Nickel allylnickel complexes

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

The following conclusions can be drawn (a) ir-Allylnickel compounds are probably not involved in the catalytic dimerization of cyclooctene, because the highest reaction rate occurs when only traces of these compounds can be detected further, the concentration of the new 7r-allyl-nickel compound (19) becomes significant only after the catalytic reaction has ceased, (b) The complex formed between the original 7r-allylnickel compound (11) and the Lewis acid is transformed immediately upon addition of cyclooctene to the catalytically active nickel complex or complexes. In contrast to 7r-allylnickel compounds, this species decomposes to give metallic nickel on treatment of the catalyst solution with ammonia, (c) The transformation of the catalytically active nickel complex to the more stable 7r-allylnickel complex occurs parallel with the catalytic dimerization reaction. This process is obviously of importance in stabilizing the catalyst system in the absence of reactive olefins. In... 

It is proposed that the reaction proceeds through (i) oxidative addition of a silylstannane to Ni(0) generating (silyl)(stannyl)nickel(n) complex 25, (ii) insertion of 1,3-diene into the nickel-tin bond of 25 giving 7r-allylnickel intermediate 26, (iii) inter- or intramolecular allylation of aldehydic carbonyl group forming alkoxy(silyl)nickel intermediate 27, and (iv) reductive elimination releasing the coupling product (Scheme 69). 

Isomerism of the but-3-enoic acids into the E-but-2-enoic acids is base-catalysed (Table 8.8), whereas the formation of the other isomers indicates the participation of rt-allylnickel complexes in the reaction. Potassium nickel tetracarbonyl is a considerably poorer catalyst, compared with nickel cyanide, whereas nickel sulphate and nickel iodide are ineffective catalysts. 

The reaction of allylic halides with nickel tetracarbonyl to form coupled products has been known for over two decades (9), but it is only in recent years that an insight into the mechanism has been obtained. Isolation of the intermediate 7r-allylnickel complexes and the discovery that these react with activated olefins and organic halides in general have led to a considerable increase in the scope of the reaction. 

Two groups of workers (40, 41) have demonstrated that the reaction proceeds through the formation of a 77--allylnickel intermediate which absorbs CO to form a nickel acyl complex. This then liberates a molecule of acyl halide which is hydrolyzed by the solvent. The presence of the intermediate nickel acyl complex in solution has been demonstrated... 

The dimerization reaction catalyzed by a nickel compound and an alkylaluminum chloride derivative was first described in a patent in 1955 [4], In 1966 Wilke et al. [5] gave crucial impetus to this reaction starting from a well-defined cationic / -allylnickel complex (Structure 1). 

The mechanism of stereoregulation, however, remained unclear for many years. Only more recently has an experimentally well-founded comprehensive reaction model been derived for the allylnickel complex-catalyzed 1,4-polymerization of butadiene, which convincingly explains the catalytic reaction mechanisms and the structure-reactivity relationships also involving the industrial nickel catalyst [26-28]. 

Deming and Novak focused their attention on -allylnickel complexes 48 and 49, bearing optically active chiral carboxylato ligands, which would be retained on the nickel center during polymerization (Scheme 38) [62]. The... 

The TT-allylnickel complex (372), obtained by the action of nickel tetracarbonyl on a 6)5-chloro-4-en-3-one (371), can be methylated at C-4, though not very efficiently, by methyl iodide. The 7r-allylpalladium complex was unreactive to alkylation. 

Takimoto and Mori reported that the nickel-catalyzed regio- and stereoselective ring-closing car-boxylation of the bis-1,3-diene 81 with dialkylzincs 82 gave the 3-allyl-4-alkenylpyrrolidines 83 in high yields (Scheme 29).108 The reaction proceeds through formation of the cyclic bis-jr-allylnickel complex 84, the insertion of CO2 into a nickel—carbon bond of 84, leading to 85, and the addition of RzZn to 85. 

A detailed mechanism of asymmetric hydrovinylation is discussed in order to explain the pathways of the asymmetric induction4- 51314. The 7t-allylnickel complex, as the catalyst precursor, is activated by phosphanes and ethylaluminum chloride and reacts with an olefin to give a catalytically active nickel hydride-olefin complex. The olefin then inserts into the metal hydride bond and after coordination and insertion of ethene a new alkylnickel compound is... 

A reversible equilibrium between nickel-olefin complexes and 7t-allylnickel hydride complexes exists at low temperature ... 

When the solution of the olefin complex XI is cooled below — 50°C, the hydrido-rt-allylnickel complex, XII, forms. The formation of a ii-allylnickel complex from allylic C—H bond cleavage is demonstrated by olefin XIII which with a dichloro complex of nickel affords the it-allylnickel complex, XIV... 

Nickel(O) complexes rapidly oligomerize aliene selectivity depends on the detailed ligand environment . Allylnickel complexes are isolable from these reactions and are involved in the catalytic cycle. Treatment of bis(cyclooctadiene)Ni at — 30°C with, sequentially, aliene and triphenylphosphine affords ... 

A nickel-catalyzed ene cyclization (sec. 11.13) has been reported that uses Ni(cod)2- The reaction proceeds by initial formation of a ac-allylnickel complex, which facilitates the intramolecular ene reaction with an allylic amine unit. l jt-Allylnickel complexes can be used in coupling reactions with both aryl and alkyl halides. Enolate anions react with nickel(O) reagents to form a complex that subsequently couples to aryl iodides. Semmelhack s final step in the synthesis of cephalotaxinone (446) treated 445 with Ni(cod)2 to... 

Many nickel-based systems catalyze the dimerization of olefins. In 1966, Wilke [5] demonstrated that the well-defined cationic ri -allylnickel complex (1) catalyzes the regioselective dimerization of propene in organic chlorinated solvent. The AICI4... 

Scheme 13 Preparation of a r -Allylnickel Complex from Nickel(ll) and...

An advance by Mackenzie has made nickel-ji-aUyl complexes accessible from enals and enones.P In a reaction that is mechanistically analogous to Method 1 in Section 1.1.2.1, enals, when treated with bis(q" -cycloocta-l,5-diene)nickel(0) (2) in the presence of chlorosilanes, afford chloro-bridged dimeric q -aUylnickel complexes such as 25 (Scheme 14). Enones are less reactive in the process and require p5n idine to facilitate the oxidative addition. Rather than using bis(ti -cycloocta-l,5-diene)nickel(0) (2), a more convenient and less expensive alternative involves the in situ reduction of dichlorotetra-lds(p3n idine)nickel(II) (26) with sodium metal in the presence of cyclooctadiene to give enone-derived q -allylnickel complexes (e.g., 27). 

Chiral (E)-enolethers. A degassed soln. of (5R)-5-cyclohexyl-2-ethenyl-l,3-dioxolan-4-one (2 1 cisjtrans) in THF added to ca. 1 eq. of a suspension of bis(l,5-cycloocta-diene)nickel(0) in the same solvent under N2, stirred until the complex dissolved (10 min), after 3h the resulting rust-coloured precipitate collected, suspended in methylene chloride, treated with MejSiCl, and stirred for 30 min intermediate 7c-allylnickel complex (Y 78%), in benzene treated with DMF and 5 eqs. 1-iodo-propane, irradiated with a sunlamp (GE 275 W Model RSW) for 2,5 h at 10°, stirred for a further 3 h, diluted with pentane to precipitate nickel halide, and stirred for a further 4 h product (Y 82% E/Z 9 1). Subsequent treatment with acetals afforded 2-p-alkoxy-l,3-dioxolan-4-ones with asym. induction, thereby providing an alternative to asym. aldol condensation. F.e. inch reaction with ar. and a,P-ethylene-bromides s. D.J. Krysan, P.B. Mackenzie, J. Am. Chem. Soc. 110, 6273 (1988). 

It has recently been suggested (19) that 7r-allylnickel complexes are intermediates in reactions involving allylic halides. Although ir-allylnickel chloride-triphenylphosphine (IX) is formed from allyl chloride and Ni(CO)3P(C6H5)3 without jrielding a carbonylation product (20), the dimeric 7r-allylnickel chloride (X) [prepared (13) by heating allyl chloride with nickel carbonyl in benzene solution] reacts rapidly with carbon monoxide to form butenoylnickel dicarbonyl chloride (XI) (Eq. 13). Moreover, this complex is converted by additional carbon monoxide into butenoyl chloride and nickel carbonyl (13), Eq. (14)... 

An allylnickel complex derived from the steroidal chloroketone (CXXXIII) and nickel carbonyl is methylated with methyl iodide to produce a mixture of CXXXIV and CXXXV (Harrison et al., 1969). 

Dienes or olefins are polymerized in the presence of a 7r-allylnickel complex and a phosphate as catalysts to afford the polymers having a Tt-allyl group at the end position as shown in eq. (19.50). Then they are able to easily copolymerize with styrene or isocyanates [61,72,92-96]. Further, the polymerization of isocyanate and cyclohexadiene has been tried in the presence of nickel catalysts [94,95]. 

The authors proposed a possible mechanism (shown in Scheme 4.20) which starts with the oxidative cycloaddition of the bis-1,3-diene to the Ni(0) complex to produce bis-allylnickel complex J, and subsequent insertion of the aldehyde into the nickel-carbon bond affords oxanickelacycle K. TYansmetallation of K with ZnMc2 provides methylnickel complex L, which can easily undergo reductive elimination to afford zinc alkoxide M. Hydrolysis of M in an aqueous workup procedure provides the final alcohol product 19. 

Allyl complexes have contributed significantly to the development of the organometallic chemistry of nickel and the applications of nickel complexes in organic synthesis, for example, nucleophilic attack on coordinated allyl ligands. In addition, allylnickel complexes have been identified as key intermediates in the oligomerization and cyclization of olefins and dienes. For example, the Ni(0)-catalyzed hydrocyanation of butadiene to adiponitrile, the main component of a major commercial process for the production of nylon, involves Ni (7r-allyl) intermediates. Moreover, the 77-rearrangements of allylnickel species have helped explain the facile isomerization of olefins in the presence of nickel complexes. The Ni-catalyzed homoallylation of carbonyl compounds with 1,3-dienes also involves Ni(7r-allyl) complexes this subject has been reviewed recently. New applications include the cleavage of G-G bonds in the deallylation of malonates, the preparation of cyclopentenones by carbonylative cycloaddi-... 

Similarly, reaction of dienes with COj in the presence of Ni(COD)2 affords Jt-allylnickel complex. Upon treatment with Me2Zn, the corresponding dicer-bonxylic acid is isolated. On the other hand, when the nickel intermediate is allowed to react with Hd, only monocarboxylation product is obtained [244]. Asymmetric induction is observed when (S)-MeO-MOP ligand is used [245,246]. 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

An obvious method to investigate the formation and the nature of the catalytically active nickel species is to study the nature of products formed in the reaction of complexes such as 3 or 4 with substrate olefins. This has been investigated in some detail in the case of the catalytic dimerization of cyclooctene to 1-cyclooctylcyclooctene (17) and dicy-clooctylidene (18) [Eq. (4)] using as catalyst 7r-allylnickel acetylacetonate (11) or 7r-allylnickel bromide (1) activated by ethylaluminum sesquihalide or aluminum bromide (4). In a typical experiment, 11 in chlorobenzene was activated with excess ethylaluminum sesquichloride cyclooctene was then added at 0°C and the catalytic reaction followed by removing... 

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