Pyridine—continued reactivity

Dewar and Maitlis143 discussed quite successfully the course of nitration in series of pyridine-like heterocycles in terms of the Dewar reactivity numbers. There is a continuing interest in the electronic structure of pyridine65, 144-140 a model of this compound has been studied by the ASP MO LCAO SCF (antisymmetrized products) method in the 77-electron approxition.146 The semi-empirical parameters146 were obtained from the most recent values of ionization potentials and electron affinities, and bicentric repulsion integrals were computed theoretically. 

Catalytic hydrogenation is the most frequently employed method of saturating the pyridine ring. Complete reduction to the piperidine normally occurs, the intermediates formed being reactive under the conditions employed. Heterogeneous catalysts continue to be the most popular method for a variety of uses ranging from the synthesis of intermediates to the denitrogenation of fossil fuels. Extensive reviews on the reduction of pyridines have been published. ... 

Continuing the analogy with pyridine reactivity, methyl groups at the 2-positions of 1,3-azoles and the... 

In the final deactivation mode reported by the authors, the active acidic sites of the catalyst are poisoned (7 = 145°C, P = 50 bar) by continuous addition of a very dilute solution of pyridine to the reacting mixture over a period of 12 h (see figure 11.10). The catalyst can be reactivated by heating and compressing the reaction mixture to conditions well within the mixture critical region (7 = 250°C, P = 500 bar). Tiltscher and coworkers report that the catalyst poison is precipitated from the product solution as pyridinium chloride. Presumably only a very small amount of pyridinium chloride is needed to deactivate the catalyst since supercritical hexene probably would not be able to solubilize much of this salt. It is surprising, however, that supercritical hexene can overcome the acid-base interactions that are occurring on the catalyst surface and, hence, remove the pyridinium chloride. 

Interest in photochromic systems other than those based on the hexa-fluorocyclopentene moiety continues to grow. The photochemical reactivity of the two photoswitches (35) is similar, and irradiation is efficient with conversions of 85% and quantum yields of around 0.6. The novel photo-chromic systems (36) undergo reversible ring closure in a reaction analogous to that observed in the bisthienyl system. Qin et al. have studied the novel pyridyl substituted cyclopentene system (37). This undergoes photocyclization with an enhanced quantum yield when the reactions are carried out in the presence of a metal. The pyridine units are capable of co-ordinating with the metal. The photochromic dithienylethene unit tethered to 3-cyclodextrin (38) has been used as a photoswitch to control the uptake of porphyrin. A series of new photochromic molecules (39) have been synthesized and studied. These exhibit the usual cyclization on irradiation. " The terthiophene derivatives (40) exhibit reversible photochemical cyclization (at 313 nm) and reversion (at wavelengths >460 nm) reactions. The cycles can be carried out many times... 

Aluminum chloride has been known for a long time to catalyze this reaction. H owever, its high acidity leads to low selectivity for alkylate. Acidic chloroaluminates proved to be interesting alternative catalysts and solvents [28] because it is possible to tune their Lewis acidity by adjusting their composition. The alkylation of ethylene or butene with isobutane has been performed in continuous-flow pilot plant operation at IFF. The feed, a mixture of olefin and isobutane, is pumped continuously into the well-stirred reactor, which contains the IL catalyst In the case of ethylene, which is less reactive than butene, [pyridine, HClj/AlClj (1 2 molar ratio) IL proved to be the best candidate. The reaction can be run at room temperature and provides good quality alkylate (2,3-dimethylbutanes is the major product) over a period of 300 h (MON = 90-94 RON = 98-101). 

There has been continued interest in developing a process for direct esterification of terephthalic acid with ethylene glycol. It does not appear, however, that this is currently practiced on a commercial scale in the U.S. In Japan, a process was commercialized where terephthalic acid is reacted with two moles of ethylene oxide to form the dihydroxy ester in situ, as the starting material. One mole of ethylene glycol is then removed under vacuum in the subsequent condensation process. Also, it was reported that the polymer can be prepared by direct esterification at room temperature in the presence of picryl chloride. The reaction can also be performed at about 120 C in the presence of diphenyl chloro-phosphate or toluenesulfonyl chloride. This is done in solution, where pyridine is either the solvent or the cosolvent. Pyridine acts as a scavenger for HCl, that is a byproduct of the reaction, and perhaps also as an activator (by converting the acid into a reactive ester intermediate). 

The reactivity of cyclohexene is much higher than that for benzene hydrogenation and cyclohexane dehydrogenation, particularly using noble metals as catalysts. Good results has been reported over 0.35 wt% Pt, Ir, Rh, Re, U, Ptir, PtRe, or PtU/y-Al203 catalysts in a pulsed MSR at 250 °C [24] or in a continuous MSR (thiophene, pyridine, and cyclohexene conversions of 94.3, 100, and 90.3%, respectively) [25]. 

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