[13463-39-3] · C4NiO4 · Tetracarbonylnickel · (MW 170.73)
(oxidation of aryl halides and alkynes; oxidative addition of alkyl and aryl halides; rearrangement and cyclization; carbonylation)
Physical Data: mp -25 °C; bp 43 °C; d 1.32 g cm-3; based on calorimetry, the average Ni-C bond strength is 35 kcal mol-1; bond distances Ni-C 1.82 Å and C-O 1.15 Å.
Solubility: insol H2O; sol most organic solvents.
Form Supplied in: colorless liquid in a lecture bottle or in a cylinder, usually under CO pressure.
Analysis of Reagent Purity: IR spectrum, nCO = 2040 cm-1; 13C NMR spectrum, d = 193 ppm; UV spectrum, l
Preparative Method: formed from finely divided Nickel and Carbon Monoxide gas to give a four-coordinate d10 metal complex, with tetrahedral geometry, that satisfies the 18-electron rule.
Handling, Storage, and Precautions: toxic; flammable in air; must be used in a well ventilated fume hood and kept under an inert atmosphere.
Upon reaction of Ni(CO)4 with Chlorine, Bromine, or Iodine, the carbonyls are released as carbon monoxide gas and the nickel is oxidized, generating NiX2 (eq 1). This reaction proves to be a very efficient method for the destruction of excess Ni(CO)4 at the end of a reaction sequence. For example, simply pouring the reaction mixture into a flask containing solid I2 or by adding an I2 solution to the reaction mixture, CO gas evolves and the nickel forms NiI2, which is water soluble.1
A second method for quenching unreacted Ni(CO)4 is to sweep all volatile compounds out of the reaction flask with a stream of nitrogen or argon gas. These volatile compounds are collected in a cold trap and then quenched with Nitric Acid. As with the halogens, the HNO3 will oxidize the Ni(CO)4, generating carbon monoxide gas and a water-soluble nickel complex.
The reaction between Ni(CO)4 and an alkali metal, a metal amalgam, or a metal hydroxide generates anionic nickel carbonyl clusters. The major two products are Ni5(CO)122- and Ni6(CO)122-, with the ratio being dependent upon the reaction conditions used.2 These clusters react with aryl halides3 (or alkynes4) to generate benzoic acid derivatives (eq 2).
There are two sites for nucleophilic attack in Ni(CO)4. The first is one of the coordinated carbonyl ligands and the second is the nickel itself, but this can occur only if one of the ligands first dissociates.
Unlike the reaction with hydroxide mentioned above, in the reaction of Ni(CO)4 with a nucleophile such as methoxide, a dialkyl amide, or an alkyllithium, the carbonyl is attacked to give a nickel acylate complex (eq 3). Because these complexes are discussed under Lithium Tricarbonyl(n-pentanoyl)nickelate, it will only briefly be mentioned here that nickel acylate complexes are very air- and water-sensitive, and they react with both alkyl and vinyl halides to form esters, amides, and ketones, respectively.5-7
Poorer nucleophiles do not attack one of the carbonyls. Instead, they coordinate to Ni(CO)3, which is formed readily by the thermal or photochemical dissociation of a carbonyl ligand from the Ni(CO)4.8 As a specific example, the reaction of Ni(CO)4 with a phosphine (PR3) is first order in nickel complex and zero order in phosphine.9 This suggests rate-limiting dissociation of a carbonyl ligand followed by a rapid attack on the Ni(CO)3 by the phosphine (eq 4).
If an excess of phosphine is used, the Ni(CO)3(PR3) complex will react in a similar manner to generate Ni(CO)2(PR3)2. Due to the increase in back-bonding from the metal into the p* orbital of the carbonyl as the number of phosphine ligands increases, removal of a third carbonyl and formation of Ni(CO)(PR3)3 does not occur.
Halide ions will also react with Ni(CO)4 to form halide-Ni(CO)3 anionic complexes.8,10 The order of nucleophilicity of the halides is the standard order of I- > Br- > Cl-. It is believed that a halide-Ni(CO)3 anionic complex is the actual species which reacts with the acid chloride in the lactone synthesis shown in eq (5).10
Ni(CO)4, or more specifically Ni(CO)3, will react with alkyl halides in an oxidative addition reaction. (There are a large number of examples of Ni0 complexes other than Ni(CO)4 undergoing oxidative addition reactions;1 however, they will not be covered here.) One of the more common reactions of this type is the reaction of Ni(CO)4 with an allyl bromide derivative to form a p-allyl nickel complex (eq 6). These complexes, unlike the analogous palladium complexes, are nucleophilic.11 In addition, the allyl groups can couple to form a 1,5-hexadiene derivative. For more of the chemistry of allyl nickel complexes, see Bis(allyl)di-m-bromodinickel.
The reaction of an aryl iodide with Ni(CO)4 generates a 1,2-diketone when THF is used as the solvent, and an ester when methanol is used as the solvent (eq 7). The exact mechanism of this reaction is not known, but a benzoyl-nickel complex has been proposed as a likely intermediate. The reaction of this intermediate with another intermediate forms the diketone or with the alcohol forms the ester.12
Similarly, the reaction of an alkyl or vinyl halide with an excess of an alkoxide (RO-) and Ni(CO)4 in an alcohol solvent at refluxing temperature, generates the corresponding ester in high yield (eq 8). In contrast to the previous reaction, it is speculated that the mechanism involves initial formation of a nickel acylate complex, followed by reaction of the acylate complex with the alkyl or vinyl halide.6 However, a mechanism in which the nickel carbonyl reacts with the alkyl or vinyl halide to form an acyl nickel complex, and then this is quenched by the alkoxide, cannot be excluded.
This general type of reaction also can be performed in the presence of an amine to form an amide as the final organic product (eq 9).6
Intramolecular variations of these two reactions have led to the syntheses of a-methylene lactones and b-lactams. Specifically, the reaction of nickel tetracarbonyl with a molecule possessing a vinyl halide on one end and an alcohol on the other, in refluxing THF with or without added base, can generate a five- or six-membered a-methylene lactone in good yield (eq 10).13
An aziridine, which is first ring-opened with Lithium Iodide, reacts with nickel tetracarbonyl in a similar cyclization reaction to generate a b-lactam (eq 11).14
These two steps must be done individually because, as discussed above, iodide and nickel carbonyl readily react with each other. Because the first step is an SN2 reaction, the less substituted carbon-nitrogen bond is carbonylated. This result is complementary to the use of a rhodium complex for the conversion of an aziridine to a b-lactam. In that reaction it is always the more substituted C-N bond which is carbonylated.15 In addition, it has been shown that when a 2,3-disubstituted aziridine is the starting material, the product b-lactam is generated with retention of stereochemistry.
The oxidative addition of nickel tetracarbonyl has been shown to be an effective method for the formation of an alkene by the removal of two halogens which are cis. For example, a cis-1,2-dichlorocyclobutane derivative upon reaction with nickel carbonyl generates the corresponding cyclobutene (eq 12).16 This reaction is compatible with a large variety of R groups; however, the chlorines must be attached to carbon atoms which contain electron-withdrawing groups. If the methoxycarbonyl groups are replaced by electron donors, the starting material is stable to the reaction conditions.17
Nickel carbonyl will catalyze the rearrangement of a vinylcyclobutene to a cyclohexadiene derivative (eq 13).17 Although a reactive intermediate has not been observed, based upon deuterium-labeling studies, it has been determined that the reaction is unimolecular and that it is the 2,3-bond (and not the 3,4-bond) which is cleaved.
Nickel tetracarbonyl has initiated a variety of other ring-formation reactions, including the lactone synthesis shown in eq 5.10 Another is the reaction between an allyl bromide and an alkyne to generate a cyclopentenone derivative (eq 14).18
This reaction is not restricted to allyl bromide itself. A large variety of allyl bromide derivatives have been used, including cyclic derivatives which generate products containing either fused bicyclic or spirocyclic ring systems. In all cases the cyclization reaction gives the five-membered ring rather than the six-membered ring. The alkyne component does not have to be symmetrical. Acetylene itself, terminal alkynes, and unsymmetrical disubstituted alkynes work well. Although the exact mechanism of this reaction is not known, it has been proposed that initially an allyl-nickel complex is formed, followed by alkyne insertion, then carbonyl insertion, and finally ring closure to produce the five-membered ring.19
In the absence of any added allyl bromide, an alkyne and nickel carbonyl will react in methanol (excess HCl also is added to this reaction to increase the rate) to give an a,b-unsaturated ester in good yield (eq 15).20
In addition, it is possible to carbonylate alkenes. This reaction proceeds at a faster rate when the double bond is incorporated into a strained ring system, but almost any alkene will work. 1,5-Hexadiene can be carbonylated to give a cis/trans mixture of 2,5-dimethylcyclopentanone, as the major product, in good yield (eq 16). Some of the methylcyclohexanone also is generated.21
A substituted dibromocyclopropane, but not the corresponding monobromocyclopropane, can be carbonylated with nickel carbonyl using DMF as the solvent to generate an ester, when the reaction is done in the presence of an alcohol, or an amide, when the reaction is done in the presence of a secondary amine. A large variety of R groups are compatible with these reaction conditions.22 Where applicable, a cis/trans mixture of products usually results (eq 17).
If one of the R groups is H and the other is CH2Cl, ring opening of the cyclopropane occurs concomitantly with the carbonylation, again to generate either the ester or the amide (eq 18).23
Allan R. Pinhas
University of Cincinnati, OH, USA