Catalytic reactions epoxide rearrangement

The well-known base-mediated rearrangement of epoxides into allylic alcohols was first reported as an enantioselective process using a chiral base in 1980. Since then, the reaction has received much attention, mostly due to the significance of chiral allylic alcohols in organic synthesis. Major breakthroughs in the area include the use of a substoichiometric amount of chiral base and the development of chiral bases for a true catalytic reaction protocol. Andersson and co-workers have reviewed this area from 1980 to 2001, with emphasis on the period 1997-2001 <2002CSR223>. 

Aggarwal VK, Winn CL. Catalytic, asymmetric sulfur ylide-mediated epoxidation of carbonyl compounds scope, selectivity, and applications in synthesis. Acc. Chem. Res. 2004 37 611-620. Li A-H, Dai L-X, Aggarwal VK. Asymmetric ylide reactions epoxidation, cyclopropanation, aziridination, olefination, and rearrangement. Chem. Rev. 1997 97 2341-2372. 

The enantioselective Wittig and epoxide rearrangements have been widely explored in the last few years. They are of interest since such processes can, in a single step, induce asymmetry and molecular complexity that would be difficult to generate otherwise. Advances in substrate scope, reaction efficiency and catalytic process constitute significant challenges for the future. 

Abstract The use of organoaluminum-based Lewis acids (A1R X3 R = alkyl, alkynyl, X = halide or pseudohalide) in the period 2000 to mid-2011 is overviewed with a focus on (1) stoichiometric reactions in which one of the organoaluminum substituents is transferred to the substrate (e.g., the opening of epoxides, 1,2-additions to carbonyl compounds, coupling with C-X, and Reissert chemistry) and (2) asymmetric, often catalytic, reactions promoted by Lewis acid catalysts derived from organoaluminum species (e.g., use of auxiliaries with alanes, Diels-Alder, and related cycloaddition reactions, additions to aldehydes and ketones, and skeletal rearrangement reactions). 

This epoxide to aldehyde rearrangement was postulated to be the first step in the silver-mediated reaction of alkylzirconocene chlorides with epoxides, in which the aldehyde is subsequently alkylated by the alkylzirconocene species (cf. Scheme 8.44) [56], In a control experiment, it was shown that zirconocene dichloride (1 equivalent or less) and silver (catalytic amounts) do indeed induce the rearrangement of an epoxide to an aldehyde very quickly. 

Electrochemical oxidation of epoxides in absence of nucleophiles, catalyses a rearrangement to the carbonyl compound. The electrolyte for this process is dichlo-romethane with tetrabutylammonium perchlorate. Reaction, illustrated in Scheme 8.7, involves the initial formation of a radical-cation, then rearrangement to the ketone radical-cation, which oxidises a molecule of the substrate epoxide. The process is catalytic and requires only a small charge of electricity [73]. 

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. 

Aliphatic or aromatic alcohols can be alkylated by epoxides under either basic or acidic reaction conditions. Reaction of aliphatic alcoholates with epoxides can be complicated by base-induced rearrangement or oligomerization of the epoxide, because alcoholates are strongly basic and because the product of epoxide ring opening is again an alcoholate. These side reactions can be suppressed by using only catalytic amounts of base (Scheme 4.78). The examples sketched in Scheme 4.78 show that under basic reaction conditions nudeophilic attack occurs preferentially at the sterically most accessible carbon atom. 

The propensity for epoxides to undergo various rearrangements under Lewis acidic conditions is also well-known. Typically, Lewis acid "catalysts", such as BF3-OEt2, must be used in quantities approaching stoichiometric amounts due to their instability under the reaction conditions. However, Yamanoto has recently demonstrated that tris(pentafluoro phenyl)boron, an air-stable and powerful Lewis acid, was not only active in catalytic quantities, but also effective in mediating the alkyl-shift rearrangement of epoxide 70 with... 

The conjugate addition of lithium peroxides on enones is a non-classical route to epoxidation. The original lithium enolate rearranges immediately in situU3. Note that a catalytic asymmetric version of this reaction was also developed (Scheme 41)184. 

The thermal rearrangement of unsaturated bicyclic 1,4-peroxides, readily available from the reaction of conjugated dienes with singlet oxygen, is a convenient route fw the preparation of bisepoxides. Epoxidation of cholesteryl acetate (176) with air in the presence of a catalytic amount of dioxo(tetra-mesitylporphyrinato)ruthenium(VI) fimiishes in 8S% yield a 99 1 mixture the epoxides (177a) aitd (177b)." >... 

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