Heck reaction pyridine ligands

Regarding bis-NHC chelating ligands, several structures that differ in the motifs used for the enlargement of the tether have been proposed as catalysts for the Mizoroki-Heck reaction. They range from non-functionalised aliphatic chains [23-25] to phenyl [26], biphenyl [27], binaphthyls [28] and to chains containing additional coordination positions like ethers [29], amines [30], and pyridines in an evolution towards pincer complexes [31-35], In most cases, the activity of aryl bromides in Mizoroki-Heck transformations was demonstrated to be from moderate to high, while the activation of chlorides was non-existent or poor (Scheme 6.7). 

By taking advantage of the C(2) activation, 2-allyloxy-3-iodopyridine (173) was prepared by an SNAr displacement of 2-chloro-3-iodopyridine with sodium allyloxide [137]. 2-Chloro-3-iodopyridine was prepared by orrto-lithiation of 2-chloropyridine followed by iodine quench. The intramolecular Heck reaction of allyl ether 173 under Jeffery s ligand-free conditions resulted in 3-methylfuro[2,3-6]pyridine (174). 

Formation of the antiasthmatic imidazoquinoline compound was achieved through the closure of the central pyridine ring in a heteroaryl Heck reaction (4.28.), The best results were obtained in the presence of tetrabutylammonium chloride without any added ligand (Jeffery s ligand free variant).32... 

A number of bidentate ligands containing pyridine and other azine binding sites were described, involving five-membered [262], six-membered [263] and macrocyclic [262] chelate rings. All these ligands afford SRPCs active in type 1 Mizoroki-Heck reactions. Similar activity was recorded for phenanthroline-derived palladium complexes [264]. 

Kawano, T., Shinomaru, T. and Ueda, I. (2002) Highly active Pd(II) catalysts with trans-bidentate pyridine ligands for the Heck reaction. Org. Lett., 4, 2545-7. 

Pfaltz and coworkers [43] developed P,N ligands 73 and 74 derived from pyridine and quinoline. They reasoned that oxazoline and pyridine/quinoline ligands, since they have different electronic effects, would induce different patterns of reactivity and selectivity in the asymmetric Mizoroki-Heck reaction. In the reaction of phenyl triflate (2) with... 

A theoretical study has been reported of the Heck reaction of electron-deficient arenes with alkenes catalysed by palladium(n) carrying a pyridine-type ligand. 

In 1975, three different protocols were available in the literature, each describing the synthesis of internal alkynes. Cassar described palladium- or nickel-mediated reactions between aryl or vinyl halides and alkynes complexes with phosphine as ligands in the presence of NaOMe [1]. As a second protocol, Heck pubhshed a variation of the Mizoroki-Heck couplings, in which the olefins were replaced by alkynes and coupled with (hetero)aryl, as weU as alkenyl bromides or iodides at 100 °C in the presence of a basic amine [2]. More than a decade earUer, Stephens and Castro had described the details of a palladium-free coupling of aryl iodides with cuprous acetylides in refluxing pyridine [3]. 

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