Nickel coupling scheme

The presence of a remote C=C double bond in the alkyl halide was critical saturated analogs underwent nickel-catalyzed halogen-zinc exchange.248 It has been suggested that the double bond coordinates to the nickel atom. As a 7r-acceptor, the C=C bond removes some electron density from the metal, thus facilitating the reductive coupling (Scheme 154).246... 

Bromothiophenes give cross-coupled products by reaction with Grignard reagents in the presence of a nickel catalyst (Scheme 71) (80TL4017). 

Another example of promising research is the efficient electrochemical dicarbo-xylations of aryl-acetylenes with C02, using an uncomplicated bimetallic redox couple as the catalytic system. In this case, metallic nickel was used as the cathode and aluminum as the anode, to generate in situ carboxylation-active nickel species (Scheme 5.20) [61]. 

Regiospecific formation of poly(3-hexylthiophene) by a nickel-catalysed cross-coupling (Scheme 6)... 

Kappe and Stadler have developed an MW procedure for rapid production of triaryl phosphines by coupling diphenylphospine with aryl halides and triflates [134]. Taking into account the importance of phosphine ligands in a variety of transition metal-catalyzed reactions, convenient procedures for their production is valuable. Both homogeneous Pd-Ni and heterogeneous Pd catalysts were explored and the more unusual substrate phenyl triflate could also be coupled swiftly by use of nickel catalysis (Scheme 15.68). Couplings with other aryl halides proceeded in 26-85% yield after 3-30 min microwave irradiation at 180-200 °C. 

In contrast to the previous methods involving reductive coupling of two 71-components, a method involving participation of the sigma bond of an epoxide has been demonstrated. Treatment of a monosubstituted epoxide and alkyne with EtaB in the presence of a nickel catalyst generated from Ni(cod)2 and PBus affords homoallylic alcohols from reductive coupling (Scheme 3-67). The process proceeds with retention of epoxide stereochemistry, and intramolecular versions are endoselective. These aspects can be explained by initial oxidative addition of Ni(0) to... 

Changing the enolate counter ion can also be beneficial. Reformatsky reagents 2.494 are zinc enolates, generated indirectly. These reagents can couple with aryl halides using either palladium or nickel catalysis (Scheme 2.141). This may also be considered as a variant of Negishi coupling (Section 2.3). 

The transition-metal-catalysed exchange of halogen atoms may also be viewed as a heteroatom coupling reaction. It is particularly useful for the conversion of the cheaper and more available chloro compounds to the more reactive iodo compounds. Nickel catalysis may be used (Scheme 2.185). Cyanide can also be coupled (Scheme 2.186). The use of hydride sources, such as formate or tri-n-butyltinhydride is a method of removing functionality (Scheme 2.187). The mechanism of formate reduction is illustrated in Scheme 5.50. 

Transmetallation of the organic group from zirconium to another metal opens up possibilities. The palladium-catalysed coupling reactions can be found in Section 2.4. Addition of dimethyl cuprate results In transmetallation to copper. The resulting cuprate then displays typical cuprate reactivity, such as addition to enones. More economically, small amounts of copper can catalytically activate the zirconium complex towards this kind of chemistry, although the precise mechanism is unclear. Additions to enones can also be achieved directly using nickel catalysis (Scheme 5.64). Transmetallation to zinc has also been demonstrated. ... 

In the asymmetric cross-coupling of the zinc reagent 36 catalyzed by Pd/(5)-(/ )-PPFA (5), ( )-/3-bromostyrene (20) " and (E)- and (Z)-l-bromo-2-(phenylthio)ethenes (39) have also been used. Enantiomerically enriched alkenyl sulfides 40 could undergo the second cross-coupling, the sulfide being replaced by the Grignard reagent in the presence of a nickel catalyst (Scheme 8). " ... 

An elegant synthesis method which is specific to sulfone polymers containing phenyl—phenyl linkages (such as PPSF) is the nickel-catalysed coupling of aryl dihahdes. The scheme for this synthesis involves a two-step process. First, an aromatic dihaUde intermediate is formed which carries the backbone features of the desired polymer. This aromatic dihahde intermediate is then self-coupled in the presence of sero-valent nickel, triphenylphosphine, and excess sine to form the biphenyl- or terphenyl-containing polymer. AppHcation of this two-step scheme to PPSF can be depicted as follows ... 

Scheme 8.30 Nickel-catalyzed reductive coupling of alkynes with epoxides.

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