Pyridine-derived transition

Transition metal complex-mediated hydroacylation of pyridine derivatives with aldehydes 99SL1. 

Transition metal complexes have been used in a number of reactions leading to the direct synthesis of pyridine derivatives from acyclic compounds and from other heterocycles. It is pertinent also to describe two methods that have been employed to prepare difficultly accessible 3-alkyl-, 3-formyl-, and 3-acylpyridines. By elaborating on reported194,195 procedures used in aromatic reactions, it is possible to convert 3-bromopyridines to products containing a 3-oxoalkyl function196 (Scheme 129). A minor problem in this simple catalytic process is caused by the formation in some cases of 2-substituted pyridines but this is minimized by using dimethyl-formamide as the solvent.196... 

Effectively, this is another example of the addition of a functional aromatic compound to an alkene, as the Murai reaction, but the mechanism is different. Alkyl substituted pyridine derivatives are interesting molecules for pharmaceutical applications. The a-bond metathesis reaction is typical of early transition metal complexes as we have learnt in Chapter 2. 

D networks based on the lanthanides with their high coordination numbers (typically 7-11) are less predictable than those based on transition metals and the high oxophilicity of the lanthanide (III) ions means that they do not bind strongly to ligands such as pyridine derivatives. As a result lanthanide coordination networks are less common in the literature however, they represent tremendous potential in terms of ever more complex topologies. 

In complexes of pyridine derivatives with non-transition and early transition elements along with the rjl(N) and rfijt) modes, rjl(C) and mixed coordination modes, e.g., rjl(N) rf n) rf n), are observed. 

Although not a subject of this chapter, Toney and coworkers have quantitated the reaction coordinate of a PLP-dependent L-alanrne racemase [15]. Despite the expectation that the cofactor provides resonance stabilization of the carbanion/enolate anion (quinonoid) intermediate derived by abstraction of the a-proton, the spectroscopic and kinetic analyses for the wild type racemase at steady-state provided no evidence for the intermediate in the reaction catalyzed by the wild type enzyme. Indeed, Toney had previously demonstrated that a kinetically competent quinonoid intermediate accumulates in the impaired R219E mutant [16] Arg 219 is hydrogen-bonded to the pyridine nitrogen of the cofactor. For the wild type racemase, the derived transition state energies for conversion of the bound enantiomers of alanine,... 

The low-lying states responsible for the visible-near UV absorption and photo physical/chemical behaviour of mixed-ligand transition metal carbonyls M(CO)5L (Cr, Mo or W in d6 configuration, L=pyridine derivative or piperidine) were usually interpreted in terms of interplaying low-lying MLCT and Metal Centred (MC) electronic transitions [6]. According to recent theoretical studies the interpretation based on the assignment of weak spectral features to MC transitions is not necessarily correct DFT [88] and CASSCF/... 

The initial product 2 loses N2 in a retro-DiELS-Alder reaction forming the 3,4-dihydropyridine 3, which aromatizes giving the pyridine derivative 4 by elimination of amine or alcohol. The geometry of the transition state of this [4+2] cycloaddition with inverse electron demand follows from the reaction of 3- or 6-phenyl-1,2,4-triazine 5 or 8 with enamines of cyclopentanone. It is apparently influenced by the secondary orbital interaction between the amino and phenyl groups. 3-Phenyl-1,2,4-triazine 5 favours the transition state 11. It leads first to the 3,4-dihydropyridine 6 which, on oxidation followed by a Cope elimination, affords the 2-phenyldihydrocyclopenta[c]pyridine 7. However, 6-phenyl-1,2,4-triazine 8 favours the transition state 12 leading to 3,4-dihydropyridine 9. Elimination of amine yields 5 -phenyldihydrocyclopenta[c]pyridine 10 ... 

Likewise when two alkyne molecules coordinate to a transition metal such as Co(I) with subsequent coupling of the C-C bond, oxidative cyclization takes place to give a metallacyclopentadiene. Further reaction of another alkyne molecule with the metallacyclopentadiene followed by reductive elimination liberates benzene derivatives. Thus cyclotrimerization of three alkyne molecules catalyzed by a cobalt complex [40,41] can be performed. If a nitrile is used as the second component, pyridine derivatives are obtained catalytically as shown in Scheme 1.13 [42]. The catalytic cyclotrimerization and cyclodimerization of alkynes and conjugated enynes have found extensive applications in synthesis of complex cyclic compounds such as steroid derivatives [43]. 

Another approach of using transmetallation process is to transfer the organic moiety bound with early transition metal complexes to late transition metal complexes such as nickel to utilize the reactivity of the diorganonickel complexes to undergo reductive elimination. Various benzene and pyridine derivatives have been prepared by the methodology [100]. 

Consequently, pyridine has a reduced susceptibility to electrophilic substitution compared to benzene, while being more susceptible to nucleophilic attack. One unique aspect of pyridine is the protonation, alkylation, and acylation of its nitrogen atom. The resultant salts are still aromatic, however, and they are much more polarized. Details for reactivity of pyridine derivatives, in particular, reactions on the pyridine nitrogen and the Zincke reaction, as well as C-metallated pyridines, halogen pyridines, and their uses in the transition metal-catalyzed C-C and C-N cross-coupling reactions in drug synthesis, will be discussed in Section 10.2. 

Although the understanding of IPTC is still limited in comparison to that of normal PTC, it is clear that this method is apt to effect organic reactions in water. As discussed in the introduction simple pyridine derivatives were introduced mainly in connection to kinetic and mechanism studies, but more recently, it was shown that the use of host compounds such as cyclodextrins and water-soluble calix[n]arenas,offers the opportunity to combine IPTC, molecular recognition, and transition metal catalysis, with additional economic and environ-metal advantages deriving from the use of water as a bulk solvent. 

The scope of various pyridine derivatives with 1-hexene was examined with L45c-Sc/[Ph3C][B(C6F5)4] as the catalyst (Table 5.8). The asymmetric alkylation of 2-Me, 2-Et, 2- Pr, 2- Bu, and 2-phenyl-substituted pyridines 188 could also be achieved similarly in high yields (83-94%) and enantioselectivity (up to 94 6 er) (entries 1-5). Notably, the C—H bond activation reaction occurred selectively at the pyridine unit rather than at the phenyl group in the case of 2-phenylpyridine, which is in contrast with the reactions catalyzed by late transition metal complexes. The iodo, bromo, and chloro substituents in the picoline substrates are well compatible (entries 6-8). No alkylation reaction was observed with unsubstituted pyridine or quinoline, probably due to the poisoning effect of the N atom of pyridine to the metal center. 

The pyridine derivatives, the monodentate l-(2-methyl-5-phenyl-3-thienyl)-2-(2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene and the bidentate 1,2-bis (2-methyl-5-(4-pyridyl)-3-thienyl)perfluorocyclopentene, incorporating the DTE moiety as the photochromic unit (4-py-DTEf [50, 51] and 4-py2-DTEf [52]) and the nonfluorinated analog (4-py2-DTE) [53] have been coordinated to various transition metals, allowing the change of physical properties of metal complexes. 

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