Ethers, aryl radical cation reactions

In summary, the literature survey provides clear evidence for a-carbonyl radical intermediates but no convincing proof for further oxidation to a-carbonyl cation in the vast majority of silyl enol ether radical cation reactions. This suggests that for most cases, silyl enol ethers are more readily oxidized than the corresponding a-carbonyl radicals. Only in oxidations of -aryl substituted silyl enol ethers, a-carbonyl cation intermediates have been invoked. For example, one-electron oxidation of 87d with TTA" " in acetonitrile/MeOH afforded 76 in analogy to the a-Umpolung of ketones via enol radical cations (Scheme 4), and oxidation of 124 with FePHEN provided benzofuran 19 [171]. 

A study of the photo-sensitized ring-opening reactions of the radical cations (76) of arylcyclopropanes (75) with methanol, water, and cyanide nucleophiles suggests a three-electron 5k2 mechanism (Scheme 11).185 The isolated products are methyl propyl ethers, derived from nucleophilic attack of methanol on the radical cation (76). They were detected by UV-VIS spectroscopy and shown to react with nucleophiles by transient kinetic methods. The benzyl radical (77) reacts with the DCB radical anion to afford monoaromatic ether (78) by oxidation and protonation or the disubstituted ether (79) by addition of DCB. Regio- and stereo-selectivity of the substitution were complete regiochemistry and rate constant were profoundly effected by the electronic nature of the aryl substituents.186 Elsewhere, a combined ab initio and CIDNP study... 

The distinction between electrophilic and electron-transfer mechanisms of addition reactions to vinyl double bonds of ArX—CH=CH2 (X = S, O, Se) has been achieved by studying substituent effects. Specifically, the effects of meta and para substituents on the rates of electrophilic additions correlated with Hammett radical cations correlates with statistical tests. The ofclcctrophilicj/o-1 (FT) dichotomy is in accord with the conventional paradigm for cr/cr+ correlations and further support has been found by ah initio calculations. Interestingly, the application of this criterion to the reactions of aryl vinyl sulfides and ethers with tetracyanoethylene indicates that cyclobutanes are formed via direct electrophilic addition to the electron-rich alkene rather than via an electron-transfer mechanism.12... 

The dimerization of butadiene, aryl olefins and ethyl vinyl ether is best rationalized by postulating a radical cation 89 (Eq. (174) ) as first intermediate. As the fi -carbon of 89 has the highest free valence, the highest positive charge density and the lowest atom localization energy radical or electrophilic reactions of 89... 

Pandey, G., Karthikeyan, M., and Mumgan, A. (1998) New intramolecular a-arylation strategy of ketones by the reaction of silyl enol ethers to photosensitized electron transfer generated arene radical cations construction of benzannulated and benzospiroannulated compounds. Journal of Organic Chemistry, 63, 2867-2872. 

A versatile strategy for efficient intramolecular oc-arylation of ketones was achieved by the reaction of silyle enol ethers with PET-generated arene radical cations. This strategy involved one-electron transfer from the excited methoxy-substituted arenes to ground-state DCN [42]. Pandey et al. reported the construction of five- to eight-membered benzannulated as well as benzospiroannulated compounds using this approach (Sch. 20) [42a]. The course of the reaction can be controlled via the silyl enol ether obtained... 

The reaction of pentafluoroiodobenzene with aromatic compounds such as anilines, pyrroles, indoles, imidazoles, aromatic ethers and phenols leads to aryl—aryl coupling477. The reactions proceed via pentafluorophenyl radicals which are generated by photoin-duced electron transfer (PET) and loss of iodide ion. Coupling between the pentafluorophenyl radical and the radical cation of the donor gives biaryl cations (138,139) which lose a proton. The reaction is illustrated for N, A-dimethylaniline (equation 125). 

The photocyclodimerization of JV-vinylcarbazole, which was reported by Ledwith and Shirota, can be accounted for by this mechanism [71-73]. A chain process is involved in this photoreaction, and the quantum yield exceeds unity (the maximum quantum yield is 66). The hole transfer from the cyclobutane radical cation to a neutral JV-vinylcarbazole is a key reaction for the chain process. Similar photodimerizations of electron-rich alkenes such as aryl vinyl ethers [74-76], indenes [29, 77, 78], styrenes [79-80] and enamines [71] have been reported by several groups. The DCA-sensitized photodimerization of phenyl vinyl ether gives cis- and trans-1,2-diphenoxycyclobutanes. This photoreaction also involves a chain process although the chain length is short [75]. 

There are many examples of such reactivity and some of these have been reviewed by Roth and coworkers, a research group that is extremely active in this area. An example that is typical of the processes encountered involves the cyclization of the diene geraniol (1). In this case the sensitizer is 9,10-dicyanoanthracene (DCA) and the reactions are carried out in methylene chloride. The authors state that a contact radical-ion parr is involved, i.e. the radical cation of the diene is in close proximity to the radical anion of the DCA. Reaction within this yields the cyclopentane derivatives 2 and 3 in the yields shown. The ring formation is the result of a five centre CC cyclization within the radical cation of 1. When a more powerful oxidant such as p-dicyanobenzene is used as the sensitizer in acetonitrile as solvent, separated radical-ion pairs are involved. This leads to intramolecular trapping and the formation of the bicyclic ethers 4 and 5 . The bicyclic ether incorporates an aryl group by reaction of the radical cation of the diene with the radical anion of the sensitizer (DCB). This type of reactivity is referred to later. Other naturally occurring compounds such as (/fj-f-bj-a-terpineol (6) and (R)-(- -)-limonene (7)... 

A radical cation is involved in the direct synthesis of chromans by an intramolecular oxidative cyclisation of 3-arylpropanols 32 brought about by a hypervalent iodine(III) reagent <04TL2293> and iodonium species catalyse the intramolecular arylation of alkenes which yields iodo-substituted chromans 33 <04JA3416>. 3-Allenylchroman-4-ols result from a one-pot reaction between salicylaldehydes and 1,4-dibromobut-2-yne in which the intramolecular cyclisation of the intermediate ether is mediated by In metal <04SL45>... 

Repeating the experiment with lb, but quenching the reaction by addition of diethyl ether as soon as the effervescence had subsided, afforded the tetrathiafulva-lenium salt 2b this compound was then subjected to solvolysis in undried deutero-acetone and afforded the corresponding alcohol 3b, consistent with its intermediacy in the reaction. The basic mechanism of the reaction can thus be represented by Scheme 2. Aryl radicals are formed following electron transfer to the diazonium cation and subsequent loss of dinitrogen. Rapid cyclization is followed by formation of the sulfonium salt 2b, and a facile solvolysis occurs to afford the alcohol 3b. Since the tertiary alcohol 3c was formed from substrate Ic, a similar pathway may have been followed, but the direct oxidation of the tertiary radical by electron transfer to a diazonium cation cannot yet be ruled out. The resistance of the primary salt to solvolysis is a classic hallmark of an Sn 1 reaction. (A refinement for the mechanism of the solvolysis step will be presented in Section 2.7.3.1 of this review, backed by very recent results). 

Electron Transfer Processes - The use of photochemical single electron transfer processes has been made in an approach to the synthesis of naphthalones. The reaction involves irradiation of the enol ether of a ketone such as (32) which yields the radical cation (33) that cyclizes onto the aryl ring. ... 

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