Nickel reactions catalyzed

A disadvantage of the nickel-catalyzed reaction is that the alkylation is primarily limited to methylation and that it often gives products with low stereo- and regioselectivity. 

Stable enolates such as diethyl malonate anions react with allyl sulfones (or acetates) in the presence of nickel complexes to give a mixture of the a- and /-product83. The regioselectivity is generally poor in the nickel-catalyzed reaction, but the molybdenum-catalyzed reaction is selective for alkylation at the more substituted allylic site, thereby creating a quaternary carbon center84. 

The regiochemistry of Al-H addition to unsymmetrically substituted alkynes can be significantly altered by the presence of a catalyst. This was first shown by Eisch and Foxton in the nickel-catalyzed hydroalumination of several disubstituted acetylenes [26, 32]. For example, the product of the uncatalyzed reaction of 1-phenyl-propyne (75) with BujAlH was exclusively ds-[3-methylstyrene (76). Quenching the intermediate organoaluminum compounds with DjO revealed a regioselectivity of 82 18. In the nickel-catalyzed reaction, cis-P-methylstyrene was also the major product (66%), but it was accompanied by 22% of n-propylbenzene (78) and 6% of (E,E)-2,3-dimethyl-l,4-diphenyl-l,3-butadiene (77). The selectivity of Al-H addition was again studied by deuterolytic workup a ratio of 76a 76b = 56 44 was found in this case. Hydroalumination of other unsymmetrical alkynes also showed a decrease in the regioselectivity in the presence of a nickel catalyst (Scheme 2-22). 

Rieke Ni-promoted (Scheme 11) and cat. NiCk/Mg/MesSiCl-mediated pinacol coupling have been investigated [38]. The similar nickel-catalyzed reaction was also reported later [39]. 

In the last decade, a new aspect of nickel-catalyzed reactions has been disclosed, where nickel serves to selectively activate dienes as either an al-lyl anion species or a homoallyl anion species (Scheme 1). These anionic species are very important reactive intermediates for the construction of desired molecules. Traditionally they have been prepared in a stoichiometric manner from the corresponding halides and typical metals, e.g., Li, Mg. In this context, the catalytic generation method of allyl anions and homoallyl anions disclosed here might greatly contribute to synthetic organic chemistry and organotransition metal chemistry. 

The disilanickela complex 21 was also found to be a good catalyst for the dehydrogenative double silylation of aldehydes. The nickel-catalyzed reactions of 1,2-bis(dimethylsilyl)carborane 11 with aldehydes such as isobutyraldehyde, trimethylacetaldehyde, hexanal, and benzaldehyde afforded 5,6-carboranylene-2-oxa-l,4-disilacyclohexane.32 34 36 The dehydrogenative 1,4-double silylation of methacrolein and tram-4-phenyl-3-buten-2-one in the presence of a catalytic amount ofNi(PEt3)4 also took place under similar conditions. In contrast, the reaction of 11 with a-methyl-tran.s-cinnamaldehyde and irans-cinnamaldehyde under... 

Scheme 6/4.18. Nickel-catalyzed reaction of oxabenzonornadiene with alkynes.

Novel transition metal-mediated strategies were also well represented this past year. Takahashi and co-workers reported a s nickel-catalyzed reaction between azaziconacyclopentadienes (9) and alkynes to form pyridines (10) of varying substitution patterns <00JA4994>. This methodology, a formal cyclotrimerization, is also noteworthy since two different alkynes can be used. In similar fashion, Eaton reported an aqueous, cobalt(II) catalyzed cyclotrimerization between two identical acetylenes and one nitrile to afford substituted pyridines . 

A nickel-catalyzed reaction of silylpropargyl alcohols 80 and Grignard reagents 81 was reported in 1985 [87]. The reaction proceeded in the presence of NiCl2(dppp) (10 mol%) and the silylallenes 82 were obtained in excellent yields (Scheme 3.41). 

Unlike thermal homo Diels-Alder reactions in which endo adducts predominate330, the nickel catalyzed reactions of acyclic electron-deficient dienophiles afford the exo isomers as the major cycloadducts. This has been explained by unfavorable steric interactions within intermediate 559 leading to the endo adduct. Cyclic dienophiles, on the contrary, give predominantly the endo isomer, which has again been explained by unfavorable steric interactions within exo 559. The preferred conformation of the dienophile, s-cis or s-trans, has also been suggested to play a role328. 

Stereoselective) additions of nucleophiles to 5-alkylidene Meldmm s acid as displayed in Scheme 17 (Section 8.11.6.1.3) <2006TA2957, 2007AGE4964> and to the carbonyl group of 2,2-dimethyl-l,3-dioxan-5-one (Scheme 47, Section 8.11.6.3.3) either in a three component transformation <2006OL3689> or in a nickel-catalyzed reaction... 

Tamao and Ito have reported a nickel-catalyzed protocol for the cyclization/hydrosilylation of 1,7-diynes to form (Z)-silylated dialkylidene cyclohexane derivatives.For example, reaction of 1,7-octadiyne with triethoxysilane catalyzed by a mixture of Ni(acac)2 (lmol%) and DIBAL-H (2mol%) in benzene at 50°G for 6h gave the corresponding silylated dialkylidene cyclohexane in 70% yield as a single isomer (Table 1). The reaction of 1,7-octadiyne was also realized with mono- and dialkoxysilanes, trialkylsilanes, and dialkylaminosilanes (Table 1). Diynes that possessed an internal alkyne also underwent nickel-catalyzed reaction, albeit with diminished efficiency (Table 1), while 1,6- and 1,8-diynes failed to undergo nickel-catalyzed cyclization/hydrosilylation. 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

General Comments. The formation of deoxy sugars by hydrogenation over Raney nickel often leads to the abnormal isomer (namely, that formed by diequatorial opening of the oxirane ring) as the major product, in contrast to the product afforded by lithium aluminum hydride this suggests that a different mechanism is involved in the nickel-catalyzed reaction. 

These palladium- or nickel-catalyzed reactions are radical reactions leading to an organometallic product. By using a precursor such as 37 as a 1 1 mixture of diastereoisomers, the palladium-catalyzed cyclization provides in a stereoconvergent way the cyclopentylmethylzinc derivative 38 which, after allylation, produces the unsaturated ester 39 in 71% yield". The intermediate radical cyclizes via a transition state A where all the substituents are in an equatorial position. Interestingly, the analogous reaction using Ni(acac)2 as a catalyst allows the preparation of heterocyclic compounds such as 40. The... 

Aldehydes constitute useful electrophilic partners in such nickel-catalyzed reactions because the condensation between alkynes, aldehydes and diorganozinc compounds can afford stereodefined cyclic or acyclic ally lie alcohols67-69, as illustrated by the stereoselective cyclization of 111 to the corresponding 3-hydroxypyrrolidine (equation 46). Allenes or 1,3-dienes instead of alkynes also lead to similar reactivity70. 

The nickel catalyzed reaction of 9 with an allyl chloride also proceeds in an Sn2 manner Eq. (42). Nickel dichloride complexed with a bidentate ligand (e.g. l,2-bis(diphenylphosphino)ethane) effects a highly selective (>99%) SN2 reaction with cinnamyl chloride [29]. 

Formally copper catalyzed couplings are analogous to palladium and nickel catalyzed reactions. Carbon-carbon and carbon-heteroatom bonds can be formed in such transformations alike. From the mechanistic point of view there is a significant difference between nickel, palladium and copper catalyzed processes however. While in the former cases the catalyst usually oscillates between the 0 and +2 oxidation states, in copper mediated transformations the common oxidation numbers are +1, +2 and +3. 

Formally, although these cyclooligomerizations can be considered as cycloaddition reactions, they are known not to occur through a direct cycloaddition process. In the best understood nickel-catalyzed reaction the dimer 12 was shown to be the key intermediate in cyclization.46,59 60 Acetylene readily undergoes cyclooligomerization in the presence of metal catalysts to form benzene and cyclooctatetraene [Eq. (13. 15)] as well as higher homologs ... 

The nickel-catalyzed reaction often proceeds in high yield under mild conditions and appears to be a useful synthetic procedure. Examples of the reactions are given in Table I. 

The stereochemistry of palladium-catalyzed hydrocyanation has been studied further using [Pd(DIOP)2] (133) as catalyst.607 It was shown that the addition of HCN to both cyclic and acyclic alkenes is cis. The mechanism is believed to be the same as for the nickel-catalyzed reaction (Scheme 58). 

Before entering into a detailed discussion of the nickel-catalyzed reactions of olefins, the convention we will use to describe the stereochemistry of the intermediate 77-allyl complexes should be mentioned. As an example, consider the bis(Tr-allyl) C8 chain postulated as an intermediate in the... 

Pyran-2-ones are accessed in good yield by a nickel-catalyzed reaction of internal alkynes with carbon dioxide under solvent free conditions (Equation 263) <2005SL2141>. 

Trialkylaluminums have shown the capability of entering into nickel-catalyzed reactions with allenic bromides, to afford direct alkylation at vinyl carbon. Inversion of configuration occurs at this carbon to a degree which is greater than with other alkylmetals (magnesium, zinc) (see equation 6O)90. 

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