Nickel jt-allyl complexes

The bond between the carbon atoms a and (3 to a C-C double bond can be broken by a transition metal with formation of a Jt-allyl intermediate providing the driving force. Whereas stoichiometric reactions of this sort are yet to appear, jt-(allyl)metal intermediates are occasionally involved in catalytic C-C bond cleaving reactions. The nickel catalyzed skeletal rearrangement of 1,4-dienes involves the formation of an olefin coordinated Jt-(allyl)nickel complex (99) [118]. 

The Jt-allyl-nickel complexes react on alkyl, allyl and even aryl halides to give the organic products resulting from the coupling. It is probable that these reactions proceed by oxidative addition followed by reductive elimination. 

A) Cross coupling reaction of allyl halides are efficiently run in the presence of nickel (or palladium (0)) complexes. The key intermediate is a jT-allyl nickel complex resulting from an oxidative addition of one of the two allylic bromides implied in the coupling process. 

A detailed mechanistic study of the coupling of jt-allyl complexes and organic halides has been carried out.l A mechanism involving the establishment of a pre-equilibrium between bis(allyl)nickel complexes and monoallyl halo-bridged dimers is proposed. A single-electron-transfer mechanism then initiates cross coupling via nickel(I) intermediates. 

An early example of the application of this oxidative addition reaction to a catalytic process is the DuPont s production of adiponitrile, which includes a nickel-catalyzed isomerization reaction of allylic cyanides (62 -> 63 in Scheme 6.11) [33]. Thorough mechanistic studies identified a jt-allyl nickel cyanide complex formed via oxidative addition of a C-CN bond as the intermediate for this isomerization reaction [34]. 

Allylic malonate 100 completely isomerizes to the thermodynamically favored linear isomer 101 on treatment with a palladium catalyst [119]. Formation of a stabilized carbanion and Jt-(allyl)palladium species facilitates the C-C bond cleavage. Analogous isomerization is also catalyzed by a nickel complex [120]. These results demonstrate that the transition metal-catalyzed nucleophilic substitution of an allylic substrate with a carbon nucleophile is reversible, if the cleaved nucleophile is sufficiently stabilized. 

Perhaps the most useful of the r 3 allyl complexes (cf. 24) are the Jt-allyls of nickel.15 The simplest type 61 are rather unstable and form the bromide-bridged complex 62 on treatment with HBr. These are stable compounds officially complexes of Ni(I) but better regarded for our purpose as dimers of r 3 complexes of allyl anions and Ni(II), much as allyl Grignard reagents 2 can be regarded as o-complexes of allyl anions and Mg(II). Direct exchange of Mg(II) for Ni(II) gives the unstable complexes 61, but the stable dimer 62 can be made by oxidative insertion of Ni(0), as its cyclo-octa-1,5-diene (COD) complex, into allyl bromide 1. 

Nickel complex compounds Nickel(I) jt-allyl complex Ni(CO),... 

A variety of nickel(O) complexes, when treated tvith allyhc electrophiles, afford jt-allyl complexes (see also Houben-Weyl, Vol. E18, pp 64 and 76).l In early studies, tetracarbon-ylnickel(O) was widely employed. However, otving to its extreme toxicity, it is now rarely used. Direct treatment of bis(T] -cycloocta-l,5-diene)nickel(0) (2) with allyl halides such as 20 is now the method of choice for the stoichiometric preparation of nickel-3t-allyl complexes. In the absence of strong donor ligands such as phosphines, halo-bridged dimers (e.g., 21) are t5T3ically obtained (Scheme 12). In the presence of phosphines, monomeric species such as 22 may be obtained. Other less-electrophilic allylic substrates such as allylic ethers and allylic alcohols also serve as precursors to nickel-Jt-allyl complexes in cataljdic procedures. However, these precursors are less widely used than allyl halides in the stoichiometric preparation of the Jt-allyl complexes. 

Electrophilic nickel(II) salts, when treated with allyl organometallics, afford nickel(ll)- t-allyl complexes. Rather than producing dimeric halo-bridged complexes as observed by the oxidative addition route (Section 1.1.2.1), monomeric bis(aUyl) complexes are instead obtained (Scheme 13). Although both the method described in Section 1.1.2.1 and this method afford structurally different jt-allyl complexes, disproportionation may, in some instances, allow the interconversion of a bis(allyl) complex 23 and the corresponding halo-bridged dimer 24 if a dihalonickel species is present. 

Scheme 16 Couplings of Nickel-Jt-Allyl Complexes with All

Enal-derived nickel-Jt-allyl complexes are efficient partners in cross-coupling reactions with alkenyl- and arylstannanes leading to products such as 39 (Scheme 23).P 1 As described for the aUylic ether derived complexes, the mechanism of the coupling process involves transmetalation followed by reductive elimination. The reaction is catal hic in nickel, unlike couplings of enals with allstoichiometric quantities of nickel. Many other variants of nickel-catalyzed conjugate additions may involve Ji-allyl complexes, but these are treated separately in a section on alkenes (Section 1.1.4.2) because the mechanism is poorly defined in most nickel-catalyzed conjugate additions. 

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... 

Carbonylative cyclizations have also been developed that likely involve the insertion of aUgmes into nickel-jt-aUyl complexes. I Chiusoli, who was the pioneer in this area, demonstrated that either cyclic or acyclic products may be isolated depending on the concentration of methanol (Scheme 27). Allyl halides are the more commonly used allyl complex precursor, and enals are also utilized an example of the latter is the formation of 44 and 45. ... 

The cyclization of allylic acetates tethered with alkenes has been extensively investigated by Oppolzer, employing a number of transition metals including nickel. The process has been termed a metallo-ene reaction however, the mechanism likely involves a sequence of oxidative addition to generate a jt-aUyl complex, alkene insertion, and P-hy-dride elimination. The process is quite general in scope and provides a very useful method for the preparation of 1,4-dienes such as 46 (Scheme 28). 

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