Oxetane radical cation

The cyclic sulfates derived from 1,3- and 1,4-diols showed a different fragmentation pattern. For example, sulfates of 1,3-diols fragment in an orderly manner via (M-SOa ), which are represented by oxetane radical cations, whereas 1,4-diol cyclic sulfates fragmented via tetrahydrofuran-like intermediates (77AJC569). 

Miranda, M.A. and Izquierdo, M.A., Stepwise cycloreversion of oxetane radical cations with initial C-O bond cleavage, /. Am. Chem. Soc., 124, 6532, 2002. 

Finally, we mention the radical cation of oxetane. Without any doubt, this is a 7t species produced by oxygen lone pair ionization [327, 329]. It has sizeable hfcs in the P and y positions (afl = 65.5 G aY = 10.8 G) and is characterized by a highly anisotropic and positively shifted g-factor (g" = 2.0046, g = 2.0135). The substantial difference between the p hfcs and g-factors of the oxetane ion and that... 

The ring opening of 2,2 -diphenyloxetane has been achieved using cerium(iv) ammonium nitrate (CAN) as a redox catalyst (Equation 7) <2003TL4585>. The first step involves oxidation of the oxetane by Ce(rv) to a cyclic radical cation. Equilibration to the ring-opened distonic version 42 with a stabilized cation, then quenching of the cation with methanol and reduction of the alkoxide radical by Ce(lll) to the anion, completes the catalytic cycle. 

Whereas a triplet 1,4-biradical has been assigned as the most probable intermediate at that time, later work on intramolecular trapping reaction favored the assumption of radical ion pairs [46]. Efficient lactonization reaction to form 51 during irradiation of pent-4-enoic acid 50 and 48 accounts for an olefin radical cation which undergoes electrophilic addition towards the carboxyl group. Another type of rearrangement has been detected in the photoreaction of tetramethylallene and 48 [47]. 5-Hydroxy-indan-2-ones 52 are formed in high yields probably via instable spiro-oxetanes 49b as intermediates. 

Bach and coworkers investigated the photocycloaddition of 7V-acyl, 7V-alkyl enamines 125 with benzaldehyde [125]. The 3-amido oxetanes 126 were formed with excellent regioselectivity (analogous to reactions with enolethers—vide supra) and good diastereoselectivity (Sch. 41). Enamines, not deactivated by acylation at the nitrogen atom are poor substrates for Paterno-Buchi reactions due to preferred electron transfer reactivity (formation of the corresponding enamine radical cation and subsequent reactions). 

Cycloadditions of carbonyl compounds to olefins generally involve exci-plexes and biradicals. (Cf. Section 7.4.4.) While normal olefins frequently yield a number of products, photoinduced electron transfer may be utilized in the case of electron-rich olefins to influence the regioselectivity. Thus, irradiation of the ketene acetal 165 and biacetyl (164) yields exclusively the oxetane 167. Since the radical cation 166 could be trapped, electron transfer... 

As can be seen from the foregoing, phthalimide photocyclizations have provided useful synthetic routes to a variety of heterocyclic products. Machida et al. have used the reaction to yield the spiro compounds (325) from the irradiation (in methanol) of the phthalimide derivatives (326). The products are presumed to be formed via bond formation in the biradical produced by the addition of methanol to the radical cation/r ulical anion pair. The indoles (327) undergo photochemical (2-f-2)-addition with A methyl phthalimide to afford the oxetan adducts (328). This work has also been the subject of a patent application. " ... 

A study of the reaction of benzoquinone with alkenes has sought to identify the nature of the transients involved. In the case of reaction with tetraphenylallene irradiation in carbon tetrachloride at 355 or 532 nm yields the indene (376) as the sole product. When methanol is added to the reaction mixture the same indene (376) is accompanied by the methoxy derivative (377), The suggested mechanism for the formation of the indene (376) involves electron transfer to yield the radical cation / radical anion pair (378). Protonation followed by combination yields the cation (379) which is the key intermediate to the final product.Benzoquinone adds photochemically to the enol lactones (380) and (381) to give the oxetans (382) and (383)... 

It was noted above that the photolysis of indole and aldehydes in the solid state leads to diindolymethanes by a mechanism postulated to involve an oxetane intermediate (Scheme 14). The reaction also proceeds when aromatic aldehydes and indole or 2-methylindole are irradiated with UV light in acetonitrile solution (Scheme 28) [61]. Under these circumstances, it has been proposed that light-induced electron transfer from the indole to the aldehyde yields a ketyl radical anion and the indole radical cation. Proton transfers, coupling, and elimination of water can then yield an electrophilic alkylidene indolenine 65 which can react with indole thermally to give the observed product. 

The sensitizer dependency for the cycloreversion of tra/7, raf7 -2,3-diphenyl-4-methyloxetane has been studied. When chloranil is used as the sensitizer, the reaction proceeds via the radical cation of tr<3f7 -(3-methylstyrene, while with pyrylium salts the trans- iiXhQxvQ radical cation is involved.Other work in this area has examined the cycloreversion of the oxetanes (34) using (35) or chloranil as the sensitizers. ... 

Cycloreversion Reactions A cycloreversion reaction is the reverse of a cycloaddition reaction and leads to the formation of the starting reactants through the cleavage of two bonds in the ring [18], A typical example is the formation of C2H4+ and neutral C2H4 from the cyclobutane radical cation. As shown in reaction (6.37), this reaction proceeds through the intermediacy of a distonic ion. The radical cations of a variety of other four-membered cyclic compounds, such as cyclobutanones (3), diketene (4), oxetane (5), cyclobutylamine (6), and thiocyclobutane (7), are known to participate in cycloreversion reactions [27]. 

Oxidative electron transfer cycloreversion of the oxetane (59) using triphenylthiapyiylium perchlorate as a photosensitizer leads to distonic 1,4-radical cations subsequent cleavage gives rise to fragmentation products, whereas nucleophilic trapping by acetonitrile affords a ring expanded oxazine (60)/" ... 

The key step in the proposed oxetane repair mechanism is the rapid fragmentation of the oxetane radical anion. This ring-opening reaction was recently studied with model compounds. Falvey and coworkers " prepared stable oxetane adducts from dimethylthymine and various benzaldehyde derivatives. Laser flash photolysis investigations in the presence of external electron donors hke dimethyl aniline showed efficient oxetane sphtting, which allowed investigation of the radical intermediates. The radical cations of the various sensitizers and the radical anions of the carbonyl products formed after... 

The chemistry of polymerization of the oxetanes is much the same as for THE polymerization. The ring-opening polymerization of oxetanes is primarily accompHshed by cationic polymerization methods (283,313—318), but because of the added ring strain, other polymerization techniques, eg, iasertion polymerization (319), anionic polymerization (320), and free-radical ring-opening polymerization (321), have been successful with certain special oxetanes. 

Some monomers with no tendency toward homopolymerization are found to have some (not high) activity in copolymerization. This behavior is found in cationic copolymerizations of tetrahydropyran, 1,3-dioxane, and 1,4-dioxane with 3,3-bis(chloromethyl)oxetane [Dreyfuss and Dreyfuss, 1969]. These monomers are formally similar in their unusual copolymerization behavior to the radical copolymerization behavior of sterically hindered monomers such as maleic anhydride, stilbene, and diethyl fumarate (Sec. 6-3b-3), but not for the same reason. The copolymerizability of these otherwise unreactive monomers is probably a consequence of the unstable nature of their propagating centers. Consider the copolymerization in which M2 is the cyclic monomer with no tendency to homopolymerize. In homopolymerization, the propagation-depropagation equilibrium for M2 is completely toward... 

By using the aluminum porphyrin-Lewis acid system, we attempted the synthesis of a narrow MWD block copolymer from oxetane and methyl methacrylate (MMA). Methacrylic monomers can be polymerized radically and anioni-cally but not cationically, so a block copolymer of oxetane and methyl methacrylate has never been synthesized. As already reported, methacrylic monomers undergo accelerated living anionic polymerization with the (TPP)AlMe (1, X= Me)-3e system via a (porphinato)aluminum enolate as the growing species. 

Degradative transfer is not limited to radical processes. It can occur in many ionic polymerizations which are not exactly living. Degradative transfer here is, of course, harder to prove an ideal standard is missing. Nevertheless, degradative transfer has been proved, for example, during the cationic polymerization of oxetane [23],... 

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