Carbanions reactions with epoxides

In Sect. 5.2.2, Example 5.12, we saw the electrophilic character of epoxide toward the alkoxide ion. The carbanion is another charged nucleophile that opens a three-membered ring in reaction with epoxides. When such an anion is unstabilized, its synthetic equivalent is an organometallic compound affording a-alkylated or arylated alcohols as products (Scheme 5.50a). 

As with epoxides, carbanions can add in a 1,4 fashion to enones or nitrosugars. Nitromethane anion has been used [64], Dithiane anion has been successfully used in the addition to nitroolefins [65] and to enones [66], Accordingly, C-5 branched-chain glucose derivatives 47 and 48 have been prepared from nitroolefin 46 (Scheme 20) [67,68], Sugar-derived enones have been also used as acceptors in free radical reactions to trap alkyl radicals as well as anomeric radicals (see Schemes 29 and 30). 

Current results indicate that stabilized arsonium ylides such as phenacylide, carbomethoxymethylide, cyanomethylide, fluorenylide, and cyclopentadienylide afford only olefinic products upon reaction with carbonyl compounds. Nonstabilized ylides such as ethylide afford almost exclusively epoxides or rearranged products thereof. However, semi-stabilized arsonium ylides, such as the benzylides, afford approximately equimolar amounts of olefin and epoxide. Obviously, the nature of the carbanion moiety of the arsonium ylide greatly affects the course of the reaction. It is reasonable to suppose that a two-step mechanism is involved in the reaction of heteronium (P, S, and As) ylides with carbonyl compounds (56). 

Fluorenyl Carbanions. Salts of fiuorene (pAa = 22.6) (6) are more hindered and less reactive than many other organometallic initiators. These carbanions can be readily formed by reaction with alkali metal derivatives as shown in equation 19 for 9-methylfiuorene (99). Carbanion salts of 9-methylfiuorene are preferable to fiuorene, since the latter generate chain ends which retain reactive, acidic fluorenyl hydrogens which can participate in chain-transfer reactions (100,101). Fluorenyl salts are useful initiators for the polymerization of alkyl methacrylates, epoxide, and thiirane monomers. 

Cyclization of 4-thiophenylbutan-2-ols 473, available by the reaction of sulfur-stabilized carbanions with epoxides, affords substituted thiochromans (Scheme 165) <2000JHC1527>. 

Oxidation of internal perfluoroolefins by alkaline solutions of hydrogen peroxide and alkaline and alkaline-earth hypohalides leads to the formation of olefin oxides, the yield of the target product being 40-50%. The reaction with sodium hypochlorite in an alkali in the presence of acetonitrile is an example of epoxidation performed by the nucleophilic attack of the OC1-anion of the multiple bond with further elimination of the chloride anion by the intermediate carbanion (79IZY2509, 79IZV2812, 79RP666176,... 

Perfluoroalkoxyanions are also generated by reaction of fluoride ion with acid fluorides and with epoxides (see Section niB, below). Reaction of the (CF3)2CO—CsF complex with tetrafluoroethene [135] gives alkoxide 8.34A, not a carbanion 8.34B (Figure 8.34). In the presence of iodine, however, ethers are formed [126], indicating the formation of intermediate hypoiodites, RpOI (Figure 8.35). 

The reactions of epoxides with dialkoxyboryl-stabilized carbanions are complicated by interaction of the oxyanion produced with the dialkoxyboryl grouping. When such an interaction is sterically inhibited, as with cyclohexene oxide, a single product results (equation 32). ... 

Scheme 4 Reactions of dimesitylboryl-stabilized carbanions with epoxides...

P-Hydroxy selenides are conveniently prepared from epoxides by treatment with sodium phenylse-lenide (Scheme 32) and by the addition of benzeneselenenic acid and its derivatives to alkenes (Scheme 33), - -" although in some cases these reactions are not regioselective. Useful phenylseleno -etherification and -lactonization reactions have been developed which can be regioselective (equation 42 and Schemes 34 and 35). -" " Selenide- and selenoxide-stabilized carbanions have been used in addition reactions with aldehydes and ketones, - and the reduction of a-seleno ketones also provides a route to P-hydroxy selenides. ... 

The reactions between a-metalloalkyl sulfoxides and electrophiles have been extensively studied. Although alkylations of the sodium or potassium salts of dialkyl sulfoxides are not always very efficient since a,a -dialkylated sulfoxides are often produced (or stilbene in the case of methyl-sulfinyl carbanion and benzyl bromide ), those employing the lithioalkyl aryl sulfoxides work more efficiently with alkyl or allyl halides " and with epoxides. " " Typical examples of these alkylations, allylations and hydroxyalkylations (from epoxides) are illustrated in Scheme 86. 

The chirality present at the sulfur atom of, for example, p-tolylsulfinyl carbanion leads to the formation of a diastereoisomeric mixture of y-hydroxy sulfoxides on reaction with prochiral epoxides (Scheme 87). This reaction, coupled with the efficient desulfurization of the resulting sulfoxide by Raney nickel, has been used ingeniously for the enantioselective synthesis of rra j-2-methylcyclohexanol from epoxycyclohexane and (H-)-p-tolylsulfinylmethyllithium (Scheme 87). 

By treatment of the carbanion of (48) with an aldehyde, the procedure may be modified to yield hydroxy carbonyl compounds (51) (Scheme 25). Such dithian carbanions also react with epoxides e.g. (52), to give hydroxy ketones, e.g. (53). In this reaction, the less hindered end of the epoxide is preferentially attacked (Scheme 26). 

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