Oxetanes decomposition

SolubiHty parameters of 19.3, 16.2, and 16.2 (f /cm ) (7.9 (cal/cm ) ) have been determined for polyoxetane, po1y(3,3-dimethyl oxetane), and poly(3,3-diethyloxetane), respectively, by measuring solution viscosities (302). Heat capacities have been determined for POX and compared to those of other polyethers and polyethylene (303,304). The thermal decomposition behavior of poly[3,3-bis(ethoxymethyl)oxetane] has been examined (305). 

Aryl ketones are often used to effect cis and tram isomerization of olefins.(118-ia0) Although this, in some cases, can be viewed as an energy transfer process where the ketone triplet transfers its energy to the olefin, which then isomerizes, the failure of noncarbonyl sensitizers of comparable triplet energy to isomerize the olefins suggests that a process other than energy transfer may be involved. Schenck and Steinmetz<118) suggested that isomerization results from decomposition of a biradical carbonyl-olefin adduct similar to that involved in oxetane formation ... 

An unexpected result was obtained when DTBB-catalyzed lithiation was applied to the vinyl-oxetane 313 . After work-up, lactone 314 was isolated, the process being explained by an elimination reaction via a radical pathway more than by reduction of the benzyl radical into the anion. Thus, this hypothetical intennediate reacted with the lithium enolate of acetaldehyde, generated in situ by reductive decomposition of THF (Scheme 92). 

The conditions for the photocycloaddition (discussed in detail in a later section of this review) can be relatively mild. There is usually a small probability of the oxetane being destroyed in dark reactions which would probably preclude isolation after preparation by any method. One mode of decomposition of oxetanes is fragmentation, either back to the starting materials or to the other possible carbonyl compound and olefin. For example, the oxetane from 4,4 -dimethoxybenzophenone and isobutylene forms readily and is easily detected and characterized by infrared and NMR spectroscopy. All efforts to purify it, however, have led to its decomposition into formaldehyde and the diarylethy-lene.17 37 In some cases, as with fluorenone and isobutylene37 or 2-methyl-2-butene,25b the oxetane is apparently too unstable for detection, but the presence of the olefin 96 attests to its formation. 

I Compound obtained as result of decomposition of oxetane (or oxetene). 

Thermal decomposition of oxetanes proceeds at elevated temperatures, usually in the range of 300-450 °C, to give an alkene and a carbonyl compound in practically quantitative yield. Gas phase thermolyses of several alkyl-substituted oxetanes have been studied in detail, because such compounds occur as intermediates in the oxidation of alkanes (Section 5.13.3.3). When the oxetane is unsymmetrically substituted, as in the case of 2,4-dimethyl-oxetane (20), two modes of cleavage, to give two different sets of products, are possible and are generally observed. In fact, when only hydrogen and alkyl substituents are present,... 

An investigation of several phenyl-substituted oxetanes and of rrani-diethyl oxetane-2,3-dicarboxylate has been carried out in hot solutions rather than in the gas phase. The solvents must, of course, be high boiling and completely free of any acidic impurities, which would cause an acid-catalyzed decomposition. 2,2-Diphenyl-3,3,4,4-tetramethyloxetane (27) pyrolyzed at 310 °C at practically the same rate in each of three solvents of quite different polarity — diphenyl ether, DMF and TMEDA — showing that the intermediate is relatively non-polar. High regioselectivity is shown in this reaction, which gave mainly benzophenone... 

The mechanism of decarboxylation of /3-lactones has attracted much attention. The gas-phase decomposition of 2-oxetanone is a unimolecular first-order process. It has a considerably lower energy of activation than the pyrolysis of oxetane and a much higher entropy of activation, indicating a loose activated complex (69JA7743). The ease of the reaction is greatly affected by the electronic effect of substituents at position-4, but not at position-3. The Hammett treatment of a series of rrans-4-aryl-3-methyl-2-oxetanones gave a good correlation with [Pg.374]

Decomposition of oxetanes is still another chemiluminescent reaction. On the basis of Woodward-Hoffman rule of conservation of orbital symmetry, the concerted bond cleavage of dioxetane, a 4-membered ring peroxide, should yield one carboxyl moiety in the excited state... 

The chemistry of methylene-substituted 2-oxetanones, and in particular diketene, has attracted a vast amount of research over the years, so other alkylene-substituted oxetanes and oxetan-2-ones, including methylene oxetanes, are discussed separately in Section 2.05.7.4. Diketene is also known as 4-methylene-2-oxetanone and a notable difference between this and other 2-oxetanones is that it undergoes thermal decomposition by a cycloreversion reaction to give ketene, rather than forming allene and C02. CHEC-II(1996) refers to a series of comprehensive reviews of the chemistry of this compound <1996CHEC-II(1)721>. 

The counterion is very stable in the polymerization of 3,3-bis-(chIoromethyl)-oxetane no termination, due to its decomposition, was observed up to 90 °C. Thus, it was proposed that the anion has a stabilized cyclic structure, formed after preliminary coordination of monomer with initiator ... 

Transfer to polymer (Sect. 5.3) has been described by Aleksiuk a.o. in polymerization of 3-chloromethyl-3-methyloxetane initiated with aluminum allQ ls. High molecular weight polymers (M = 6 to 8 10 ) with narrow molecular weight distribution (Mw/M = 1.29) were formed from very beginning of reaction and were independent of conversion. These data suggest slow initiation, fast propaption and unimolecular decomposition process. Authors propose the termination on the own polymer backbone and formation of nonreactive branched oxonium ion. Similar mechanism described earlier for 3,3-bis(chloromethyl)oxetane is discussed in Sect. 3.2.11. 

Early reports by Kossanyi and coworkers illustrate the variety of strained products that can be obtained from intramolecular photoreactions. For example, irradiation of 2-allylcyclohexanone gave a mixture of (186 32%), (188 22%) and (189 14%), the latter two arising presumably from decomposition of the strained tricyclic oxetane (187). In a related azulene synthesis,Kossanyi obtained high yields of oxetanes (190) and (191), the latter comprising 80% of the reaction mixture. When thermolyzed, compound (191) formed a mixture of isomeric dihydroazulene compounds that could be dehydrogenated (Pd/AhOs) to form azulenes. 

The addition of [2- " C] but-2-ene to reacting n-butane + oxygen mixtures at 315 °C [56] showed that the reverse isomerization reaction (— 14a) does not occur to any appreciable extent, since the 2-methyloxetan found in the products was inactive. It can be safely concluded, therefore, that the formation of derivatives of oxetans, furans and pyrans is diagnostic of alkylperoxy radical isomerization and subsequent decomposition, reactions (14) + (16). Table 1 thus presents a considerable volume of evidence for the wide occurrence of this chain-propagation step. 

The estimated values of fe 14 for oc, /3, 7 and 6-H transfer are given in Table 3 (p. 275) from which it can be seen that the reverse reaction is faster than the forward reaction. However, it can be seen by compEirison with Table 4 (p. 278) that the reverse isomerization of 7 and 5-hydro-peroxyalkyl radicals is much slower than their decomposition to tetra-hydrofurans and tetrahydropyrans, respectively. Reactions (—147) and (—145) will therefore be of little significance. On the other hand, the reverse isomerization of a- and j3-hydroperoxyalkyl radicals would be expected faster than either their decomposition to oxirans and oxetans, respectively or than their decomposition to the conjugate alkene and j3-scission products respectively. Reactions (—14a) and (—14j3) would thus be expected to be important. As discussed in Sect. 3.2.2, however,... 

In contrast, compounds arising from /3-scission of C—C bonds are the major products formed during the oxidation of 3-ethylpentane, their yields being ca. 3 times as large as those of the corresponding oxetans. /3-Scission decomposition of /3-hydroperoxyalkyl radicals competes effectively, therefore, with their decomposition by cyclization to oxetans. 

The reason that esters frequently give better yields of oxetane is not known, but it seems likely that reaction of hydroxide ion would u> primarily on the carbonyl carbon atom, to form an Intermediate of i fa> usual type- This might reasonably decompose either (a) with con carted attack on the halogeii-subetituted carbon atom, or (b) inii. -i 3-halogenoalkoxide ion which is thermally activated by tlw he i.l decomposition of the intermediate, thus fadlitating ring cloeure. 

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