Dichlorobis(triphenylphosphine)nickel(II)1

NiCl2(PPh3)2 (R1 = Ph, R2 = Ph)

[14264-16-5]  · C36H30Cl2NiP2  · Dichlorobis(triphenylphosphine)nickel(II)  · (MW 654.18) NiCl2(PEt3)2 (R1 = Et, R2 = Et)

[17523-24-9]  · C12H30Cl2NiP2  · Dichlorobis(triethylphosphine)nickel(II)  · (MW 365.91) NiCl2(PBu3)2 (R1 = Bu, R2 = Bu)

[15274-43-8]  · C24H54Cl2NiP2  · Dichlorobis(tributylphosphine)nickel(II)  · (MW 534.23) NiCl2(dppe) (R1 = Ph, R2,R2 = -(CH2)2-)

[14647-23-5]  · C26H24Cl2NiP2  · Dichloro[1,2-bis(diphenylphosphino)ethane]nickel(II)  · (MW 528.02) NiCl2(dppp) (R1 = Ph, R2,R2 = -(CH2)3-)

[15629-92-2]  · C27H26Cl2NiP2  · Dichloro[1,3-bis(diphenylphosphino)propane]nickel(II)  · (MW 542.05) NiCl2(dppb) (R1 = Ph, R2,R2 = -(CH2)4-)

[18750-53-3]  · C28H28Cl2NiP2  · Dichloro[1,4-bis(diphenylphosphino)butane]nickel(II)  · (MW 556.07) NiCl2(dppf) (R1 = Ph, R2,R2 = -(C5H4)Fe(C5H4)-)

[67292-34-6]  · C34H28Cl2FeNiP2  · Dichloro[1,1-bis(diphenylphosphino)ferrocene]nickel(II)  · (MW 683.99)

(catalysts for the cross-coupling reactions of various RX with Grignard reagents,1-34 C-X bond reduction,35,36 homocoupling of Csp2 halides,37-40 displacement of aryl halides,41 and oligomerization of dienes47)

Physical Data: NiCl2(PPh3)2: dark green; mp 205-206 °C. NiCl2(PEt3)2: dark red; mp 112-113 °C. NiCl2(PBu3)2: red; mp 48-49 °C. NiCl2(dppe): orange; mp 263-265 °C. NiCl2(dppp): red; mp 213 °C (dec). NiCl2(dppb): light violet; mp 270-272 °C. NiCl2(dppf): dark green; mp 283-284 °C.

Solubility: sol benzene, acetone, THF, hot alcohol.

Preparative Methods: most nickel chloride phosphine complexes are commercially available. They can also be conveniently synthesized by mixing the stoichiometric amount of NiCl2 with the appropriate phosphine ligand.48

Handling, Storage, and Precautions: nickel chloride phosphine complexes are corrosive and cancer suspect agents. Most of these complexes are air stable in solid form. Handle in a fume hood.

Cross-Coupling Reactions.

Transition metal catalyzed cross-coupling reactions of R-X with organometallic nucleophiles are well documented.1 R can be an aryl, vinyl, or allyl group, and X can be any leaving group ranging from the more reactive triflates and halides to less labile thioalkoxides, sulfoxides, sulfones, hydroxides, alkoxides, and even amido groups. Aryl, vinyl, as well as alkyl Grignard reagents are commonly used organometallic nucleophiles. These reactions provide extremely versatile carbon-carbon bond formation processes. Eqs 1-4 summarize representative examples.2-4 Even acid chlorides undergo the cross-coupling reaction (eq 5).5

Palladium and nickel complexes are the most useful catalysts for this purpose. Although nickel compounds without phosphine ligands can mediate cross-coupling reactions, those bearing phosphine ligands occasionally give much better results, with bidentate phosphine ligands providing the greatest catalytic activity for simple aryl, vinylic, or allyl substrates.

The choice of catalyst is particularly important for reactions of secondary alkyl Grignard reagents. When secondary Grignard reagents that bear b-hydrogens are employed, competitive b-hydride elimination results in the formation of both reduced and isomerized byproducts. These side reactions are particularly bothersome in reactions with hindered alkenyl halides. It is noteworthy that the use of the NiCl2(dppf) catalyst can minimize these side reactions by accelerating the rate of desired reductive elimination relative to undesired b-elimination.6 For example, NiCl2(dppf) is the most selective catalyst for the cross-coupling reaction of b-bromostyrene with t-BuMgCl, giving (E)-b-t-butylstyrene exclusively (eq 6).6 Other nickel complexes, NiCl2(PPh3)2, NiCl2(dppe), NiCl2(dppp), and NiCl2(dppb), also catalyze the coupling reaction, but it is accompanied occasionally by t-butyl isomerization to give (E)-b-isobutylstyrene as a byproduct. Moreover, there are only limited cases which are known for the cross-coupling reactions of alkyl iodides with Grignard reagents. NiCl2(dppf) again demonstrates a unique reactivity for this transformation.7 It is interesting to note that only neopentyl iodide can undergo coupling reactions with aromatic Grignard reagents (eq 7). Other coupling reactions including the activation of aliphatic carbon-sulfur bonds will be discussed later.

In general, reactions of alkenyl halides exhibit predominant retention of alkene geometry (eq 8),8 while reactions of alkenyl Grignard reagents are complicated by competing (Z) to (E) isomerization.1

Coupling reactions of unsymmetric allylic substrates normally gives a mixture of isomers. However, when a zinc reagent is used, allyl chlorides undergo regioselective displacement reactions giving SN2-like products predominantly, if not exclusively (eq 9). It is noted that copper-catalyzed reactions give SN2 substitution products selectively.9

Vinyl and aryl ethers also react with Grignard reagents under nickel catalysis, although they are less reactive than the corresponding bromides.10 Reactions of dihydrofuran and dihydropyran derivatives with Grignard reagents, giving homoallylic and bishomoallylic alcohols, respectively, are particularly noteworthy (eqs 10 and 11).11 Retention of alkene geometry is predominantly observed. Silyl enol ethers12a and enol phosphates12b,c behave similarly.

Allylic alcohols also give the corresponding coupling products upon treatment with a Grignard reagent in the presence of the nickel catalyst (eq 12),1e,13 and propargylic ethers yield the allene products (eq 13).14a It is noteworthy that soft nucleophiles can also displace the allylic ethers in the presence of the nickel catalyst (eq 14).14b

The nickel-catalyzed substitution of vinyl sulfides by Grignard reagents forms the basis of a procedure for the stereoselective synthesis of alkenes by sequential cross-coupling reactions (eq 15).1c-g,15,16 The cross-coupling reactions of thiophenes under the same conditions lead to stereoselective synthesis of substituted butadienes (eq 16).15d,e Unlike halide and triflate leaving groups, where palladium is also effective, the nickel catalyst plays a unique role in the displacement of the carbon-sulfur bonds. Thiols, sulfoxides, sulfones, as well as sulfonates also undergo coupling reactions smoothly (eq 17).15 Among vinylic substrates, the reactivity trend of various leaving groups in these nickel-catalyzed cross-coupling reactions appears to be in the following order: I > Br > SR > Cl > OR.

Optically active alkenylsulfoximines undergo nickel-catalyzed cross-coupling with organozinc reagents in the presence of an additional magnesium, lithium, or zinc salt. The reaction proceeds with more than 98% retention of alkene geometry (eq 18).17

Dithioacetals have two carbon-sulfur bonds attached to the same carbon atom. Sequential displacements of these two bonds in allylic substrates (eq 19) and o-amino-substituted benzylic substrates (eq 20) result in the net conversion of a carbonyl equivalent into a geminal dimethylated product.1f,g,18 It is noteworthy that only nickel complexes containing bidentate ligands catalyze the desired geminal dimethylation. When NiCl2(PPh3)2 is used, a mixture of alkenation and dimethylation products is obtained.

As mentioned earlier, b-hydride elimination is a major side reaction in the cross-coupling. However, this elimination process has been used in cross-coupling reactions of dithioacetals leading to alkenation products.1g In this case, one of the carbon-sulfur bonds is substituted by a carbon-carbon bond, and subsequently the second carbon-sulfur bond is eliminated to produce an alkene (eq 21).1f,g,19 In this respect, NiCl2(PPh3)2 is the best catalyst and the sulfur leaving group may coordinate to the nickel catalyst throughout the catalytic process.20 The reaction has particularly useful applications for the synthesis of various silyl-substituted alkenes and dienes (eqs 22 and 23).21 When the cyclopropyl Grignard reagent is employed, the reaction provides an interesting ring opening route for the synthesis of substituted dienes (eq 24).22 This reaction demonstrates an interesting example using cyclopropyl anion as an allyl anion synthon. Reactions of dithioacetals with allylmagnesium halides under similar conditions, on the other hand, afford 1,4-pentadienes as the predominant, if not exclusive, products.21b Benzylic and allylic orthothioesters also give the corresponding alkylative alkenation products in satisfactory yields.21c,e

Whereas simple aliphatic dithioacetals do not undergo the cross-coupling reaction, tetrathioorthocarbonate interestingly serves as a C4+ synthon under these reaction conditions.23 For example, tris(trialkylsilyl)isobutene is conveniently synthesized as shown in eq 25. When an appropriately located heteroatom is present for chelation of the nickel catalyst, aliphatic dithioacetals can also undergo cross-coupling reactions. Depending on the nature of the substrates, bisdithioacetals undergo either selective monoalkenation or a tandem coupling process via an allylic dithioacetal intermediate (eqs 26 and 27).24

Since the reaction medium is basic, benzylic dithioacetal-S-oxides undergo a Pummerer-type rearrangement followed by the reaction with the Grignard reagent to give the dithioacetals which then undergo the nickel-catalyzed alkenation process to yield the corresponding alkylative alkenation products (eq 28).25

Carbon-tellurium bonds can be replaced26 and quarternary ammonium ion can also serve as a leaving group in the nickel-catalyzed cross-coupling reactions (eq 29).27

Cross-coupling reactions of Si-O and Si-H bonds with Grignard reagents are also catalyzed by nickel complexes (eqs 30 and 31).28,29

Cross-Coupling Reactions Using Chiral Ligands.

When chiral phosphine ligands are used, the coupling reaction can be enantioselective.1l-n Thus the cross-coupling reaction of 3-cyclohexenyl ether with EtMgBr yields the corresponding hydrocarbons with excellent selectivity when [(S,S)-chiraphos]NiCl2 is employed (eq 32).30

The cross-coupling reaction between 1-phenylethylmagnesium bromide and vinyl bromide in the presence of a [(S,S)-norphos]NiCl2 catalyst yields the corresponding coupling product in 95% yield with 67% ee (eq 33).31

Higher enantioselectivities are achieved when certain chiral aminophosphines derived from amino acids are used. In general, ligands derived from (S)-amino acids give rise to the (S)-product, but it has been reported that this trend is reversed in the presence of ZnX2. It is worth noting that a sulfide-containing side chain appended to the dimethylaminophosphine ligands improves the enantioselectivity. The sulfide moiety offers a third site for coordination to the metal, and such intramolecular participation would accelerate the rate of reductive elimination, minimizing the loss of enantiomeric purity resulting from competitive b-elimination (eq 34).32

It is particularly noteworthy that cross-coupling reactions of 1-bromonaphthalenes with 1-naphthyl Grignard reagents in the presence of a ferrocene-based chiral nickel catalyst [(S)-(R)-PPFOMe]NiCl2 furnish a useful entry for the preparation of a various chiral binaphthyl derivatives (eq 35).33 Chiral ternaphthyl derivatives are obtained similarly.33 It is interesting to note that other ferrocene-based chiral ligands do not behave well under the reaction conditions.34

Reduction.

Nickel hydride species, generated in situ from the reaction of secondary Grignard reagents and NiCl2(PPh3)2 catalyst, are active for the reductive cleavage of the carbon-sulfur bonds (eq 36).35 Vinylic or aryl thiols, sulfides, sulfoxides, and sulfones can be reduced with ease under these conditions. The reduction is stereospecific, with no over reduction being detected. Si-X bonds are also reduced under similar conditions.36

Homocoupling.

A Ni0 catalyst generated in situ from the reduction of NiCl2(PPh3)2 with zinc promotes homocoupling of aryl or vinyl halides in good yields (eq 37).37,38 DMF is the most suitable solvent. Esters, amides, ketones, aldehydes, and ether functionalities are stable under the reaction conditions. Bicinnamyl is obtained from cinnamyl chloride in 50% yield.38 (E/Z) isomerization may occur in the homocoupling of alkenyl halides.38b Et4NI or KI or thiourea have been shown to assist the reduction of NiII with zinc and also to convert the alkenyl bromides to the corresponding alkenyl iodides.38e,f The extension of this reaction to the synthesis of heterocyclic biaryls, such as bipyridyls and bithienyls, has been executed.39 The generation of a Ni0 species by electrochemical reduction of NiCl2(PPh3)2 is useful for the reductive homocoupling of aryl bromides.40

Displacement.

Aryl bromides are readily displaced by iodide in the presence of nickel catalysts.38e,f In a similar manner, carbon-sulfur bonds are produced when an aryl iodide is treated with thiourea in the presence of a Ni0 catalyst generated in situ from the reaction of NiCl2(PEt3)2 and Sodium Cyanoborohydride (eq 38).41 Hydrolysis of the thiouronium salt gives the corresponding thiols.

Reductive Heck-Like Reactions.

In the presence of the zinc, NiCl2(PPh3)2 promotes reductive coupling reactions of aryl bromides with excess a,b-unsaturated esters in the presence of pyridine and a trace amount of water (eq 39).42 Alkyl bromides also give similar coupling products.

Miscellaneous.

Octa-1,7-diynes bearing ester groups on the terminal positions react with propargyl alcohols in the presence of stoichiometric amounts of a nickel(0) reagent derived from NiCl2(PPh3)2 and n-Butyllithium to give tetralin lactones in moderate yields (eq 40).43

The formation of ketones from reaction of a carboxylic acid with a Grignard reagent is catalyzed by NiCl2(dppe) (eq 41).44 The cis addition of the Grignard reagent to triple bond is also catalyzed by NiCl2(PPh3)2 (eq 42).45 Elimination of HBr from a terminal bromo compound is reportedly promoted by NiCl2(PPh3)2 (eq 43).46

Although low-valent nickel complexes are very useful for dimerization and oligomerization of alkenes, the use of NiCl2(PPh3)2 or related complexes for this purpose is relatively rare (eq 44). Occasionally, a number of isomeric products are obtained.47


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Tien-Yau Luh & Tien-Min Yuan

National Taiwan University, Taipei, Taiwan



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