Alkenes hydrodimerization

Zimtaldehyd und 3-Oxo-1-phenyl-1-alkene liefern ebenfalls Hydrodimere z. B. ... 

The formation of dimers by reduction of a,p-unsaturated ketones in aqueous media is well documented in the early literature of electrochemistry. Reductants include sodium or aluminium amalgams [58], dissolving zinc and a lead cathode in both acid and alkaline conditions [59,60]. Mixtures of dimers and dihydro derivatives were isolated. As the concept of the hydrodimerization of activated alkenes... 

Hydrodimerization of olefinsIn addition to dehydrodimerization of alkanes 15. 198), hydrodimerization of alkenes can be effected by mercury-photosensitiza- jon, and has the advantage that it is applicable to a wide range of unsaturated wbstrates alcohols and derivatives, ketones, and others. Since the hydrogen adds to ae alkene to give the most stable intermediate (tert > sec > primary), this dimeriza-son can be regioselective. The last example shows that cross-dimerization is possible In this case the hydrodimer of both components is also formed, but in lower ld. 

Various diphenylethylenes are reduced in high yield by magnesium in methanol. Expectedly, reductions of conjugated double bonds via one-electron transfer processes can also be carried out electro-chemically. Scheme 87 illustrates both the reduction of a conjugated alkene and partial trapping of the intermediate anion radical to yield a mixed hydrodimerization product. 

Hydrodimerization of activated alkenes is a well-established process. 7r-Electron-deficient heteroaromatic compounds activate a double bond similarly to a cyano or car-bethoxy group, and in accordance with that analogy vinylpyridines can be hydrodimer-ized. 4-Vinylpyridine [380] forms l,4-bis-(4-pyridyl)butane in 82% yield on electrolysis in a mildly alkaline solution containing methyltriethylammonium / -toluenesulfonate and some DMF. The mechanism is discussed in Chapter 21. 

This section concerns the classical hydrodimerization of alkenes activated by electron-withdrawing substituents, as in Eq. (1). The literature in this area is extensive and this chapter cannot be exhaustive. The focus will be on typical reactions and general conclusions, which may serve as guidelines for further work. Special emphasis will be put on the effect of reaction conditions on the mechanisms, product selectivity, and stereochemistry. Section II.A deals with the monoactivated alkenes, that is, structures of the type 1 where R and R" are H, alkyl, or aryl Sec. II.B deals with intramolecular coupling reactions where two identically activated alkenes are linked together within the same molecule. The reactions of alkenes activated by two electron-withdrawing groups either in a, a- or in a,yS-positions, are treated in Sec. II.C. 

Hydrodimerization of unsymmetrically Q ,y6-diactivated alkenes may in principle give rise to three types of coupling products the a.a -LHD formed by coupling between the two a-carbons, the )6,)6 -LHD formed by coupling between the two y6-carbons, and the a,0-LYiT> formed by coupling between the a-carbon in one molecule and the y6-carbon in the other. Examples are given in Table 12. 

If the cathode potential used is more negative, more of the symmetrical dimers are obtained in competition. Typical conditions for hydrodimerization involve the use of aqueous solutions containing tetraalkylammonium tosylates (with a cosolvent, if necessary) and alkene. Mercury and lead cathodes give highest yields. 

Conjugated, electron-deficient alkenes can be reductively coupled in the presence of Sml2, providing hydrodimerized products virtually instantaneously in excellent yields [93], Both inter- and intramolecular versions of the reaction have been established, the latter leading to the construction of 3- and 6-membered rings (Eq. 83). Alkynes also take part in the reaction, albeit in modest yields. 

In an interesting adaptation of this chemistry, cyclopropanes serve as pseudo alkenes, resulting in homologous hydrodimerizations [95]. Yields in these cases can be quite high, but the process is rarely stereocontrolled (Eq. 85). 

Conjugated Alkenes. - By far the largest and best known industrial electro-organic reduction is the hydrodimerization of acrylonitrile to adiponitrile, an important precursor in nylon manufacture. Plants where this process... 

Exciplex formation initially prevented application of the method to compounds such as amines or alkenes which fail to undergo simple C-H homolysis with Hg. The method was therefore extended [22a] by running the reaction under H2 in which case H atoms are formed and either abstract H atoms directly from the substrate or add to C=C double bonds. The first makes it possible to dehydrodimerize amines, the second allows hydrodimerization of alkenes. In the alkene case, we do not get C-C bond formation at the weakest C-H bond as usual, but at the most substituted of the two vinyl carbons of the double bond. For example, n-alcohols tend to give 1,2-diols because the weakest CH bond is a to O (eq. 37), but unsaturated alcohols give other isomers, for example the 1,4-diol tend to be formed from allyl alcohols (eq. 38). 

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