Pyridines solvent-free approach

A reversible solid-state HCl elimination reaction from a Cu(ii) pyridinium coordination complex has been reported. The reaction proceeds with a colour change from yellow (pyridinium complex) to blue (pyridine complex). This reaction suggests that other protic ligands may successfully be coordinated to metals using a solvent free approach in the future. 

As CH-acids in the MCRs with aldehydes and aminoazoles, other classes of organic compounds were used as well. Cyanoacetic acid derivatives, acetoyl(aroyl) acetonitriles, ketosulfones, acetophenones, and other reagents were successfully introduced into these three-component heterocyclizations. For example, synthesis of pyrazolo[3,4-b]pyridine-5-carbonitriles 40 was carried out as the multicomponent treatment of 5-aminopyrazole, aldehyde, and benzoylacetonitriles solvent-free by fusion either in ammonium acetate at 120°C or in boiling ethanol with EtsN (Scheme 17) [69]. The second approach gave the worst results from the viewpoint of yields and purity of the target compounds. 

Two novel protocols have been developed for the direct conversion of 1,2,4-triazines 27 into highly substituted pyridines 28 via the inverse-electron demand Diels-Alder reaction a microwave-assisted solvent-free route and a tethered irnine-enarnine approach <05JOC10086>. 

Recently this approach has been extended to parallel combinatorial library synthesis. One example is the solvent-free synthesis of a 96-member library of substituted pyridines via a one-step Hantzsch-3CC conducted in 96-weU micro titer filter plates (Scheme 2.3-12). Here the p-ketoester and aldehyde reagents were impregnated onto a 5 1 bentonite clay-ammonium nitrate mixture. Irradiation for 5 min in a domestic MW oven, followed by washing the product off the support into a receiver daughter plate gave substituted-pyridine products in >70% purity overall. 

It has to be assumed that these processes are occurring on the boundary between SN1(P) and SN2(P) mechanisms in whose transition states considerable P—0(—Ar) bond cleavage takes place. The lifetime of the resulting, more or less free metaphosphate anion 102 then depends upon the nucleophilicity of the surrounding solvent. With pyridine, for example, a very fast reaction occurs so that the overall process approaches an SN2 reaction. Acceleration of the reaction by amines such as 2,6-lutidine, which are disqualified from acting as nucleophiles by steric hindrance, or by solvents such as dioxane, whiche are presumably too... 

In a supramolecular approach to fullerene-porphyrin hybrids, the assembly of a rigidly connected dyad, in which a zinc tetraphenylporphyrin, Zn(TPP), is noncovalently linked to a C60 derivative via axial pyridine coordination to the metal, was reported [219-222]. Photo excitation of the dyad Zn-complex led to electron transfer with very long lifetimes of the charge-separated pairs, as revealed by optical spectroscopy and confirmed by time-resolved electron paramagnetic resonance spectroscopy. Accordingly, two different solvent-dependent pathways can be considered for the electron-transfer processes. Either the excitation of the porphyrin chromophore is followed by fast intramolecular electron transfer inside the complex, or alternatively the free porphyrin is excited undergoing intermolecular electron transfer when the acceptor molecules ap-... 

To further test pyridine-N-oxide as an indicator, the solvatochromic data are correlated versus micelle radius in Figure 5. The Xmax values for pyridine-N-oxide were determined in solutions of. 025 M AOT in two solvents, n-bctane d n-hexane. For a given Wq the values are very similar in the two solvents. The micelle radii are from the photon correlation experiments of Zulauf and Eicke (, at the same AOT concentration, for a similar solvent, isooctane. It is widely accepted that Wo is a good indicator of the size of reverse micelles, as is evident in the relationship for the two horizontal axes in the figure. There is a relatively linear relationship between micelle size and Xmax for a Wq up to 15. The Xmax this point approaches that of pure water. This is in accord with Eicke and Kvita (28). who indicate that at a Wq of approximately 15, the water pool has the characteristics of free water. These results supply additional evidence of pyridine-N-oxide s hydrophilic nature and utility as an indicator. 

Reaction of the hydrochloride derivative of the PEI fraction of a MW of 1800 liberated 94% of the anticipated hydrogen within a few hours. In contrast with this the 10000 MW fraction produced only 86% of the theoretical amount of H2. These two materials were obtained as free flowing powders after solvent evaporation which retained the sodium chloride by-product. Attempts to separate the salt by ultrafiltration or dialysis were partially effective because the polymers behaved as ion exchangers and retained chloride ions. An alternate synthetic approach to the cyanoborane derivatives which would not produce salt as a byproduct was explored. In this case, previously formed pyridine.BH2CN was used in a transamination reaction with neutral PEI producing a derivative that showed 59% derivatization. Attempts to derivatize polyallylamine hydrochloride, PAA, by reaction with NaBH3CN failed even in boiling water under reflux. Apparently this marked difference with PEI stems from differences in pKa values of the two hydrochlorides which was measured to be about 5.0 for PEI and 7.0 for PAA. 

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