Oxirane, radical cation

This contrasts to the situation for the oxirane radical cation, for which the nature of the species observed experimentally in frozen matrices was long disputed [63], Tomasi and his group [64] first reported calculations on the fate of the oxirane radical cation produced by vertical excitation. Later calculations [65-67] all agree that the C- C ring-opened product 15 is formed from the oxirane radical cation with a low activation energy and that this is the structure observed in frozen matrices. Remarkably, the prototype W.-H. electrocyclic... 

Several gas-phase reactions were known to involve bifunctional radical cations preceding the definition of the term distonic we mention only the ring-opened oxirane radical cation, which will be discussed presently. ... 

Arnold first discovered that PET reactions of cis- and rrans-2,3-diphenyloxiranes (1 and 2) with electron-accepting sensitizers such as DCN, 1,4-dicyanobenzene, dimethyl terephthalate, and methyl 4-cyanoben-zoate produce the isomeric tetrahydrofuran derivatives 7 and 8 (Scheme 2). The proposed mechanism involves the CC bond cleavage of the oxirane radical cations to give ylide radical cations 3 and 4, followed by back electron transfer (BET) to generate carbonyl ylides 5 and 6. The resulting carbonyl ylides are trapped with dipolarophiles, such as acrylonitrile, maleonitrile, and fumaronitrile, to produce the observed products. In the absence of dipolarophiles, C)S-/ir s-isomerization (1/2 = 0.19-0.28) was observed, which is consistent with the formation of carbonyl ylides 5 and 6, as well as their radical cations 3 and 4. This method was successfully apphed to the synthesis of various tetrahydrofurans (Scheme 3) and dihydrofurans (Scheme 4). ... 

The type of O2 adduct depends on the donor structure. For example, tetraphenyl-oxirane forms an ozonide (78), " 1,4-bifunctional radical cations form dioxanes (79) conjugated dienes form cyclic adducts (80) and ergosteryl acetate (81) forms the 5a,8fl-peroxide (82) at —78... 

Redox photosensitization or co-sensitization by aromatic hydrocarbons has been utilized for enhancement of the efficiency of photoinduced electron transfer reactions. For example, the efficiency of the 9,10-dicyanoanthracene-sensitized photooxygenation of 1,2-diphenyloxirane in acetonitrile is enhanced appreciably by adding biphenyl as a co-sensitizer, giving 3,5-diphenyl-1,2,4-trioxolane in good yield [31-32]. This photoreaction does not take place in the absence of biphenyl. Schaap proposed that in this photoreaction the primary electron transfer reaction occurs from biphenyl (BP) to DCA to produce biphenyl radical cation BP and DCA . The secondary electron transfer from the oxirane to BP produces BP and the radical cation of the oxirane which is converted into the trioxolane (Scheme 5). 

The ring-opening reaction is not limited to conventional nucleophiles. Ceric ammonium nitrate in the presence of excess nitrate ion converts oxiranes to / -nitrato alcohols <1995T909>. The reaction is believed to proceed via a one-electron transfer to form an oxiranium radical cation that is subsequently captured by a nitrate ion. Nitric oxide adds to 2,3-epoxy phenyl ketones <2004TL1565>. 

Simple amines in the presence of Oxone oxidize alkenes to oxiranes. For example, Oxone, pyridine, and a 2-pyrrolidine derivative in a medium of aqueous acetonitrile selectively converts the triene in Equation (72) to a single epoxide. This process also proceeds using noncyclic alkenes. The mechanism is believed to proceed via a single-electron transfer (SET) process involving radical cation intermediates <2000JA8317>. 

Irradiation of 2,5-dimethylbenzoyl oxiranes results in formation of p-hydroxy functionalized indanones (74) that structurally resemble biologically active pterosines. Nanosecond laser flash photolysis and quantum-chemical calculations based on density functional theory support a photoenolization mechanism.Reversible photochemical transformations of cis- and tra v-2,3-dimethyloxirane radical cations in freonic matrices at 77 K has been observed and explained by the interconversion between the open (C2-C3 broken) and the cyclic forms. Photocatalytic ring opening of a-epoxy ketones in cyclopentanone, using 2,4,6-triphenylpyrylium tetra-fluoroborate as electron acceptor, results in the diastereoselective formation of 2,5-dioxaspiro[4,4]nonanes through Ca-O bond cleavage and subsequent nucleophilic attack by cyclopentanone to the radical cation intermediates. ... 

However, it has been recently suggested that oxirane intermediates also play a part in this reaction, producing some of the minor products (Scheme 59)152. Dienes do not appear to be good substrates for this reaction, at least not with triplet oxygen, as the cation-radical Diels-Alder dimerization is much faster unless the alkene is sterically hindered153. 

Heterocycles form a specific class of monomers. They do not usually undergo radical polymerization, and the kind of ionic polymerization mechanism is determined by the kind of heteroatom, substituent and ring size. Oxiranes and, aziridines are polymerized by both ionic mechanisms. With the exception of lactone, four-membered and larger heterocycles with oxygen and with substituted nitrogen can only be polymerized cationically heterocycles with unsubstituted nitrogen can also be polymerized anionically. 

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