Cyclic ethers scission

A linear phenylene-ethylene copolymer is obtained by a Wurtz-type condensation of 1,2-dibromoethane and 1,4-dibromobenzene. A cross-linked variation can be obtained on addition of 1,3,5-tribromobenzene to the reaction mixture. Equation 119 illustrates the reductive scission of a cyclic ether, catalyzed by such polymers in the presence of lithium, followed by quenching with cyclohexanone . 

UV irradiation of the resulting prepolymers caused a-scission, and benzoyl and polymer bound electron donating radicals are formed in the same manner as described for the low-molar mass analogues. Electron donating polymeric radicals thus formed may conveniently be oxidized to polymeric carboeations to promote cationic polymerization of cyclic ethers. It was demonstrated that irradiation of benzoin terminated polymers in conjuction with pyridinium salts as oxidants in the presence of cyclohexene oxide makes it possible to synthesize block copolymers of monomers with different chemical natures [75] (Scheme 19). 

Cyclic ethers from cyclic ketones The replacement of the carbonyl group of a cyclic ketone by oxygen can be effected in four steps Baeycr-Villiger oxidation to a lactone, DIBAH reduction to a lactol, (3-scission to an iodoformate, and finally cyclization. 

Catalytic hydrogenation of dihydrothebaine methine [ix] with colloidal palladium in dilute acetic acid causes saturation of the 9 10 double bond, scission of the cyclic ether, and hydrolysis of the enol ether group giving dihydrothebainone dihydromethine [xvm] [3], and this is the most satisfactory method of preparing the latter. The dihydromethine [x] may be reduced to [xvm] under the same conditions [3]. [xvm] can also be prepared by hydrolysis of dihydrothebaine dihydromethine [x] to dihydrocodeinone dihydromethine [xix] (also accessible by the chromic acid oxidation of a-tetrahydrocodeimethine [xx] [3]) followed by aluminium amalgam reduction [3]. 

Cyclic ethers also give good results (Table 12), wliile acyclic ethers undergo partial /5-scission with formation of carbonyl and alkyl derivatives [Eq. (18)]... 

Polyoxymethylenes have a marked tendency to undergo thermal depolymerization with loss of formaldehyde. To prevent thermal depolymerization, polyoxymethylenes are structurally modified, the two possibilities being acetylation to block the reactivity of the end groups of co-polymerization with cyclic ethers, e.g., ethylene oxide. Polyacetals are also sensitive towards autoxidation, which invariably leads to depolymerization as a result of chain scission. The formaldehyde released by depolymerization is very likely to be oxidized to formic acid, which can catalyze further depolymerization. 

Ring-type monomers such as cyclic ethers, lactams, and lactones can be polymerized via ring scission reactions to high-molecular-weight polymers. 

Another possibility is the observation of j9-scission of a cyclic ether... 

Scheme 13). Trichlorosilane reduces y-, and e-lactones to the corresponding cyclic ethers /5-lactones suffer ring scission, ... 

A tertiary radical can be formed by elimination during AF of ter/-butyl methyl and ethyl ethers thus, isolation of the respective perfluoro-/erf-butyl ethers, e.g. 1, occurs in only 36 and 42% yield.28 Significant quantities of perfluoro(2-methylpropane) (2) are also isolated. The longer alkyl chains (ethyl and larger) appear to be slightly less prone to scission than the methyl group. Apparently, carbonyl fluoride is more readily eliminated than trifluoroacetyl fluoride, a phenomenon observed during AF of esters.29 Elimination becomes most serious in the special class of polyethers called ortho esters, e.g. 3-5.30 Cyclic ortho esters, acetals and ketals are much less affected than acyclics. 

Ring scission of cyclic p-halogeno ethers to olefinic alcohols mediated by Sml2 has been studied. Scission in the tetrahydrofuran series could be accelerated by addition of HMPA or DMPU with only a small deterioration in diastereo-selectivity, but in the tetrahydropyran series there was a drastic change in the stereochemistry of the product when DMPU was used. Obviously, DMPU has a substantial effect on the formation and conformation of complexes in solution [88,89]. 

Robinson, G. M. Robinson, R. Scission of certain substituted cyclic catechol ethers. J. Chem. Soc., Trans. 1917, 111, 929-940. 

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