Thus, imidazoles and 1-substituted imidazoles are alkylated exclusively at C-2, albeit in rather low yields. The use of isopropyl and r-butyl radicals gives improved yields (80-90%) but benzyl and allyl radicals tend to dimerize in preference. Benzimidazoles are also alkylated at C-2, and with isopropyl and t-butyl radicals yields of 50-80% can be achieved (80AHC(27)24l, 74AHC(16)123). 

Addition of the radical (2) to butadiene (in CH3OH) gives the allylic radical (7), which dimerizes to a mixture of unsaturated diesters. 

The electrochemical reduction of a,p-unsaturated ketones and related compounds in aprotic media in the absence of metal cations can, in some cases, lead to relatively stable anion radicals.However, in the presence of proton donors the latter are protonated to form hydroxyallyl radicals, which tend to dimerize more rapidly than they diffuse back to the electrode to undergo further reduction (Scheme 17). Although these allyl radicals prefer to dimerize by coupling at the -position, if this position is sterically hindered, as in the case of cholest-4-en-3-one, coupling at the carbonyl carbon may be observed, yielding pinacols. ... 

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