Nickel allylation

Nickel-allyl complexes prepared from Ni(CO)4 and allyl bromides are useful for the ole-fination of alkyl bromides and iodides (E.J. Corey, 1967 B A.P. Kozikowski, 1976). The reaction has also been extended to the synthesis of macrocycles (E.J. Corey, 1967 C, 1972A). 

The most important use is the hydrocyanation of butadiene to adiponitrile, NC—(CH2)4—CN, a precursor to hexamethylenediamine for the synthesis of nylon. The process goes stepwise. The first addition of HCN involves nickel allyl intermediates and gives a mixture of linear and branched products in a ratio of —70 30. 

Nickel allyl complexes in the presence of chiral bidentate ligands catalyze the enantioselective codimerization of ethylene with norbornene and with styrene 129... 

Tetracarbonylnickel is an inexpensive (although toxic) source of Ni° and its use for the in situ generation of allylnickel intermediates dates back at least 25 years. In fact, the nickel allylation/methoxycarbo-nylation of alkynes with allyl chloride and an excess of Ni(CO)4 in methanol has b n reviewed (Scheme 15). Accordbgly, dienoates (68) (35-80%) were obtained in a regio- and stereo-selective manner corresponding to a metallo-ene cis addition of an allylnickel compound at the less-substituted termirud C-1 of (67), followed by a CO insertion into the resulting vinylnickel species. The dienyl esters (68) were accompanied by variable amounts of cyclopentenones, indicating the possibility of consecutive CO insertion processes. 

The choice of the anion is also cmcial in systems where the active cationic species is formed from a neutral precursor, as in the case of the nickel allyl chloride catalyst used for asymmetric hydrovinylation (eq. (6) cf. also Section 3.3.3). The previously optimized conditions for this reaction involved the use of highly flammable Al2Et3Cl3 as chloride-abstracting agent and required the use of CH2CI2 at -78 °C. Using NaBARF in compressed CO2, the C-C bond coupling occurs around room temperature with excellent chemo-, stereo-, and enantioselectivity [73]. This example demonstrates nicely that the application of CO2 can have environmental benefits for catalytic processes far beyond the solvent replacement. 

Nickel, palladium and copper catalysts can effect a variety of C — C bond-forming reactions P to the ester carbonyl via j8-zinc esters. These include 1,4-addition to a,jS-unsaturated carbonyl compounds (copper), arylation with aryl halides (palladium, nickel), allylation with allyl chloride (copper), and acylation with acyl chlorides (palladium, copper). " ... 

This last example succeeds partly because nickel catalyses the union of the three reagents in the second step. Similar things happen in the combination of nickel allyls with zinc borates 76. The allyls are derived from unsymmetrical allylic acetates 75 and react at the end remote from the phenyl group (R1 is an alkyl group and R2 is an aryl or alkyl group).19... 

Cationic nickel allyl complexes [(allyl)NiL2]BF4 are catalyst precursors for the stereospecific polymerization of 1,3-butadiene [L = CNBu, P(OPh)3,... 

By hydrolysis of the nickel-allyl-carboxylato complexes with mineral acids usually 3-pentene-carboxylic acids are formed [40]. Instead of mineral acids also acetylacetone can be used as displacing agent [44]. 

Coupling of Allyl Halide Derived Nickel-Allyl Complexes with Alkyl Halides and Other Electrophiles... 

Coupling of Enal-Derived Nickel-Allyl Complexes... 

Coupling of Nickel-Allyl Complexes with Main Group Organometalllcs... 

Extensive studies of the oligomerization of dienes by nickel catalysts were conducted by WUke and co-workers. More recently, Brookhart and co-workers reported the synthesis, isolation, and structural characterization of the active intermediate in nickel-catalyzed butadiene oligomerization (Equation 22.39). This species is formed from a cationic nickel-allyl species in the presence of butadiene. The coordination sphere of tlais complex contains one allyl and two olefin donors in a single hydrocarbon ligand that was formed from three butadiene ligands and the starting allyl group. 

Rhodium catalyzes retro-allylation of homoallylic alcohols to generate nucleophilic allylic rhodium species. A combination of [RhCl(cod)]2, PMcj, and cesium carbonate catalyzes crotyl transfer from homoallylic alcohol to benzaldehyde in dioxane at 100 "C to afford the corresponding homoallylic alcohol (Scheme 5.37) [26]. Like allylic nickel, allylic rhodium experiences rapid and E-Z interconversions under the reaction conditions, which prevents stereoselectivity of the crotylation. Interestingly, a similar crotylation is followed by isomerization into saturated ketone with the aid of a bulkier catalyst [RhCl(cod)]2/PtBu3 at a higher temperature. 

The reaction of norbornene with A -methylallylnickel bromide or iodide gives linear polymers of the alkene via cw-insertion. Evidence for the stereochemistry of insertion is provided by the reaction of norbornene with h -methylallylnickel chloride, which gives the alkene complex (6). Various types of reaction of (6) then result in cis-insertion of the norbornene into the nickel-allyl bond. Thus treatment of (6) with sodium acetate gives (7), which is shown by A"-ray studies to be formed by an ejcu-c/s-insertion of the alkene. A similar cw-insertion occurs with the corresponding palladium complex. ... 

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