Nickel complexes, reaction with pyridines

The most important by-product formed in the reaction of pyridine with degassed Raney nickel is an organonickel complex which has been shown to be a complex of one molecule of 2,2 -bipyridine, two molecules of 2,2 -pyrrolylpyridine (17), and one nickel II ion. It is significant that, although the formation of 2,2 -bipyridine ceases after 50 hr refluxing, the formation of this complex continues for at least another 140 hr. 

Pyridine compounds 45 can also be produced by the NHC-Ni catalysed cycloaddition between nitriles 43 and diynes 44 (Scheme 5.13) [16]. The SIPr carbene was found to be the best ligand for the nickel complex in this reaction. The reaction required mild reaction conditions and low catalyst loadings, as in the case of cycloaddition of carbon dioxide. In addition to tethered aUcynes (i.e. diynes), pyridines were prepared from a 3-component coupling reaction with 43 and 3-hexyne 23 (Scheme 5.13). The reaction of diynes 44 and nitriles 43 was also catalysed by a combination of [Ni(COD)J, NHC salts and "BuLi, which generates the NHC-Ni catalyst in situ. The pyridines 45 were obtained with comparable... 

Reduction of both nickel porphyrins and thiaporphyrins to Ni1 species has been studied by EPR and 2H NMR spectroscopy.179, 2 58 The Ni1 complex of 5,10,15,20-tetraphenyl-21-thiaporphyrin has been isolated and characterized. Reaction of this complex with sulfur dioxide produced a paramagnetic five-coordinated Ni1 S02 adduct, while reaction with nitrogenous base ligands (amines, pyridines, imidazoles) yielded five- and six-coordinate complexes. In addition, the crystal structure of Ni1 diphenyldi-p-tolyl-21-thiaporphyrin has been determined. The coordination geometry about the nickel center is essentially square planar with extremely short Ni—N and Ni—S bonds (Ni—N = 2.015(2) A, 2.014(12) A, and 1.910(14) A and Ni—S = 2.143(6) A).2359... 

Most studies on nickel-catalyzed domino reactions have been performed by Ikeda and colleagues [287], who observed that alkenyl nickel species, obtained from alkynes 6/4-41 and a (jr-allyl) nickel complex, can react with organometallics as 6/4-42. If this reaction is carried out in the presence of enones 6/4-43 and TM SCI, then coupling products such as 6/4-44 are obtained. After hydrolysis, substituted ketones 6/4-45 are obtained (Scheme 6/4.12). With cyclic and (5-substituted enones the use of pyridine is essential. Usually, the regioselectivity and stereoselectivity of the reactions is very high. On occasion, alkenes can be used instead of alkynes, though this is rather restricted as only norbornene gave reasonable results [288]. 

Recent developments are ring-cleavage reactions of the heterocycles [R2M-SbR2]w with 4-(dimethylamino)pyridine leading to base-stabilized monomers, [L — R M-SbR, (R = Me, SiMe3 R = Me, Et, i-Bu M = A1, Ga).83,84 Reaction of [L-Al(Me2)-Sb(SiMe3)2] [L = 4-(dimethylamino)pyr-idine] with [Ni(CO)4] leads to the corresponding tricarbonyl nickel complex (Equation 4).85... 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Reaction of bis(l,2-diaminoethane)nickel(II) perchlorate with acetone allows the isolation of the P-amino ketone complex (42), which can be converted in pyridine to the trans macrocyclic complex (41). A similar reaction occurs with methyl ethyl ketone (Scheme 7).85... 

These coordinatively unsaturated complexes can undergo coordination reactions with compounds having a free electron-pair donor group, particularly pyridine, amines, ammonia, and water. Coordination is associated with a marked hyp-sochromic color change. For example, the unsaturated nickel complex36 is green and its pyridine adduct 37 is violet [74],... 

The equilibrium constant for the monomer trimer reaction of bis[(2,6-dimethyl-heptane-3,5-dione)nickel(ii), in benzene at 30°C, is 2 x 10312 mol 2. Adduct formation with pyridine to form Ni2L4py and NiL2py2 was also examined.518 The reactions between Lewis bases and the square-planar bis-(2,2,6,6-tetramethylheptane-3,5-dione)nickel(n) to form complexes were examined in solution and equilibrium constants and enthalpies determined.519... 

The solution to this synthesis problem goes back to the work of Reiner Sust-mann, who successfully carried out the formal, nickel(0)-mediated addition of alkyl or aryd halides to electron-deficient alkenes, like acrylate esters or acrylonitrile. [101] Sub-stoichiometric amounts of nickel(ll) chloride hexahydrate (15-20 mole %) are reduced with zinc in presence of pyridine to nickel(O), which forms together with pyridine a complex with the alkene. Oxidative addition of the alkyl halide leads to an alkyl-nickel species, the carbon-metal bond of which undergoes an alkene insertion. Hydrolysis gives ultimately the product. Heck reaction products are not observed. 

Similar to the iron chemistry (compare Chapter 2.3), also nickel complexes allow the reaction of one molecule of butadiene with two molecules of CO2 yielding a,u-dicarboxylic acids [48]. In the reaction of butadiene and CO2 in the presence of nickelbis(cyclooctadiene) and tetramethylethylenediamine first a nickelamonocarboxylate is formed (Figure 19). By further treatment with carbon dioxide and by addition of pyridine a nickeladicarboxylate complex is obtained in yields up to 72 %. Decomposition of the complex with methanol/hydrochloric acid gives cis-dimethyl-3-hexenedioate. 

In 1975, three different protocols were available in the literature, each describing the synthesis of internal alkynes. Cassar described palladium- or nickel-mediated reactions between aryl or vinyl halides and alkynes complexes with phosphine as ligands in the presence of NaOMe [1]. As a second protocol, Heck pubhshed a variation of the Mizoroki-Heck couplings, in which the olefins were replaced by alkynes and coupled with (hetero)aryl, as weU as alkenyl bromides or iodides at 100 °C in the presence of a basic amine [2]. More than a decade earUer, Stephens and Castro had described the details of a palladium-free coupling of aryl iodides with cuprous acetylides in refluxing pyridine [3]. 

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