A- pyridinium halide

The initially formed chlorides PyX ionize to a pyridinium halide while PyR represents mixed 1,4- and 1,2-dihydropyridines.233,234 4-Nitrobenzyl halides react by a different mechanism the initial step here is electron transfer from 66 to the halide to give a nitro anion-radical which subsequently loses halide. Since this reaction involves ionic intermediates it is much more susceptible to solvent effects than the atom transfer reaction.235,236... 

In a slight variation of the above synthetic approach, 2-phenylimidazo[l,2-a]pyridine (290) was prepared by the reaction of l-phenacyl-2-chloropyridinium bromide (288) with ammonia (289) (55CBIII7). The use of primary amines (291) resulted in l-alkylimidazo[l,2-a]pyridinium halides (292) (69JOC2129), whereas hydroxylamine (293) gave imidazo[l,2-a]pyridine 1-oxides (294) (78JOC658). 

An interesting formation of highly substituted trihydroxybenzenes (36) and (37) together with the simpler phenol (38) involves the transfer of a methyl group from O to induced by a pyridinium halide, and the similarly congested 2,4,6-tris-(4-t-butylphenyl)-phenol (39) results from the pyrylium salt (40) (Scheme 6) formed by the reaction of 2 moles of the aldehyde Bu C6H4CHO with 1 of the ketone Bu CeH4COMe. 

A wide class of aiyl-based quaternary surfactants derives from heterocycles such as pyridine and quinoline. The Aralkyl pyridinium halides are easily synthesized from alkyl halides, and the paraquat family, based upon the 4, 4 -bipyridine species, provides many interesting surface active species widely studied in electron donor-acceptor processes. Cationic surfactants are not particularly useful as cleansing agents, but they play a widespread role as charge control (antistatic) agents in detergency and in many coating and thin film related products. 

A -(l-Haloalkyl)pyridinium halides have been advantageously employed in the Hantzsch multicomponent synthesis, yielding alkyl 1,4-dihydropyri-dine-3,5-dicarboxylates, which are a well-known class of calcium channel modulators (81AGE762). Tire halides readily interact with an excess of an ethyl 3-aminobut-2-enoate 82 (R = H) in dichloromethane at room temperature to afford the heterocycles 83 (R = H) in good to excellent yields (65-95%) (92T1263). Tliis observation has been exploited to perform a quantitative study of the reactivity of the salts (93CB1251).Tlie results have... 

Some pyridinium halides 33 have been used as a starting material for the synthesis of a novel 5/6/5 heterocyelie system (98EJOC2923). A multistep... 

Primary alkyl amines RNHi can be convertedto alkyl halides by (1) conversion to RNTs2 (p. 447) and treatment of this with I or Br in DMF, or to N(Ts)—NH2 derivatives followed by treatment with NBS under photolysis conditions, (2) diazotization with terr-butyl nitrite and a metal halide such as TiCU in DMF, or (3) the Katritzky pyrylium-pyridinium method (pp. 447,489). Alkyl groups can be cleaved from secondary and tertiary aromatic amines by concentrated HBr in a reaction similar to 10-71, for example,... 

The novel heterocyclic system 92 has been prepared by reaction of 2-amino-1,3,4-thiadiazoles 91 with either l-(haloalkyl)pyridinium halides 89 or N,N -methylenebis(pyridinium) dihalides 90. A mechanism for the formation of 92 was proposed and involved a series of specific proton migrations, bond-breaking and bond-forming processes <98EJOC2923>. 

The main consensus seems to be that the first major studies of room temperature molten salts were made in the 1940s by a group led by Frank Hurley and Tom Weir at Rice University. When they mixed and gently warmed powdered pyridinium halides with aluminum chloride, the powders reacted, giving a clear, colorless liquid [4-7]. These mixtures were meant to be used in electrochemistry, particularly in electroplahng with aluminum. 

Pyridinium halides were prepared by bubbling anhydrous hydrogen halide gas through a solution of pyridine in anhydrous ethyl ether. The precipitates then were filtered, washed with ethyl ether, and dried under high vacuum at room temperature. 

Intensities estimated visually relative to a value of 10 for most intense line. b Also observed in powder pattern of respective pyridinium halide. 

The last step in the proposed mechanism, which shows the displacement of coordinated l-(4-pyridyl)pyridinium ion, is required, since only the tetrahalodi-(pyridine) adducts were found in the reaction mixtures. The acetonitrile extractions also demonstrated that the l-(4-pyridyl)pyridinium ion could be easily washed from the reaction mixtures, and indicated the presence of this ion as a free halide rather than as a constituent of the complex. 

Commercially important surfactants based on heterocyclic structures are relatively few in number (B-80MI11510) the most important are the cationic compounds derived from imidazoline (48). Cetyl pyridinium halides (49) are used as germicides and sanitizing agents. Piperidinium (50) and morpholinium (51) compounds find application in hair conditioning formulations as antistatic agents. Substituted oxazolines (52) are a class of cationic dispersants and corrosion inhibitors. 

The outline procedure involves the initial reaction of the 2,4,6-triphenyl-pyrylium halide with the primary amine to yield the corresponding 2,4,6-triphenylpyridinium halide (see Section 8.4.1, and also Section 5.15.3, p. 768) this reaction proceeds either at room temperature in a suitable solvent, or more efficiently under reflux in benzene with azeotropic removal of water. Pyrolysis of the pyridinium halide under controlled conditions then yields the alkyl (or aralkyl) halide in good yield. The mechanism of the reaction in this case is probably of the Sn2 type. 

Spectroscopic methods can be used to specify the position of donors and acceptors before photoexcitation [50]. This spatial arrangement can obviously influence the equilibrium eomplexation in charge transfer complexes, and hence, the optical transitions accessible to such species [51]. This ordered environment also allows for effective separation of a sensitizing dye from the location of subsequent chemical reactions [52], For example, the efficiency of cis-trans isomerization of A -methyl-4-(p-styryl)pyridinium halides via electron transfer sensitization by Ru(bpy) + was markedly enhanced in the presence of anionic surfactants (about 100-fold) [53], The authors postulate the operation of an electron-relay chain on the anionic surface for the sensitization of ions attached electrostatically. High adsorptivity of the salt on the anionic micelle could also be adduced from salt effects [53, 54]. The micellar order also influenced the attainable electron transfer rates for intramolecular and intermolecular reactions of analogous molecules (pyrene-viologen and pyrene-ferrocene) solubilized within a cationic micelle because the difference in location of the solubilized substances affects the effective distance separating the units [55]. 

The recent chemistry of A-aminopyridinium halides is dominated by the reactions of the corresponding pyridinium ylides generated by deprotonation of the amino group. These 1,3-dipoles undergo cycloaddition with dipolarophiles and react with electrophiles at the nitrogen of the amino group. 

Z values are both temperature- and pressure-dependent. The CT absorption band of substituted pyridinium halides is shifted hypsochromically as the temperature of the solution is decreased [59]. Thus, Z values decrease with increasing temperature due to a lowering of the solute/solvent interactions at the higher temperature. Furthermore, it has been shown that the CT absorption band of l-ethyl-4-(methoxycarbonyl)-pyridinium iodide is shifted bathochromically for solutions in methanol and ethanol with increasing pressure (up to 1920 bar), while for other solvents such as acetone and A, A -dimethylformamide it is shifted hypsochromically [60]. Except for the lower alcohols, the bulk solvent polarity generally increases with pressure [60]. 

Pyridinium halides are easily reduced to pyridinyl radicals (e in place of hv, eq. 1) with scKiium amalgam in oxygen-fr acetonitrile using a vacuum line and the apparatus shown in Fig. 1 After filtration, the pure pyridinyl irulicals are isolated by several distillations. Magnesium or zinc have been used on a preparative scale various other metals (e.g., calcium, strontium and barium) are suitable for preparing solutions of pyridinyl diradicals... 

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