Nickel allylnickel halides

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

Examples of w-allylnickel-X compounds (X = anionic ligand) other than 77-allylnickel halides which have been used in combination with (alkyl)aluminum halides as olefin oligomerization catalysts are 7r-allyl-nickel acetylacetonate (11) (Section III), 7r-allylnickel aziridide (4, 56), and bis(7r-allyl)nickel (6) (59). In addition to ir-allylnickel halides, organo-nickel halides such as tritylnickel chloride (60, 61) and pentafluoro-phenylbis(triphenylphosphine)nickel bromide (62), or hydridonickel halides, e.g., trans-hydridobis(triisopropylphosphine)nickel chloride (12) (Section III), give active catalysts after activation with aluminum halides... 

Polymerization was carried out in benzene in the presence of bis-(7r-allylnickel halides). The latter were prepared from nickel carbonyl and allyl halide (allyl bromide, crotyl chloride, bromide, or iodide etc.). The results of the polymerization runs are reported in Table I. The data indicate that all of the bis(7r-allylnickel halides) initiate by themselves the stereospecific butadiene polymerization yielding a polymer with 97-98% 1,4-units. The cis-l,4/trans-l,4 ratio depends on the halide in the dimeric r-allylnickel halide but not on the nature of allylic ligand. The case of bis(7r-crotylnickel halides) shows the effect of halide on microstructure, for whereas (C4H7NiCl)2 initiates cis- 1,4-polybutadiene formation, trans-1,4 polymers are produced by (C4H7NiI)2. The reactivity increase in the series Cl < Br < I. 

This article describes further progress in the chemistry of 7r-allylnickel compounds. First, preparative methods for 7r-allylnickel halides, alkoxides, amides, and alkyls are described. Next, some chemical properties of these compounds—e.g., a recently observed disproportionation reaction—are discussed. Then, the use of 7r-allylnickel halides as homogeneous catalysts is discussed. Whereas bis (7r-allyl) nickel is a catalyst for butadiene cyclotrimerization, 7r-allylnickel halides combined with Lewis acids, such... 

The common preparative method for 7r-allylnickel halides is at the moment the reaction of nickel(O) olefin complexes like bis(cycloocta-1,5-diene) nickel, (IV), with allylic halides (23). The olefin complex, IV, can be prepared easily by reducing nickel(II) salts (like nickel acetyl-acetonate) with aluminum organic compounds in the presence of cycloocta-1,5-diene (5). 7r-Allylnickel halides and substituted 7r-allylnickel halides prepared according to this method are listed in Table I. 

Table I. 7r-Allylnickel Halides Prepared from Bis ( cycloocta-1,5 -diene) nickel ( 0 )...

TT-Allylnickel halides are more stable, and thermal disproportionation is not observed even at higher temperatures. Recently, we found that TT-allylnickel halides can be disproportionated easily by treating them in solution with excess gaseous ammonia (2). Bis(7r-allyl)nickel and ammonia adducts of nickel dihalides are obtained in quantitative yields and can be separated easily. In fact, the disproportionation reaction represents at the moment the easiest way to synthesize bis (7r-allyl) nickel type compounds since as mentioned, all types of 7r-allylnickel halides can be prepared easily. The advantage of the new method lies in the fact that bis (TT-allyl) nickel type compounds can be prepared without prior preparation of organometallic allyl compounds, such as Grignard compounds, which are sometimes diflBcult to prepare. The disproportionation of TT-allylnickel halides has an analog in the chemistry of alkyl-mercuric halides, some of which disproportionate under the influence of ammonia (12). 

The transfer of halide from 7r-allylnickel halide to aluminum to give a haloaluminum anion results in a strong decrease in the electron density and formation of free coordination positions on nickel. The presence and number of free coordination positions can be demonstrated by reaction with carbon monoxide. The catalytically active complex, XIII, reacts at —40°C. with 2 moles of carbon monoxide to give the inactive XIV which can be isolated. In XIII, there are accordingly two free coordination positions. During catalysis, these positions are presumably occupied by olefins in the form of 7r-complexes. A stable olefin 7r-complex, 7r-allylnickel (7T-cycloocta-l,5-diene) aluminum tetrabromide, XV, can actually be isolated by adding cycloocta-l,5-diene to a solution of the active complex, XIII. 

The dimerization reaction has been carried out under two different conditions. In laboratory experiments, the reaction is conveniently carried out under 1 or less than 1 atmosphere and at a temperature of —20° to — 10°C. These relatively low temperatures are necessary to obtain a sufficient concentration of ethylene or propylene in the catalyst solution. The dimerization catalyst for laboratory experiments is usually prepared by mixing, for example, chlorobenzene solutions of a 7r-allylnickel halide and an aluminum halide (or alkylhalide) in molar ratio of at least 1 1. The phosphine-modified catalyst is obtained by simply adding 1 mole of a phosphine per mole of nickel to the solution of the catalyst. When ethylene or propylene is introduced into the catalyst solution, reaction starts immediately, as evidenced by a sudden rise in temperature. Dimerization is exothermic to the extent of about 28 kcal./mole propylene dimer. Hence, the mixture must be stirred and cooled intensively during the reaction. Under these conditions (Table V), reaction rates of about 6 kg. 

The dimeric ir-allylnickel halide complexes are most conveniently prepared from allylic halides and nickel(O) species, such as Ni(COD)2 and Ni(CO)4. Although other allylic systems have been used, the most common procedure involves treatment of an allylic bromide with Ni(CO)4 at 50-70 C. ° This gives rise to the moderately air-sensitive dimeric ir-allyl species (equation 46). In polar coordinating solvents, (DMF, HMPA, N-methylpyrrolidone, etc.) these dimeric species are converted into highly reactive monomeric species (equation 47). ... 

Asymmetric modifications of hydrovinylation are one of the earliest examples of successful asymmetric transition metal catalysis. After optimization of various dimerization and codimerization reactions using phosphane modified nickel catalysts, the first examples of asymmetric olefin codimerization were reported with n-allylnickel halides activated by organoaluminum chloride and modified by chiral phosphanes7. Thus, codimerization of 2-butene with propene using n-allylnickel chloride/A]X, (X = Cl, Br) in the presence of tris(myrtanyl)phosphane gives low yields of (—)-( )-4-methy 1-2-hexene (I) with 3% ee7,7 . 

Similarly, asymmetric metal-catalyzed hydrovinylation of various other olefins has been investigated, usually catalyzed by nickel systems prepared from Ni(IJ) salts or / -allylnickel halides, activated by Lewis acids such as ethylaluminum dichloride or diethylaluminum chloride, and modified by chiral phosphanes. Surveys of these results are found in more general reviews11-12 and reviews dealing preferentially with this subject -5-7-13 - A... 

The generality of the carbon monoxide insertion reaction is clear from reports that methylcyclopentadienyliron dicarbonyl (16), ethylcyclopentadienylmolylbde-num tricarbonyl (66), alkylrhenium pentacarbonyls (50), alkylrhodium dihalo carbonyl bisphosphines (34), allylnickel dicarbonyl halides (35), and mono-and di-alkyl derivatives of the nickel, palladium, and platinum bisphosphine halides (P), also undergo the reaction. The reaction of Grignard reagents (24), and of boron alkyls (51) with carbon monoxide probably takes place by the same mechanism. 

Another clear example of an acetylene insertion reaction was reported by Chiusoli (15). He observed that allylic halides react catalytically with nickel carbonyl in alcoholic solution, in the presence of CO and acetylene, to form esters of cis-2,5-hexadienoic acid. The intermediate in this reaction is very probably a 7r-allylnickel carbonyl halide, X, which then undergoes acetylene insertion followed by CO insertion and alcoholysis or acyl halide elimination (35). Acetylene is obviously a considerably better inserting group than CO in this reaction since with acetylene and CO, the hexadienoate is the only product, whereas, with only CO, the 3-butenoate ester is formed (15). [See Reaction 59]. 

The reaction of allylic halides with nickel tetracarbonyl to form coupled products has been known for over two decades (9), but it is only in recent years that an insight into the mechanism has been obtained. Isolation of the intermediate 7r-allylnickel complexes and the discovery that these react with activated olefins and organic halides in general have led to a considerable increase in the scope of the reaction. 

Understanding of the mechanism of the apparently simple coupling of two allylic groups in the presence of nickel carbonyl is largely due to the investigations of E. J. Corey and his co-workers (10). The first step is formation of a 7r-allylnickel carbonyl halide (I). Corey suggests that (I)... 

Aromatic halides are reported to give only carbonylated products with nickel tetracarbonyl. In contrast, pentafluorophenyl iodide in DMF gives decafluorobiphenyl in 70% yield (27). From the other products obtained (pentafluorobenzene, decafluorobenzophenone) it has been suggested that a radical mechanism is involved. The reactions of benzyl halides with nickel carbonyl in various solvents have been reported (28). The main reaction involves carbonylation, as discussed in Section III. Using benzene as solvent, a 33% yield of bibenzyl may be obtained. Here again a mechanism involving a 7r-allylnickel derivative should perhaps be considered, particularly since such a system is known to exist in (XVI) (29). 

Two groups of workers (40, 41) have demonstrated that the reaction proceeds through the formation of a 77--allylnickel intermediate which absorbs CO to form a nickel acyl complex. This then liberates a molecule of acyl halide which is hydrolyzed by the solvent. The presence of the intermediate nickel acyl complex in solution has been demonstrated... 

K-Allylnickel ImBiles (2, 291), Walter and Wilke reported briefly that Ni(COD)2 is more reactive than nickel carbonyl in the reaction of allylic halides to form 71-allyl-nickel halides. The reaction proceeds below 0° and in quantitative yields. The method has not been used extensively since nickel carbonyl is commercially available. However, Semmelhack recommends use of Ni(COD)2 for the preparation of thermally sensitive a-allyinickel halides such as it-(2-carbocthoxyallyl)nickel bromide," which was prepared from ethyl 2-bromomethylacrylate with this nickel reagent in 76% yield. 

The thermal stability of 7r-allylnickel amides, alkoxides, and halides is considerably higher than that of 7r-allylmethylnickel. With the exception of some TT-allylnickel alkoxides, these compounds are stable at ordinary temperatures. The reason for the increased stability may be, aside from the greater polarity of the bond between nickel and the nonmetal, the... 

A new reaction observed with 7r-allylnickel methyl, alkoxides and halides is disproportionation. 7r-Allylnickel methyl decomposes spontaneously in solution even at — 60°C. to bis (7r-allyl) nickel, elemental nickel, and presumably transient methyl radicals. The reaction proceeds quantitatively accordingly to the reaction shown below since after removal of elemental nickel by filtration, exactly half of the nickel is found in the filtrate in the form of bis (7r-allyl) nickel. 

A nickel-catalyzed ene cyclization (sec. 11.13) has been reported that uses Ni(cod)2- The reaction proceeds by initial formation of a ac-allylnickel complex, which facilitates the intramolecular ene reaction with an allylic amine unit. l jt-Allylnickel complexes can be used in coupling reactions with both aryl and alkyl halides. Enolate anions react with nickel(O) reagents to form a complex that subsequently couples to aryl iodides. Semmelhack s final step in the synthesis of cephalotaxinone (446) treated 445 with Ni(cod)2 to... 

Chiral (E)-enolethers. A degassed soln. of (5R)-5-cyclohexyl-2-ethenyl-l,3-dioxolan-4-one (2 1 cisjtrans) in THF added to ca. 1 eq. of a suspension of bis(l,5-cycloocta-diene)nickel(0) in the same solvent under N2, stirred until the complex dissolved (10 min), after 3h the resulting rust-coloured precipitate collected, suspended in methylene chloride, treated with MejSiCl, and stirred for 30 min intermediate 7c-allylnickel complex (Y 78%), in benzene treated with DMF and 5 eqs. 1-iodo-propane, irradiated with a sunlamp (GE 275 W Model RSW) for 2,5 h at 10°, stirred for a further 3 h, diluted with pentane to precipitate nickel halide, and stirred for a further 4 h product (Y 82% E/Z 9 1). Subsequent treatment with acetals afforded 2-p-alkoxy-l,3-dioxolan-4-ones with asym. induction, thereby providing an alternative to asym. aldol condensation. F.e. inch reaction with ar. and a,P-ethylene-bromides s. D.J. Krysan, P.B. Mackenzie, J. Am. Chem. Soc. 110, 6273 (1988). 

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