Bromide-zinc exchange

Finally some transition-metal (Ni, Mn)-catalyzed iodine- or bromide-zinc exchange reactions using Et2Zn constitute a convenient approach to organozinc halides under mild reaction conditions [32,33] (Scheme 9-8). 

E-1,4-bis(trimethylstannyl)-1,3-butadiene 174 bis(triphenylphosphane) palladium(O) 100 bombykol 64, 65 a-bond metathesis 37 9-borabicyclo[3.3.1]nonane 49 boron-zinc exchange 389, 396 (5)-(/ )-BPPFA 29 Bredt s rule 243 bromide-zinc exchange 404 1,4-bromoacetoxylation 366 1 -bromo-1 -alkenes 63 bromoboration of acetylene 51... 

By removing electron density from the metal center, such reductive eliminations should be made easier [79-81]. Thus, it was observed that, whereas the unsaturated alkyl bromide 50 undergoes a smooth cross-coupling reaction with Et2Zn in the presence of Ni(acac)2 leadin to 52, the corresponding saturated alkyl bromide 51 does not undergo the cross-couplin reaction, but instead produces the bromide-zinc exchange product 53 (Scheme 9-43) [82]. 

Halide exchange, sometimes call the Finkelstein reaction, is an equilibrium process, but it is often possible to shift the equilibrium." The reaction is most often applied to the preparation of iodides and fluorides. Iodides can be prepared from chlorides or bromides by taking advantage of the fact that sodium iodide, but not the bromide or chloride, is soluble in acetone. When an alkyl chloride or bromide is treated with a solution of sodium iodide in acetone, the equilibrium is shifted by the precipitation of sodium chloride or bromide. Since the mechanism is Sn2, the reaction is much more successful for primary halides than for secondary or tertiary halides sodium iodide in acetone can be used as a test for primary bromides or chlorides. Tertiary chlorides can be converted to iodides by treatment with excess Nal in CS2, with ZnCl2 as catalyst. " Vinylic bromides give vinylic iodides with retention of configuration when treated with KI and a nickel bromide-zinc catalyst," or with KI and Cul in hot HMPA." ... 

An example for the use of the boron-zinc exchange reaction for copper-mediated SN2 -substitutions of allylic electrophiles is the hydroboration of nitroolefin 130 with diethylborane, followed by successive transmetallation of the borane 131 with diethylzinc and CuCN-2LiCl, and final trapping with allyl bromide to give the product 133 with 83% yield over four steps (Scheme 34).34,34a This transformation again demonstrates the tolerance of the method towards functional groups and acidic hydrogen atoms. 

As mentioned in Section 9.12.2.1.1, the boron-zinc exchange can be performed stereoselectively if diisopropyl-zinc instead of diethylzinc is used. For example, hydroboration of the chiral, racemic endocyclic olefin 134 with diethylzinc, followed by twofold transmetallation and electrophilic capture of the resulting copper intermediate with allyl bromide was used for the highly diastereoselective formation of the stereotriad in product 136 (Scheme 35).35,35a 103 QorreSp0nding enantioselective transformations were carried out with chiral boranes and catalytic amounts of copper salts (see Section 9.12.2.2.2).36... 

A few modifications of this reaction are the reaction between alkyl iodide and copper-zinc alloy, the reaction between zinc and a mixture of alkyl iodide and alkyl bromide, the transmetalation between zinc and lithium, magnesium, or tin, and the iodine-zinc exchange. ... 

Aminoheteroaryl chlorides can also be coupled in good yields with dialkylzincs or alkylzinc chlorides in dioxane at 65 °C in the presence of NiCl2(dppp). In addition, as this nickel complex also catalyzes the halogen-zinc exchange, generation of alkylzincs from alkyl bromides and diethylzinc can be achieved in situ (Scheme 4.90) [283]. 

The diastereoselectivity of the zinc iodide catalyzed reaction of the azetidinone I with the trimethylsilyl enolate derivatives of the chiral 3-(l-oxopropyI)oxazolidinones 6 was considerably lower (about 60 40), although independent generation of the zinc enolate, via exchange of the lithium enolate with zinc bromide, afforded the /9-Iactam carboximide derivatives in a ratio (RIS) 80 20177. 

A PRP -1 (Hamilton Reno, NV) reversed phase column was coated with cetylpyridinium and eluted with tetramethylammonium salicylate acetoni-trile water.89 The separation was comparable to that observed on conventional ion exchange. Coated phases were also used to separate oxalate complexes of manganese, cobalt, copper, and zinc.90 Reversed phase silica supports were coated with poly(N-ethyl-4-vinylpyridinium bromide), poly(dimethydiallylammonium chloride), poly(hexamethyleneguanidinium... 

Although less common than zinc chlorides, there has been much synthetic and structural work carried out on zinc bromide and iodide complexes. For example, the 1 1 adduct of N,N,N, A -tetramethyl-o-phenylenediamine with zinc bromide has been structurally characterized.630 The ion exchange properties of zinc bromide and iodide have been studied in ethylene glycol.631 The electrolytic behavior of zinc bromide in propylene carbonate has been studied and the conductance data shows the existence of dimers.632... 

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