Reactivity allylic organozinc reagents

Reactivity of allylic organozinc reagents towards metallated alkynes 936... 

Allylic organozinc reagents display a unique reactivity towards alkenyl-, allenyl- and alkynylmetals. Allenic organozinc reagents which are in metallotropic equilibrium with their propargylic counterparts (equation 150) also share some similarities with allylic organozinc reagents in their behavior towards alkenes and alkynes. 

Based on the observation that alkenylmetals were highly reactive partners in carbo-zincations involving allylic organozinc reagents (see Section III.D), Nakamura and... 

A prototypical study for this section has been obtained as early as in 1983 for carbonylative cross-coupling of the mixture of aryl iodide and alkyl iodide in the presence of Zn metal and palladium catalyst. This system apparently works due to differences of reactivity of aryl versus alkyl iodide toward metallation by Zn. Further studies were rather scarce to involve only preformed functionalized alkylzincs. Carbonylative cross-coupling of functionalized organozinc reagents with allylic esters and GO (1 atm) can be carried out in THF in the presence of HMPA, which suppresses side-reactions (Scheme 4). ... 

Several methods have been described for preparing allylic zinc derivatives. In contrast to alkylzincs, allylic zinc reagents are much more reactive due to the more ionic nature of the carbon-zinc bond in these organometallics. The chemistry displayed by these reagents is not representative of the usually moderate reactivity of organozinc derivatives. Tamaru and coworkers have converted various allylic benzoates to the corresponding organozinc intermediates in the presence of palladium(O) as catalyst. 

The CO ligands are electron withdrawing and these allyl complexes are electrophilic. The iron complexes, being cationic, are the more reactive. Very weak nucleophiles such as enol ethers, allyl silanes and electron-rich aromatics may be used (Scheme 9.6). Malonate anions, amines and organozinc reagents have also been used. The resulting t -complexes are usually quite unstable 9.18 and decomplex on exposure to air to give the allylation product 9.19. 

Organozinc reagents are insuffieiently reactive to react with carbonyl derivatives and double bonds, exeept allyl and propargylzinc reagents that react with carbonyl derivatives, nitriles and alkynes. 

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). 

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