[1295-35-8]  · C16H24Ni  · Bis(1,5-cyclooctadiene)nickel(0)  · (MW 275.08)

(Ni(cod)2 is a source of nickel(0) useful for the preparation of p-allylnickel halides,2 for coupling of aryl and alkenyl halides,3 and for the oligomerization and cycloaddition of strained alkenes,4 of alkynes,5 and of 1,3-dienes6)

Physical Data: mp 60 °C dec (N2).

Solubility: sol benzene, toluene, THF, ether, DMF, HMPA, N-methylpyrrolidinone.

Form Supplied in: yellow-orange crystals of 98+% purity.

Analysis of Reagent Purity: 1H NMR: d 4.31 (br, =CH), 2.08 (br, CH2).1

Preparative Methods: the standard preparation2b is a modification of the original procedure by Wilke and co-workers7 and involves the reduction of Nickel(II) Acetylacetonate, Ni(acac)2, with Triethylaluminum in the presence of 1,5-cyclooctadiene (cod) and 1,3-butadiene in toluene. A more convenient preparation utilizes Diisobutylaluminum Hydride (DIBAL) as the reducing agent.8 In a typical reaction, 45.4 mL of a 1.0 M THF solution of DIBAL was added to a 250 mL Schlenk flask containing 4.67 g of Ni(acac)2, and 7.93 g cod in THF solution under a nitrogen atmosphere at -78 °C. The resulting dark, reddish-brown solution was warmed to 0 °C and treated with diethyl ether to give a light yellow precipitate. Filtration under nitrogen gave a 72% yield of Ni(cod)2 which was suitable for immediate use. Optional recrystallization from toluene under inert atmosphere gave bright yellow-orange needles with 40% recovery.

Handling, Storage, and Precautions: highly oxygen sensitive. Special inert-atmosphere techniques must be used.9 Should be stored at 0 °C.

p-Allylnickel Halides.

p-Allylnickel halide complexes are prepared by reaction of allylic halides with Ni(cod)2 in a nonpolar solvent (eq 1).10 The resulting dimeric species, isolated as a red solid, can be purified by crystallization, stored in the absence of air, and weighed out for reaction like any other moderately air-sensitive compound.2b Ni(cod)2 is preferred over Tetracarbonylnickel for this transformation due to the mildness of conditions required, the extreme toxicity and thermal instability of Ni(CO)4, and the absence of byproducts caused by CO insertion.

In polar, coordinating solvents such as DMF, HMPA, and N-methylpyrrolidinone, p-allylnickel halide complexes react with a wide range of alkyl, alkenyl, and aryl halides11 to replace a halogen with the allyl group, as shown by the reactivity of p-(2-methallyl)nickel bromide (1) (eq 2).12 These complexes also react with aldehydes and ketones to give homoallylic alcohols,13 with quinones to produce allylquinones,14 and with 2-pyridylcarboxylates to give b,g-unsaturated ketones.15 However, they do not react readily with esters, acid chlorides, amides, nitriles, or alcohols. Due to this significantly lower reactivity in these reactions, they offer a greater degree of selectivity over their allyllithium, -magnesium, and -zinc counterparts.16 Mechanistic aspects of the coupling reaction between organic halides and p-allylnickel halides have been investigated and a radical-chain pathway has been proposed.17

The p-allylnickel halide formation reaction (eq 1) is tolerant to substitution on the allyl halide starting material. 1,1-Dimethylallyl,13 2-ethoxycarbonylallyl,13 2-methoxyallyl,18 and 2-trimethylsilylmethyl19 nickel halide complexes, to name a few, have all been synthesized and utilized in subsequent coupling reactions. Alkene-containing allyl halides have been also used to form dienes.20 Allyl iodides are more reactive than allyl bromides for the generation of p-allylnickel halides and allyl chlorides are unreactive. Allyl mesylates and trifluoroacetates do not work as well as bromides due to extensive homocoupling of the substrates.19 a,b-Unsaturated aldehydes and ketones react with Ni(cod)2 in the presence of a trialkylsilyl chloride to give [1-[(trialkylsilyl)oxy]allyl]nickel chloride dimers (2) (eq 3).21 These complexes couple to alkyl halides at the g-position upon irradiation under a sun lamp to give silyl enol ethers (3), making the overall transformation a useful reversed-polarity complement to organocuprate conjugate addition chemistry. Chiral allyl acetals (4) have also been utilized to form chiral (E)-enol ethers (5) which serve as homoenolate equivalents of a new type (eq 4).22

Complexes having p-allylnickel halide-like reactivity may also be used as transient intermediates in polar media without the need for isolation. This type of reactivity is especially important for the intramolecular coupling of allyl groups (see Tetracarbonylnickel). In these cases, allylic tosylates and carboxylates may also be used for the initial oxidative addition of the allyl substrate to nickel.23 In one interesting group of reactions (eq 5),24 a p-allyl intermediate undergoes an intramolecular alkene insertion to form a ring followed by b-hydride elimination to give diene (6). If carbon monoxide and methanol are included in the reaction mixture, CO insertion, followed by intramolecular insertion of the newly-formed double bond, followed by a final methoxycarbonylation gives rise to keto ester (7). The palladium catalyst Bis(dibenzylideneacetone)palladium(0) also catalyzes these reactions, but with poor diastereoselectivity. An allylic sulfonium ion (8) has shown a similar reactivity and has been utilized in the total synthesis of confertin.25 Intramolecular coupling with an aldehyde followed by spontaneous lactonization yielded the tricyclic a-methylene-g-lactone (9) (eq 6). Insertion of terminal alkynes into p-allylnickel complexes derived from allylic esters (10) has led to a catalytic synthesis of nonconjugated alkenynes (11) (eq 7).26

Coupling of Aryl Halides.

Ni(cod)2 reacts with a variety of aryl halides in DMF to give diaryl compounds in generally high yields (80-90%) (eq 8).27

Ortho substituents on the aryl halide drastically reduce the rate of the reaction and an increase in temperature leads only to decomposition of the nickel catalyst. The reaction is tolerant of most functional groups with both electron-donating and electron-withdrawing substituents allowing efficient coupling. Nitro groups, however, destroy the catalytic activity of the nickel complexes.28 Acidic functionalities such as alcohols, phenols, or carboxylic acids cause reduction of the aryl halide in preference to coupling. The order of reactivity of substrates is I > Br > Cl with phenol p-toluenesulfonate esters being completely unreactive. This reaction may be contrasted with the copper-catalyzed Ullmann reaction,29 which typically requires high temperatures (&egt;200 °C), or a two-step coupling procedure requiring intermediate arylmagnesium or -lithium reagents which are incompatible with many functional groups.

Attempted cross-coupling usually leads to mixtures of products due to extensive symmetrical coupling. Nevertheless, this strategy was utilized in the synthesis of a 2-benzazepine.30 Intramolecular coupling reactions,3,31 on the other hand, are generally quite efficient (eq 9)32 and often benefit from added phosphine ligand (see also Tetrakis(triphenylphosphine)nickel(0)).

Coupling of Alkenyl Halides.

Ni(cod)2 reacts with alkenyl halides to produce symmetrical 1,3-dienes.33 These reactions may be carried out in DMF, or in ether with an added ligand such as Ph3P. Coupling of simple alkenyl halides gives only moderate yields (48-70%) with mixtures of geometrical isomers. Reactions of alkenyl halides bearing electron-withdrawing substituents such as a- and b-haloacrylates, however, are more efficient and highly stereoselective (eq 10).3 The intramolecular coupling of a simple diiodide has also been shown to be effective (eq 11).3 Finally, there is one example of the intramolecular coupling of alkyl halides in which a,o-dihaloalkanes are cyclocoupled to give cycloalkanes by a Ni(cod)2-bipyridyl complex.34

Oligomerization of Strained Alkenes and Alkanes.

3,3-Disubstituted cyclopropenes are known to react with electron-deficient alkenes in the presence of catalytic Ni(cod)2 to give vinylcyclopropanes (eq 12).35 The mechanism of the reaction begins with oxidative addition of Ni0 into the C-1-C-3 bond, but there is some dispute as to whether a carbenoid intermediate is involved.4 Methyl, phenyl, and methoxy groups have all been used as geminal substituents on the cyclopropene; however, the stereochemistry of the starting alkene is not preserved when methoxy is used.36 Alkyl group substitution on the electron-deficient alkene disfavors this reaction pathway and leads to [2 + 2] cyclodimerization of the cyclopropene.37 Addition of ligands has also been shown to affect the distribution of products. When the catalyst is modified by the bulky P(i-Pr)2(t-Bu), a [2 + 2 + 2] cycloaddition is observed (eq 13).38

Cyclobutenones react with alkynes in the presence of Ni(cod)2 to give substituted phenols. Many substituents are tolerated, but regioselectivity is poor (eq 14). The reaction proceeds by oxidative addition of nickel at the C-1-C-4 bond followed by insertion of the alkyne.39 Cyclopropenones react to form benzoquinones.40 This mechanism may involve a nickel-catalyzed [2 + 2] cyclodimerization followed by a thermal isomerization or an alkene methathesis.

Norbornene derivatives41 and cyclobutenes42 have also been shown to react with alkenes under the influence of Ni(cod)2 catalyst to give [2 + 2] cycloadducts. Norbornadiene substrates can react with electron-deficient alkenes in a [2 + 2 + 2] cycloaddition pathway to give homo-Diels-Alder products (eq 15) (see also Bis(acrylonitrile)nickel(0)).43 Norbornadiene also forms [2 + 2] cycloadducts with the exceptionally reactive methylene cyclopropane.44 When this reaction was carried out in the presence of a chiral phosphine ligand, the product was obtained in an enantiomerically enriched form.

In addition to the ability to undergo [2 + 2] cycloadditions, methylenecyclopropanes have two additional and more useful [3 + 2] modes of reactivity available under nickel catalysis. As shown for the cycloaddition of methylenecyclopropane with an alkene (eq 16), distal ring opening by nickel leads to products of Type A, whereas Type B products are not a direct result of proximal ring opening, but are formed indirectly via reductive dimerization of two alkene units to give a metallacyclopentane followed by a cyclopropylmethyl/3-butenyl rearrangement. The course of the reaction is determined by many factors including the stoichiometry and physical properties of the ligands bonded to nickel, the number, type, and position of substituents on the methylenecyclopropane, and the nature of the substituents on the participating alkene. Because of these many contributing effects, it is difficult to predict which reaction pathway will be followed under any given condition; however, some general reactivity patterns can be deduced.4

While the dimerization and trimerization of methylenecyclopropanes have been investigated,45 these reactions are not useful from a synthetic standpoint. For this reason the present treatment will consider only the cycloadditions between methylenecyclopropanes and alkenes. The use of low valent nickel complexes, which have the ability to catalyze reactions of both Type A and Type B should be contrasted with the use of palladium(0) catalysts such as Bis(dibenzylideneacetone)palladium(0)46 or Pd(h3-C3H5)(h5-C5H5)47 which lead exclusively to products of Type A.4b

Unsubstituted methylene cyclopropane can form 1:1 adducts with acrylates, crotonates, and maleates to give Type B cycloaddition products in high yields (eq 17)48 (see also Bis(acrylonitrile)nickel(0)). The use of enantiomerically pure alkyl acrylates in this type of reaction gave products with up to 64% de.49 Addition of ligands requires higher reaction temperatures and results in decreased stereoselectivity.50 Ligand addition also leads to Type A cycloadducts when highly electron-deficient alkenes such as dialkyl fumarates and maleates are used. It is clear that the outcome of these reactions is highly dependent upon the electronic properties of catalyst and substrate.

Substitution at the three-membered ring generally gives rise to cycloadducts of Type B.4b When (-)-camphorsulfamylacrylate was reacted with 2,2-dimethylmethylenecyclopropane, a methylenecyclopentane was obtained with a diastereomeric excess of 98% (eq 18).51 When the substituents are phenyl groups, however, Type A cycloaddition results.4

Substitution at the double bond almost always leads to cycloaddition of Type A. In these reactions there is an added complication of regioselectivity brought about by the intermediacy of a trimethylenemethane-like species. Thus ethyl crotonate reacts with isopropylidenecyclopropane to give a mixture of the alkylidenecyclopentane (12) and the 2-substituted methylenecyclopentane (13) (eq 19).52 The use of bulky phosphite ligands and a high ligand:metal ratio favors the formation of 2-substituted methylenecyclopentanes. Alkenes without electron-withdrawing groups favor the formation of alkylidenecyclopentanes.4,53 The use of triisopropylphosphine/palladium(0) catalysts also leads to this type of product.4,54 Disubstituted alkynes can also be used in the cycloaddition to produce alkylidenecyclopentenes.55 When Triethylborane was added along with the catalyst, systems that normally reacted along a Type A pathway were induced to react via Type B.56 An interesting transannular cycloaddition was utilized to prepare a [3.3.3]propellane, although a palladium catalyst was found to be more suitable than nickel (eq 20).57 Allene can also be oligomerized by Ni(cod)2.5a,58

Finally, some strained alkanes are known to undergo oligomerization in the presence of Ni0 catalysts (see also Bis(acrylonitrile)nickel(0)). Bicyclo[1.1.0]butanes react by suffering a geminal two-bond cleavage to form an allylcarbene intermediate which may be trapped stereoselectively by an electron-deficient alkene (eq 21).59 Additionally, a nickel-catalyzed asymmetric vinylcyclopropane-cyclopentene rearrangement has been reported using Ni(cod)2 with chiral phosphine ligands.60

Oligomerization of Alkynes.

The nickel catalyzed intermolecular oligomerizations of alkynes are some of the oldest and best-studied reactions in organometallic chemistry.1,5a Tetramerization and trimerization lead to cyclooctatetraenes and aromatic molecules, respectively. The cycloaddition of two equivalents of alkyne with one equivalent of alkene provides an interesting route to cyclohexadienes.61 Addition of isocyanides leads to iminocyclopentadienes,62 insertion of CO2 gives pyrones,63 and insertion of isocyanates yields 2-oxo-1,2-dihydropyridines.64 Finally, hydroacylation of monoalkynes with aldehydes yields a,b-enones.65

The synthetic utility of these reactions is greatly increased when they are used intramolecularly with tethered alkynes since more than one ring can be formed and regiochemical problems are drastically reduced.5b Tethered diynes react with Ni(cod)2 and CO2 in the presence of trialkylphosphine ligands to give bicyclic a-pyrones (eq 22).66 The reaction is catalytic in nickel and works well with three- and four-atom tethers, but the yield suffers when the tether length is raised to five. Tricyclohexylphosphine was found to be the best choice of ligand. When other ligands were used, dimerization of the starting material was observed. It is believed that an electron-donating ligand may be required for strong CO2 coordination. The reaction is tolerant of many groups on the alkyne terminus including alkyl, hydrogen, and trimethylsilyl. The choice of these groups helps determine the regiochemistry of the CO2 addition.67 The use of aldehydes in the reaction allows for the catalytic generation of bicyclic a-pyrans (eq 23).68 In these cases the structure of the added phosphine ligand does not play a crucial role; however, the length of the tether and the choice of alkyne substituents exert a great influence on the outcome of the reaction. Other products from this reaction such as oxoalkyl-substituted cyclopentenes are explained by various hydrogen transfer isomerizations of a strained 1,2-bis(alkylidene)cycloalkane intermediate.

Diynes undergo cyclization with 2,6-dimethylphenyl isocyanide in the presence of a stoichiometric amount of Ni(cod)2 to yield polycyclic iminocyclopentadienes.69 In contrast to the previous cycloadditions, this reaction does not require a phosphine ligand, and seven-membered rings can be formed in moderate yields (47%). The products of these reactions could be hydrolyzed to the corresponding cyclopentadienones by 10-Camphorsulfonic Acid, used as diene moieties in Diels-Alder reactions, or stereoselectively substituted at the angular position by a 1,4-addition of an alkyllithium reagent.5b Enynes may also take part in this reaction to produce iminocyclopentenes with significant diastereoselectivity (eq 24).70 No reaction occurs when carbon monoxide is used as the cyclization partner; however, there are titanium, zirconium, and cobalt catalysts that are able to carry out such a transformation directly.71

Oligomerization of 1,3-Dienes.

Extensive studies of the catalytic cyclooligomerization of butadiene have shown that many different products can be obtained from the reaction, depending upon the conditions chosen.6 These products result from the variety of s- and p-allylnickel intermediates that can be formed during the catalytic cycle (eq 25).72 Low-temperature NMR studies of the reaction of butadiene with stoichiometric amounts of Ni(cod)2 have shown that the h1,h3-octadienyl complex (14) is formed after reductive coupling of the two diene units.73 This intermediate, which is probably important in the catalytic reaction as well, can isomerize to the bis-p-allyl complex (15). Reductive elimination to regenerate a nickel(0) species from these and other allylnickel(II) complexes can lead to 4-vinylcyclohexanes (VCH), 1,2-divinylcyclobutanes (DVCB), or 1,5-cyclooctadienes (cod). Insertion of alkenes or dienes into these intermediates leads to larger ring systems.6a These intermediates can also be induced to react with carbonyl compounds, but they are much less nucleophilic than p-allylhalide-like complexes.74

Due to the lack of general methods for the production of medium-sized rings, the synthetic value of these reactions lies especially in their ability to generate cyclooctane ring systems. Control of the cyclodimerization of 1,3-dienes to give cyclooctadiene products preferentially is contingent upon the substitution of the diene as well as the composition of the catalyst system, with both of these factors being mutually dependent. The choice of modifying ligand is extremely important in the selectivity of the reaction, with both electronic and steric parameters being important.75 It has also been shown for some functionalized dienes that the method of generation of the Ni0 catalyst is important to this selectivity. Catalysts generated from the reduction of nickel(II) salts such as Ni(acac)2 with alkylaluminum derivatives favor formation of VCH derivatives, while aluminum-free reagents such as Ni(cod)2 give preference to the cod products.76 Several iron and cobalt catalysts have been reported to selectively catalyze cod production, while titanium, iron, and manganese catalysts have led to VCH products. Palladium catalysts, on the other hand, give DVCB products.5a

When [4 + 4] cycloadditions occur, simple alkyl substituents on the diene usually give 1,5-disubstituted cod products as a result of initial head-to-tail linking, but mixtures of products are inevitable.1 Functional groups that can coordinate to nickel act as internal ligands to guide the reaction. Thus methyl 2,4-pentadienoate cyclodimerizes regio- and stereoselectively to give a trans-1,2-disubstituted cyclooctadiene (eq 26).77 While ester and silyl ether substituents worked well in this reaction, amino and amido groups were not tolerated.

The synthetic utility of these reactions is significantly enhanced when the cycloadditions are carried out intramolecularly. When the diene units are connected by a tether, the production of byproducts can be essentially eliminated.78 Terminally linked dienes undergo Type I [4 + 4] cycloadditions, giving rise to cis-fused bicycles when the tether length is three atoms, and trans-fused systems when the tether length is increased to four atoms. Diastereoselectivity is observed in these cycloadditions when substitution is made at an allylic position, and the degree of selectivity is related to the size of the substituent (eq 27).79 A nickel catalyzed intramolecular [4 + 4] cycloaddition was used as the key step in the synthesis of (+)-asteriscanolide (eq 28).80 Furthermore, when one of the diene fragments is connected at an internal position, bridged ring-systems can be assembled (eq 29).81 This Type II class of cycloaddition was used in an approach to the taxane skeleton.82 When trisubstituted dienes are used as substrates for the reaction, angularly substituted bicycles can be obtained.

These reactions can also be tailored to give 1,4-cyclohexadiene products if an alkyne is tethered to the diene (eq 30).83 These catalytic [4 + 2] cycloaddition reactions are an attractive alternative to the Diels-Alder reaction because they are very mild, tolerant of a wide array of functional groups, and are exempt from the restrictive electronic substitution requirements of the thermally activated reaction. If the alkyne portion of the molecule is replaced by an alkene, however, a rhodium catalyst must be used.84 When an allene is used the regiochemistry of the cycloaddition can be controlled by the choice of catalyst (eq 31).85

Linear dimerizations of 1,3-dienes may be induced by the addition of a hydrogen donor such as an amine, alcohol, or an aminophosphinate ligand to the catalyst.5a,86 When a chiral aminophosphinate ligand was used, piperylene was dimerized to a 21:70 ratio of the two head-to-head 1,3,6-octatriene isomers having ~90% and 35% ee, respectively.87

Other Uses.

Sulfur heterocycles can undergo hydrodesulfurization and ring contraction in the presence of nickel(0) complexes.88 Symmetrical aromatic ketones may be prepared from aromatic carboxylic acids through the coupling of their S-(2-pyridyl) derivatives.89 Alkenes can be formed from 1,2-diols by the stereospecific cleavage of thionocarbonates.90 Alkane- and alkenecarboxylic acids can be generated from the reaction of alkenes with a nickel(0) catalyst and CO2.91 Geminal dihalides react with nickel(0) catalysts and electron deficient alkenes to give cyclopropanes.92

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Paul A. Wender & Thomas E. Smith

Stanford University, CA, USA

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