Dienyl radical cations

The reaction proceeds by an ET pathway giving the I9e organoiron radical cation and the organic radical R which couple in the cage after escape ofX. The cationic Fe1 intermediate is noted at low temperature by its characteristic purple color and the classical spectrum of Fe1 species with rhombic distortion (g = 2.091, 2.012, 2.003 at —140 °C in acetone) before collapse to the orange substituted cyclohexa-dienyl Fe11 complexes. 

Many studies used radiation chemistry to produce the radical and radical cations and anions of various dienes in order to measure their properties. Extensive work was devoted to the radical cation of norbomadiene in order to solve the question whether it is identical with the cation radical of quadricyclane . Desrosiers and Trifunac produced radical cations of 1,4-cyclohexadiene by pulse radiolysis in several solvents and measured by time-resolved fluorescence-detected magnetic resonance the ESR spectra of the cation radical. The cation radical of 1,4-cyclohexadiene was produced by charge transfer from saturated hydrocarbon cations formed by radiolysis of the solvent. In a similar system, the radical cations of 1,3- and 1,4-cyclohexadiene were studied in a zeolite matrix and their isomerization reactions were studied. Dienyl radicals similar to many other kinds of radicals were formed by radiolysis inside an admantane matrix. Korth and coworkers used this method to create cyclooctatrienyl radicals by radiolysis of bicyclo[5.1.0]octa-2,5-diene in admantane-Di6 matrix, or of bromocyclooctatriene in the same matrix. Williams and coworkers irradiated 1,5-hexadiene in CFCI3 matrix to obtain the radical cation which was found to undergo cyclization to the cyclohexene radical cation through the intermediate cyclohexane-1,4-diyl radical cation. 

MacMillan and co-workers then developed the a-arylation of aldehydes and showed it to be ortfto-selective. The single electron oxidant in this case was [Fe(phen)3] (PFg)3 in place of CAN, as it was shown to provide a superior catalyst/ oxidant combination. The general applicability of this procedure was demonstrated with a wide variety of substrates [33, 34], The mechanism proposed by MacMillan and co-workers is an overall two electron oxidation whereby the 3- Ji-radical cation reacts with the benzene ring to form a bicyclic radical cation 107 that upon further oxidation, generates a dienyl cation 108. Rearomatisation and hydrolysis of the iminium species generates the product and regenerates the catalyst (Scheme 1.27). 

As for the mechanism, initially, the reaction of exited photocatalyst (PC ) and benzene gave benzene radical cation and photocatalyst radical (PC ). The later species may undergo an electron transfer to the metal cocatalyst Co to produce Co and ground state photocatalyst to complete the photocatalysis cycle. The former species reacts with amine to give a dienyl radical. This adduct may be oxidized by Co° to furnish Co and dienyl cation, which afford the desired substituted benzene after deprotonation. The Co may reduce two protons produced during the reaction to a molecule of H2 (Scheme 2.4). 

The properties of the 7-norbornyl (18), 7-nobomenyl (19), and 7-norboma-dienyl (20) ions and radicals have aroused much interest in view of the evidence that the cations 19 and 20 have nonclassical -complex structures. We... 

The presence of a 4-methoxy substituent on the 2-phenylethyloxyl or 3-phenylpropyl-oxyl side chains radically altered the course of these cyclizations (Scheme 4). 31a and 31b afforded the spiro-fused ring systems 32 and 33 in 26 and 69% yields, respectively, as the only cyclization products. With this substituent, cyclization onto the activated ipso positions was favoured over direct attack, even where the strained transition state for Atj -5 cyclization of 31a to 32 was involved. Demethylation of the intermediate spirocyclohexa-dienyl cation is favoured over rearrangement in these cases. Kikugawa and coworkers effected the formation of 32 (82%) and 33 (39%) with reverse efficiencies using AgaCOs in TFA . 

A similar case of concurrence of one-electron transfer and nucleophilic addition is observed in the thermal ion-pair annihilation of CpMo(CO)3 anion with (dienyl)Fe(CO)3+ cations [84, 179]. Thus, spontaneous electron transfer (A et) occurs upon mixing of ( / -cyclohexadienyl)Fe(CO)3 with CpMo(CO)3 in acetonitrile to afford the transient 19-electron iron radical and the 17-electron molybdenum radical which both rapidly dimerize (Eq. 51). 

For cationic 7r-ligands, the usual ionic suffix, for example, dienium, becomes -dienyl as in cyclohexadienium becoming 7i-cyclohexadienyl. Cationic ligands sometimes behave as anionic or radical species because of the ligand structure [1 -20-1 -22]. 

Since 1958 a considerable amount of research activity has centered around these systems, both in the acetylene-iron carbonyl reactions and the direct reactions of olefins with iron carbonyls. The types of unsaturated ligands which are now known to occur in stable iron carbonyl complexes include substituted and nonsubstituted cyclic, acyclic, and nonconjugated dienes as well as some aromatic systems. Furthermore, what may be formally regarded as dienyl cations as well as allyl cations and radicals are found to... 

---