Carbon, quaternization

A related effect is observed on quaternization, but in this case the operation of a (3-substituent effect results in the overall change at the a carbon atom normally being small (Table 5). A further important general trend in the azines arises on A-oxidation, which... 

A nitrogen atom at X results in a variable downfield shift of the a carbons, depending in its extent on what else is attached to the nitrogen. In piperidine (45 X = NH) the a carbon signal is shifted by about 20 p.p.m., to ca. S 47.7, while in A-methylpiperidine (45 X = Me) it appears at S 56.7. Quaternization at nitrogen produces further effects similar to replacement of NH by A-alkyl, but simple protonation has only a small effect. A-Acylpiperidines show two distinct a carbon atoms, because of restricted rotation about the amide bond. The chemical shift separation is about 6 p.p.m., and the mean shift is close to that of the unsubstituted amine (45 X=NH). The nitroso compound (45 X = N—NO) is similar, but the shift separation of the two a carbons is somewhat greater (ca. 12 p.p.m.). The (3 and y carbon atoms of piperidines. A- acylpiperidines and piperidinium salts are all upfield of the cyclohexane resonance, by 0-7 p.p.m. 

Quaternization of the enamine has been the more frequent result of alkylation reactions with cyclic enamines 114,277-279), but some successful carbon alkylations have been reported 280-282). 

Similar results were obtained when 4,4 -bipyridyl was quaternized with p-bis(bromo-acetyl) benzene in refluxing benzene to give a 92% yield of the blue-green salt. Potassium carbonate treatment of their blue-green salt gave a greenish black polymeric ylide (Scheme 11), which was similar in appearance and properties to (Scheme 12). 

The C6 position of a V,./V-dialkyl-3//-azepin-2-amine, e.g. 1, displays remarkable reactivity towards electrophilic substitution, and with dimethyl(methylsulfanyl)sulfonium tetrafluorobor-ate undergoes methylsulfanylation and quaternization to yield 6-(methylsulfanyl)-3//-azepin-ium tetrafluoroborates, e.g. 2, identical to those obtained by quaternization of the free bases with trityl tetrafluoroborate.64 Basification of the salts with potassium carbonate yields the free bases, e.g. 3, which decompose during distillation or on exposure to air. 

Synthesis of Compound I. As shown in Scheme II, 3-(thiophene-3-yl)propyl bromide can be prepared by a two-carbon homologation(2 ) of 3-thenyl bromide via reaction with diethyl malonate to form diethyl 3-thenylmalonate. This is followed by saponification, decarboxylation, reduction of acid to alcohol, (2 ) and replacement of the hydroxyl group with bromide by reacting with PBr3.(22) Compound 2 is synthesized by mono-quaternization of an excess of 4,4 -bipyridine with 3-(thiophene-3-yl)propyl bromide followed by N-methylation with CH3I. All the intermediates in Scheme II have been identified by NMR spectroscopy. 2 has been characterized by NMR and high resolution mass spectroscopy and by electrochemistry. 

The direct quaternization of chloromethylated polystyrenes by tertiary amines or phosphines represents the easiest way to obtain polymer-supported quaternary onium salt (12,13). A lipophilic character of quaternary cation and a topology allowing sufficient cation-anion separation also play an important role (35,36). A linear spacer chain (of about 10 carbon atoms) between the catalytic site and the polymer backbone substantially increases the reaction rates. The loading of quaternary onium groups also affects catalytic efficiency, the influence being different for directly bonded and spaced groups, e.g. 10 and 11, respectively (37). 

Fig. 16a,b. a Surface pressure - area/molecule isotherms of Langmuir films of (PS)2so (RPVP+I )24o, where the P4VP block was quaternized with different n-alkyliodides R = Cj, C4, Q, Cio- b Metal shadowed TEM micrographs of the LB films of P(S26o-b-VP7i/C101) deposited on a carbon-coated surface at 2 mN/m from a pure water surface [149]... 

Reactivity is normally enhanced by quatemization [77HC(30)179]. In this case a methyl group at C-7 seems to be the most reactive toward both benzaldehydes (74KGS565) and orthoesters or malonic bisacetal (88UKZ880). Bis-quaternized methylene 2,2 -bis-TPs are attacked by benzaldehydes at the methylene carbon instead of the methyl group (82UKZ79). 

Bearing in mind that chemical shifts are usually not dependent on the solvent or concentration, but may be partly dependent on the anion37, quaternization of alkyl-phosphines causes an upfield shift (1-7 ppm) for the near carbons of the alkyl chain, the effect becoming only slightly detectable on C(y) carbons37,94. Where triarylphosphines are quaternized, the chemical shift of the phenyl ipso carbon Cf moves upfield whereas that of the Cpara carbon moves downfield, as expected from the polarization of the 71-electron density in the phenyl ring36,37. For the aromatic carbon atoms, the increments... 

Solvent effects on relative rate constants are also usually small. When a heteroaromatic compound quaternizes at more than one site, for example, the product ratio can be insensitive to solvent variations. A constant isomer ratio is recorded for methylation (Mel) of 3-/erf-butyl-6-dimethylaminopyridazine (10), in hexane, benzene, carbon tetrachloride, acetone, and acetonitrile, but not in dimethoxyethane or tetrahydrofuran. The suggestion was made that Mel may have reacted with the last two ether solvents to give an oxonium ion. Since the identity of the quaternizing agent changes, the product ratio varies as well.13... 

Steric effects on the rates of quaternization of quinoline are magnified as the size of the alkylating agent is increased in the series methyl, ethyl, and isopropyl iodide. In order to reveal the changing steric effect, it is first necessary to eliminate the reduction in rate that naturally occurs as the electrophilic carbon atom of the alkylating agent undergoes sub-... 

Hart et al,172 observed that pyridines cannot be alkylated on carbons bonded to positions 2, 4, or 6 unless the nitrogen atom is first quaternized. 

As expected, carbon-13 shifts of phosphines (Table 4.49) are enhanced by increasing alkylation (a effect) and increasing number of jS alkyl carbons ([1 effect). In contrast to amines, however, quaternization of phosphines causes shieldings in a and fi posititions (Table 4.49). A particularly large a shielding is induced by quaternization of triphenylphosphine. 

N-Quaternization and the formation of N-oxides of alkaloids cause strong paramagnetic deshielding effects for the carbon atoms neighboring the nitrogen atom however the carbons located at /(-position usually become more shielded [600, 606, 634, 643],... 

Naphthyridine chemistry has been well reviewed by Allen1 (up to 1947), and by Weiss and Hauser2 (up to 1958). It has also been considered incidentally by Duffin8 (quaternization reactions) and by Campbell4 in The Chemistry of Carbon Compounds. ... 

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