[12012-90-7]  · C6H10Br2Ni2  · Bis(allyl)di-m-bromodinickel  · (MW 359.33)

(reacts with organic halides by a nickel-radical chain reaction; couples allyl groups, forming macrocycles; generates alcohols from carbonyls)

Physical Data: red solid; sublimation point 80 °C; dipole moment 1.31 D.1

Solubility: sol benzene, DMF, and other organic solvents.

Preparative Method: the reagent may be prepared from allyl bromide and Ni(CO)4 or Ni(cod)2 as described below.

Purification: by sublimation.

Handling, Storage, and Precautions: is air and moisture sensitive. It is generally prepared and used in situ. See entries for Allyl Bromide, Tetracarbonylnickel, and Bis(1,5-cyclooctadiene)nickel(0) for further information.

Synthesis and General Considerations.

The reagent was originally generated from the reaction of Ni(CO)4 with allyl bromide (eq 1).1 Later, it was found that Ni(cod)2 also works well as the nickel source in the synthesis of (allyl)nickel halides.2

The geometry of the complex consists of two allyl groups bonded through their p-systems to the nickel atoms. Because the center core of Ni2Br2 is planar, there are two possible geometries for the complex. In one, the center carbons of the allyl groups are on the same side (cis) of the Ni2Br2 plane, and in the other, they are on opposite sides (trans). Typically, the mixture of these two complexes is used for all subsequent reactions.

The synthesis of the allylnickel halide complex is not restricted to allyl bromide itself. A variety of methyl substituted and cyclic allyl derivatives will form complexes. In contrast to the better known allylpalladium complexes, which are electrophilic, these allylnickel halide complexes are nucleophilic. With unsymmetrically substituted allyl complexes, it is usually the less substituted of the carbons which forms the bond to the electrophile.3

Reactions with Alkyl, Aryl, and Vinyl Halides.

Allylnickel halide complexes react with a large variety of organic halides, including primary, secondary, tertiary, vinyl, and aryl compounds, and will do so smoothly, even in the presence of many common functional groups, including alcohols and esters (eq 2). The reactivity order for the X group is I > Br >> Cl. These reactions work only in polar coordinating solvents such as DMF or HMPA.3-6

Initially it was believed that a coordinating solvent was needed because the first step is cleavage of the dimer, with the solvent filling-in the now vacant coordination site. Subsequent steps of this reaction are oxidative addition to the RX bond, which proceeds smoothly regardless of the substitution pattern of the R group, followed by reductive elimination of the R-allyl product.3

More recently, it has been shown that the mechanism is much more complicated.7 The reaction goes by a radical chain process; however, it is a nickel and not a carbon radical which is the chain carrying species. This mechanism nicely explains the observed initiation by light, heat, or a reducing agent, the inhibition by an electron acceptor, the reactivity of a large variety of different organic halides, and the racemization observed for secondary halides.

Reaction with Allyl Halides.

One of the more common reactions of an allylnickel halide complex is with an allyl halide to generate a 1,5-hexadiene. A variation of this reaction has been used to couple two allyl groups together to form a macrocyclic ring system (eq 3).8 This method has proven successful for rings as large as 18. In addition, the reaction conditions are compatible with a large variety of substitution patterns and functional groups.

Reaction with Carbonyls.

The allylnickel halide complex will attack an aldehyde or a ketone to generate an alcohol (eqs 4 and 5).9 However, it will not attack an ester or nitrile. The reaction with a quinone generates an allyl-substituted quinone product, through an initial Michael addition to the a,b-unsaturated system.10

Other Reactions.

Tetracarbonylnickel, an allyl bromide derivative, and an alkyne react to generate a cyclopentenone derivative (eq 6).11,12 It is believed that these reactions go through an allylnickel halide complex as an intermediate.

1. Fischer, E. O.; Burger, G. B 1961, 94, 2409; ZN(B) 1961, 16b, 77.
2. Wilke, G.; Bogdanovic, B.; Hardt, P.; Heimbach, P.; Keim, W.; Kroner, M.; Oberkirch, W.; Tanaka, K.; Steinrucke, E.; Walter, D.; Zimmermann, H. AG(E) 1966, 5, 151.
3. (a) Semmelhack, M. F. OR 1970, 19, 115. (b) Baker, R. CR 1973, 73, 487. (c) Jolly, P. W.; Wilke, G. The Organic Chemistry of Nickel; Academic: New York, 1974; Vol. 1. (d) Jolly, P. W.; Wilke, G. The Organic Chemistry of Nickel; Academic: New York, 1975; Vol. 2. (e) Wilkinson, G.; Stone, F. G. A.; Abel, E. W. Comprehensive Organometallic Chemistry; Pergamon: Oxford, 1982; Vol. 6, pp 1-231.
4. Corey, E. J.; Semmelhack, M. F. JACS 1967, 89, 2755.
5. Corey, E. J.; Semmelhack, M. F.; Hegedus, L. S. JACS 1968, 90, 2416, 2417.
6. (a) Sato, K.; Inoue, S.; Morii, S. CL 1975, 747. (b) Sato, K.; Inoue, S.; Watanabe, K. JCS(P1) 1981, 2411.
7. Hegedus, L. S.; Thompson, D. H. P. JACS 1985, 107, 5663.
8. (a) Corey, E. J.; Wat, E. K. W. JACS 1967, 89, 2757. (b) Corey, E. J.; Hamanaka, E. JACS 1967, 89, 2758. (c) Dauben, W. G.; Beasley, G. H.; Broadhurst, M. D.; Muller, B.; Peppard, D. J.; Pesnelle, P.; Suter, C. JACS 1974, 96, 4724.
9. Semmelhack, M. F.; Wu, E. S. C. JACS 1976, 98, 3384.
10. Hegedus, L. S.; Wagner, S. D.; Waterman, E. L.; Siirala-Hansen, K. JOC 1975, 40, 593.
11. (a) Chiusoli, G. P. ACR 1973, 6, 422. (b) Chiusoli, G. P.; Cassar, L. AG(E) 1967, 6, 124.
12. (a) Pagès, L.; Llebaria, A.; Camps, F.; Molins, E.; Miravitlles, C.; Moretó, J. M. JACS 1992, 114, 10449. (b) Camps, F.; Coll, J.; Moretó, J. M.; Torras, J. TL 1987, 28, 4745. (c) Camps, F.; Liebaria, A.; Moretó, J. M.; Pages, L. M. TL 1992, 33, 109, 113. (d) Camps, F.; Llebaria, A.; Moretó, J. M.; Pages, L. M. TL 1992, 33, 113.

Allan R. Pinhas

University of Cincinnati, OH, USA

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