Olefins from -eliminations

Olefins by elimination from quaternary ammonium salts (less substituted olefin preferred). 

A c/5-elimination may also explain the predominant formation of A -olefins from 4,4-dimethyl-5a-3-alcohols irrespective of hydroxyl configuration/ ... 

The reductive elimination of halohydrins provides a means of introduction of double bonds in specific locations, particularly as the halohydrin may be obtained from the corresponding a-halo ketone. This route is one way of converting a ketone into an olefin. (The elimination of alcohols obtainable by reduction has been covered above, and other routes will be discussed in sections IX and X.) An advantage of this method is that it is unnecessary to separate the epimeric alcohols obtained on reduction of the a-bromo ketone, since both cis- and tran -bromohydrins give olefins (ref. 185, p. 251, 271 cf. ref. 272). Many examples of this approach have been recorded. (For recent examples, see ref. 176, 227, 228, 242, 273.) The preparation of an-drost-16-ene (123) is illustrative, although there are better routes to this compound. 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Yields of chlorides are good to excellent for primary and secondary alcohols, but a competing olefin-forming elimination process renders the method of limited value for preparing tertiary chlorides.12 An adaptation of the procedure using carbon tetrabromide allows the synthesis of alkyl bromides. Some examples are the preparation of rt-C5H11Br (97%) and C H6CH2Br (96%).12 Farncsyl bromide has been prepared in 90% yield from fame sol.23... 

A method for the preparation of olefins from primary amines is shown in equation 120. Treatment of 2-(4-bromophenyl)ethylamine (358) with acetic acid, acetic anhydride and sodium nitrite generates the nitroso amide 359, which decomposes to 4-bromostyrene in the presence of rhodium(II) acetate. The procedure is thus a mild, non-basic alternative to the classical Hofmann elimination of amines396,397. 

Another, and quite telling, example concerns the structure-specific elimination of olefins from acyclic trienes (Scheme 6). Bestmann and coworkers70 found that the El spectra of (8 ,10Z)-l,8,10-dodecatriene (23, R = H), (11 ,13Z)-1,11,13-pentadecatriene and some of their homologues display characteristic peaks at m/z 68, 82, 96 etc., corresponding to the formation of ionized alkadienes C5 Hs 12n 1L along with a neutral diene. The peaks... 

Co(CO)4] ) dehydrohalogenation [Eq. (15)] followed by addition of HCo(CO)4, or whether splitting out of HCo(CO)4 occurs from the alkyl-cobalt [Eq. (14)], which is the malonate precursor, followed by HCo(CO)4 addition in the opposite direction. In one case [Eq. (15)], olefin formation proceeds directly from the bromide and no reversibility of any steps is required, while according to Eq. (14) olefin formation proceeds from elimination of HCo(CO)4. 

The interaction of an ester and an olefin was also the basis of another early polymerization mechanism (Berthelot, 55). It was postulated that the acid portion of the ester and a hydrogen atom from the olefin are eliminated in the following manner ... 

Olefins from Aliphatic Diazonium Salts Hydro-diazonio-elimination... 

A. C. Cope and E. R. Trumbull, Olefins from amines the Hofmann elimination reaction and amine... 

Silyloxide eliminations (Petersen olefination) also proceed readily and regiospecifically to give olefins. When base is used to produce die oxyanion, the elimination occurs widi syn stereochemistry. If an acid is used to promote the elimination, it occurs in an anti fashion, leading to die opposite olefin stereochemistry. This is a very useful way to generate either a Z or E olefin from die same starting material. 

J. K. Crandall, M. Apparu, Base-Promoted Isomerizations of Epoxides, Org. React. 1983, 29, 345—443. A. C. Cope, E. R. Trumbull, Olefins from Amines The Hofmann Elimination Reaction and Amine Oxide Pyrolysis, Org. React. 1960, 11, 317-493. 

The cyclo addition of the alkene to the ruthenium vinylidene species leads to a ruthenacyclobutane which rearranges into an allylic ruthenium species resulting from / -elimination or deprotonation assisted by pyridine and produces the diene after reductive elimination (Scheme 16). This mechanism is supported by the stoichiometric C-C bond formation between a terminal alkyne and an olefin, leading to rf-butatrienyl and q2-butadienyl complexes via a ruthenacyclobutane resulting from [2+2] cycloaddition [62]. 

However, reactions like this are of limited use—their success relies on the base s lack of choice of protons to attack provide an alternative H and we are back with the situation in the reaction on p. 810. Logic dictates, therefore, that only trisubstituted double bonds can be made stereospecifically in this way, because the reaction must not have a choice of hydrogen atoms to participate in the elimination. The answer is, of course, to move away from eliminations involving H, as we did with the Julia olefination. We shall look at this type of reaction for much of the rest of this chapter. 

The evidence for the mechanisms of the mass-spectrometric and photochemical reactions leading to the eliminations of an olefin from a ketone [equation (120)] have been summarized (Section VIIDl). If it is accepted that the structure of the fragment ion from this process has an enolic structure, it is possible to discuss the mechanism of the reaction theoretically. The reaction appears to consist of two parts, first the transfer of hydrogen and second, the elimination of olefin. There has been considerable conjecture as to whether these parts of the mass-spectrometric McLafferty rearrangement are stepwise or concerted. Prom their self-consistent field calculations, Boer et al. (1968) have concluded the reaction is step-wise. From perturbation and valence-electron molecular orbital calculations, Dougherty (1968b) has concluded the reaction is concerted. The above results depend on the adjustable parameters fed into the equations one set of parameters may eventually prove to be better. 

Many steroid secondary alcohols are resistant to acid-catalysed dehydration, but can be converted into olefins by elimination reactions of their sulphonate esters, usually the toluene 3 -sulphonate ( tosylate ) or methanesulphonate ( mesylate ). These elimination reactions always accompany attempted nucleophilic substitution of the sulphonate (p. 41), and can be made to predominate by appropriate choice of reaction conditions. Activated alkaline alumina has been used to prepare A - and AH-olehns from the sulphonates of 3 - [131] and i2a-alcohols [132]. In the latter case the naphthalene-i-sulphonate was most effective, and the methanesulphonate distinctly less so, suggesting that steric acceleration occurs with the more bulky naphthalenesulphonate [ 132c]. This view is strengthened by the enhanced efficiency of elimination of a i2a-sulphonate in the presence of a 17a-methyl group, which increases the compression at C(i2) [132b]. The mechanism and stereochemical features of these heterogeneous reactions have not been described in detail. 

Julia-Lythgoe olefmation is probably the most important method for synthesizing acceptor-free, -configured alkenes, starting from an aldehyde and a primary alkylphenyl sulfone. In this two-step procedure, first the sulfone reacts with the aldehyde to form an acetyl-protected alcoholate and second this species undergoes Elcb elimination to afford the desired alkene. (Sylvestre) Julia olefination is a one-step procedure. It also affords -configured olefins from an aldehyde and an alkylsulfone as substrates, but is limited to base-resistant aldehydes. The most advanced variant is (Sylvestre) Julia-Kocienski olefination, which is also a one-step procedure and is applicable to all kinds of aldehydes. The mechanism is shown below. 

Casanova and Rogers [59] as well as Fry [69] postulate that the reduction of vicinal dihalides is a concerted process in which both carbon-halogen bonds are partially cleaved as a carbon-carbon double bond starts to form. Nonelectrochemical evidence [70] suggests that a vicinal dihalide undergoes one-electron reduction to a radical anion, which loses the first halide ion to form a neutral radical, after which the neutral radical accepts an electron to become a carbanion that eliminates the second halide ion to yield an olefin. From a study of the behavior of meso- and c/,/-l,2-dibromo-l,2-diphenylethane, Fawell and coworkers [71,72] concluded that the reduction of vicinal dihalides is a stepwise process. Andrieux and coworkers [73] have examined the reductive elimination of vicinal dibromides at carbon in MeCN. 

The method involving a-heterosubstituted cyclopropyllithiums and a-selenenylalde-hydes provides31,3S,77) quite exclusively the (E) stereoisomers of 1-heterosubstituted-1-vinylcyclopropanes after treatment of the l-(l-heterosubstituted)-l -hydroxy-2 -selenoalkyl)cyclopropanes with PI3/NEt3 (Schemes 60, 62b, 63 b). This implies that the two steps involved in the transformations are stereoselective (Schemes 60, 62b, 63b). The stereochemical result of the first step can be rationalized123 on the basis of the Cram-Karabatsos-Felkin rules124,125 and it is well-known that the synthesis of olefins from (P-hydroxyalkyl)selenides occurs by a formal anti elimination of the hydroxyl and selenyl moieties 4 9 11,12 123 163 . 

Chugaev reaction. Formation of olefins from alcohols without rearrangement through pyrolysis of the corresponding xanthates via cis elimination. 

Corey-Winter olefin synthesis. Synthesis of olefins from 1,2-diols and thiocarbonyldiimidazole. Treatment of the intermediate cychc thionocarbo-nate with trimethylphosphite yields the olefin by els elimination. 

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