Carbometallation alkenylmetals

Products of the carbometallation 442 and 443 contain very reactive metal carbon bonds, and can not be isolated in many cases. They undergo further reactions such as protonation, transmetallation and nucleophlic attack. The preparation of tri- or tetrasubstituted alkenes 444 and 445 from alkynes via the alkenylmetals 443 is particularly useful. Only catalysed carbometallations are treated here. 

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. 

One particular example of carbometallation of alkenes is the addition of 2-aUcenylmagnesium halides and bis(2-alkenyl)zinc to ethylene under pressure (20-100 bar) to give the corresponding 4-alkenylmetal compounds [95] (Scheme 7-84). 

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... 

Since the preparation of /3,/3-substituted alkenylmetals as the first-generation organometals has been achieved mostly by Zr-catalyzed carboalumination " and carbocupration, the exploitation of Protocol I has largely resorted to these two carbometallation reactions, as indicated by the results summarized in Table 13. Moreover, since it is impractical to carry out methylcupration of alkynes requiring several days at -25 the synthesis of natural products represented by 1 and 2 by the use of Protocol I has mostly been limited to those cases where Zr-catalyzed carboalumination is... 

Since most of the facile and general hydro- and carbometallation reactions involve syn-addition, the preparation of trans-a.jS-substituted alkenylmetals via 5yn-addition of alkynes would require carbometallation of terminal alkynes placing the metal in the internal position. Although such reactions exemplified by carbopalladationf are known, they are still more exceptional than normal. From the perspective of the current discussion, more commonly used are (i) some anfi-hydrometallation reactions of proximally heterofunctional internal alkynes and (ii) the hydroboration-migratory insertion tandem process of 1-haloaIkynes. Whereas the H migration produces (Z)-/3-substituted alkenylboranes (Sect. D.iii), the corresponding C migration provides trans-a,/3-substituted alkenyhnetals. (See Table 15.)... 

---