Nickel catalysis substitution

Several reports have been made of a successful catalyzed addition/ substitution reaction resulting in direct attachment of phosphorus to aromatic rings. The preparation of mixed triarylphosphines has been accomplished by the reaction of tin- or silicon-substituted diphe-nylphosphines with aryl halides catalyzed by palladium reagents.74 A similar transformation has also been reported using nickel catalysis.75 The addition/substitution of diphenylphosphine to triflate functionalized phenolic linkages has been of use for the preparation of substances as analogues of tyrosine-related amino acid derivatives, accomplished with catalysis by palladium acetate (Equation 4.29).76... 

A remarkable product selectivity is also observed in the case of methylenecyclopropanes with geminal diphenyl substitution. Whereas under nickel catalysis [Ni(cod)2 at SO-TO C] 18 is selectively dimerized to tra7w-l,l,6,6-tetraphenyldispiro[2.1.2.1]octane, which can be obtained in 28% yield at a conversion of 40%, substrate 18 reacts in a completely different manner with palladium(O) catalysts derived from (t/ -allyl)( -cyclopentadienyl)palladium(II) and triisopropylphosphane. Besides isomerization to 19, proceeding at temperatures above 85 °C, the monospiro derivative 20 is formed as the major product. Additionally, minor amounts of a formal [3 + 3] dimer 21 can be isolated. The latter probably arises from a palladium-mediated, stoichiometric reaction as the yield of 21 could not be improved under any conditions in catalytic runs. On prolonged heating, thermal isomerization of the methylenecyclopropanes to form... 

Of course, it must be pointed out that the reaction of benzonitrile with the amine needs the nickel catalysis the reaction without NiBr2 affords only tiny amounts of the N-substituted benzamide. 

Nickel catalysis has also been used in the formation of biaryls, such as (51), by substitution of the methoxy group in 1-methoxynaphthalene by tolylmagnesium bromide. It is also reported that the reaction of aryl or heteroaryl tosylates with phenylmag-nesium bromide to give biaryl derivatives is catalysed by palladium complexed with heteroatom-substituted secondary phosphine oxide ligands. 

The nickel-catalyzed Suzuki-Miyaura-type C-O bond arylation has been successfully applied to steroidal architecture (Scheme 4). A hydroxyl moiety in estrone can readily be substituted by an array of aryl groups under nickel catalysis via conversion into pivalate instead of the typically used triflate [38]. The carbonyl moiety in estrone also serves as a suitable precursor for an alkenyl C-O electrophile. Treatment of estrone with 2-propenyl acetate affords the compounds bearing two acetate groups, both of which are potentially reactive toward nickel-catalyzed cross-coupling. However, selective arylation took place at the alkenyl position, and... 

As classically observed in coupling reactions, electron-poor arenes were more reactive. Also, steric hindrance and ort/ro-substitution were not really deleterious, as generally observed in nickel catalysis. 

In recent years there has been a tendency to assume that the mechanisms of substitution reactions of metal complexes are well understood. In fact, there are many fundamental questions about substitution reactions which remain to be answered and many aspects which have not been explored. The question of associative versus dissociative mechanisms is still unresolved and is important both for a fundamental understanding and for the predicted behavior of the reactions. The type of experiments planned can be affected by the expectation that reactions are predominantly dissociative or associative. The substitution behavior of newly characterized oxidation states such as copper-(III) and nickel (III) are just beginning to be available. Acid catalysis of metal complex dissociation provides important pathways for substitution reactions. Proton-transfer reactions to coordinated groups can accelerate substitutions. The main... 

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

Moderate to good enantioselectivities were obtained for nearly all examples, but the products from 83a-c could be recrystallized to higher enantiomeric purity. Addition of iodine was critical for catalysis as was the use of a ligand with electron-poor para-fluorophenyl groups on the phosphorous atom. Substitution at the 3 position of the pyridine ring was described as being difficult for both the quinolines and pyridine systems. The resulting hydrazine derivatives could be easily converted to piperdines by reduction with Raney nickel or under Birch conditions. 

Two other Ni(CO)4 substitutes, Ni(CO)3PPh3 and Ni(COD)2/dppe, prove to be appropriate for the catalysis of tandem metallo-ene/carbonylation reactions of allylic iodides (Scheme 7)399. This process features initial oxidative addition to the alkyl iodide, followed by a metallo-ene reaction with an appropriately substituted double or triple bond, affording an alkyl or vinyl nickel species. This organonickel species may then either alkoxycar-bonylate or carbonylate and undergo a second cyclization on the pendant alkene to give 51, which then alkoxycarbonylates. The choice of nickel catalyst and use of diene versus enyne influences whether mono- or biscyclization predominates (equations 200 and 201). 

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