Radical cations subsequent nucleophilic

Oxidative electron transfer cycloreversion of the oxetane (59) using triphenylthiapyiylium perchlorate as a photosensitizer leads to distonic 1,4-radical cations subsequent cleavage gives rise to fragmentation products, whereas nucleophilic trapping by acetonitrile affords a ring expanded oxazine (60)/" ... 

No single mechanism accounts for all the reactions. One pathway involves a concerted one-step process involving a cyclic transition state. This of necessity affords a c -product. Another possibility, more favoured in polar solvents, involves a cationic 5-coordinate intermediate [IrX(A)(CO)L2]+, which undergoes subsequent nucleophilic attack by B-. Other possibilities include a SN2 route, where the metal polarizes AB before generating the nucleophile, and radical routes. Studies are complicated by the fact that the thermodynamically more stable isolated product may not be the same as the kinetic product formed by initial addition. 

Moreover, one should mention that in spite of similar electronic structures, PBN and the isoquinoline nitrone (278) react in a different way. Under no circumstances does PBN give an oxidative methoxylation product, whereas nitrone (278) reacts readily to form a,a-dialkoxy-substituted nitroxyl radical (280) (517). Perhaps this difference might be due to the ability to form a complex with methanol in aldo-nitrones with -configuration. This seems favorable for a fast nucleophilic addition of methanol to the radical cation (RC), formed in the oxidation step. The a-methoxy nitrone (279), obtained in the initial methoxylation, has a lower oxidation potential than the initial aldo-nitrone (see Section 2.4). Its oxidation to the radical cation and subsequent reaction with methanol results in the formation of the a,a-dimethoxy-substituted nitroxyl radical (280) (Scheme 2.105). 

The homopolymerization ofl consists of a room-temperature reaction of the monomer dissolved in nitrobenzene in the presence of anhydrous ferric chloride. Polymerizations were carried out under a stream of dry nitrogen. As depicted in Scheme 2, the homopolymerization of 1 to form 6FNE takes place by means of the Scholl reaction. The mechanism of the Scholl reaction was assumed to proceed through a radical-cation intermediate derived from the single-electron oxidation of the monomer and its subsequent electrophilic addition to the nucleophilic monomer. The reaction releases two hydrogens, both as protons, to form the... 

Electron transfer from the alkene leads to a radical cation that can undergo coupling (Scheme la). The radical cation can also react with the nucleophilic heteroatom of a reagent to afford addition or substitution products (Scheme lb). Adducts can be likewise obtained by oxidation of the nucleophile to a radical that undergoes radical addition. Reactions between alkenes and nucleophiles can be realized too with chemical oxidants that are regenerated at the anode (mediators) (see Chapter 15). Finally, cycloadditions between alkenes can be initiated by a catalytic anodic electron transfer. These principal reaction modes are subsequently illustrated by selected conversions. 

Allylic CH bonds Aliphatic alkenes frequently undergo allylic substitution by oxidation of the double bond to a radical cation that undergoes deprotonation at the allylic position and subsequent oxidation of the resulting allyl radical to a cation, which finally combines with the nucleophiles from the electrolyte [21, 22]. The selectivity is mostly low. Regioselec-tive allylic substitution or dehydrogenation is, however, found in some cases with activated alkenes, for example, -ionone that reacts to (1) (Fig. 5) as a major product [23], menthone enolacetate that yields 90% (2) [24], and 3,7-dimethyl-6-octen-l-ol... 

McCleland has reported that 3-phenylpropan-l-ol [125] and 3-(p-methyl-phenyl)propan-l-ol 99 [126] cyclize to chromans when oxidized by the radical anion SO4, generated by redox decomposition of S20 with Fe. The intermediate arene radical cation 100 is attacked by the nucleophilic hydroxy group. Whereas 1,6-cyclization yields 7-methylchroman 102, 1,5-cyclization with subsequent C-migration leads to the regioisomer 6-methylchroman 105. A dependence of the isomeric ratio and the combined yields to the pH value is determined. While 7-methylchroman 102 is the main product over a wide pH range, 6-methylchroman 105 is only formed at low pH. When the pH is lowered, the combined yields decrease due to the formation of an a-oxidized non-cyclized product. 

Products from the electrochemical oxidation of cyclohexene (Scheme 2.1) illustrate the general course of reaction [28, 29]. The radical-cation either undergoes loss of an allylic proton or reacts, at the centre of liighest positive charge density, with a nucleophile. Either reaction leads to a carbon radical, which is oxidised to the carbonium ion. A Wagncr-Meerwein rearrangement then gives the most stable carbonium ion, which subsequently reacts with a nucleophile. 

Electron density calculations are less successful in accounting tor the reactions of benzenes with substituents such as methoxy, and there is strong evidence with these for a different pathway that involves ejection of an electron to form a radical cation (3.7) this is in keeping with the greatly enhanced electron-donor properties of an excited state. Flash photolysis studies support therormation of radical cations for methoxybenzenes on irradiation, and solvated electrons have also been detected in scavenging experiments. Subsequent attack by the nucleophile on the radical cation can then be rationalized by calculations based on this species rather than on the excited state. 

One electron transfer from the highest filled MO of a neutral substrate 170 (Eq. (236) ) to the anode yields a radical cation 171 as product. This may be either a transient intermediate or a stable, long-lived species depending on its substituents and the nucleophilicity of the solvent. The reaction paths of radical cations have been expertly and comprehensively reviewed by Adams 2 5 2 9 so that a short summary seems sufficient at this place. Deprotonation and 1 e-oxidation of 171 with a subsequent Sj l reaction of the resulting cation yields side-chain substitution products 172 (path a), see 9.1. Solvolysis of 171 followed by le-oxidation... 

Enol radical cation intermediates have rerantly been invoked in the ribonucleotide reductase process. According to a hypothesis by Stubbe [127], they are formed through water loss from the 3 -ribonucleotide radical. They are supposed to react subsequently with H (or alternatively via a very unlikely two-electron reduction followed by protonation) to an intermediate 3 -hydroxy radical that is finally transformed to the deoxyribonucleotide. The above mechanistic evidence on simple enol radical cation chemistry, however, argues against this mechanistic model, since deprotonation should be much faster than nucleophilic attack even under physiological conditions. 

These results, however, do not imply that mechanism C is impossible in general [228]. Recently, sterically hindered enol acetates, where nucleophilic attack (mech. A) and deprotonation (mech. B) on the radical cation stage are suppressed, were synthesized and studied by cyclic voltammetry as well as by product analysis [229]. Accordingly, enol acetates 146-149 undergo loss of CH3CO upon one-electron oxidation and open up a novel route to a-carbonyl cation chemistry. 150-153 rearrange subsequently to the benzofurans 19-21,23. The C-O bond cleavage reaction in 147 is rather slow (k < 10 s as derived from fast-scan cyclic voltammetry studies. 

The ring-opening reaction is not limited to conventional nucleophiles. Ceric ammonium nitrate in the presence of excess nitrate ion converts oxiranes to / -nitrato alcohols <1995T909>. The reaction is believed to proceed via a one-electron transfer to form an oxiranium radical cation that is subsequently captured by a nitrate ion. Nitric oxide adds to 2,3-epoxy phenyl ketones <2004TL1565>. 

The second modification is Umpolmg of the carbonyl group via conversion into the corresponding enol ethers or enols and subsequent oxidation to give the radical cations of enol ether and enol, respectively [192]. The oxidation potentials of these substrates are approximately 1 V (relative to the SCE) and thus oxidation is feasible even with moderately active oxidants or via anodic oxidation. Subsequent reactions of the enol radical cations and radical cations of enol ethers can result in a-substitution products (e.g. by running the reaction in the presence of nucleophiles) and re-formation of the carbonyl group (Scheme 47). Thus, the overall process corresponds to a-activation of carbonyl substrates via intermediate tautomeric enols (and sometimes also enolates) [193, 194]. 

The corresponding silyl enol ethers are likewise readily available carbonyl umpolmg substrates which can be oxidized by a variety of chemical oxidants and also by cathodic oxidation. If not trapped by nucleophiles, the radical cations can dimerize and subsequently hydrolyze to give 1,4-dicarbonyl (homo)coupling products [195]. 

A third possibility of chemical modification is conversion into an acylsilane which reduces the oxidation potential of the corresponding ketone by approximately 1 V. A peak potential of 1.45 V (relative to Ag/AgCl) for the oxidation of undecanoyltrimethylsilane has been reported. Preparative electrochemical oxidations of acylsilanes proceed in methanol to give the corresponding methyl esters. A two-step oxidation process must be assumed because of the reaction stoichiometry —oxidation of the acylsilane results in the carbonyl radical cation which is meso-lytically cleaved to give the silyl cation and the acyl radical, which is subsequently oxidized to give the acyl cation as ultimate electrophile which reacts with the solvent. A variety of other nucleophiles have been used and a series of carboxylic acid derivatives are available via this pathway (Scheme 49) [198]. 

Two alternative routes (with the same final product and electrochemical characteristics) were also considered65. They include an irreversible proton transfer from 68a+ to 68a resulting in the deprotonated radical that is coupled in a subsequent step with parent 68a or its radical cation 68a1 However, in further work107 it was shown that the oxidation peak potential Epa does not depend on the concentration of the added base. vym-collidine (which is a stronger base in water than 68a). This indicates that 68a is not able to deprotonate its radical cation 68a1 and the mechanism involves only a true nucleophilic attack of 68a onto its radical cation 68a+ , as shown in Scheme 8. 

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