Substitution Reactions with Nickel Carbonyl

The substitution of CO in metal carbonyls by olefinic and acetylenic compounds is one of the chief methods for preparing tt complexes of transition metals. Unfortunately this procedure fails almost completely when applied to nickel carbonyl, and this may be one of the reasons why until recently no tt complexes of nickel with olefinic or acetylenic ligands were known. The reasons for this behavior of nickel carbonyl will become clearer, if both its electronic structure and the mechanism of the ligand exchange reactions are considered. 

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As discussed in Section I, the reaction of ally lie halides with nickel carbonyl at atmospheric pressure leads to coupling products or in some cases, in hydroxylic solvents, to substitutive hydrogenation (34). Under... 

The role of the solvent in practically all of the reactions so far discussed is decisive. For example, allylic halides having electron-attracting substituents, such as methyl bromocrotonate, upon treatment with nickel carbonyl in hydroxylic solvents do not react with CO. Instead substitutive hydrogenation of the halogenated carbon atom occurs (55), while in ketonic solvents the products which might be expected from the carbonylation of normal allylic halides are obtained (50). 

This scheme implies addition reactions followed by substitution reactions, but the intermediate stages have not been identified. The NO2 substitution reactions are no doubt similar in principle to, but rather more complicated than, the reaction between nickel carbonyl and nitrogen dioxide (see below). The structure of the product, and the bonding of the NO2 groups to the metal, are discussed in connection with the nickel compound, which has been more fully investigated. 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

Reaction of l,3-bis(phenylmethyl)imidazol-2-ylidene with nickel tetra carbonyl gives [(t (C)-1,3-bis(phenylmethyl)imidazol-2-ylidene)Ni(CO)3] (970M2472). Complexes of composition [Ni(CO)2L2] with imidazol-2-ylidenes are also known (93JOM(459)177). Another species to be mentioned in this respect is bis(l,3-dimesitylimidazol-2-ylidene)nickel(0) (94JA4391). 1,3-Dicyclohexylimidazol-2-yUdene substitutes triphenylphosphine or THF from [NiX LJ (X = Cl, Br L PPhj, THF) to yield the stable nickel(II) complexes 69 (X = C1, Br R = Cy) (97OM2209). Another preparation of nickel(II) derivatives is the interaction of... 

It has been pointed out that the types of solvents which are used here, are not generally such as would enter into strong association with the substrate. The molecularity of the substitution reaction may then stand more chance of being an operational concept. Amongst the binary carbonyls, the only systems which have been extensively studied have been nickel tetracarbonyl and the hexacarbonyls of group VI. For the former, the observation of a first-order rate is at least consistent with a rate-determining dissociation of one carbonyl ligand followed by reaction of the intermediate with whichever nucleophile should be available. 

A nickel-catalysed alkyne insertion between the carbonyl carbon and the -carbon of the cyclobutanone was achieved by combining a ketone-alkyne coupling reaction with a /3-carbon elimination process (Scheme 79).121 The reaction uses cyclobutanones as a four-carbon unit and provides access to substituted cyclohexenones. 

Since 1948-50, by using as reactants isonitriles, phosphorus trihalides, and tertiary phosphines, we have gained important insight into the dependence of reactions of metal carbonyls with bases upon the nature of the ligand. Organophosphines were introduced into carbonyl chemistry even prior to 1948 by Reppe and Schweckendiek (7). In general, these ligands react only by substitution of CO, and do not cause disproportionation. Thus nickel carbonyl frequently reacts with complete displacement of carbon monoxide, as we were first able to demonstrate in the reaction with phenyl isonitrile (8). 

Copper(II) polyamine complexes are substitutionally labile, in a manner similar to the corresponding nickel(II) complexes. This means that individual donor atoms may at times decoordinate and thus be available for derivatization reactions, e.g., with suitable carbonyl compounds. More complex ligands may thus be constructed, including macrocycles (35), polymacrocycles (36 38), and concave chelators (35). The copper(II) complex of 1 was synthesized as a starting material for reactions aiming at the derivatization of the pentaamine ligand (24-28). 

Two other Ni(CO)4 substitutes, Ni(CO)3PPh3 and Ni(COD)2/dppe, prove to be appropriate for the catalysis of tandem metallo-ene/carbonylation reactions of allylic iodides (Scheme 7)399. This process features initial oxidative addition to the alkyl iodide, followed by a metallo-ene reaction with an appropriately substituted double or triple bond, affording an alkyl or vinyl nickel species. This organonickel species may then either alkoxycar-bonylate or carbonylate and undergo a second cyclization on the pendant alkene to give 51, which then alkoxycarbonylates. The choice of nickel catalyst and use of diene versus enyne influences whether mono- or biscyclization predominates (equations 200 and 201). 

The reduction of 3,4-dichlorocyclobutene (222) in the presence of metal carbonyls has been utilized to prepare the parent complex [223, MLn = Cr(CO)4, Mo(CO)3, W(CO)3, Fe(CO)3, Ru(CO)3 orCo2(CO)6] (equation 32) .Morerecently, reaction ofNi(CO)4 with 3,4-dihalocyclobutenes (X = Br or I) or with 222 in the presence of AICI3 produced the corresponding (cyclobutadiene)nickel dihalides . Methodology for the preparation of 1,2- or 1,3-disubstituted (cyclobutadiene)Fe(CO)3 complexes from 1,2- or 1,3-disubsli-tuted-3,4-dibromocyclobutenes has been presented - In turn, the substituted dibromo-cyclobutenes are prepared from squaric esters. The reaction of cz5-3,4-carbonyldioxycy-clobutene and substituted variants with l c2(CO)9 orNa2Fe(CO)4 also produces (cyclobu-tadiene)Fe(CO)3 complexes . Photolysis of a-pyrone generates 3-oxo-2-oxabicyclo [2.2.0]hex-5-ene (224) which undergoes photolysis with a variety of metal carbonyls to afford the parent cyclobutadiene complex 223 [MLn = CpV(CO)2, Fe(CO)3, CoCp. or RhCp] (equation 33) 2 0. 

Nickel(II) phosphine complexes have been reported to he efficient catalysts in carbonylation reactions. To investigate this reaction mechanism, we have studied the reaction of CO on the related Ni(II) complexes NiX2(PMes)n (n = 2,3) and [NiX(PMes)m]BFj> (m = 3,4). Pentacoordinate carbonyl nickel(II) species (without reduction of Ni(II) to Ni(0)) were isolated (1) by direct substitution of PMcs by CO in the pentacoordinate complex and (2) by addition of CO on the trans square-planar tetracoordinate complex. These compounds are trigonal-bipyramidal complexes with CO in equatorial position. The Ni-CO distance (1.73 A) is the shortest reported Ni-CO distance. Since these carbonylation reactions can be viewed as substitution of an equatorial PMes by CO in a TBP, they can be related to the substitution reactions in square-planar d metal complexes. 

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