Radical cations sigmatropic

The first evidence that the radical cation generated by a single-electron transfer (SET) of an unsymmetrical 1,5-diene 408 can undergo a [3,3]-sigmatropic shift (Cope reaction)... 

Stereorigid radical cations may undergo stereospecific sigmatropic shifts for example, the puckered ions, anti- and syn- 5-methyl-19 ", undergo stereospecific hydride or methyl migration, respectively, forming the 1-methylcyclopentene ion (46 ) and the 3-methyl isomer (47 ). ... 

The ubiquitous and reversible formation of radical cations in photoelectrochemical transformations allows pericyclic reactions to take place upon photocatalytic activation since the barriers for pericyclic reactions are often lower in the singly oxidized product than in the neutral precursor. For example, ring openings on irradiated CdS suspensions are known in strained saturated hydrocarbons [176], and formal [2 -I- 2] cycloadditions have been described for phenyl vinyl ether [ 177] and A-vinyl carbazole [178]. The cyclization of nonconjugated dienes, such as norbomadiene, have also been reported [179]. A recent example involves a 1,3-sigmatropic shift [180]. 

The radical cations of conjugated systems can also take part in pericyclic reactions. Examples are known of cycloadditions, electrocyclic reactions and sigmatropic rearrangements. One noticeable feature of some of... 

The reaction can be analyzed by assuming that a homolytic cleavage in a [1,2] shift results in the production of a vinyl radical cation and in a [1,6] shift results in the production of a hexatriene radical cation. In case of a [1,2] shift, the HOMO of the rr-framework of the transition state is and in the [1,6]-shift it is W3. In either case the HOMO has mirror symmetry, and, therefore, under thermal conditions, the [1,2] or [1,6] sigmatropic shift occurs via a suprafacial pathway without inversion at the migrating center (Figure 3.18). 

A homolytic cleavage in a [1,4] shift results in the production of a butadiene radical cation, whose HOMO is W2 having C2 symmetry. Therefore, under thermal conditions, the [1,4] sigmatropic shift occurs via a suprafacial pathway with inversion at the migrating center (Figure 3.19). 

A review of the reactions and cationoid skeletal transformations in the series of polyfluoroarenes has been reported. The monoprotonated mechanism of the benzidine acid-catalysed rearrangement of hydrazobenzene has been compared with the diproto-nated mechanism monoprotonation results in concerted closed-shell sigmatropic shift, whereas diprotonation leads to a stepwise radical cation recoupling. ... 

The close association between metal ions and p-benzoquinones catalyzes their Diels-Alder reactions with anthracenes. The efficiency of the metal cations correlates with their Lewis acidity171. A mechanism proceeding via radical-anions for a [3,3] sigmatropic rearrangement was established172. 

Important general aspects of the cation-radical Diels-Alder reaction and other cation-radical sigmatropic reactions are summarized below ... 

Bauld, N. (1989) Cation radical cycloadditions and related sigmatropic reactions. Tetrahedron, 45, 5307—5363. 

One of the earliest, and certainly one of the more impressive examples of apparent cation radical sigmatropic shifts involves the corrin series [97], Electrochemical oxidation of a nickel(II)-A/D-secocorrinate in acetonitrile containing a trace of water was found to provide an almost quantitative yield of a secocoriinoxide in which a [1,16] hydrogen shift, from the methylene group at C19 in the D ring to the methylidene carbon in the A ring, had occurred in the intermediate cation radical. 

Many methods for the preparation of both the a- and )8-C-glycosides have been developed, and new approaches are still appearing in the literature. This chapter covers the literature from the end of 1994 to early 1999, with other inclusions as deemed appropriate. Following the organization of earlier work [1], we select the mode reactivity of the anomeric center to define the category. The chapter discusses free radical chemistry. Cl anions, Wittig chemistry-cyclization chemistry, transition metal mediated chemistry, sigmatropic chemistry, and approaches based on cationic chemistry. The chapter focuses on the most recent and novel developments in the field, but all the relevant references are included whenever possible. 

The yield and ratio of the two Stevens rearrangement products are dependent on both the base and the solvent however, the ortho-rearrangement product was only detected when butyllithium was used in hexane. Variation of the halide ion had only a small effect on the ratio of rearrangement products. Several mechanisms were considered 93) as routes to the formation of the rearrangement products, 1) ion-pairs, 2) cationic rearrangement, 3) carbenoid, 4) dimetallated intermediates, 5) free-radical, 6) predissociation of the ammonium salt and, 7) sigmatropic shift. 

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