Trisubstituted alkenylmetals

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. 

TABLE 6. Pd-Catalyzed Cross-Coupling of Trisubstituted Alkenylmetals with Aryl Electrophiles ... 

B.iv.c. trans-a,pSubstituted Alkenylmetals. The accessibility of trani-a,/3-substituted alkenyhnetals is still rather limited, and only a few examples of this type of crosscoupling reactions have been reported (Table 6). Notably, trtmi-a,/3-substituted alkenylboronates can be prepared by hydroboration of 1-bromo-l-alkynes followed by subsequent treatments with organolithiums and bases, a procedure similar to that for the preparation of (Z)-/3-substituted alkenylboron compounds.t " Et The Pd-catalyzed crosscoupling reaction of the resultant organoboranes with aryl halides proceeds with complete retention of alkenyl stereochemistry to give trisubstituted alkenesP (Scheme 33). 

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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