Nickel complexes carbene

The reaction of alkenylcarbene complexes and alkynes in the presence of Ni(0) leads to cycloheptatriene derivatives in a process which can be considered as a [3C+2S+2S] cycloaddition reaction [125]. As shown in Scheme 77, two molecules of the alkyne and one molecule of the carbene complex are involved in the formation of the cycloheptatriene. This reaction is supposed to proceed through the initial formation of a nickel alkenylcarbene complex. A subsequent double regioselective alkyne insertion produces a new nickel carbene complex, which evolves by an intramolecular cycloprop anation reaction to form a nor-caradiene intermediate. These species easily isomerise to the observed cycloheptatriene derivatives (Scheme 77). 

The synthesis of this salt started with the enantiomerically pure 1,2-diamine 106, that was converted into the corresponding thiourea derivative 107 (Scheme 12). Exposure of the thiourea 107 to oxalyl chloride in toluene at 60 °C cleanly afforded the desired imidazolinium chloride 108. These two salts were used to produce new palladium and nickel carbene complexes. The structure of both palladium carbene complexes 96a and 96b has been elucidated by X-ray diffraction <2005CEJ1833>. 

Table 3.2. Cyclopropanation with stoichiometric amounts of cationic iron and nickel carbene complexes.

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

Nickel/carbene complexes have also been successfully employed in the Suzuki-Miyaura cross-coupling reaction. One of the first successful applications of this was demonstrated by Blakey and MacMillan, wherein boronic acids were coupled with aryltrimethylammonium salts [52]. It was found that the transformation could be accomplished using 10 mol % Ni(COD)2,... 

Because of the extraordinary strength of the carbon-fluorine bond, transition metal-mediated activation of fluoroalkanes and arenes is not easy to achieve. Nevertheless, activation of the C-F bond in highly electron-deficient compounds such as 2,4,6-trifluoropyrimidine, pentafluoropyridine, or hexafluorobenzene is possible with stoichiometric amounts of bis(triethylphosphano) nickel(O) [101] (Scheme 2.45). More recently Herrmann and coworkers [102] have described a variant of the Kumada-Corriu cross-coupling reaction [103] between fluorobenzene and aryl Grignard compounds which uses catalytic amounts of nickel carbene complexes. Hammett analysis of the relative kinetic rate constants indicated that the reaction proceeds via initial oxidative addition of the fluoroaromatic reactant to the nickel(O) species. 

Perfluorooxiranes in which a different perfluoroalkyl group substitutes for the CF3 of HFPO can also function as sources of difluorocarbene. In the presence of nickel powder, for example, oxirane 32 reacts with iodine to give difluorodiiodomethane (33) in high yield, accompanied by small amounts of oligomeric diiodides." Yields are very low in the absence of nickel, and it is suggested that the reaction occurs on the surface of the metal with a nucleophilic nickel-carbene complex. 

In our group nickel carbene complexes (Fig. 5.3-16) dissolved in slightly acidic, buffered chloroaluminate ionic liquids have been applied for the dimerization of propene and 1-butene [242]. 

Alkyl aiyl and diaiyl ethers also can be hydrogenated using a nickel-carbene complex as catalyst under 1 bar of hydrogen at temperatures of 80 to 120 °C (Scheme 14.29). The relative reactivity of ether substrates is Ar-OAr Ar-OMe > ArCHa-OMe. ... 

Diazoalkanes are decomposed by nickel carbonyl yielding nitrogen and reaction products indicative of the presence of carbenes as intermediates (25). Although carbenes usually show little tendency to combine with carbon monoxide, formation of ketenes was detected by decomposing the diazoalkanes in the presence of excess nickel carbonyl. This carbonylation of carbenes undoubtedly occurs via nickel-carbene complexes (25)... 

Catalytic carbon-halogen bond activation with nickel carbene complexes... 

Nickel complexes with carbene ligands will be discussed later in this section. However, several nickel carbene complexes also contain carbonyl ligands, and they will be considered here. The reaction of A-heterocyclic carbene (NHG) reagents with Ni(GO)4 typically results in the simple monosubstitution reaction to give Ni(GO)3(NHG). However, a 2,5-dimethyl-substituted NHG ligand combines with Ni(GO)4 to give the disubstituted dicarbonyl-dicarbene complex (Equation... 

While the advent of NHC ligands brought much new activity to the field of nickel carbene chemistry, important progress was also made with more traditional Fischer-type carbene complexes. The typical route to methoxy(amino) or bis(amino) Fischer-type carbene complexes is the nucleophilic attack of alcohols or amines on coordinated isocyanides. " A new and efficient route to heteroatom-stabilized carbene nickel(ii) complexes was recently reported to occur by a protonation reaction of the nickel(O) complex Ni(GNXyl)(triphos)." Addition of 2 equiv. of HBF4 to solution of Ni(CNXyl)(triphos) in THF affords the stable dicationic nickel carbene complex [Ni C(H)N(H)Xyl (triphos)]2+(BF4-)2 (Equation (16)). 

Other than the dinuclear iV-heterocyclic carbene Ni(ii) complex 16 described in the preceding section on mononuclear nickel carbene complexes, no further examples of dinuclear nickel complexes with carbenes were reported beyond those summarized in GOMG(1995). 

In 1982, Casey and co-workers reported the first reactions that could be considered hydrocarbations because they involved the direct C-H bond addition across the C-C double bond of alkenes. They showed that the cationic bridging iron methylidyne complex undergoes this type of reaction with alkenes with anti-Markovnikov regioselectivity. No other hydrocarbation reactions had been reported until recently, when Kubiak and co-workers reported hydrocarbation reactions of a nickel carbene complex with alkenes. Thus, the dicationic aminocarbene complex 31 reacts with ethylene, resulting in a complete conversion to the ethylcarbene complex (Scheme 1). 

The catalytic syntheses of seven-membered rings has long been an elusive goal, achieved only rarely and in poor yields.Significant advances have been made in the last decade through the use of nickel carbene complexes. Thus, using a diazoalkane carbene precursor in the presence of Ni(cod)2, an efficient [4 + 2+ 1]-cycloaddition to a seven-membered ring was reported. (Equation (23)). 

Isomerization of unactivated vinyl cyclopropanes to cyclopentanes using nickel carbene complexes has been accomplished. The nickel carbene catalyst was generated in situ from Ni(cod)2, PPBF4 salt and base. These reactions constitute a simple protocol for the preparation of cyclopentenes by the isomerization of vinyl cyclopropanes. This result, combined with recent developments in the preparation of vinyl cyclopropanes,may provide a powerful new approach to the preparation of five-membered ring structures. 

These novel mixed nickel carbene complexes were found to effect Kumada cross-couplings of -methoxyphenylmagnesium bromide with chloro- or bro-mobenzene, and 2-chloropyridine, but accompanied with significant amounts of 4,4 -dimethoxybiphenyl (15-25%). 

In the presence of a large excess of tetracarbonyl nickel and ethanol diphenyldiazo-methane, diazofiuorene, bis-(4-methoxyphenyl)diazomethane, and ethyl diazoacetate give carbonylation products trapped by ethanol and processed into the corresponding carboxylic acid in 74,38,26, and 8.5% isolated yields, respectively, which presumably arise from a nickel carbene carbonyl intermediate that releases a substituted ketene upon decomposition at 50-66 °C. In the absence of ethanol, by refluxing a solution of 1 mol diphenylketene with 6.4 mol tetracarbonyl nickel in diethylether 35% isolated yield of diphenylketene was observed [86]. The r -(C,C)-ketene complex of nickel (31) was isolated in 17% yield from the reaction of nickelacyclobutane (29) with carbon monoxide (3 bar) at 50 °C (reaction 8.54) [87]. Complex 29 is believed to be in equilibrium with the nickel-carbene-olefrn complex 30 [88]. The nickel-ketene complex 31 was also obtained either by direct reaction of Ni(PPh3)4 with ketene or by carbonylation of the nickel-carbene complexes presumably formed from the reaction of Ni(PPh3)4 and CH2Br2 in the presence of metallic zinc [89]. 

Miyashita, A. and Gmbbs, R.H. (1981) Reactions of nickel-carbene complexes generated from nickelacycle complexes. Tetrahedron Letters, 22, 1255—1256. 

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