7r-Allylnickel complexes

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... 

The reaction of CO2 with 1,3-butadienes in the presence of Ni catalysts usually gave an isomeric mixture of carboxylic acids 89 and 90 after hydrolysis (Scheme 32).47,48 The oxa-7r-allylnickel complexes 87 and 88 might be the reaction intermediates, which could be formed through oxidative cyclization of Ni(0) with C02 and the dienes. When Me2Zn was used as a transmetallation agent to react with the oxa-7r-allylnickel intermediates under a C02 atmosphere, further carboxylation took place at the 7r-allylnickel unit. Thus, the 1,4-diesters 95 were obtained after acidic hydrolysis and treatment with diazomethane as shown in Scheme 32.47... 

Cyclization of butadiene catalysed by Ni(0) catalysts proceeds via 7r-allylnickel complexes. At first, the metallacyclic bis-7i-allylnickel complex 6, in which Ni is bivalent, is formed by oxidative cyclization. The bis-7r-allyl complex 6 may also be represented by cr-allyl structures 7, 8 and 9. Reductive elimination of 7, 8 and 9 produces the cyclic dimers 1, 2 and 3 by [2+2], [2+4] and [4+4] cycloadditions. Selectivity for 1, 2 and 3 is controlled by phosphine ligands. The catalyst made of a 1 1 ratio of Ni and a phosphine ligand affords the cyclic dimers 1, 2 and 3. In particular, 1 and 3 are obtained selectively by using the bulky phosphite 11. 1,2-Divinylcyclobutane (1) can be isolated only at a low temperature, because it undergoes facile Cope rearrangement to form 1,5-COD on warming. Use of tricyclohexylpho-sphine produces 4-vinylcyclohexene (2) with high selectivity. 

We have restricted ourselves to discussing stoichiometric and catalytic reactions which clearly involve 7r-allylnickel complexes and have not considered polymerization reactions, the Reppe synthesis, or template reactions. Fortunately there is an abundance of books and reviews which cover these fields as well as the similarities, and dissimilarities, with the other transition metals (1-8). The literature up to the end of 1968 has been surveyed but no attempt has been made to include all the material available. 

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. 

Since the 7r-allylnickel complexes are tolerant to polar amide and ester groups, the catalysts mentioned in the previous sections could polymerize allene derivatives with functional groups. The polymerization of N-allenyl-... 

Hydroxymethylallene undergoes living polymerization catalyzed by the 7r-allylnickel complex([Eq. 36) [137]. This preparation of polymer with hy-... 

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)... 

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 unpredictable nature of insertion reactions of carbon monoxide into bis-7r-allylnickel complexes has frustrated the development of this otherwise attractive approach to a general synthesis of cyclic ketones. In investigations of insertion... 

Nickelocene and tetrafluoroethylene in tetrahydrofuran at 80° C yield air-stable, volatile, red crystals, m.p. 93° C 143). NMR 143) and other studies 123) establish the structure of this complex as (XII). The compound belongs to a family of recently characterized 7r-allylnickel complexes 132). A similar compound is formed by chlorotrifluoroethylene and nickelocene 143). 

Method B2 A modification of method B4 for the preparation of catalysts starting from organonickel halides consists in the exchange of the halide anion by an anion of a strong complex acid, HY. This has been accomplished by reacting 7r-allylnickel halides (1) or their phosphine adducts (2) with silver salts (65) ... 

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... 

It has been known that 7r-allylnickel halides are catalysts for polymerization of butadiene (50, 51). When the halide is chloride, the polymer formed is cis-polybutadiene when the halide is iodide, the polymer is trans-polybutadiene. Porri and co-workers (50) interpret this effect in terms of the ease of dissociation of the dimeric complex 29 by butadiene. The chloride complex... 

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). 

In the case of a 7r-allylnickel(NHC)chloride complex (9) a different reaction with oxygen was observed [75]. The active complex is formed in a one-pot reaction (Scheme 8). Solutions of complex 9 which are exposed to molecular oxygen undergo rapid color changes under precipitation of a purple solid, which has been identified as a yu,-hydroxo Ni(II) dimer which is air-stable, but moisture-sensitive. It also was reported to be unstable in solution [75]. 

Both stoichiometric and catalytic reactions of allylic compounds via 7r-allyl complexes are known. Reactions of nucleophilic 71-allyl complexes with electrophiles involve oxidation of metals and hence constitutes stoichiometric reactions. 7i-Allyl complexes of Ni, Fe, Mo, Co and others are nucleophilic and undergo the stoichiometric reaction with electrophiles. However, electrophilic 71-allyl complexes react with nucleophiles, accompanying reduction of metals. For example, 71-allylnickel chloride (2) reacts with electrophiles such as aldehydes, generating Ni(II), and hence the reaction is stoichiometric. In contrast, electrophilic 7i-allylpalladium chloride (3) reacts with nucleophiles such as malonate and Pd(0) is generated. Thus repeated oxidative addition of allylic compounds to Pd(0) constitutes a catalytic reaction. 

Avery active form of Ni(0) is generated from bis-7r-allylnickel (12) and Ni(cod)2 (13) in the absence of phosphine ligand, and is often called naked Ni(0). Three butadienes coordinate to naked Ni(0) and the 18-electron trimeric complex 10 is... 

Interesting synthetic applications of the [4+4] and [4+4+4] cycloadditions are reported. A novel, short-step synthetic method of muscone (24) has been developed using complex 10 as a starting compound [7]. Insertion of allene to the Ni-carbon bond in 10 at low temperature gives the bis-7r-allylnickel 22. Then isonitrile is inserted to 22. When the reaction mixture is warmed, the 15-membered cyclic compound 23 is formed by reductive elimination, and conversion of 23 to muscone (24) is achieved by hydrolysis and subsequent hydrogenation in 43 % overall yield. 

From the reaction of acrylonitrile with 7r-allylnickel bromide an intermediate, (CH2 CHCH2CH2CHCN)2Ni, may be isolated which is believed to have a polymeric structure (24). This complex is easily decomposed either by water or acids to give 5-hexenonitrile, or, thermally, to a mixture of 2,5-hexadienonitrile and 5-hexenonitrile. 

A second mode of reaction is available in which unreacted allyl halide or 7r-allylnickel halide complex apparently reacts with either (XXII), or the acyl chloride formed from (XXII), and with (XXIII) to give the... 

The stability of halide bridges in dimeric r-allylic complexes against the action of bases depends on the nature of the halogen the chloride complexes are the least stable. According to Burger (3) the dimers of bis(7r-allylnickel halides) are hydrolyzed with destruction of bridges and monohydrate formation. 

The term 7r,7r-transfer has been proposed for 7r-allyl ligand transfer from one transition metal to another. Such processes differ from those giving a-allyl derivatives 160), which are termed Tr.a-transfer processes. The latter have been studied in detail 161) for 7r-allylnickel, -palladium, and -platinum complexes interacting with metallic mercury. It has been shown that binuclear 7r-allylpalladium complexes rapidly react with mercury in... 

The common preparative method for 7r-allylnickel halides is at the moment the reaction of nickel(O) olefin complexes like bis(cycloocta-1,5-diene) nickel, (IV), with allylic halides (23). The olefin complex, IV, can be prepared easily by reducing nickel(II) salts (like nickel acetyl-acetonate) with aluminum organic compounds in the presence of cycloocta-1,5-diene (5). 7r-Allylnickel halides and substituted 7r-allylnickel halides prepared according to this method are listed in Table I. 

Some experiments designed to obtain information about the nature of the catalytically active species have been carried out (15, 16), Complex formation between 7r-allylnickel halides and aluminum halides can be followed by the shift of the Trallyl protons in the H-NMR spectrum. By adding n electron donors, such as ethers or amines, the catalytically active complexes, XIII, are decomposed. 

The transfer of halide from 7r-allylnickel halide to aluminum to give a haloaluminum anion results in a strong decrease in the electron density and formation of free coordination positions on nickel. The presence and number of free coordination positions can be demonstrated by reaction with carbon monoxide. The catalytically active complex, XIII, reacts at —40°C. with 2 moles of carbon monoxide to give the inactive XIV which can be isolated. In XIII, there are accordingly two free coordination positions. During catalysis, these positions are presumably occupied by olefins in the form of 7r-complexes. A stable olefin 7r-complex, 7r-allylnickel (7T-cycloocta-l,5-diene) aluminum tetrabromide, XV, can actually be isolated by adding cycloocta-l,5-diene to a solution of the active complex, XIII. 

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. 

The cyclization of butadiene, as illustrated, involves the coupling of the 7r-allylnickel moiety. Various cyclic 1,5-dienes can also be synthesized by the coupling reaction of allyl dibromides using Ni(CO)4 via a 7r-allyl complex. 

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-... 

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