Iodoalkenes Heck coupling

Fig. 16.36. Stereoselectivity and stereospecificity of Heck coupling reactions with isomeric iodoalkenes.

A haloalkene that contains a stereogenic C=C double bond can usually be coupled with alkenes via the Heck reaction without isomerization. This is illustrated with the three reaction pairs in Figure 16.36. As can be seen, both the as- and the /raw-configured iodoalkenes react with acrolein or methyl vinyl ketone or acrylic acid methyl ester with complete retention of the C=C double bond configuration. These coupling reactions are thus stereoselective and— when considered as a pair—stereospecific. 

A major achievement was the discovery that Heck reactions are greatly accelerated in the presence of phase-transfer catalysts using quaternary ammonium salts and solid bases [ Jeffery conditions Pd(OAc)2, MHCO3 (M = K, Na), nBu4NX (X = Br, Cl), DMSO or DMF] [100]. Under these conditions, iodoarenes and iodoalkenes can be coupled to alkenes... 

Another intriguing cascade of an intramolecular Heck-type reaction followed by an intermolecular Stille coupling produces the dienyne 98 starting from the enyne 96 and the stannane 97 [322] (Scheme 8.25). The reversed sequence of a Stille-type reaction followed by a Heck reaction has also been applied. It is also possible to terminate such a sequence after transmetallation from a zincate [180a,c], either added to the reaction mixture [180a] or generated in situ from an iodoalkene [180c]. 

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. 

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