Radicals nucleophilic capture

By-products from capture of nucleophilic anions may be observed.53 Phenols can be formed under milder conditions by an alternative redox mechanism.98 The reaction is initiated by cuprous oxide, which effects reduction and decomposition to an aryl radical, and is run in the presence of Cu(II) salts. The radical is captured by Cu(II) and converted to the phenol by reductive elimination. This procedure is very rapid and gives good yields of phenols over a range of structural types. 

The table shows the effect on product ratio of ultrasonic irradiation (Kerry Pulsatron cleaning bath 35 kHz 50 W) during electrolysis. Here there is only 8% of the bicyclohexyl dimeric one-electron product, with approximately 41 % of the two-electron product from nucleophilic capture of the intermediate carbocation. The preponderance of cyclohexene (32 %) over cyclohexane (> 3 %) shows its formation is by proton loss from the carbocation intermediate, since free-radical routes to cyclohexene (i. e. hydrogen atom abstraction) also produce cyclohexane in equal if not greater amounts... 

Radical cations as well as cations are good electrophiles dependent on steric and electronic effects and, therefore, potent targets for nucleophilic capture. 

Concerning their structure and reactions, organic radical cations have been the focus of much interest. Among bimolecular reactions, the addition to alkenes and their nucleophilic capture by alcohols, which lead to C—C and C—O bond formation, respectively have been investigated in detail. Unimolecular reactions like geometric isomerization and several other rearrangements have also attracted attention. 

NUCLEOPHILIC CAPTURE OF CYCLOPROPANE RADICAL CATIONS CORRELATIONS BETWEEN RADICAL CATION STRUCTURE AND REACTIVITY... 

However, electron transfer-induced photoreactions in the presence of nucleophiles have attracted by far the greatest attention a rich variety of cyclopropane systems have been subjected to these reaction conditions. We will consider several factors that may affect the structure of the radical cations as well as the stereo- and regiochemistry of their nucleophilic capture. Factors to be considCTcd include (1) the spin and charge density distribution in the cyclopropane radical cation (the educt) (2) the spin density distribution in the free-radical product (3) the extent of... 

With ions or dipolar substrates, radical ions undergo nucleophilic or electrophilic capture. Nucleophilic capture is a general reaction for many alkene and strained-ring radical cations and may completely suppress (unimolecular) rearrangements or dimer formation. The regio- and stereochemistry of these additions are of major interest. The experimental evidence supports several guiding principles. 

The nucleophilic capture of radical cations forms (free) radicals, one H atom shy of the adduct. The missing H may be introduced in one step, by hydrogen abstraction, or in two, involving successive reduction by the sensitizer radical anion and protonation. Both mechanisms have been observed, sometimes in competition with each other. 

Many of these reactions support a measure of thermodynamic control in nucleophilic capture Conjugated radicals or products formed with release of ring strain are favored. For example, the addition of ethanol to radical cation 110 + is regiospecific, forming the more stable (benzylic) intermediate 111 + the capture of 112 + likewise forms a benzylic radical (113 ). Radical cation 48 + generates a... 

The nucleophilic capture of tricyclane radical cations 115 " and 117 " supports the role of conventional steric hindrance 115 reacts at the 3° carbon ( 116 ), whereas the chiral isomer 117 + is captured by backside attack at the less hindered 3° carbon ( 118 ). " Both reactions are regio- and stereospecific and avoid attack at the neopentyl-type carbon (denoted by an asterisk). 

The ET photochemistry of (IR, 35)-(+)-c/i-chrysanthemol (c/i-127) proceeds via nucleophilic attack of the internal alcohol function on the vinyl group with simultaneous or rapid replacement of an isopropyl radical as an intramolecular leaving group, forming 128. This reaction is a mechanistic equivalent of an Sn2 reaction the mode of attack underscores the major role of strain relief in governing nucleophilic capture in radical cations. 

In addition to nucleophilic capture by alcohols, nonprotic nucleophiles also react with these intermediates. For example, the distonic dimer radical cation 96 + can be trapped by acetonitrile a hydride shift, followed by electron return, gave rise to the pyridine derivative 131. Similar acetonitrile adducts are formed in the electron-transfer photochemistry of terpenes such as ot- and (3-pinene ° or sabinene. ... 

RBSctions of Radical Anions With Radicals. The coupling of arene or alkene radical anions with radicals is an important reaction, and one that has significant synthetic potential. For example, radicals formed by nucleophilic capture of radical cations couple with the acceptor radical anion, resulting in (net) aromatic substitution. Thus, the l-methoxy-3-phenylpropyl radical (113 R = H) couples with dicyanobenzene radical anion loss of cyanide ion then generates the substitution product 132.2 + ... 

Coupling of alkyl radicals resulting from nucleophilic capture of alkenes with sensitizer radical anions in acetonitrile-methanol (3 1) was studied in detail. 

In addition to nucleophilic capture of alkene or cyclopropane radical cations (see above) radicals may be generated by cleavage of C—X bonds, particularly C—Si bonds. Such cleavage is often assisted by a nucleophile. Because the radical is generated near the radical anion, to which it couples, the resulting C—C bond formation may be considered a reaction of a modified radical (ion) pair. 

The oxidative method is often conducted on enol (or enolate) derivatives and a simplified mechanism is shown in Scheme 71. Initial chemical or electrochemical oxidation gives an electrophilic radical (68 that may be free or metal-complexed) that is relatively resistant to further oxidation. Addition to an alkene now gives an adduct radical (69) that is more susceptible to oxidation. Products are often derived from the resulting intermediate cation (70) by inter- or intra-molecular nucleophilic capture or by loss of a proton to form an alkene. The concentration and oxidizing potential of the reagent help to determine the selectivity in such reactions. 

B2. With Neutral Molecules Electron Transfer Addition to Olefins Cycloaddition to Diolefins Complex Formation B3. With Ions or Dipolar Substrates Nucleophilic Capture B4. With Radicals Spin Labeling Oxygenation (302)... 

When electron transfer reactions of olefins are carried out in nucleophilic solvents (alcohols) or in the presence of an ionic nucleophile (KCN/acetonitrile/2,2,2-trifluoroethanol), the major products formed are derived by anti-Markovnikov addition of the nucleophile to the olefin. In several cases, nucleophilic capture completely suppresses dimer formation [122, 143]. It is important to realize that the observed mode of addition reflects the formation of the more stable (allylic) intermediate and cannot be interpreted as evidence for the charge density distribution in the radical cation. 

In some cases the nucleophilic capture of a radical cation is followed by coupling with the radical anion (or possibly with the neutral acceptor), resulting ultimately in an aromatic substitution reaction. Thus, irradiation of 1,4-dicyanobenzene in acetonitrile-methanol (3 1) solution containing 2,3-dimethylbutene or several other olefins leads to capture of the olefin radical cation by methanol, followed by coupling of the resulting radical with the sensitizer radical anion. Loss of cyanide ion completes the net substitution reaction [144]. This photochemical nucleophile olefin combination, aromatic substitution (photo-NOCAS) reaction has shown synthetic utility (in spite of its awkward acronym). 

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