Radical ions, alkenes

The groups R2N and Cl can be added directly to alkenes, allenes, conjugated dienes, and alkynes, by treatment with dialkyl-V-chloroamines and acids. " These are free-radical additions, with initial attack by the R2NH- radical ion. " N-Halo amides (RCONHX) add RCONH and X to double bonds under the influence of UV light or chromous chloride. " Amines add to allenes in the presence of a palladium catalyst. ... 

Electron transfer sensitization allows either the radical cation or the radical anion of an aromatic alkene to form as desired, which finally results in nucleophile addition with Markovnikov and anti-Markovnikov regiochemistry. In an apolar solvent, the tight radical ion pair undergoes a stereoselective reaction when the electron-accepting sensitizer is chiral (Figure 3.10). ... 

Electrodimerization of activated alkenes in aprotic solvents occurs by radical-ion, radical-ion coupling. There is ample evidence for steric inhibition to this process. In contrast to the low reactivity of 11,4-methylbenzabnalononitriIe radical-ion dimeiises with a rate constant of 5.8 x 10 M s in dimethylformamide containing tetraalkylammonium ions [48]. Dimethyl maleate radical-anion diraerises faster than dimethylftimarate radical-anion by a factor of lO in dimethylformamide [49]. 

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

Radical cations of n donors are derived typically from substrates containing one or more N, O, or S atoms they are substituted frequently with alkene or arene moieties. Among these systems, we mention only a few examples, including two radical ions derived from l,4-diazabicyclo[2.2.2]octane (2) and the tricyclic tetraaza compound (3). For both ions, ESR as well as OS/PES data were measured. The bicyclic system (an = 1-696 mT, 2N ah = 0-734 mT, 12H) ° shows... 

The majority of radical ion reactions are bimolecular in nature, although some of these are merely variations of the unimolecular reactions discussed above, and many occur as pair reactions, albeit with a modified partner. Radical ions may react with polar or nonpolar neutral molecules, with ions, with radicals, or with radical ions of like or opposite charge (Scheme 6.3). Alkene radical ions undergo a particularly rich variety of reactions, including additions and cycloadditions. 

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. 

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 photoinduced electron transfer (PET) initialed cyclodimerization was first studied with 9-vinylcarbazole as substrate1 and characterized mechanistically as a cation radical chain reaction.2 The overall reaction sequence3-4 consists of a) excitation of an electron acceptor (A), b) electron transfer from the alkene to the excited acceptor (A ) with formation of a radical ion pair, c) addition of the alkene radical cation to a second alkene molecule with formation of a (dimeric) cation radical, and d) reduction of this dimeric cation radical by a third alkene molecule with formation of the cyclobutanc and a new alkene cation radical. Steps c) and d) of the sequence are the chain propagation steps. The reaction sequence is shown below. 

Cycloadduct formation is not observed upon irradiation of t-1 with fumaronitrile, maleic anhydride, or tetracyanoethylene. Irradiation of t-1 and maleic anhydride results in the formation of an alternating copolymer (96). The radical-ion pair or free radical ions obtained upon irradiation of the charge-transfer complex in polar solvent are presumed to be the initiating species. Irradiation of the ground state complex of t-1 and tetracyanoethylene at 580 nm in solution or the solid state results in neither adduct formation or t-1 isomerization (76). Irradiation of t-1 at 313 nm in the presence of tetracyanoethylene results in rapid isomerization followed by slow but quantitative formation of phenanthrene and tetracyanoethane (97). Product formation is proposed to occur via a dark reaction of dihydrophenanthrene with the electron-poor alkene. 

It is hardly surprising that different chemical reactivity might be expected from the exciplex and the radical ion pair formed by complete electron exchange. Lewis observation (50) that in the excited state interaction of trans-stilbene with either electron-rich or electron-poor alkenes cycloaddition is more efficient from the relatively less polar exciplex than from radical ion pairs is typical for many such cycloadditions. 

In contrast to radical ions generated from alkenes or carbonyl compounds, substantially fewer recent reports have appeared which describe the chemistry of radical ions generated from the >C=N— functional group. This situation likely results from the relative obscurity of the >C=N— group (compared to >C=0 and >C=C<), rather than specific problems with the chemistry, per se. Based upon the limited data available, and as might be anticipated, >C=N— + chemistry appears to be analogous to that of >C=C< +, while >C=N— chemistry is reminiscent of >C=0. ... 

Another extensively investigated system involves the interaction of two alkenes, each capable of geometric isomerization, viz., the system stilbene-dicyanoethylene, which also illustrates the involvement of ground-state charge-transfer complexes. Excitation of the ground-state complex results in efficient Z - E isomerization of the stilbene exclusively, because the stilbene triplet state lies below the radical ion pair, whereas the dicyanoethylene triplet state lies above it (Fig. 11) [163-166]. 

A PET reaction between excess phthalimide (in equilibrium with its conjugate base) and an alkene led to a clean phthalimidation of nonactivated double bonds. Here, the singlet excited state of phthalimide acts as the oxidant and a radical ion pair is formed. The olefin cation radical is trapped by the phthalimide anion, and back electron transfer, followed by protonation, affords the photoaddition products [40], Protected phenethylamines are readily accessible in this way. This reaction has been carried out by using NaOH as the base it has been shown that the amounts (usually equimolar with the alkene) must be carefully chosen in order to avoid the undesired competition with [2 + 2] photocycloaddition. 

In this chapter, recent developments in the regioselective, site-selective, and stereoselective preparation of oxetanes have been summarized. The relative nudeophilicity of the alkene carbons was seen to be important for regioselectivity, in addition to the well-known radical stability rule. Likewise, the three-dimensional structures of the triplet 1,4-biradicals were seen to play an important role in stereoselectivity. For photochemical reactions that proceed via radical ion pairs, the spin and charge distributions are crucial determinants of regioselectivity. It follows that the concepts used in selective oxetane synthesis should stimulate future investigations into the mechanistically and synthetically fascinating Paterno-Bitchi-type reactions. 

Halofuran and halothiophene derivatives undergo photochemical reactions with arylalkenes and arylalkynes and with benzo[6]furan513,514. With the arylalkenes and aryl-alkynes, heteroarylation takes place at the terminal alkene or alkyne carbon atom, while benzo[6]furan is substituted at position 2. The experimental results are interpreted in terms of solvent-separated or contact radical ion pairs. Iodothiophene and iodofuran derivatives can also be used to synthesize derivatives of benzimidazole by means of photochemical coupling515. The reaction of 5-iodothiophene-2-carboxaldehyde (157) with benzimidazole (158) giving the coupling product 159 is illustrated in equation 131. 

The PET-induced dehalogenation of aromatics has been extensively investigated. It occurs in the presence of various donors, such as aliphatic and aromatic amines [82-92], alkenes and dienes [91, 93-94], mixed hydrides [95-97], hydroxides [98], carbanions [99-101]. In the absence of an added donor, a viable mechanism involves the excimer (singlet or triplet) and its collapse to oppositely charged radical ions followed by fragmentation of the radical anion [93, 102-104] (a micellar medium has an important effect in this case [103]). 

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