Olefination, torquoselective

The mechanism of olefination can be deduced by consideration of orbital interactions to proceed via torquoselective olefination, rather than chelation control, for the following reasons (1) the sterically hindered siloxy and phenoxy groups are also effective for high -induction (2) in the presence of a crown ether, the selectivity still remains high and (3) an axially oriented siloxy group induces a high Z-selectivity. Theoretical calculations indicate that the transition state of inward rotation is stabilized by an orbital interaction between a(C-O) and ct (C-OR) (157) . 

Fig. 17 Torquoselectivity in the transition states of the ring-opening for olefination of acetophenones...

In contrast, the C-C triple bond shows excellent effect on torquoselectivity. Alkynyl aUcyl ketones 81 are olefinated by ynolates to afford the tetrasubstituted olefins 83 in good -selectivity (Fig. 28) [72]. In the electrocyclic reaction of 82, the alkynyl group prefers outward rather than inward rotation over the alkyl and alkenyl... 

The importance of the torquoselective olefination is illustrated in Fig. 34 for the particular case in which a multisubstituted alkenylsilane is converted to various kinds of multisubstituted olefins. The silyl-substituted allyl alcohol 93 is allylated to give the 1,4-diene 94, and the iodoalkene 95, prepared by desilyliodination of 93, is subjected to palladium-catalyzed cross-coupling reactions (the Heck reaction, Stille coupling) to afford the dienes 96 and 97 without ElZ isomerization. 

The ( )-p-alkoxy divinyl ketones 129, potential Nazarov reaction precursors, are prepared according to the torquoselective olefination methodology with ynolates (Fig. 45). For example, ethyl 3-phenylpropionate (127) is olefinated by the ynolate to afford the p-alkoxy-ot,p-unsaturated acid 128 with high -selectivity. The acid 128 is converted into the Weinreb amide, which is subjected to alkenylation to provide the p-alkoxy divinyl ketone 129 in good overall yield. 

A silyl-substituted ynolate 141 undergoes cycloaddition to A-sulfonyl aldimines 142, followed by ring opening, to afford the a,p-unsaturated amide 143 at 20 °C (Fig. 52) [39]. This stereoselectivity is unexpected for the torquoselective olefination. The steric interaction between bulky MeaSi and the phenyl groups may be critical. A-o-Methoxyphenylaldimines 144 with ynolates at room temperature produce a,(3-unsaturated amides 145 in good yield with high -selectivity (Fig. 53) [122], Since the double adduct 146 is produced as an intermediate, the process involves the retro-Mannich reaction. 

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). 

Stereoselective Synthesis of Tetrasubstituted Alkenes via Torquoselectivity-Controlled Olefination of Carbonyl Compounds... 

Review Shindo M, Mori S. Torquoselective olefination of carbonyl compounds with ynolates highly efficient stereoselective synthesis of tetrasubstituted alkenes. Synlett 2008 2231-2243. 

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