Pyridinium salt - alkene

Reactive halogen compounds, alkyl haUdes, and activated alkenes give quaternary pyridinium salts, such as (12). Oxidation with peracids gives pyridine Akoxides, such as pyridine AJ-oxide itself [694-59-7] (13), which are useful for further synthetic transformations (11). 

The 1,3-dioxolane group is probably the most widely used carbonyl protective group. For the protection of carbonyls containing other acid-sensitive functionality, one should use acids of low acidity or pyridinium salts. In general, a molecule containing two similar ketones can be selectively protected at the less hindered carbonyl, assuming that neither or both of the carbonyls are conjugated to an alkene. ... 

The utilization of solid-support pyridinium salts in the synthesis of bicyclic pyridines has been reported. Yue et al. synthesized 1,2,3,7-tetrasubstituted indolizines using poly(ethyleneglycol)bound pyridinium salts <06JHC781>. The PEG-bound pyridinium salts 53 were reacted with alkenes or alkynes in the presence of Et3N, via 1,3-dipolar cycloaddition, to give polymer-bound indolizines 54 and 55, respectively. Liberation of the heterocycle with KCN/MeOH afforded 1,2,3,7-tetrasubstituted indolizines 56 and 57 in good to excellent yield. 

A library of 1,2,3,7-tetrasubstituted indolizines was synthesized using poly(ethylene glycol) (PEG) bound pyridinium salts reacting with alkenes or alkynes in the presence of Et3N <2006JHC781>. 

Attack on Unsaturated Carbon. The annual addition of phosphites to every variety of activated double bond continues. These include nitro-alkenes,9 a/S-unsaturated carboxylic acid derivatives,10 maleimides,11 fulvenes,12 and pyridinium salts.13 The reaction of diethyl phosphite with keten 0,N-, S,N, and Al,AT-acetals has been used to prepare the enamine phosphonates (19).14... 

Support-bound alkylating agents have been used to N-alkylate pyridines and dihydropyridines (Entries 7 and 8, Table 15.21). Similarly, resin-bound pyridines can be N-alkylated by treatment with a-halo ketones (DMF, 45 °C, 1 h [267]) or other alkylating agents [246]. Polystyrene-bound l-[(alkoxycarbonyl)methyl]pyridinium salts can be prepared by N-alkylating pyridine with immobilized haloacetates (Entry 8, Table 15.21). These pyridinium salts react with acceptor-substituted alkenes to yield cyclopropanes (Section 5.1.3.6). Pyridinium salts have also been prepared by reaction of resin-bound primary amines with /V-(2,4-dinitrophenyl)pyridinium salts [268,269]. 

Radicals generated at the cathode by either direct or indirect reduction (see also Section 2.6.5.2) have been added to alkenes, aromatic compounds, pyridinium salts, nitriles, alkynes, carbon and metals. 

Benzoxazolo[3,4-a]pyridinium salts (116) are known to give cycloadditions of the [4-1-2] and [3 -I- 2] type with alkenes and alkynes. The primary adducts were readily converted to alternative heterocyclic systems (121) and (122) which were usually the isolated major products (Scheme 24) <79JOCill, 82JOC3098>. The formation of oxazoles from the reaction of alkynic aldehydes with (116) was claimed to result from the Diels-Alder pathway with the carbonyl group acting as the dienophile <93JCS(P1)1839>. 

The pyridinium salt is a bit hke an iminium ion, so sodium borohydride attacks it at the C=N+ bond to make a neutral enamine. Looking at the product, you can see that another reduction must take place as well, but enamines are nucleophilic, so the borohydride can t atttack directly. What must happen instead is that the enamine is protonated to make another iminium, which can then be reduced. The final double bond is safe from attack, since it is an isolated, electron-rich alkene. 

A catenane is one of the MIMs consisting of two or more interlocked macrocycles. We recently reported the first pillar[5]arene-based [2]catenane. A pyridinium salt with alkene moieties at both ends (8.44) was prepared. Pil-lar[5]arene formed a stable host-guest complex with the molecule... 

In 2011, Sanford et al. developed an elegant Pd(II)-catalyzed, pyridine-directed aerobic olefination of C(sp )-H bonds. The use of air as an external oxidant renders this protocol very promising. It constitutes an extremely rare example of Pd(II)-catalyzed aerobic C(sp )-H bond functionalization in the absence of copper salts additives. The resulting cyclic pyridinium salt can easily afford 6,5-iV-fiised bicyclic framework or alkene product under reductive conditions or organic base. 

Alkenes can be aziridinated using a variety of nitrogen sources. Among the recently reported systems are Chloramine T (iV-chloro-A -sodio-p-toluenesulfonamide) with pyridinium hydrobromide perbromide catalyst (c.g., 119 120) <99OL705>, the A -chloramine salt of t-... 

Another way to use pyridine in brominations is to make a stable crystalline compound to replace the dangerous liquid bromine. This compound, known by names such as pyridinium tribromide, is simply a salt of pyridine with the anion Brj. It can be used to brominate reactive compounds such as alkenes (Chapter 20). 

Appropriately functionalized alkenes (enamines, a-haloalkenes, silylenoi ethers, acylvinyl phosphonium salts) are frequently employed as the CC fragment in reaction with amidines, guanidines and ureas. An -chloro-A -alkylamidine will react with the enamine derived from an aldehyde to give in the first instance a 2-imidazoline (4). Such compounds deaminate readily to produce the fully aromatic imidazoles (5), especially when heated with pyridinium or triethylammonium chlorides or 50% sulfuric acid (Scheme 4.3.3). 

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