Olefins rearranged products

Hexafluoropropylene oxide (HFPO), which decomposes reversibly to di-fluorocarbene and trifluoroacetyl fluonde with a half-life of about 6 h at 165 °C [30], is a versatile reagent. Its pyrolysis with olefins is normally carried out at 180-2(X) °C, and yields are usually good with either electron-nch or electron-poor olefins [31, 32, 33, 34, 35, 36, 37] (Table 2). The high reaction temperatures allow the eyclopropanation of very electron poor double bonds [58] (equation 10) but can result in rearranged products [39, 40, 41] (equations 11-13)... 

Isomerization of fluoroolefins by a shift of a double bond is catalyzed by halide 10ns [7] The presence of crown ether makes this reaction more efficient [74] Prolonged reaction time favors the rearranged product with an internal double bond (equations 3-5) Isomerization of perfluoro-l-pentene with cesium fluoride yields perfluoro-2-pentenes in a Z ratio of 1 6 [75] Antimony pentafluoride also causes isomenzation of olefins leading to more substituted products [76]... 

From 5 the formation of alkene 2 is possible through loss of a proton. However, carbenium ions can easily undergo a Wagner-Meerwein rearrangement, and the corresponding rearrangement products may be thus obtained. In case of the Bamford-Stevens reaction under protic conditions, the yield of non-rearranged olefins may be low, which is why this reaction is applied only if other methods (e.g. dehydration of alcohols under acidic conditions) are not practicable. 

The Chugaev elimination is of synthetic value, because it proceeds without rearrangement of the carbon skeleton. Other non-thermolytic elimination procedures often lead to rearranged products, when applied to the same substrates. However applicability of the Chugaev reaction is limited if the elimination is possible in more than one direction, and if a /3-carbon has more than one hydrogen. Complex mixtures of isomeric olefins may then be obtained. For example the thermolysis of xanthate 12, derived from 3-hexanol yields 28% S-hex-3-ene 13, 13% Z-hex-3-ene 14, 29% -hex-2-ene 15 and 13% Z-hex-2-ene 16 ... 

However, coUisional deactivation in solution is so effective that no vibration-ally excited species is present. The reaction of photochemicaUy generated methylene with 2-methylpropene-l-)- C yields, 2-methyl-butene, which is formed by allylic insertion. In the liquid phase 2 % of the rearranged product labeled in the 3-position are formed, whereas in the gas phase 8% of this olefin can be isolated. This can be interpreted as follows 4% of 2-methyl-butene in solution and 16% of 2-methyl-butene in the gas phase are formed by an abstraction-recombination mechanism involving triplet methylene 96). 

Phenol esters of a,(3-unsaturated carboxylic acids have an interesting reactivity due to the synthetic utility of the resulting hydroxychalcones (Scheme 19). This aspect will be illustrated in Section IV. However, from the basic point of view, it is worth mentioning that the cis or trans configuration of the olefinic part of the acyl moiety can have a marked influence on the photochemical reactivity of the ester. When para-methoxyphenyl fumarates are irradiated, the normal ortho-rearranged products are obtained. By contrast, irradiation of para-me-thoxyphenyl maleates does not lead to rearrangement. Instead, cyclization products are obtained (Scheme 20). 

Other Reactions of Olefinic Steroids.—Reaction of cholest-5-en-3-one with air and acetic acid shows that isomerization to the A -3-oxo-compound is accompanied by autoxidation to the 6a- and 6/8-hydroxy-3-oxo-A -compounds and the 3,6-dioxo-A -compound. The oxidation appears to be controlled by heterolysis of the 4/3-proton and formation of the intermediate ion pair (73). Sitosterol was autoxi-dized at C-7 to give the 7-oxo- and the epimeric 7-hydroxy-derivatives. Oxidation of a 17-methylene steroid with Pb, Tl" , and Hg acetates in methanol gave a wide variety of products. The reaction with Pb(OAc)4 gave the rearranged products (74), (75), and (76) whereas the Tl and Hg products retained the... 

Olefins can be hydrated quickly under mild conditions in high yields without rearrangement products by the use of oxymercuration144 (addition of oxygen and mercury) followed by in situ treatment with sodium borohydride145 (2-24). For example, 2-methyl-1-butene treated with mercuric acetate,146 followed by NaBH4, gave 2-methy 1-2-butanol ... 

TABLE 6 Product Distribution Upon Photolysis of at-Alkyl Dibenzylketones in Zeolites Yields of Olefin, Coupling Product (AB) and Rearrangement Product 7 (see Scheme l)a b... 

These methods were extended to 0-hydroxy orthothioesters and p-hydroxy thioacetals [438], leading to ketenethioacetals and vinyl sulfides. With secondary alcohols (R2 - H) a side reaction, which can become preponderant when SOCl2 is used instead of P2I4 in the elimination step, was observed a rearranged product (R2 = SMe in the olefin) was formed. 

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). 

The addition of HX to double bonds in the dark and in the absence of free-radical initiators is closely related to hydration The orientation of the elements of HX in the adduct always rnrrrsponds to Markownikoff addition 16 no deuterium exchange wish solvent is found in unreacted olefins recovered after partial reaction, nor is recovered starting material isomerized after partial reaction.17 However. the addition of HX apparently can proceed by a number of different mechanisms depending on the nature Ol the substrate and on the reaction conditions. Thus when HC1 is added to f-butylethylene in acetic acid, the rate is first-order in each reactant and the products are those shown in Equation 7.5.le Since 4 and 6 were demonstrated to be stable to the reaction conditions, the rearranged product (5) can be formed only if a carbocationic intermediate is formed during reaction. However, the carbocation exists almost solely in an intimate ion pair, and the rate of collapse of the ion pair to products must be faster than, or comparable to, the rate of diffusion of Cl- away from the carbocation. This must be so because the ratio of chloride to acetate products is unaffected by... 

Oxidation of organic substrates. This hydroperoxide converts 2,3-dimethyl-2-butcne into tetramethylethylene oxide with simultaneous formation of 3-bromo-4,5-dihydro-5-hydroxy-4,4-dimethyl-3,5-diphenyl-3H-pyrazole (2). Dialkyl olefins, however, are not epoxidized by I. Enol ethers are converted to a variety of epoxide rearrangement products.2... 

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