Hydrometallation conjugated dienes

Although zirconium is only one out of over 50 potentially usable metals in this class (including the lanthanides and actinides), virtually all synthetic applications of hydrometallation with transition metals involve zirconium Why is this so The primary reason derives from the near requirement of a d -metal center for hydrometallation of a general alkene or alkyne. For later transition metals, hydrometallation to give a stable organometallic product can usually be achieved only for special cases—conjugated dienes, alkenes with electronegative substituents, etc. This is due to the relative stability of the ti -complex, as discussed previously. 

Generation of stereo- and regio-defined alkenylmetals via hydrometallation or carbometallation of alkynes followed by cross-coupling (Scheme 1-11) is a synthetically attractive notion for preparing arylated alkenes, conjugated dienes, and conjugated enynes. Its feasibility was demonstrated in 1976 [14,15] in the prototypical examples shown in Schemes 1-4 and 1-5. In these processes, hydroalumination was employed for generating the required alkenylmetals. 

Alkenylcoppers have not been frequently used. However, generation of (Z)-/3-substi-tuted alkenylcoppers via carbocupration followed by their Pd-catalyzed cross-coupUng appears to be the method of choice in the synthesis of 1,4-disubstituted (Z,Z)-conjugated dienes. It also nicely complements the synthesis of 1,4-disubstituted ( , Z)-conjugated dienes by the hydrometallation-cross-coupling tandem process involving B, Al, and Zr. 

Conjugated dienes, such as those represented by 1 and 2 in the previous subsection, can, in principle, be synthesized also by hydrometallation-cross-coupling tandem processes shown in Scheme 66. This is indeed one of the most significant cases of the use of cis-ct, /3-subslituted alkenylmetals in Pd-catalyzed alkenyl-alkenyl coupling. 

Before leaving this section we briefly examine hydrometallation mechanisms completely different from the classic one depicted in Scheme 2. One class involves metal hydrides which do not have a readily available vacant site and requires conjugated alkenes — 1,3-dienes, styrenes these proceed via a... 

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