Cyclobutanation cation radical chain

A cation radical chain cycloaddition-polymerization catalysed by tris(4-bromophenyl)aminium hexachloroantimonate has been reported to afford polymers with an average molecular weight up to 150000. Both cyclobutanation and Diels-Alder polymers were obtained. " The reactivity of the phospine radical cation towards nucleophiles was studied. Tributylphosphine reacted with l,l-dimethyl-4,4-bipyridinium (methyl viologen, MV) in the presence of an alcohol or thiol (RXH X = O, S), which resulted in the gradual formation of the one-electron reduced form... 

Hence, cation-radical copolymerization leads to the formation of a polymer having a lower molecular weight and polydispersity index than the polymer got by cation-radical polymerization— homocyclobutanation. Nevertheless, copolymerization occnrs nnder very mild conditions and is regio-and stereospecihc (Bauld et al. 1998a). This reaction appears to occnr by a step-growth mechanism, rather than the more efficient cation-radical chain mechanism proposed for poly(cyclobutanation). As the authors concluded, the apparent suppression of the chain mechanism is viewed as an inherent problem with the copolymerization format of cation-radical Diels-Alder polymerization. ... 

Scope of the cation radical chain cyclobutanation reaction... 

In a reaction quite analogous to cation radical cyclobutanation, cation radicals of some relatively ionizable, but sterically hindered alkenes have been found to add to ground state dioxygen in a very efficient cation radical chain process (Scheme 13) [37]. 

The cyclodimerization depicted in Scheme 7.19 is one of the many examples concerning cation-radicals in the synthesis and reactions of cyclobutanes. An authoritative review by Bauld (2005) considers the problem in detail. Dimerization is attained through the addition of an olefin cation-radical to an olefin in its neutral form one chain ends by a one-electron reduction of the cyclic dimer cation-radical. Unreacted phenylvinyl ether acts as a one-electron donor and the transformation continues. Up to 500 units fall per one cation-radical. The reaction has an order of 0.5 and 1.5 with respect to the initiator and monomer, respectively (Bauld et al. 1987). Such orders are usual for branched-chain reactions. In this case, cyclodimerization involves the following steps ... 

Transition to the final product, the cyclobutane LV, can be greatly facilitated by the use of electron-donating substituents R. In this case, the electron transfer from substituted ethylene to the orbital of the cation-radical LIV occupied by one electron switches on the chain mechanism represented on the above scheme, similar to that for the reactions of the SrnI type (Sect. 9.1). Such mechanisms are a characteristic feature for the class of reactions, catalyzed by electron transfer, in which the sequence 1) ionization (electron capture), 2) reaction in the ion-radical, 3) electron capture (ionization) - is a very convenient route for effecting the needed transformation [96]. 

Intramolecular bond formations include (net) [2 + 2] cycloadditions for example, diolefin 52, containing two double bonds in close proximity, forms the cage structure 53. This intramolecular bond formation is a notable reversal of the more general cycloreversion of cyclobutane type olefin dimers (e.g., 15 + to 16 +). The cycloaddition occurs only in polar solvents and has a quantum yield greater than unity. In analogy to several cycloreversions these results were interpreted in terms of a free radical cation chain mechanism. 

A [2 + 2] photocycloaddition with two alkenes can also be induced by photochemical electron transfer [16,17]. In such cases, sensitizers are frequently used and the reactions therefore occur under photocatalysis [18]. Under photochemical electron transfer (PET) conditions, the diene 10 yielded in an intramolecular reaction the cyclobutane 11 (Scheme 5.2) [19], such that in this reaction a 12-membered cyclic polyether is built up. The reaction starts with excitation of the sensitizer 1,4-dicyanonaphthalene (DCN) only 0.1 equivalents of the sensitizer are added to the reaction mixture. Electron transfer occurs from the substrate 10 to the excited sensitizer, leading to the radical cation I. This intermediate then undergoes cycli-zation to the radical cation of the cyclobutane (II). Electron transfer from the radical anion of the sensitizer to the intermediate II leads to the final product 11, and regenerates the sensitizer. In some cases, for example the cydodimerization of N-vinylcarbazole, the effidency is particularly high because a chain mechanism is involved [20]. 

The photocyclodimerization of JV-vinylcarbazole, which was reported by Ledwith and Shirota, can be accounted for by this mechanism [71-73]. A chain process is involved in this photoreaction, and the quantum yield exceeds unity (the maximum quantum yield is 66). The hole transfer from the cyclobutane radical cation to a neutral JV-vinylcarbazole is a key reaction for the chain process. Similar photodimerizations of electron-rich alkenes such as aryl vinyl ethers [74-76], indenes [29, 77, 78], styrenes [79-80] and enamines [71] have been reported by several groups. The DCA-sensitized photodimerization of phenyl vinyl ether gives cis- and trans-1,2-diphenoxycyclobutanes. This photoreaction also involves a chain process although the chain length is short [75]. 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

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. 

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... 

Scheme 5.3 Tr/s(p-bromophenyl)aminium hexachloroantimonate (l SbClj)-mediated radical cation chain reaction ofJrans-anethole (2) to give head-to-head trans,anti,trans-l,2-ii/s- 4-methoxyphenyl)-3,4-dimethyl cyclobutane (3) via the reactive intermediates 2 and 3 .

The reverse Q—reaction is thermally forbidden by the Woodward-Hoffmann rules, but this transformation can be readily accomplished via a radical-cation chain reaction pathway initiated by chemical, electrochemical, or photosensitized single-electron oxidations. The metastable Q structure contains a highly strained cyclobutane and two cyclopropane rings, and the enthalpy of the 2—>1 isomerization in the pure liquid state has the value of AH = -89 kj mob, as obtained from direct calorimetric measurement. Results of other experiments and computed values vary in the range of -81 to -112 kJ mol". - - ... 

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