Cross coupling substituted alkenylmetals

TABLE 3. Pd-Catalyzed Cross-Coupling of (E)-/3-Substituted Alkenylmetals with Aryl Electrophiles ... 

Some representative examples of the cross-coupling of three different types of trisubstituted alkenylmetals with aryl electrophiles are shown in Table 6 and arranged in the order /3,/3 -, cis-a, -, and lra i-Q ,j8-substituted alkenylmetals. 

B.iv.a. p, P -Substituted Alkenybnetals. Carbometallation reactions of 1-alkynes, most notably carboalumination and carbocupration,t collectively provide a powerful route to the synthesis of /3,j8 -substituted alkenylmetals. The alkenylmetals generated in situ may be used directly for Pd- or Ni-catalyzed cross-coupling with aryl electrophiles (Protocol 1). One prototypical example of the carbometallation-cross-coupling tandem process is shown in Scheme... 

At present, (Z)-/3-substituted alkenylmetals containing other metals are less readily accessible than those mentioned above, even though the Zr-catalyzed carboalumination of ethyne has been shown to produce (Z)-/S-substituted alkenylalanesJ They are generally prepared via metallation-transmetallation of (Z)-/S-substimted hahdes. Despite this drawback, (Z)-/3-substituted alkenylzincs generated by this procedure have been shown to be superior reagents in the subsequent Pd-catalyzed cross-coupling reactiont (Table 11). 

The a-substituted alkenylmetals used in Pd-catalyzed cross-coupling have been mainly those containing Mg, Zn, B, and Sn, as shown in Table 12 as well as Schemes 54-57. Of these, a-substituted alkenylmetals containing Mg and Zn can readily be prepared by direct oxidative metaUation of 2-halo-l-alkenes that are easily accessible by Markovnikov addition of HX to 1-alkynes (Scheme 54). a-Substituted alkenyltin compounds have been prepared and used in the construction of bicyclic diene systems via intramolecular Stille coupling, as shown in Scheme 55. ... 

The synthesis of alkene-substituted arenes by cross-coupling can, in principle, be achieved either by the reaction of alkenylmetals with aryl halides and related electrophiles or by that of aryhnetals with alkenyl electrophiles. Eor the sake of simplicity, the former reaction is termed the alkenyl-aryl coupling, while the latter is termed the aryl-alkenyl coupling in this Handbook. Similar terms may be devised by linking the carbon groups of the organometal and organic electrophile with a dash in this order. 

Haloboration of 1-aIkynes followed by chemoselective cross-coupling can provide j8,/3-substituted alkenylborons that undergo Pd-catalyzed coupling with alkenyl halides to give stereodeflned trisubstituted alkenes. The use of other /3,/3-substituted alkenyl-metals has also been demonstrated. For example, exocyclic alkenylmetals containing Zn and Sn have been nsed in Pd-catalyzed cross-coupling approach to the vitamin D skeleton as shown in Scheme 65. It shonld be noted that the alkenylzinc appeared to be distinctly snperior to the corresponding alkenyltins. 

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