Towards Tetrasubstituted Olefins

As might be expected from the results discussed in the previous paragraphs, coupling reactions between secondary a-sulfonyl carbanions and ketones are rarely encountered in the literature [12, 51, 57]. Even the use of sulfoxides proved to be of no avail [52, 53]. It is thus even more remarkable that the method reported by Falck and Mioskowsky affords tetrasubstituted yS-hydro-sulfones in excellent yields [56]. It is quite possible that the unique properties of the Sm salt, which acts as a 

In this context, it is worthy of note that a modification of the Julia sequence, introduced in 1982, allows the preparation of tetrasubstituted olefins from vinyl sulfones [58]. This modification, which concerns mainly the reductive elimination step, will be discussed in detail in the relevant section (Section 3.4.5). 

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Rh complexes with ChiraPhos, PyrPhos, or ferrocenyl phosphines lacking amino alkyl side chains (such as BPPFA) are much less active toward tetrasubstituted olefins. Table 6-1 shows that in asymmetric hydrogenations catalyzed by 5a-d, the coordinated Rh complex exerts high selectivity on various substrates. It is postulated that the terminal amino group in the ligand forms an ammonium carboxylate with the olefinic substrates and attracts the substrate to the coordination site of the catalyst to facilitate the hydrogenation. 

Both of these complexes can be used in ADMET polymerizations at temperatures up to approximately 55 °C, although decomposition certainly occurs over the time scale of a typical ADMET polymerization (days). A structure-reactivity study was performed on complexes 1 and 2 that revealed a number of features of these complexes [68]. Notably, 2 will polymerize dienes containing a terminal and a 1,1-disubstituted olefin, but never produces a tetrasubstituted olefin. One of the substituents of the 1,1-disubstituted olefin must be a methyl group. In contrast, complex 1 will not react with a 1,1-disubstituted olefin. The tungsten complex is more reactive towards internal olefins than external olefins [23, 63] indicating that secondary metathesis, or trans-metathesis, probably dominates the catalytic turnovers in ADMET with complex 1. 

Stereoselective synthesis of tetrasubstituted alkenes remains a challenging task. A unique solution to this problem is to use torquoselectivity-controlled oletina-tion of carbonyl compounds with ynolates, which is summarized in Chapter 1. In general, the stereoselective synthesis of Z-alkenes, which are thermodynamically less favorable, is more difficult than the synthesis of corresponding -isomers. In Chapter 2, various methods for stereoselective synthesis of Z-alkenes are reviewed. Finally, the C=C double bond formation through catalytic carbene transformation has recently emerged as a new approach toward olefin synthesis. Two chapters covering olefin synthesis based on catalytic carbene transformations are included (Chapter 5 and Chapter 8). 

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