Nickel codimerization

In one example of an asymmetric codimerization, a pronounced effect on the optical yield has been observed by increasing the phosphine/nickel molar ratio (94, 95). This effect may be ascribed to suppression of a dissociation process or to complexation of a second molecule of the inducing phosphine to the nickel atom. 

Codimerization of butadiene with dicyclopentadiene (example 8, Table II) was shown to proceed via a crotyl-nickel complex (62). Ring contraction of cyclooctadiene (example 10, Table II) appears to be a hydride promoted reaction. The hydride-promoted dimerization of norbomadiene to -toly 1 norbornene (example 9, Table II) appears to be quite different from dimerization via a metallacycle (see Table I, example 16). 

The essential steps in the nickel-catalyzed 1 1 codimerization reaction, which involve hydride addition to butadiene and ethylene coordination to the metal atom, were first proposed by Kealy, Miller, and Barney (35) and were later demonstrated by Tolman (40) using a model complex. Tolman prepared the complex H—Ni+L PFe [L = (EtO)3P] and showed that, after prior dissociation to form H—NiL3, it can react with butadiene to form a 7r-crotyl complex 19. 

Scheme 7. Nickel-catalyzed codimerization—model system. Data from Tolman (40).

The nickel catalyst under the condition for the 1 1 codimerization is not known to dimerize or polymerize ethylene, although a similar catalyst system has been known to dimerize propylene (26, 27) via a w-allyl intermediate. 

In the literature there are many reports of the formation of active catalyst for the 1 1 codimerization or synthesis of 1,4-hexadiene employing a large variety of Co or Fe salts, in conjunction with different kinds of ligands and organometallic cocatalysts. There must have been many structures, all of which are active for the codimerization reaction to one degree or another. The scope of the catalyst compositions claimed to be active as the codimerization catalysts is shown in Table XV (69-82). As with the nickel catalyst system discussed earlier, the preferred Co or Fe catalyst system requires the presence of phosphine ligands and an alkylaluminum cocatalyst. The catalytic property can be optimized by structural control of these two components. 

Asymmetric hydrovinylation has been pioneered by Bogdanovic [30] and Wilke [31] using nickel catalysts. Of special interest is the reaction between vinylarenes and ethylene, as enantioselective codimerization provides a convenient route to... 

Codimerization of ethylene and 1,3-butadiene is also of commercial significance since the product 1,4-hexadiene is used as a comonomer in the manufacture of ethylene-propylene-diene elastomers. Rhodium and nickel catalysts are used since they are the most active and selective, bringing about the formation of the required... 

A tremendous amount of work concerning metal-induced cycloadditions of methylenecyclopropane with olefins and alkynes has been done in recent years since the first reported nickel(0) catalyzed 3+2 cycloaddition of methylenecyclopropanes with electron-poor olefins (equation 352)415 and the analogous palladium(O) codimerization (equation... 

Nickel(0)-catalyzed codimerization of methylenecyclopropanes with electron-deficient olefines are highly regiospedfic, but show a rather poor stereoselectivity. Thus the asymmetric nickel(0)-catalyzed codimerization of methylenecyclopropanes with the chiral bomane derivatives of acrylic acid leads to the optically active 3-methylenecyclopen-... 

In Table XIII we have summarized the results obtained by codimerization of various substituted 1,3-dienes with butadiene. In all cases the highest yields were obtained using the nickel-tri(o-phenylphenyl)phosphite catalyst. 

Nickel allyl complexes in the presence of chiral bidentate ligands catalyze the enantioselective codimerization of ethylene with norbornene and with styrene 129... 

The nickel(0)-catalyzed codimerizations of methylenecyclopropane (26) or 2,2-dimethylmethylene-cyclopropane with the chiral derivatives of acrylic acid lead to optically active 3-methylenecyclopen-tanecarboxylic esters or amides (39 equation 16) in good yields (Table 3). When (-)-camphorsultam acrylate is used, 3-methylenecyclopentanecarboxylic amides are obtained in up to 98% de. °... 

Additions of methylenecyclopropanes to alkynes (59 equation 24) give 4-methylene-1-cyclopentenes (60), (61) and (62) in the presence of phosphite-coordinated nickel(O) catalysts (Table S). Aftynylsilanes are particularly suitable for these codimerizations. In the reactions with 1-alkynes or dialkylalkynes, oligomerization of the alkynes cannot be avoided. When alkynes with electron-attracting substituents are used, cyclotrimerization is so rapid that cross addition no longer occurs. ... 

Iron complexes favor the codimerization of BD with alkynes in a 1 1 ratio to (substituted) cyclohexadienes [39]. Two BD molecules and one alkyne give cyclodecatrienes with zerovalent nickel catalysts and good electron-donating ligands such as Ph3P [7, 40]. Ten-membered rings are in fact the principal products of such a reaction the variety of dienes seems to be limited to BD, iso-prene and 1,3-pentadiene, whereas numerous alkynes - simple alkyl-substituted alkynes, alkynes with aprotic functional groups, dialkynes, and cyclic alkynes... 

The nickel-catalyzed hydrovinylation of bicycloheptene has been used as a standard reaction to test the efficacy of a new ligand. The reaction occurs with complete diastereoselectivity to give exo-2-vinylbicycloheptane (16) and none of the endo-isomer is formed. The same species, however, catalyze the isomerization of the primary product to cis- and franv-2-ethylidenebicycloheptane (17) and the codimerization with further ethylene to the butenyl derivatives 18 and 19. The product distribution is dependent upon the nature of the ligand [3, 8 c, 40]. 

During our investigation of these codimerizations it turned out that with Ni(0) catalysts not only the metal determines the course of the reaction, but also several other factors e.g. the kind and number of the ligands bonded to the nickel, the kind, number and position of substituents on the methylenecyclopropane and the electronic properties of the second olefme. These observations make it nearly impossible to predict, whether methylenecyelopentanes of Type A or B will be the products. 

In the presence of naked nickel , methylenecyclopropane can be codimerized with alkyl acrylates, alkyl crotonates and alkyl maleates 175-186-187) giving Type B cycloadducts in moderate to excellent yields (Eq. 80). 

When phosphane-free nickel complexes, such as bis(cycloocta-l,5-diene)nickel(0) or te-tracarbonylnickel, are employed in the codimerization reaction of acrylic esters, the codimer arising from [2-1-1] addition to the electron-deficient double bond is the main product. The exo-isomer is the only product in these cyclopropanation reactions. This is opposite to the carbene and carbenoid addition reactions to alkenes catalyzed by copper complexes (see previous section) where the thermodynamically less favored e Jo-isomers are formed. This finding indicates that the reaction proceeds via organonickel intermediates rather than carbenoids or carbenes. The introduction of alkyl substituents in the /I-position of the electron-deficient alkenes favors isomerization and/or homo-cyclodimerization of the cyclopropenes. Thus, with methyl crotonate and 3,3-diphenylcyclopropene only 16% of the corresponding ethenylcyc-lopropane was obtained. Methyl 3,3-dimethylacrylate does not react at all with 3,3-dimethyl-cyclopropene, so that the methylester of tra 5-chrysanthemic acid cannot be prepared in this way. This reactivity pattern can be rationalized in terms of a different tendency of the alkenes to coordinate to nickel(O). This tendency decreases in the order un-, mono- < di-< tri- < tet-... 

A suitable approach to the synthesis of spiro[2.3]hexanes is the [2-1-2] cycloaddition of alkenes to the double bond of methylenecyclopropanes. This reaction is often described as a codimerization and usually requires catalysis by a nickel(O) complex such as bis(cycloocta-l,5-diene)nickel. l,l-Dimethyl-2-methylenecyclopropane reacts with alkyl acrylates to give a mixture of alkyl cis- and tranj-l,l-dimethylspiro[2.3]hexane-5-carboxylates 1 (19-40%) and alkyl 3,3-dimethyl-4-methylenecyclopentanecarboxylate 2 (60-81 %). The proportion of spiro [2.3]hexane derivative was highest when rer/-butyl acrylate was used as the activated alkene. 

Under similar conditions, methylenecyclopropane reacted with norbornene to yield a mixture of cycloadduct (2, 78%) and dimer (7, 21%). With norbornadiene and a nickel catalyst only the [2-1-2] cycloadduct was formed. " The codimerization of 2-phenylmethylenecyclo-propane and ethylene in the presence of a triisopropylphosphane modified palladium(O) catalyst... 

Most remarkably, no codimerization involving the unsaturated ester molecule is observed, even though nickel compounds such as the acrylonitrile complex are efficient catalysts for such codimerization reactions vide infra). An additional homodimer, l,3-bis(methylene)cy-clohexane (6), is formed as a minor product with phosphane-modified nickel catalysts. Only trace amounts of product 6 are obtained with maleic anhydride as cocatalyst. 

Only a limited number of vinyl sulfones, e.g. phenyl ( )-2-phenylvinyl sulfone (14), undergo codimerization with MCR Homocyclodimerization of MCP is the most efficient side reaetion. Interestingly, yields and product distributions are solvent dependent. No reaction takes place with catalytic amounts of bis(t -cycloocta-l,5-diene)nickel(0)/triphenylphosphane. In this case the vinyl sulfones are strongly coordinated to the catalyst metal, thus preventing interaction with MCP. When the sulfones bear alkyl-substituted vinyl groups, isomerization to yield allyl sulfones usually proceeds faster than cycloaddition, at least in the case of palladium(O) catalysis. 

Increased stereoselectivity is found at low reaction temperatures but is then accompanied by a decrease in reaction rate. Below — 20"C no codimerization at all occurs with the nickel(O) catalyst, thus further improvement in the stereoselectivity could not be achieved. 

Transition metal complexes with cyclopropene derivatives as ligands have been postulated as intermediates in the nickel(0)-catalyzed codimerization of 3,3-dimethylcyclopropene (1). ... 

---