Nickel complexes conjugate addition

Enal-derived nickel-Jt-allyl complexes are efficient partners in cross-coupling reactions with alkenyl- and arylstannanes leading to products such as 39 (Scheme 23).P 1 As described for the aUylic ether derived complexes, the mechanism of the coupling process involves transmetalation followed by reductive elimination. The reaction is catal hic in nickel, unlike couplings of enals with allstoichiometric quantities of nickel. Many other variants of nickel-catalyzed conjugate additions may involve Ji-allyl complexes, but these are treated separately in a section on alkenes (Section 1.1.4.2) because the mechanism is poorly defined in most nickel-catalyzed conjugate additions. 

Enantioselectivities were found to change sharply depending upon the reaction conditions including catalyst structure, reaction temperature, solvent, and additives. Some representative examples of such selectivity dependence are listed in Scheme 7.42. The thiol adduct was formed with 79% ee (81% yield) when the reaction was catalyzed by the J ,J -DBFOX/Ph aqua nickel(II) complex at room temperature in dichloromethane. Reactions using either the anhydrous complex or the aqua complex with MS 4 A gave a racemic adduct, however, indicating that the aqua complex should be more favored than the anhydrous complex in thiol conjugate additions. Slow addition of thiophenol to the dichloromethane solution of 3-crotonoyl-2-oxazolidinone was ineffective for enantioselectivity. Enantioselectivity was dramatically lowered and reversed to -17% ee in the reaction at -78 °C. A similar tendency was observed in the reactions in diethyl ether and THF. For example, a satisfactory enantioselectivity (80% ee) was observed in the reaction in THF at room temperature, while the selectivity almost disappeared (7% ee) at 0°C. 

With the success in Lewis acid-catalyzed thiol conjugate addition reactions mentioned above, we further tried to apply the J ,J -DBFOX/Ph-nickel(II) aqua complex catalyst to the catalyzed asymmetric conjugate addition reactions of hydroxyl-amines [88, 89]. However, after some preliminary examinations, we found that... 

Combination of nickel bromide (or nickel acetylacetonate) and A. A -dibutylnorephcdrinc catalyzed the enantioselective conjugate addition of dialkylzincs to a./Tunsaturated ketones to afford optically active //-substituted ketones in up to ca. 50% ee53. Use of the nickel(II) bipyridyl-chiral ligand complex in acetonitrile/toluenc as an in situ prepared catalyst system afforded the //-substituted ketones 2, from aryl-substituted enones 1, in up to 90% ee54. 

The addition of allcenes to alkenes can also be accomplished by bases as well as by the use of catalyst systems consisting of nickel complexes and alkylaluminum compounds (known as Ziegler catalysts), rhodium catalysts, and other transition metal catalysts, including iron. These and similar catalysts also catalyze the 1,4 addition of alkenes to conjugated dienes, for example. 

In order to obtain adiponitrile, 2 should isomerize to 4, and not to the thermodynamically more stable 3 (stabilised by the energy of conjugation). The thermodynamic ratio is 2 3 4 = 20 78 1.6 [6], The isomerization of 2 to 4 happens to be favorably controlled by the kinetics of the reactions the reaction 2 to 4 reaches equilibrium, but the reaction 2 to 3 does not. Note that the nickel complex not only is responsible for the addition of HCN but that it is also capable of catalysing selectively the isomerisation. The final step is the addition of HCN to 4 to give 5, adiponitrile. 

Alkynes in combination with a catalytic amount of a nickel complex have been found to catalyse the conjugate addition of arylboron reagents to ,/3-unsaturated carbonyl compounds.238... 

Concerning the catalytic enantioselective conjugate addition reaction, conjugate addition of dialkylzinc to chalcone in the presence of a catalytic amount of the chiral nickel complex derived from norephedrine affords 3-substituted ketones with up to 90% ee (eq 16). ... 

Catalytic Enantioselective Conjugate Addition of Dialkylzincs to Enones. A chiral nickel complex modified with DBNE and an achiral ligand such as 2,2 -bipyridyl in acetonitrile/toluene is an highly enantioselective catalyst for the addition of dialkylzincs to enones. p-Substituted ketones with up to 90% ee are obtained (eq 23). The method is the first highly enantioselective catalytic conjugate addition of an oiganometallic reagent to an enone. 

Asymmetric conjugate addition of thiols to 3-crotonyl-2-oxazolidinone is effectively catalyzed by the aqua nickel(II) complex of (i ,l )-DBFOX/Ph to produce conjugate adducts in high chemical yields and with high enantioselectivity [125] (Sch. 27). 

In the first stage Lewis acids are absent and further hydroeyanation of the monoolefm products 3-PN 40 and 2M3BN 41 does not readily oeeur. The monoeyanation of butadiene is similar to HCN addition to olefins. An individual feature of hydrocyanation of conjugated dienes is the intermediate appearance of TT-allylic complexes 43, which participate in the successive carbon-carbon coupling. Equations (12) and (13) demonstrate the reaction of butadiene with the hydrido-nickel complex 42 leading to formation of nitrile 40 (a) and explain the generation of byproducts, i.e., the branched nitrile 41 via an alternative pathway (b) [68-70]. 

The asymmetric conjugate addition of diethylzinc with chalcone was also catalyzed by nickel and cobalt complex (Eq. (12.31)) [71]. A catalytic process was achieved by using a combination of 17 mol% of an aminoalcohol 34 and nickel acetylacetonate in the reaction of diethylzinc and chalcone to provide the product in 90% ee [72, 73]. Proline-derived chiral diamine 35 was also effective, giving 82% ee [74]. Camphor-derived tridentate aminoalcohol 36 also catalyzes the conjugate addition reaction of diethylzinc in the presence of nickel acetylacetonate to afford the product in 83% ee [75]. Similarly, the ligand 37-cobalt acetylacetonate complex catalyzes the reaction to afford the product in 83% ee [76]. 

HCN also adds to alkenes in the presence of an appropriate catalyst (Arthur et al., 1954 Jackson and Level, 1982). Thus, cobalt carbonyl leads to Markownikov addition, for example, 1-propene yields isopropyl cyanide in approximately 75% yield. HCN adds to alkynes in the presence of metal complexes, and the use of a nickel complex may lead to syn addition (Jackson and Level, 1983 Jackson et al., 1988). Hydrogen cyanide reacts with conjugated dienes, the mechanism involving a TT-allyl intermediate. The course of addition is complex and may lead to more than one product (Keim et al., 1982). 

Nickel complexes, prepared by the combination of Ni(acac)2 with a chiral P-ami-no alcohol [24,25] or a P-hydroxysulfoximine [26], catalyze the conjugate addition of diethylzinc to chalcone. Some asymmetric ampHfications have been observed. 

The camphor-derived tridentate amino alcohol 32 (Scheme 17) also catalyzes the conjugate addition reaction of diethylzinc in the presence of nickel acetylac-etonate to afford the product in 83% ee [75]. Similarly the Hgand 33-cobalt acety-lacetonate complex catalyzes the reaction to afford the product in 83% ee [76]. 

OXIMES AND SEMICARBAZONES Ceric ammonium nitrate. CLEMMENSEN REDUCTION Zinc dust. COMPLEX OF BENZYNE. Nickel carbonyl. n-COMPLEXES Diiron nonacarbonyl. CONDENSING AGENT Calcium carbide. CONJUGATE ADDITION Dimethylcop-perlithium. Nickel carbonyl. Tetrakisjiodo-(tri-ff-butylphosphine)copper(I)]. CONVERSION OF PHENOLS TO ARYL BROMIDES Triphenylphosphine dibromide. 

Conjugate addition of acyl groups to a,fl-unsaturated carbonyl compounds.3 Nickel carbonyl forms unstable complexes with organolithium compounds (ether,... 

Nickel complexes also exhibit high catalytic activity toward the hydrosiiyiation of conjugated dienes under mild conditions. The reaction usually occurs by 1,4-addition, but the regio- and stereoselectivity is low compared with that achieved by palladium catalysts. 

Various other examples of nickel-catalyzed asymmetric addition of organozinc compounds to enones have been reported using different chiral auxiliaries111-117. Complexes of other transition metals, such as rhodium118-119, lanthanum120, and titanium121, have also been found to promote asymmetric conjugate addition of enolates to a,/i-unsaturated ketones. 

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