Hydrozincation

A more convenient, but less general, synthesis of diorganozincs from olefins is the nickel-catalyzed hydrozincation, shown in Scheme 6. The yields (40-63%) are independent of the amount of diethylzinc added, but dependent on the nature of the olefin. For allylic and homoallylic alcohols and amines, yields as high as 85-95% are possible.32,33... 

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. 

The direct hydrozincation of olefins14 is possible using Et2Zn in the presence of catalytic amount of Ni(acac)2 and 1,5-cyclooctadiene (COD). The resulting diorganozincs can be used for asymmetric additions to aldehydes. The best hydrozincation reactions are obtained with allylic alcohols or amines. In these cases the raction is driven to completion by the formation of a zinc-heterocycle of type 7 (Scheme 5.10). 

Preparation of bfs(4-pivaloxybutyl)zinc by hydrozincation and its enantioselective addition to an aldehyde preparation of (4S)-1-(triisopropylsiloxy)-4-hydroxy-8-pivaloxyoctane13... 

Preparation of 3-(3-pivaloxypropyl)-1-zinca-2-oxacyclopentane by hydrozincation and its copper-mediated addition to an electrophile preparation of ethyl 2-(4-hydroxy-7-pivaloxy)acrylate13... 

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... 

HydroZinc A biological process for leaching zinc from heaps of zinc sulfide ore. Naturally occurring bacteria attack the ore, and the leachate is then neutralized and extracted with a solvent. The metal is produced from this liquor by electrowinning. The process was developed by Teck Cominco in British Columbia and piloted between 2000 and 2002. 

Diorganozincs can be obtained by the hydrozincation of olefins using diethylzinc and catalytic amount of Ni(acac)2. The resulting diorganozincs are well suited for applications in asymmetric synthesis [33,34] (Scheme 9-9). 

The presence of a heteroatom in the olefin at an allylic or homoallylic position considerably stablizes the zinc organometallic compound obtained after the hydrozincation of the olefin. Thus allylic alcohols and amines are especially well suited as substrates for this hydrozincation procedure. 

H. L. Lizama, J. R. Harlamovs, S. Belanger, The Teck Cominco Hydrozinc process in Hydrometallurgy 2003 - Fifth International Conference in Honor of Professor Ian Ritchie, Vancouver, (Eds. C. A. Young, A. M. Alfantazi, C. G. Anderson et al.),TMS (TheMiner-als, Metals Materials Society), Vancouver, Canada, 2003, pp. 1503-1516. 

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-... 

Organocopper reagents prepared from the more readily available organozincs have found much use in the synthesis of highly functionalized molecules. For example, Ni-catalyzed hydrozincation of allylic alcohols initiates the preparation of a-(4-hydroxyalkyl)acrylates when the cuprate intermediates are used in the coupling with the a-bromomethylacrylic esters." ... 

---