Pyrrolidinium radical

The X-ray crystal structure of the hexafluoroantimonate salt of 1,4-diithin radical cation stabilized by bicyclo[2.2.2]octane annelation revealed a planar ring and was in agreement with theoretical calculations. Tertiary aminium radical cations underwent facile 5-exo-cyclization to give distonic 2-substituted pyrrolidinium radical cations. 

Radical polymerization of diallylamine derivatives produce water-soluble polymers of low molecular weight [22,55-57]. In order to increase the molecular weight, acrylamide has been copolymerized with these diallylamine derivatives to produce cationic polymers with variable charge density depending on the content of the structural units of pyrrolidinium rings and acrylamide in polymeric chains [22,55,58-61]. 

The cyclization process can be promoted by using a single electron transfer mediator. Electron transfer from the mediator generates the carbonyl radical-ion away from the electrode surface so that cyclization can occur before there is opportunity for a second electron transfer. Thus reduction of 16, R = Me, in dimethyl-forraaraide at mercury in the presence of tetraethylammonium fluoroborate leads only to conversion of the ketone function to the secondaiy alcohol. However addition of a low concentration of N,N-dimethyl pyrrolidinium fluoroborate alters the course of reaction and the cyclized tertiary alcohol is now formed. This pyrrolidinium salt is reduced at -2.7 V vs. see at mercuiy to yield a complex DMP(Hg5) which is thought to act as a single electron transfer mediator [94]. Cyclization can... 

Figure 12.23 Structure and conformations of N-butyl-N-methylpyrrolidinium cation [CjC4pyr]+. In [P14]+, P denotes pyrrolidinium and the digits denote the number of carbon atoms in radicals and R. Also, the ion is commonly called [bmpy]+ (N is atom number 1). The ring of the (CH2)4NR R pyrimidinium ion is not planar and has two stable (twist and envelope) forms.

The lifetimes of hypervalent radicals have been found to depend rather dramatically on isotope substitution. For example, dimethyloxonium, (CH3)2OH, dissociates completely on a 1 -ps time scale when formed by collisional reduction of the stable cation (CH3)2OH+. By contrast, (CH3)2OD furnishes an abundant survivor ion in the +NR+ mass spectrum that is evidence that the deuterated hypervalent radical is metastable [178,179]. From the time scale of the NR measurements and the survivor ion relative intensities one can estimate that (CH3)2OH dissociates >5 times faster than (CH3)2OD. Similar isotope effects have been observed for CH3OH [180], C2H5OH [181], and hypervalent ammonium radicals, e.g., CH3NH [182], (CH3)2NH [60], (CH3)3NH [183], and [pyrrolidinium] [184], which are metastable only as deuterated species. 

The complete cis stereoselectivity which is generally observed can be attributed to the adsorption of the radical species by its unhindered face on the electrode surface. The stereochemistry of 1 was assigned on the basis of comparison with authentic c/.v-isomers, prepared by hydrogenation of 2,5-disubstituted pyrroles, and mixtures of cis- and trans-isomers prepared by mercury-mediated cyclization of the same 4-alkenylamines, and also by X-ray analysis of a derived quaternary pyrrolidinium iodide24. 

Kariv-Miller and coworkers have developed indirect electroreductive cyclizations with the dimethyl-pyrrolidinium ion (DMP") as a mediator. Preparative electrolysis of 6-hepten-2-one (9) at a graphite cathode afforded cu-dimethylcyclopentanol (10) in 90% yield (equation 5). The reduction is believed to occur via the ketyl radical anion, which cyclizes onto the alkenic bond. In the absence of DMP simple reduction to 6-hepten-2-ol takes place.Very recently it was shown that instead of DMP several aromatic hydrocarbons can be used as mediators to initiate the cyclization reaction. The carbonyl group can also be cyclized onto an alkynic bond and even an aromatic ring. - ... 

Two findings are particularly noteworthy. First, the experiments in which the reactivity of water-soluble fullerene derivatives in aqueous media was probed (62-64) Not only, that the intermolecular reactions with hydrated electron and various radicals provided unequivocally evidence for the presence of fullerene clusters. But, furthermore, these investigations helped, in reference to the kinetics of the fullerene monomers, to estimate the agglomeration number for, for example, the mono pyrrolidinium salt in the respective fullerene cluster. Secondly, the intermolecular electron transfer reactions between radiolytically generated arene tt-radical cations and higher fullerenes (25,51) The noted parabolic dependence of the rate constants on the thermodynamic driving force is one of the rare confirmations of the existence of the Marcus-Inverted region in forward electron transfer. 

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