Cycloadditions, radical cation mediated

Stearically hindered or electronically disfavoured reactions as well as thermally forbidden cycloadditions are possible in the radical-cation-mediated cycloadditions. Generally, this type of reaction starts with electron transfer from one of the reagents to a one-electron oxidant (Equation (36)) ... 

Radical cation-mediated intramolecular [4+2] cycloaddition proceeds efficiently only when the diene and dienophile are separated by three atoms... 

Cycloaddition reactions of alkene radical cations have been the subject of a number of mechanistic studies and are potentially useful synthetic reactions. - - - Most of the initial work on radical cation mediated cycloadditions focused on the dimerization of arylalkenes. with one of the first examples being Ledwith s report of the chloranil-sensitized dimerization of M-vinylcarba-zole to generate a diarylcyclobutane. This work led to the development of the mechanism outlined in Scheme 2, in which addition of the radical cation to neutral alkene generates an acyclic 1,4-radical cation as the primary intermediate. This intermediate cyclizes to a cyclobutane radical cation that is then reduced by the neutral alkene and regenerates a second radical cation to carry the chain. 

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. 

The development of mass spectrometric ionization methods at atmospheric pressures (API), such as the atmospheric pressure chemical ionization (APCI)99 and the electrospray ionization mass spectrometry (ESI-MS)100 has made it possible to study liquid-phase solutions by mass spectrometry. Electrospray ionization mass spectrometry coupled to a micro-reactor was used to investigate radical cation chain reaction is solution101. The tris (p-bromophenyl)aminium hexachloro antimonate mediated [2 + 2] cycloaddition of trans-anethole to give l,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane was investigated and the transient intermediates 9 + and 10 + were detected and characterized directly in the reacting solution. However, steady state conditions are necessary for the detection of reactive intermediates and therefore it is crucial that the reaction must not be complete at the moment of electrospray ionization to be able to detect the intermediates. 

A formal iron-catalyzed [3 + 2]-cycloaddition of styrene derivatives with benzoqui-none was reported by Itoh s group [96]. The process is believed to proceed via electron-transfer reactions mediated by a proposed Fe3+/Fe2+ couple, which generates a styrene radical cation and a semiquinone. These intermediates undergo stepwise addition to yield the benzofuran product 51 (Scheme 9.38). The reaction seems to be limited to electron-rich alkoxy-functionalized styrenes, as the Fe3+/Fe2+ redox couple is otherwise unable to transfer the electrons from the styrene to the quinone. 

Styrenes can also participate as the diene component in photo-chemically-mediated radical cation [4+2] cycloadditions, as well as in... 

An interesting application of PET mediated bond cleavage reaction from azirine 63 has been reported by Mattay et al. [67] for synthesizing N-substituted imidazoles (65) via the (3 + 2) cycloaddition reaction of resultant 2-azaallenyl radical cations with imines 64. Synthesis of pyrrolophane 3,4-dimethyl ester (68) has been reported recently [68] by the ring opening of 66 followed by inter-molecular cycloaddition with dimethyl acetylene dicarboxylate (67) as shown in Scheme 13. 

Electron-transfer-mediated cycloadditions of 2-(l-cyanovinyl)indoles can be carried out with cyclohexadienes and electron-rich styrenes the reactions are catalysed by photoexcited pyrylium salts or triarylaminium radical cations (Equation (91)) <93AG(E)90i>. 

Meyer and Metzger [29, 40a[ studied the tris(p-bromophenyl)aminium hexachlor-oantimonate (l SbClg) mediated [2 + 2[-cycloaddition of trans-anethole (2) to give l,2-fois-(4—methoxyphenyl)-3,4-dimethyl cyclobutane (3) (Scheme 5.3). The reaction proceeds as a radical cation chain reaction via transients 2 and 3 that were unambiguously detected and characterized by ESI-MS/MS directly in the reacting solution. At first, the reaction was studied by APCI-MS, because substrate 2 and product 3 are not ionized by ESI. A solution of l SbCla and a solution of 2, both in... 

Cycloadditions of alkenes and alkynes onto imine cation radicals have been reported, with the cation radicals generated by either PIET mediated by DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone)180, or by TIET mediated by FeCb1066. The reaction is shown in Scheme 77. 

A cycloaddition methodology has been exploited in the cation radical-mediated reactions between electron-rich chalcone epoxides 287 and A -aryl imines 286 using tris(4-bromophenyl)aminium hexachloroantimonate (TBPA -SbCle ) as the radical initiator to generate substituted 1,3-oxazolidines 288a and 288b in good yields (Equation 21) <2005SL161>. 

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