Isomerization epoxides

However, this process requires a reaction time of 2 days and is inapplicable to unsymmetrical couplings (two different epoxides). As the study described in equation 24 revealed a dramatic solvent effect on similar Brook isomerizations in the adduct of lithio dihalo(triaIkylsilyl)methanes with epoxides (isomerization did not occur following metalation and initial alkylation in THF but proceeded readily upon addition of HMPA), the effect of HMPA for promoting the Brook isomerization was studied once the first alkylation was complete (equation 28) . ... 

In the 177-300 °C temperature range studied, epoxide isomerization, oxidation and homopolymerization can occur followed by complex degradation reactions. There have been numerous studies on the homopolymerization of epoxides including the effects of catalysts, alcohols, cure temperature and epoxide-amine ratio on the... 

A number of workers have reported epoxides isomerize to allylic alcohols (5) 30, 35-38, o,4i,5i-56) Epo de isomerization to... 

Hence, the most plausible explanation of our FTIR observations of the simultaneous appearance of hydroxyl, carbonyl and ether groups upon TGDDM epoxide consumption is epoxide isomerization and/or oxidation followed by epoxide-hydroxyl chain extension reactions. 

A particularly significant aspect of hypohalous arid addition is tlu-fact that it can lead to epoxides isomeric in configuration to thcompare Tables 6 and 0 fo illustrations of this principle). 

Important related studies have been published by House, tJ- u and by Naqvi end co-workers,1218 on the subject of magnesium bromide-initiated epoxide isomerization. Their results, discussed in section IV.2.A, suggest that abnormal addition of Qrignard reagents to dicyelio a-haloketones does not proceed by way of an ephemeral poxide intermediate, in accordance with the view expressed previously by Gciesman and Akawie.4 4... 

Where a very stable oarbonium ion is possible, bridged intermediates are less likely to participate in epoxide isomerization. With 2-feri-butyl-l,2-epoxy-3,3-dimethylbutane no lees than Beven prmLuclK are farmed, according to Newman and oo-workera.IW7 These presumably ariee from the initially generated di-ferf-butylcarboruim ion. The chief product, ieolable in 40-60% yield, is 2-f rt-butvl-J>3-dimetbyl-3-buten-l-ol (Eq. 430). 

Soveral 1,1-diarylethylcne oxides have been subjected to Ihr action of magnesium bromide or boron trifluoride (Eq. 453). Again, migratory aptitude in the epoxide-isomerization process appears to l -a function of the overall substitution pattern. Thus, 1,1-diphenyi ethylene oxide and 2-methyl-1,1 -diphenylethylene oxide -J suffer hydride shifts exclusively. On the other hand, 2-ethyl-i.i-diphenylethylene oxide reportedly undergoes hydride shift and cthvl migration... 

Brief mention may be made here, incidentally, of an interesting photochemical epoxide isomerization reported by Bodforafi18 to give -di ketones in email yield from several substituted bcnzalaoetophonone oxides, as shown in Eq. (483). 

Several instructive illustrations taken from the field of natural products will serve to conclude the present discussion of acid-catalyzed epoxide isomerization. Much has been said about the occurrence of simple i,2-hydride transfers in boron trifhioiide-catajyzed epoxy steroid rearrangements. Although initiated by a mineral acid rather than a Lewis acid, an instance of traneatmular hydride transfer has boon reported far a 9/3,110-epoxy steroid,1824 as shown in Eq. (484). Such migrations can in principle occur elsewhere. 

Base-catalyzed rearrangement of ethylene oxides is a topic that baa, until now, received only limited attention in the literature, chiefly because epoxides undergo simple nudeophilio attack rather than isomerisation with most bases. Strictly speaking, a base-catalysed epoxide isomerization is one in which the initial event is direct proton abstraction from the oxide ring. This may bo followed by redistribution of bonding electrons in any of several possible ways, to give ultimately one or more carbonyl compounds. For the general case the course i>f such a reaction may be depicted a in Eq. (480),... 

Washerman and Gorbonoff1808 have published a good illustration of baae-oetalyxed epoxide isomerization in which the activated proton... 

The ability of ethylene oxide to undergo rearrangement to acetaldehyde was mentioned (see section. L2.) in connexion with the thermal decomposition and photolysis of ethylene oxide, and also (see section m.l.C.) in connexion with catalytic ethylene oxidation at elevated temperatures. This characteristic property is discussed, again below with regard, to reactions of epoxides with Qrignard reagents (see section IV.4.F,). For the purposes of this section the subject of epoxide isomerization can be divided into two parts. The first, and most extensive, is concerned with thermal and acid-catalyzed ethylene oxide isomerisation the second involves base-catalyzed rearrangement. 

Culvenor and co-workers 88 contributed two interesting illuetea-lionn of base-catalyzed epoxide isomerization (Eq. 4H3J The first consists of the rearrangement of glyoidonjtrile to 3-cyanoallyl alcohol... 

If the proton donor is an alcohol or a phenol, the active centre is formed directly in reactions (56)—(58) without epoxide isomerization. The propagation steps are the same as in the previous mechanism (Eqs. (59) and (60)). Antoon and Koenig61) proposed a copolymerization scheme for the curing of epoxy resins by anhydrides as a refinement of the mechanisms suggested by Tanaka52) and Lustoft 45 74). However, copolymerization again occurs in the presence of proton donors. The complete sequence proposed by Antoon and Koenig 67) is as follows ... 

At present, we can say that copolymerization initiated by various salts proceeds by an anionic mechanism, after dissociation of the initiators in the reaction medium. The primary step is the addition of the initiator anion to the epoxide. In the initiation by Lewis bases, i.e. by tertiary amines, initiation involves formation of a primary active centre of an anionic character. This active centre is probably generated by interaction of the tertiary amine with the anhydride and an allyl alcohol. The allyl alcohol can be formed by a base-catalyzed isomerization of the epoxide. In the presence of a proton donor, the formation of active centres is possible through interaction of tertiary amine, anhydride and proton donor without epoxide isomerization. Another way of initiation consists in a direct reaction of epoxide with tertiary amine yielding an anionic primary active centre. We believe that in both kinds of initiation in the strict absence of proton donors, the growing chain end has the character of a living polymer. The presence of proton donors, however, gives rise to transfer reactions. 

Subsequent work [55-65], in particular by Asami [56-60] and Andersson [61-65], has led to the development of catalytic methods in which a sub-stoichiometric amount of a chiral diamine such as 61 or 62 is used with an over-stoichiometric quantity of an achiral lithium amide base such as LDA (Scheme 13.29). Examples of catalytic epoxide isomerizations using the Asami diamine 61 or the Andersson... 

Conversion of C3H6 in the ratio interval between 1 1.5 and 1 2 is decreased by 3 wt.%, whereas for epoxide this value equals 10 wt.%. These data show that the increase in yields of other oxidation products is caused by epoxide isomerization and the parallel proceeding of other catalytic reactions. It should be noted that chromatographic tests have detected only unreacted C3H6 in the gas products, whereas CO, C02 and other products of C3H6 degradation were not observed. 

Molecular hydrogen can reduce cyclic peroxo complexes, for example Reaction 30 (111), and Sheldon and Van Doom (33) had envisaged epoxide production via a similar process, Reaction 31. A mechanism based on such reactivity (with subsequent epoxide isomerization) is a possibility for our catalysis using H2/02, although Reaction 31 did not occur under mild conditions (33). Isolation of a related peroxometallocyclic rhodium complex, (Ph3As)2Rh[02C2(CN)4] + (92), allows for a testing of Reaction 31 at a Rh center. 

Epoxides can isomerize under the influence of transition metal catalysts. This formal 1,2-hydride shift is a method to prepare unsaturated carbonyl compounds from epoxides (Equation 54) <1998T1361>. This method has been extended as a double epoxide isomerization-intramolecular aldol condensation (Equation 55) <1996JOC7656, 1998TL3107>. m-Epoxides are isomerized to /ra r-epoxides under ruthenium catalysis <2003TL3143>. 

Cobalt(iii) diketonate complexes generate alkyl peroxo adducts that can oxidize alkenes to oxiranes <1999IC1603>. 0-Phenylenebis(oxamate)-Iigated square-planar cobalt(iii) complexes catalyze high-yield epoxida-tions of unfunctionalized tri- and disubstituted alkenes <1997TL2377>. Low yields are obtained with terminal alkenes. Terminal alkenes can be converted smoothly to aldehydes using an epoxidation-isomerization with ruthenium(ii) porphyrin catalysts <2004AGE4950>. 

Beside the Grignard and other C-C bond-forming reactions, a number of functional group transformations may also serve as an entry into allylic systems. Some of them, namely the reduction of a, -unsaturated carbonyl compounds (products of crotonic condensation), halogenation of alkenes at the allylic position with Af-bromosuccinimide (NBS) and epoxide isomerization, are shown in Scheme 2.56. 

Wasserman and published a good illuBtratioji of baae-oatalyxed epoxide isomerization in whicb the activated proton... 

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

Epoxide isomerization accompanying pH-independent reactions 283 10 Benzylic epoxides that exhibit complicated pH-rate profiles 286... 

Another example of an epoxide isomerization occurring by a similar mechanism is the pH-independent reaction of cts-anethole oxide 121, which yields (n-methoxy-phenyl)acetone 125 along with threo and erythro l-(p-methoxyphenyl)-l,2-pro-panediols 126.107 During the course of the reaction, trans-anethole oxide (124) builds up to a level that is detected by JH NMR (Scheme 35). The mechanism for this reaction involves benzylic C-0 bond breaking, rotation about the Cx-Cp bond via zwitterionic structures 122 and 123, and epoxide ring closure to form trans-anethole... 

Epoxide isomerization through C-O bond cleavage promoted by Rh(I) and Ir(I) complexes and its application in homogeneous catalysis have been also reported [105]. 

Toluene or dichloromethane (Run 14) solutions of Ni Br2(PPh3)2 do not exhibit A or B type EPR signals in the presence of Zn. We propose that if Ni I is reduced to Ni, under such conditions, it cannot be stabilized. This could explain the low activity of these samples for epoxide isomerization and show once more the influence of the solvent on catalyst performance. 

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