Pyridine ligands coupling reactions

Pyridine compounds 45 can also be produced by the NHC-Ni catalysed cycloaddition between nitriles 43 and diynes 44 (Scheme 5.13) [16]. The SIPr carbene was found to be the best ligand for the nickel complex in this reaction. The reaction required mild reaction conditions and low catalyst loadings, as in the case of cycloaddition of carbon dioxide. In addition to tethered aUcynes (i.e. diynes), pyridines were prepared from a 3-component coupling reaction with 43 and 3-hexyne 23 (Scheme 5.13). The reaction of diynes 44 and nitriles 43 was also catalysed by a combination of [Ni(COD)J, NHC salts and "BuLi, which generates the NHC-Ni catalyst in situ. The pyridines 45 were obtained with comparable... 

Various phenyl-substituted ketimines and aldimines react with metallocenes 1 and 2, in a manner that depends on the substituents present [41]. In all cases, elimination of the al-kyne is observed. Complex 2b reacts with PhN=CMePh to give the r 2-complex 64, which is stabilized by an additional pyridine ligand [41a], In the reactions of 1 or 2a with the ketimine HN=CPh2, hydrogen transfer generates complexes 65. Two molecules of the aldimine PhN=CHPh are coupled by 2a to give the cyclic diamido complex 66 [41b]. 

ESMS has been used to characterize the intermediate Nin-complexes formed in the coupling reaction of 2-bromo-6-methylpyridine in the presence of Raney nickel (Scheme 1) [45]. The composition of the intermediate had already been determined previously by elemental analysis, but the ES mass spectra, showing a strong peak for the ion [Ni2(dmbp)2Br3]+, pointed to a dimeric structure. It was concluded that this ion was formed by the loss of Br from the dimeric structure 1. An alternative explanation is that the intermediate has the more common four-coordinate structure 2, and that the observed peak was due to the ion-paired species [Ni2(dmbp)2Br2]2++Br. The dimeric nature of the intermediate was confirmed by a cross experiment when mixtures of differently substituted pyridines were reacted, mixed ligand dinickel species were observed in the ES mass spectra. 

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