Oxonium salts, alkylation

In oxonium salts (alkylated ethers), the positive charge is located formally on the oxygen atom. It has to be remembered, however, that this is only the formal notation. In fact, as follows from CNDO-2 calculations, the charge is located mainly on carbon and hydrogen atoms [22],... 

Oxonium salts alkylate diamides of dicarboxylic acids twice, provided the amide groups are separated by at least two carbon atoms,thus the bis(dimethylamides) of maleic acid or terephthalic acid have been converted to dications (69 Scheme 4) by means of oxonium salts. ... 

Diem, M.J., Burow, D.F. and Fry, J.L. (1977) Oxonium salt alkylation of structurally and optically labile alcohols. The Journal of Organic Chemistry, 42, 1801-1802. 

Oxonium salt formation. Shake up 0 5 ml. of ether with 1 ml. of cone. HCl and note that a clear solution is obtained owing to the formation of a water-soluble oxonium salt. Note that aromatic and aliphatic hydrocarbons do not behave in this way. In general diaryl ethers and alkyl aryl ethers are also insoluble in cone. HCl. 

Direct alkylation or acylation of the oxygen of THF by exchange or addition occurs with the use of trialkyl oxonium salts, carboxonium salts, super-acid esters or anhydrides, acyhum salts, and sometimes carbenium salts. 

S.06.3.3.4 Alkyl halides, oxonium salts and related compounds... 

Examples of polyfunctional carboxylic acids esterified by this method are shown in Table I. Yields are uniformly high, with the exception of those cases (maleic and fumaric acids) where some of the product appears to be lost during work-up as a result of water solubility. Even with carboxylic acids containing a second functional group (e.g., amide, nitrile) which can readily react with the oxonium salt, the more nucleophilic carboxylate anion is preferentially alkylated. The examples described in detail above illustrate the esterification of an acid containing a labile acetoxy group, which would not survive other procedures such as the traditional Fischer esterification. 

Alkyl halides can be alkylated by ethers or ketones to give oxonium salts, if a very weak, negatively charged nucleophile is present to serve as a counterion and a Lewis... 

Diazo compounds and oxonium salts are the most efficient alkylating agents in the synthesis of alkyl nitronates. It is assumed that diazo compounds are inserted into the O-H bond in the aci forms of the corresponding AN, whereas oxonium salts generally react with AN anions. 

Alkylation with Oxonium Salts Oxonium salts are the most efficient alkylating agents and can react with both activated and nonactivated AN, including nitromethane and 2-nitropropane (Scheme 3.4). 

Akylation, of acids, 50, 61 by oxonium salts, 51,144 Alkyl bromides, from alcohols, benzyl bromide, and triphenyl phosphite,... 

One of the major advantages of oxonium salts is that alkylations can be effected under reaction conditions that are generally much milder than those necessary with the more conventional alkyl halides or sulfonates. Triethyloxonium tetrafluoroborate, for example, has usually been employed at room temperature in dichloromethane or dichloroethane solution. Occasionally chloroform16-22 or no solvent at all4-20 is used. Difficult alkylations can be effected in refluxing dichloroethane.29 30 The less soluble trimethyloxonium tetrafluoroborate has been used as a suspension in dichloromethane or dichloroethane, or as a solution in nitromethane or liquid sulfur dioxide. Reports of alkylations in water23 and trifluoroacetic acid21 have also appeared. Direct fusion with trimethyloxonium tetrafluoroborate has succeeded in cases where other conditions have failed.25-30... 

Alkylations by oxonium salts have added several new weapons to the synthetic chemist s armamentarium. For example, the O-alkylated products from amides [R1C(OR)=NR2R3]+ (R == CH3 or C2H5) may be hydrolyzed under mild conditions to amines and esters,14-34 reduced to the amines RjCH-jNRaRa by sodium borohydride,13 converted to amide acetals RiC(OR)2NR2R3 by alkoxides,4-16 and (for R3 = H) deproton-ated to the imino esters R1C(OR)=NR2.16-18 Amide acetals and imino esters are themselves in turn useful synthetic intermediates. Indeed, oxonium salts transform the rather intractable amide group into a highly reactive and versatile functionality, a fact elegantly exploited in recent work on the synthesis of corrins.34... 

Other reagents which approach or exceed the oxonium salts in alkylating ability include dialkoxycarbonium ions,35 alkyl... 

O -Alkylations of azepin-2-ones, and benzazepin-2-ones, are most efficiently brought about by trialkyloxonium tetrafluoroborates (Meerwein s reagents) (B-69MI51600,73JOC1090, 81HCA373). S-Alkylation of the thiones is effected similarly. These oxonium salts have also proved useful for the alkylation of azepinedione precursors of azatropones and azatropolones (72JOC208) (see also Section 5.16.3.1.2). 5//-Dibenz[f>,e]azepine-6,11 -dione with triethyloxonium tetrafluoroborate O-ethylates at the amide carbonyl and not at the ketonic carbonyl as was first proposed (72AJC2421). 

In contrast to pyridine derivatives, aryl- and alkyl-substituted A -phosphorins cannot be protonated by strong, non-oxidizing acids such as trifluoroacetic acid. Addition of trifluoroacetic acid to cyclohexane solutions of various A -phosphorins fails to produce any change in the UV spectra Similarly, alkylation by such strong agents as oxonium salts or acylation by acylchlorides cannot be induced at the P atom or any ring C atom. This behavior has also been discussed theoretically 55a)... 

The course of the alkylation was investigated in detail by Markl and Merz It was found that alkyl iodides attack by an SnI mechanism, primarily at the phosphorus, to form 144, while oxonium salts prefer the C—2 position. Nonpolar solvents favor SN2-alkylation at C—2 to 145. The X -phosphorins 144 are thermodynamically more stable than the isomeric 1,2-dihydro-X -phosphorins... 

Method J Alkylation of 2-Hydrophosphinic Acids and Esters with Oxonium Salts... 

Only those 1.1-diphenyl-X -phosphorins in which the phosphorin ring is unsubstituted could be alkylated or acylated at the ring (see p. 77). The 2.4.6-tri-phenylated 1.1-disubstituted A -phosphorins cannot be alkylated by oxonium salts or acylated by acylchlorides under normal conditions. 

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