Furan radical cation

The mass spectrum of 2-pyrone shows an abundant molecular ion and a very prominent ion due to loss of CO and formation of the furan radical cation. Loss of CO from 4-pyrone, on the other hand, is almost negligible, and the retro-Diels-Alder fragmentation pathway dominates. In alkyl-substituted 2-pyrones loss of CO is followed by loss of a hydrogen atom from the alkyl substituent and ring expansion of the resultant cation to the very stable pyrylium cation. Similar trends are observed with the benzo analogues of the pyrones, although in some cases both modes of fragmentation are observed. Thus, coumarins. 

Pyridone undergoes fragmentation by loss of CO and formation of the pyrrole radical cation. 3-Hydroxypyridine, on the other hand, loses HCN to give the furan radical cation while 4-pyridone... 

The AMI-calculated structure and charge distribution of radical the trication of ) -carotene have been reported and its UV absorption spectrum estimated from INDO/S methods.The decomposition of the furan radical cation proceeds by two separate pathways according to a recent theoretical study, one via formation of propene radical cation and CO, the other a lower energy process via acetylene and a ketene radical cation.As a result of a reflection mass spectrometric study, a likely mechanism is... 

The form of the potential hypersurface for a molecular species changes drastically upon ionization. As an example, whereas azacyclohexatriene-2-ylidene 7 is largely destabilized (by ca 50 kcal mol ) relative to isomeric pyridine, it has been calculated that the difference is reduced to a few kcal mol for the corresponding radical cation 7 (see Scheme 7) [40]. Isomerization of the ions is prohibited by an energy barrier, evaluated at 40-60 kcal mol , which is lower, however, than the dissociation threshold, so interconversion does occur under mass spectrometric conditions. The effect of aromaticity even at the radical cation stage can, on the other hand, be evaluated because the furan radical cation is the main fragment formed (along with a minor amount of vinylketene) from the decarbonylation of the 2- and 4-pyrone radical cation (8+ , see Scheme 8) [41]. 

A single electron transfer- (SET) induced ring enlargement has been found in the conversion of substituted 2-furylmethanols (10) into 6-hydroxytetrahydropyran-3-ones (12) by the action of the binary reagent PhI(0Ac)2-Mg(C104)2 it has been suggested that the reaction occurs via a furan radical cation (11) as illustrated in Scheme 10 [44],... 

Fig. 6.5 Cut through the lowest PES of the furati, pyrrole, and thiophene radical cations along the coordinate of an effective mode. The latter is given by a straight line connecting the minimum of the A2 ionic ground state (taken to be the zero of energy in all cases) to the minimum of the Coin between the ground and first excited ( Si) ionic states. The dotted lines refer to thiophene, the dashed ones to pyrrole and the full lines to the furan radical cation...

Fig. 6.6 Electronic populations of the ground and first excited electronic states of the three radical cations given in the panels, following a vertical excitation to the upper Bi) electronic state. Consistent with the energetic positions of the surface crossings in the preceding figure, the transition occurs within a single vibrational period for pyrrole and thiophene, but is considerably slower in the case of the furan radical cation...

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

Julid investigated the behavior of terfuran 22 and bis(thienyl)furan 23 by cyclic voltammetry as well as the EPR spectra of the radical cations derived from these two compounds. Condensation of the diketone 20 with sulfuric acid furnished furan 22 in 18% yield, while reaction of diketone 21 with hydrochloric acid produced 23 in 84% yield.In a related report, Luo prepared oligomeric bis(thienyl)furans via similar methodology. ... 

Substituted 1,4-dimethoxybenzenes [76], furan [77], and dienes [78] undergo exclusively to predominantly 1,4-diaIkoxylation upon electrolysis in alcohol or an inert solvent with alcohol added. The reaction proceeds via an intermediate radical cation that undergoes solvolysis at the positions of the highest positive charge density. Anodic se-lenoalkoxylation proceeds regioselectively... 

Homo- and copolymerizations involving the monomers depicted in Scheme 1 are chain reactions which can be initiated, at least potentially, by typical free-radical, cationic or anionic promoters. TTie object of the studies reported below is to establish first of all which monomers adapt best to each type of initiation, then what peculiarities (if any) are caused by the presence of the furan ring, compared to the known behaviour of the corresponding aliphatic and/or aromatic homologues and finally to establish the structure-properties relationships of the materials obtained. 

In methanol, the radical-cation intermediates from oxidation of thiophenes and N-methylpyrroles can be trapped to give low molecular weight products. Reactivity resembles that of furan but with additional consequences because of the properties of thioethers and amines. 

Examples of photochemically driven carbon-carbon bondforming reactions emanating from the radical cations formed by photoinduced electron transfer include the addition of diphenyl-ethylene to furan, eq. 26 (80),... 

Analogous experimental conditions (i.e. Cl, 0.1-1 Torr) allowed for the detection by tandem mass spectrometry of the collision complexes formed in the ion-molecule reactions of several aromatic radical cations M+ (M = C6H5X, X= Me, N02, Cl pyridine, furan, thiophene) and neutral iodoalkanes RI (R= n-Pr, 2-Pr, n-Bu, 2-Bu, etc.) en route to areni-um ions34,35. The collision complexes are covalently bonded species, namely nonisomeriz-ing iodonium radical positive ions 4 which dissociate to arenium ions 5 via reductive elimination of I (Scheme 7)34. 

The reaction of phenanthrene and unsaturated compounds, such as furan, 1,1-diphenylethylene and indene, in the presence of electron acceptors and a nucleophile leads to products incorporating the nucleophile (Majima et ai, 1981). Thus furan gives [96], Excitation generates the radical cation of phenanthrene (Phent) and the radical anion of 1,4-dicyanobenzene. Phent oxidises furan to its radical cation which reacts in the manner as shown in Scheme 16. 

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