Alkene Hydrozincation

Diorganozincs can also be prepared by a nickel-catalyzed hydrozincation. The reaction of Et2Zn with Ni(acac)2 may produce a nickel hydride that adds to an alkene leading. 

Hydrozincation is not as widely observable as that involving , A1 and Zr. This is one of the main reasons why the indirect hydrometallation-transmetallation procedures shown in Schemes 8 and 12 have been developed and used. It is nevertheless highly desirable to directly generate organozincs to be used for the Pd- or Ni-catalyzed cross-coupling from alkenes and alkynes via hydrozincation. Indeed, such reactions have been developed, as shown in Scheme 17. However, further developments are clearly desirable. 

Hydrozincation of terminal alkene with Et2Zn, catalysed by Ni(acac)2, yields the dialkylzinc 525, which reacts with the nucleophile 526 via transmetallation involving Cu salt to give 527 [199]. In this reaction, the Ni hydride is generated by / -elimination of EtNi(acac), and the hydronickellation product 524 is formed by alkene insertion to the hydride H—Ni(acac). Then transmetallation of 524 with Et2Zn affords 525. 

Diorganozincs can also be prepared by a nickel-catalyzed hydrozincation. The reaction of Et2Zn with Ni(acac)2 may produce a nickel hydride that adds to an alkene leading after transmetallation with Et2Zn to a diakylzinc (Scheme 9). This reaction proceeds in the absence of solvent and at temperatures of 50-60 °C. A number of functionalized olefins like allylic alcohols or amines can be directly used. This method is especially well suited for the preparation of functionalized diorganozincs for the asymmetric addition to aldehydes (Equation (45)).108,50... 

Hydrotitanation of 1-silyl- and I-stannyl-1-alkynes leads to P-silyl (or p-stannyOalkenyltitanates, thus showing opposite regioselectivity to other related processes (hydroboration, hydroalumination, hydromagnesiation, hydrotitanation, hydrozincation, hydrozirconation).Well-defined alkenes are obtained on further reaction of the alkenylti-tanates, for example, with allylic carbonates an 8 2 displacement occurs to afford 1,4-di-... 

The nickel-catalyzed hydrozincation of alkenes has been developed as a useful entry to functionalized organozincs (Scheme 3-85). Simple terminal olefins, when treated with EtaZn and catalytic quantities of Ni(acac)2 and cyclooctadiene, are efficiently converted into the corresponding dialkylzinc reagents from hydrozincation of the alkene. A wide variety of copper-catalyzed alkylations are accessible... 

The nickel-catalyzed hydrozincation of alkenes provides a novel method for the preparation of functionalized organozincs such as (97) (Scheme 77).H74.i75] resulting ftmction-alized organozincs may be transformed into cuprate reagents and then alkylated with a variety of electrophiles. A hydronickelation step is likely involved in the reaction pathway. 

The Ni-catalyzed hydrozincation of alkenes has been reported, leading to functionalized diorganozinc reagents (Scheme 4.21) [89,94]. This reaction is particularly... 

Scheme 4.21 Preparation of dialkylzinc reagents by nickel-catalyzed hydrozincation of alkenes [89, 94].

The nickel-catalyzed hydrozincation or carbozincation of double or triple bonds are worthy of note. Thus, treatment of an alkene with diethylzinc in the presence of a catalytic amount of Ni(acac)2 and COD affords the corresponding organozinc reagent. Since diethylzinc will react with aliphatic bromide or iodide, such zinc-... 

---