Nickel catalysis addition

For this specific task, ionic liquids containing allcylaluminiums proved unsuitable, due to their strong isomerization activity [102]. Since, mechanistically, only the linkage of two 1-butene molecules can give rise to the formation of linear octenes, isomerization activity in the solvent inhibits the formation of the desired product. Therefore, slightly acidic chloroaluminate melts that would enable selective nickel catalysis without the addition of alkylaluminiums were developed [104]. It was found that an acidic chloroaluminate ionic liquid buffered with small amounts of weak organic bases provided a solvent that allowed a selective, biphasic reaction with [(H-COD)Ni(hfacac)]. 

Several reports have been made of a successful catalyzed addition/ substitution reaction resulting in direct attachment of phosphorus to aromatic rings. The preparation of mixed triarylphosphines has been accomplished by the reaction of tin- or silicon-substituted diphe-nylphosphines with aryl halides catalyzed by palladium reagents.74 A similar transformation has also been reported using nickel catalysis.75 The addition/substitution of diphenylphosphine to triflate functionalized phenolic linkages has been of use for the preparation of substances as analogues of tyrosine-related amino acid derivatives, accomplished with catalysis by palladium acetate (Equation 4.29).76... 

In an extension of an early work on the nickel-catalyzed addition of hydrogen cyanide to unsaturated compounds, a basic reaction in various large-scale processes in the polymer industry, the hydrocyanation of butadiene (equation 15) and the efficiency of catalysis of this reaction by low-cost copper salts has been studied extensively by Belgium researchers47,48. 

Multiple-component difunctionalization reactions of a,/ -unsaturated carbonyl systems have been achieved by catalytic conjugate addition/aldol sequences. As Scheme 8.13 illustrates, an efficient method reported by Montgomery [46] allows regioselective addition of an aryl iodide to the /i-position of an unsaturated ester under nickel catalysis and subsequent trapping with an aldehyde to give / -hydroxyesters (e.g. 33). Significantly, premature termination of the sequence by the /Miydride elimination process that is usually observed in Pd-catalyzed Heck reactions does not occur here. 

Nickel catalysis is a very active field in organometallic and organic chemistry (selected reviews [3-7]). Complexes of all oxidation states are active in two-electron transfer processes, such as oxidative addition or reductive elimination as well as in single electron transfer initiating radical reactions. Through these processes, oxidation states from Ni(0) to Ni(III) can be easily accessed under mild conditions. Occasionally, Ni(IV) intermediates were also proposed. Apart from the vast number of Ni(II) complexes, a number of organonickel(I) complexes were characterized by X-ray crystallography and their potency as active species in catalytic cycles tested [8-10]. Either radical or two-electron reactivity was observed. Recently, the structure of some alkylnickel(III) complexes was also structurally elucidated [11]. 

A remarkable product selectivity is also observed in the case of methylenecyclopropanes with geminal diphenyl substitution. Whereas under nickel catalysis [Ni(cod)2 at SO-TO C] 18 is selectively dimerized to tra7w-l,l,6,6-tetraphenyldispiro[2.1.2.1]octane, which can be obtained in 28% yield at a conversion of 40%, substrate 18 reacts in a completely different manner with palladium(O) catalysts derived from (t/ -allyl)( -cyclopentadienyl)palladium(II) and triisopropylphosphane. Besides isomerization to 19, proceeding at temperatures above 85 °C, the monospiro derivative 20 is formed as the major product. Additionally, minor amounts of a formal [3 + 3] dimer 21 can be isolated. The latter probably arises from a palladium-mediated, stoichiometric reaction as the yield of 21 could not be improved under any conditions in catalytic runs. On prolonged heating, thermal isomerization of the methylenecyclopropanes to form... 

Ni(COD)2/DPEphos was demonstrated to catalyse polyfluoroaiylcyanation of norbornene and norbornadiene in the presence of BPhs (Scheme 14.47). Ni(COD)2/PMc2Ph catalyses the reaction of 1,2-dienes with ethyl cyano-formate affording kinetically favoured p-cyano-a-methylenealkanoates regioselectively without the requirement of a Lewis-acid additive. In this reaction, thermodynamically favoured a-cyanomethyl-a,p-unsaturated carb-oxylates were also obtained as byproducts (Scheme 14.47), which are formed via isomerisation of p-cyano-a-methylenealkanoates at high temperatures under the nickel catalysis. ... 

In addition to simple hydroamination processes, nickel-catalyzed alkene diaminations have been developed under oxidative conditions (Scheme 3-110). Treatment of sulfamide substrates with PhI(OAc)2 in the presence of Ni(acac)2 as catalyst directly provides diamination products. This key report suggests that further related developments in oxidative nickel catalysis are likely. 

The same group reported an extension of the direct alkylation of (benz)oxazoles with various alkyl bromides and chlorides by using the stronger base lithium tert-butoxide (Scheme 19.23) [38]. 5-Aryloxazoles containing electronically diverse substituents such as CFj and OMe were also alkylated successfully. Various linear alkyl chains were introduced, such as phenylpropyl, citronellyl, or octyl, affording interesting lipophilic molecules. The optimized reaction conditions failed to apply to benzothiazoles, and the authors had to turn their attention to nickel catalysis to achieve the corresponding alkylation (Section 19.2.3). Experiments were run to understand the reaction mechanism that presumably involves Sj 2-type oxidative addition of the alkyl halide to palladium(O) followed by transmetallation by the in situ-lithiated (benz)oxazole (Scheme 19.24). 

In addition to benzyl zincates, thymidine derivatives bearing carbonyl functionalities were successfully employed in this reaction. However, according to a recent report, nickel catalysis might be superior to palladium catalysis for such coupling reactions. ... 

In summary, the introduction of versatile classes of Af-heterocyclic carbene ligands has transformed the field of nickel catalysis [108]. Their use has increased the scope and selectivity of many classes of transformations, while often making the processes proceed more efficiently at lower temperatures and with lower catalyst loadings. Additionally, new reactions have been developed that are... 

Transmetallation of the organic group from zirconium to another metal opens up possibilities. The palladium-catalysed coupling reactions can be found in Section 2.4. Addition of dimethyl cuprate results In transmetallation to copper. The resulting cuprate then displays typical cuprate reactivity, such as addition to enones. More economically, small amounts of copper can catalytically activate the zirconium complex towards this kind of chemistry, although the precise mechanism is unclear. Additions to enones can also be achieved directly using nickel catalysis (Scheme 5.64). Transmetallation to zinc has also been demonstrated. ... 

Even cyclopropanes lacking a methylene or vinyl group can be involved in formal cycloadditions. Cyclopropyl ketones 11.137 undergo cycloaddition to enones under nickel catalysis with an NHC ligand (Scheme 11.47) in the absence of the enone, their dimerization is observed. The reaction is proposed to proceed via a metallacyclic enolate complex 11.140, perhaps after initial oxidative addition to one of the cyclopropyl C-C bonds. 

On the other hand, Matsunaga and Shibasaki have observed an opposite syn diastereoselectivity in the enantioselective conjugate addition of a-keto anilides to nitroalkenes under dinuclear nickel catalysis. Indeed, the use of 10 mol% of dinuclear chiral Schiff base nickel catalyst 9 in the presence of HFIP and 5 A MS as additives in 1,4-dioxane as solvent allowed the corresponding Michael adducts to be achieved in moderate to good yields, with good syn diastereoselectivities of up to >90% de and combined with good to excellent enantioselectivities of up to 98% ee (Scheme 2.15). The substrate... 

---