Radical cation distonic

Recent results of Bentrude et al. [44] suggest that a vinyl radical cation is first generated, and that the 1,3-distonic radical cation is reduced to a diradical involving a phosphoranyl radical moiety. However, because the phosphite and styryl moieties of phenylallylphosphites exhibit very close oxidation potentials [45], the presence of a phosphoniumyl radical cannot be totally ruled out. 

The photoinduced anti-Markovnikov addition of methanol to 1,1-diphenylethene reported by Arnold and co-workers in 1973 provides the first example of the addition of a nucleophile to an arylalkene radical cation. There are now a number of studies that demonstrate the generality of nucleophilic addition of alcohols, amines, and anions such as cyanide to aryl- and diaryl-alkene radical cations. Product studies and mechanistic work have established that addition occurs at the 3-position of I-aryl or 1,1 -diarylalkene radical cations to give arylmethyl or diaryl-methyl radical-derived products as shown in Scheme I for the addition of methanol to 1,1-diphenylethene. For neutral nucleophiles, such as alcohols and amines, radical formation requires prior deprotonation of the 1,3-distonic radical cation formed in the initial addition reaction. The final product usually results from reduction of the radical by the sensitizer radical anion to give an anion that is then protonated, although other radical... 

It can be assumed that in the domino process of, for example 5-3, a reactive radical cation intermediate 5-5 is initially formed [5]. The intramolecular cyclization then proceeds almost exclusively through a stable, chair-like, six-membered transition state 5-8 to give a distonic radical cation 5-9, which is trapped by the aromatic... 

Hammerum, S. Distonic Radical Cations in the Gaseous and Condensed Phase. Mass Spectrom. Rev. 1988, 7, 123-202. 

Yates, B.F. Bouma, W.J. Radom, L. Distonic Radical Cations. Guidelines for... 

Gassman and co-workers have recently investigated the photoinduced cy-clization of y,5-unsaturated carboxylic acids to y-lactones [100]. 5-Methyl-4-hexenoic acid 51 is converted into the radical cation 52. The cyclization of 52 is expected to yield the distonic radical cations 53 and 54, which leads to the anti-Markonikow products 55 and 56 in ratio 5 1. Fine-tuning of the sensitizer improved combined yields up to 89% [101],... 

Two possible mechanisms are proposed. Primarily the enol radical cation is formed. It either undergoes deprotonation because of its intrinsic acidity, producing an a-carbonyl radical, which is oxidized in a further one-electron oxidation step to an a-carbonyl cation. Cyclization leads to an intermediate cyclo-hexadienyl cation. On the other hand, cyclization of the enol radical cation can be faster than deprotonation, producing a distonic radical cation, which, after proton loss and second one-electron oxidation, leads to the same cyclo-hexadienyl cation intermediate as in the first reaction pathway. After a 1,2-methyl shift and further deprotonation, the benzofuran is obtained. Since the oxidation potentials of the enols are about 0.3-0.5 V higher than those of the corresponding a-carbonyl radicals, the author prefers the first reaction pathway via a-carbonyl cations [112]. Under the same reaction conditions, the oxidation of 2-mesityl-2-phenylethenol 74 does not lead to benzofuran but to oxazole 75 in yields of up to 85 %. The oxazole 75 is generated by nucleophilic attack of acetonitrile on the a-carbonyl cation or the proceeding enol radical cation. 

Transformations of the radical cations of 2,3- and 2,5-dihydrofuran (DHF), radi-olytically generated in Freon matrices, were investigated by low-temperature EPR. The 2,3-DHF+ radical cation is stable at 77 K but at higher temperatures is transformed into dihydrofuryl radical, DHF. The oxygen-centred radical cation 2,5-DHF+ is unstable at 77 K and transforms via an intramolecular H-shift into a transient distonic radical cation 2,4-DHF+ which at higher temperatures yields the DHF radical. 

Cleavage of a C—C bond gives a distonic radical cation as an intermediate, while concerted cleavage of two C—C bonds yields the corresponding ArO and ArO in cycloreversion of aryl-substituted cyclobutane Therefore, the cycloreversion mechanism is related to dimerization of ArO where tt- and a-dimers are detected during PR of ArO such as... 

Cyclobutanes disubstituted in the 1,2-positions should favor strucmre-type C or a related distonic structure with one broken C—C bond. Calculations [QCISD-(T)/ 6-31G //UMP2/6-31G ] suggest a trapezoidal structure for frawi-1,2-dimethyl-cyclobutane radical cation.This expectation is born out by experimental results such as the ET induced cis-trans-isomerization of 1,2-diaryloxycyclobutane (Ar = aryl), leading to IS " ", and likely involving the distonic radical cation (14 +) formed via a type C ion. ... 

Not surprisingly, bifunctional species preceded the definition of the term distonic, both in the gas and condensed phases. For example, Hammond et al. studied the cage effect for radical pairs generated by decomposition of azo compounds. Among their targets was the doubly protonated amidine (27), whose decomposition yielded a pair of distonic radical cations (28 +).Attachment of the spin-bearing carbon in the 2-position of the diazaallyl function ensures minimal delocalization of unpaired spin into the latter. 

The distonic radical cation (14 ) stabilized by delocahzation of spin and charge into one aryl group each, was discussed above as a potential intermediate in the geometric isomerization of 1,2-diaryloxycyclobutane (13). ... 

In some cases, a stepwise mechanism is indicated by randomization of the dienophile stereochemistry. For example, addition of cw-anethole radical cation (100 +) to cyclopentadiene produces comparable yields of four possible diastereoi-someric adducts (102) clearly supporting a distonic radical cation intermediate (lOl ). Only products supporting the stepwise mechanim, that is, trans,endo-and trans,exo-lQ2, are shown. " ... 

R. L. Smith, R K. Chou, and H. I. Kenttamaa, Structure and reactivity of selected distonic radical cations , in Structure, Energetics and Dynamics of Organic Ions, T. Baer, C. Y. Ng, and I. Pawis, Eds., John Wiley Sons, Inc., New York, 1996, p. 197. 

The mechanistic aspects of aromatic121 and alkene122 radical cation reactions have been reviewed. A second review article covers the structure and properties of hydrocarbon radical cations, as revealed by low-temperature ESR and IR spectroscopy.123 A review of the reactivity of divalent phosphorus radical cations has appeared which discusses ionic and SET processes and their kinetics.124 The structure and reactivity of distonic radical cations have been reviewed, including experimental and calculated heats of formation, structures, reactivity, and mechanisms.122125... 

The vinylcyclopropane radical cation, generated at 77 K by X-irradiation of (139) in a Freon-113 matrix, was shown to rearrange at 105-110 K to afford two ring-opened distonic radical cationic species.300 The rearrangement reactions of the radical cations of 1,3- and 1,4-pentadiene and cyclopentene and the formation of spin adducts with 2.4.6-tri-/-butylnitrosobenzene (BNB) are discussed. The pulse radiolysis of 1,1 -binaphthyl-2,2,-diyl hydrogenphosphate (BiNPCUH) (140) in deaerated f-butanol at... 

The structure of diphosphallenic radical cations, generated from the allene ArP=C=PAr by electrochemical oxidation, has been examined using EPR spectroscopy. Ab initio calculations including correlation effects at the MP2 and MCSCF levels have determined that two rotamers exist compatible with Jahn-Teller distortion of the allene.146 Anodically generated radical cations of alkyl phosphites [(RO P] and silylphosphites [(RO)2POSiMe3] reacted with alkenes by initial attack at the C=C bond followed by electron transfer, deprotonation, and elimination of an alkyl or trimethylsilyl cation to form identical alkyl phosphate adducts.147 The electron ionization-induced McLafferty rearrangement of n-hexylphosphine afford the a-distonic radical cation CTEPH, the distinct reactivity of which suggests there is no... 

In summary, a variety of interesting bifunctional radical cation structures have been established. At least one species (152) features an orthogonal arrangement of spin and charge, whereas another (155) clearly meets the definition of a distonic radical cation. The fulvene radical cations (154), on the other hand, are bifunctional and may deviate to some extent from planarity however, they appear to be ordinary olefin radical cations rather than perpendicular ones. 

The cycloreversion of the cyclobutane radical cation Pyr +oPyr could proceed in either a concerted or stepwise manner, and many attempts were made to determine the mechanism of this cleavage step. Because the radical cation is delocalized, it is not unreasonable that both the C(5)-C(5 ) and the C(6)-C(6 ) bonds are weakened by oxidation of PyroPyr. The observation of a substantial secondary deuterium isotope effect for the cleavage of the first bond [C(6)-C(6 )] and a small isotope effect for the cleavage of the second bond [C(5)-C(5 )] in various deu-terated uracil-derived cyclobutane dimers was, however, taken as an indication of a stepwise splitting mechanism via the distonic radical cation Pyr+-Pyr [9]. Theoretical studies performed by Rosch, Michel-Beyerle et al. also strongly support the assumption of a successive cycloreversion [10]. 

Bauld and coworkers studied the [2+2] cycloaddition of A-vinyl carbazoles 86a and electron-rich styrenes 86b catalyzed by iron(III) catalysts A or B in the presence of 2,2 -bipyridine as a ligand, which was reported originally by Ledwith and coworkers (Fig. 21) [142, 143]. Deuterium-labeling studies provided support for the stepwise nature of the process, consisting of reversible SET oxidation of the electron-rich olefin to a radical cation 86 A. Nucleophilic addition of excess 86 leads to distonic radical cation 86B, which cyclizes to cyclobutane radical cation 86C. Back electron transfer affords cyclobutanes 87 and regenerates the catalyst. Photoelectron transfer catalysis gave essentially the same result, thus supporting the pathway. 

Distonic radical cation — is a radical cation in which... 

CH2(CH2) NH2. Distonic radical cations are more stable than their conventional isomers CH3(CH2) NHJ. With the increase of n the energy difference converges... 

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