Quinolinium salts oxidation

In a study concerning the kinetics and mechanism of osmium tetroxide-catalyzed ferricyanide oxidation of pyridinium and quinolinium salts it was found that the rates of reaction are first order with respect to substrate, alkali, and catalyst, but zero order with respect to ferricyanide concentration (82IJC(A)517). 

Quinolinium salts react similarly to pyridinium compounds when oxidized. N-Alkylated salts 142, for example, yield only 2-quinolones 143 after Decker oxidation. 4-Quinolones 144 cannot be isolated (1892CB443 70HCA1903). 

Oxidative imination of quinolinium salts and aza analogues with subsequent hydrolysis to cyclic amides occurs as in the pyridine series (85JHC765). 

A range of cis-substituted alkenes have been epoxidized with a new dihydroiso-quinolinium salt catalyst, using tetraphenylphosphonium monoperoxysulfate as the stoichiometric oxidant, giving ees of up to 97%.94... 

Reaction of pyridine-N-oxides with triethyloxonium salt in methylene chloride or chloroform gives directly AT-alkoxy pyridinium salts with high yield. Quinolinium salts can also be prepared from the corresponding N-oxides [46], The synthetic procedure does not require the anion exchange reaction since triethyl oxonium salt possesses a non-nucleophilic counter anion, i.e. PFg ... 

The reaction has been used to synthesize libraries of benzonaphthyridines 196, in high diastereoselectivity, from the cycloaddition of 1,4-dihydrop3Tidines with imines formed from aldehydes and anilines. When cyclic enol ethers were used as dienophUes, mixtures of diastereomers 197 were obtained. These compounds were oxidized to the corresponding quinolines 198 and were further transformed to the quinolinium salts 199 as shown in Scheme 36 [76]. Compounds 196 and 198 were tested for their ability to inhibit human propyl oligopeptidase (POP) and were found to have modest potencies. Much better results were obtained when the quinoline nitrogen was methylated to provide adducts 199. The cationic center improved the inhibitory activity of these compounds (Fig. 23). 

The predominant property of these salts is the ease with which nucleophiles add to the quinolinium-2-and the isoquinolinium-1-positions. Such additions are favoured in these bicyclic compounds since the products retain a complete aromatic benzene ring. Hydroxide, hydride and organometaUic nucleophiles aU add with facility, though the resulting dihydroaromatic products require careful handling if they are not to disproportionate or be oxidised. " This approach can give 2-nifluoromethyl-quinolines CFj carbanion (from trifluoro(trimethylsilyl)methane and fluoride) is added to an A -(para-methoxybenzyl)-quinolinium salt, then the A -substituent is removed and oxidation to the aromatic level achieved with ceric ammonium nitrate. ... 

N-Alkoxy Pyridinium Salts /V-Alkoxy Pyridinium salts are obtained with relatively high yields by a reaction of pyridine /V-oxides with a triethyloxonium salt in methylene chloride or chloroform [13]. Quinolinium salts can also be prepared from the corresponding /V-oxides [40], In both cases, an anion exchange is not necessary since the triethyl oxonium salt is available with nonnucleophilic counter anions. The most frequently used photoinitiators of this type are shown in Chart 11.8. The spectral response of these salts is in 260-310 nm range [13],... 

It has recently been discovered that2-(l-methylindol-3-yl)quinoline is formed in a good yield on reacting quinoline with Al-methyUndole in the presence of hydrogen chloride in dioxane. Two equivalents of quinoline substrate (quinolinium hydrochloride + quinoline) are necessary for this reaction, since one equivalent of quinoline (or the NH-protonated quinolinium salt) acts as oxidant to abstract hydrogen atoms from the intermediate o -adduct, thus giving the Sn product and a mixture of 1,2,3,4-tetrahydroquinoline and l, 2, 3, 4 -tetrahydro-2,6 -biquinoline (Scheme 30) [118]. 

Similarly produced trifluoromethyl carbanion, as well as perfluoroisopropyl carbanion generated by addition of fluoride anion to perfluoropropene, is able to add to a variety of iV-(4-methoxy-benzyl) pyridinium and quinolinium salts. The obtained dihydroazines can easily be transformed into 2-perfluoroalkyl azines through oxidative dealkylation-aromatization with CAN or DDQ (Scheme 31) [110, 111],... 

The reaction mechanism for A-oxidation by performic acid has been studied by AMI calculation methods.174 The iminium salt A-mcthyl-3,4-dihydroisoquinolinium p-toluenesulfonate has been used to catalyse the oxidation of the azo dye calmagite by peracetic acid. The mechanism at pH 10 involves peracid oxidation of the quinolinium ion to form an oxaziridinium salt, which then acts as an oxygen transfer agent for oxidation of cahnagite.175 The presence of lithium salts affects the course of the reaction determining the formation of benzoyl peroxide and benzoic acid as final products in the oxidation of benzaldehyde by perbenzoic acid.176,177... 

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