Allylic organozinc compounds, addition reactions

One of the important new directions in the study of addition reactions of organozinc compounds to aldehydes is the use of ionic liquids. Usually, application of these compounds in reactions with common organometallic reagents has a serious problem ionic solvents are usually reactive toward them, particularly Grignard and organolithium derivatives. It has been recently reported that carbonyl compounds react with allylzinc bromide formed in situ from allyl bromide and zinc in the ionic liquid 3-butyl-l-methylimidazolium tetrafluoroborate, [bmim][BF4].285 Another important finding is that the more reactive ZnEt2 alkylates aldehydes in a number of ionic liquids at room temperature.286 The best yields (up to 96%) were obtained in A-butylpyridinium tetrafluoroborate, [bpy][BF4] (Scheme 107). 

Functionalized organozinc halides are best prepared by direct insertion of zinc dust into alkyl iodides. The insertion reaction is usually performed by addition of a concentrated solution (approx. 3 M) of the alkyl iodide in THF to a suspension of zinc dust activated with a few mol% of 1,2-dibromoethane and MeaSiCl [7]. Primary alkyl iodides react at 40 °C under these conditions, whereas secondary alkyl iodides undergo the zinc insertion process even at room temperature, while allylic bromides and benzylic bromides react under still milder conditions (0 °C to 10 °C). The amount of Wurtz homocoupling products is usually limited, but increases with increased electron density in benzylic or allylic moieties [45]. A range of poly-functional organozinc compounds, such as 69-72, can be prepared under these conditions (Scheme 2.23) [41]. 

Several papers are concerned with the threo-erythro stereoselectivity of the reaction of allylic organozinc reagents with carbonyl compounds. The addition (involving allylic rearrangement) of crotylzinc derivatives to various aldehydes occurs stereoselectively, and the relative amount of tAreo-alcohol increases with increasing steric demand of the group R tert-Bu, S4% f-Pr, 707o n-Bt, 46%) and in the sequence of metals Mg < Zn < Cd 3, 7). The temperature or the polarity of the solvent... 

An indirect addition converts alkynes to an organozinc compound using a palladium catalyst, which then reacts with allylic halides. " Similarly, the reaction of an alkyne with Ti(0/Pr)4/2 /PrMgCl followed by addition of an alkyne leads to a conjugated diene. 

Bis(alk-2-enyl)zinc compounds 12, on addition to 3,3-dimethylcyclopropene, yield more than 90% of organozinc addition compounds 13 that can, by acidolysis, be converted into substituted cyclopropanes. Depending on the substitution pattern of the starting material, the reaction can be seen to proceed with allylic inversion. In contrast to addition reactions with unstrained double bonds, the reaction proceeds smoothly at — 20 to — 5 °C. Diethyl ether or tetrahy-drofuran can be employed as solvent and the cA-addition products are formed stereoselectively and in high yield. 

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