Alkenylzinc derivatives

Although alkylzinc derivatives add only slowly to aldehydes, alkenylzinc derivatives display a higher reactivity (Equation (74)).142 Reactive benzylic or related zinc reagents smoothly add to aldehydes, providing the allylic alcohol in almost quantitative yield (Equation (75)).143... 

Carbozincation of alkynes. A dialkylzinc in combination with EZrCp, undergoes regioselective cw-addition to terminal alkynes to provide alkenylzinc derivatives (equation I). The reaction is catalytic in l2ZrCp2, but at least 1 equiv. of I2Z1CP2 is required for satisfactory results. Cl2ZrCp2 or Br2ZrCp2 is less effective. ... 

Dienes.1 (E)-l-Alkenylalanes, readily obtained by hydroalumination of 1-al-kynes,2 couple with vinyl iodides in the presence of this Pd(0) catalyst and zinc chloride to give 1,3-dienes with retention of the configuration. Alkenylzinc derivatives may be the actual reactants. Nickel catalysts are less stereoselective than palladium-phosphine catalysts. 

The formation of (24) is >98% stereoselective. Efforts are currently being made to apply this methodology to the synthesis of carbacyclin (2S) A model study indicates that the Pd-catalyzed cross-coupling reaction of alkenylzinc derivatives is a promising method for the attachment of the so-called prostanoid (i>-side chain, as shown in equation (20). 

Addition to alkynes and dienes. Organozincation of alkynes and dienes generates new organozinc species that are exploited synthetically. Eor example, alkenylzincs derived from propargyltrimethylsilane are useful for the synthesis of 1,3-disubstituted 1,3-dienes. ... 

Alkylation is much less satisfactory and generally inferior to the corresponding reaction of aUcylcoppers. The use of /3,/3-disubstituted alkenylzinc derivatives can also be problematical, as indicated by the results shown in Scheme However,... 

More recently, direct conversion of a-iodoenones into the corresponding a-zinco derivatives has been achieved nsing either an excess of Zn dnst ° or Zn-Ag couple (4 equiv) in the presence of TMEDA (1 equiv).t The Zn reagents thus generated are less reactive than the more usual alkenylzinc derivatives. Nonetheless, some satisfactory results have been obtained with aryl hahdes. On the other hand, variable results were obtained with alkenyl halides (Table In summary, the a-zincoenone-based pro-... 

---