Trialkyloxonium salts, as alkylating agents

Tosylhydrazones, with methyllithium to give olefins, 51, 69 Trialkyloxonium salts, as alkylating agents, 51,144... 

Trialkyloxonium salts are generally prepared by the alkylation of dialkyl ethers.1 90 Alkyl halides can be used as alkylating agents in the presence of strong halide acceptors such as BF3, PF5, SbCl3, and SbF5 [Eq. (4.13)].91... 

The alkylation of aromatic 1,2,4-triazines has been studied by various groups.251 -259 Alkylation with iodomethane affords the red 1-methyl-1,2,4-triazinium salts 4c-j predominantly and in a few cases yields the colorless 2-methyl-l,2,4-triazinium salts 5. The best results are obtained when nitromethane is used as the solvent for the alkylation. Similar results are obtained when trialkyloxonium tetrafluoroborates or dialkyl sulfates are used as alkylation agents. The formation of either the 1- or the 2-isomers depends on the substituents bound to the 1,2,4-triazine ring. In cases with a bulky substituent in the 3-position, alkylation occurs exclusively at Nl, in other cases mixtures of N1 and N2 alkylated triazinium salts are obtained. Introduction of a more bulky substituent, such as isopropyl or aryl substituents, in the 6-posi-tion hinders quaternization at Nl. 

Studying alkylations, we developed a series of selective ionic alkylating agents. Although Meerwein s trialkyloxonium and dialkoxycar-benium salts are widely used as transfer alkylating agents, they lack selectivity and generally are incapable of C-alkylation. 

Other examples of esterification with trialkyloxonium salts have been reported.7,8 The present procedure offers the advantages that the reactive carboxylate ion is generated in sitv and that a low-boiling, nonaqueous solvent is employed, whereby the experimental procedure is considerably simplified. A related method has been reported which utilizes a hindered amine wdth dimethyl sulfate [Sulfuric acid, dimethyl csterj as the alkylating agent.9 The present procedure is carried out under somewhat milder conditions and avoids the use of highly toxic reagents. 

A7-Acyl derivatives of heterocycles can often be quatemized by hard alkylating agents, such as trialkyloxonium fluoroborates406 or methyl fluorosulfonate.364 The quaternary salt is not usually isolated, but N-alkylated derivatives are obtained whose structures are generally different from those obtained by alkylation of anions (Sections IV,C,2). This method, due to Olofson and Kendall,406 has been used with success in azapentalenes [Eqs. (34) and (35)1, where the products 384 and 385 are different from those obtained by alkylation of the anions 361 and 364, respectively (Scheme 16). 

Trialkyloxonium fluoroborates give better yields of lactim ethers than other alkylating agents because of the selectivity of these reagents in the O-alkylation of lactams. This was borne out by Meerwein at al.,42 who arranged carbonyl compounds according to their capacity to undergo alkylation with oxonium salts as follows lactams > acyclic amides > lactones > carboxylic esters > ketones > aldehydes. 

Additional flexibility in the control over the selectivity of heterolytic reactions is provided in the diversity of electrophilic reagents that formally correspond to the same electrophile. For example, reagents such as RCO BF, RCOO-SO2CF3, RCOCl, and (RC0)20 are employed in synthesis as equivalents of the acyl cation RCO. However, a tremendous difference in the reactivity of these acylating species enables one to choose a reagent specifically adjusted to the peculiarity of the nucleophilic counterpart. In a similar way, such unlike compounds as trialkyloxonium salts, R30 BF7, alkyl halides, tosylates, or acetates can serve as transfer agents of the same alkyl cation, R, but they differ drastically in their activity and pattern of selectivity toward various nucleophiles. 

Trialkyloxonium salts with the more stable PFg or SbFg anions can be prepared, according to Olah, by using orthoformates as the alkylating agents ) ... 

Dialkylhalonium fluoroantimonate salts can be isolated as crystalline salts and are versatile, very reactive alkylating agents. As the differing halogen atoms effect a range of selectivity, they were found to be more versatile than trialkyloxonium salts. Several bicyclic halonium ions also have been prepared and studied151. ... 

Meerwein s salts, also known as the Meerwein reagent, refer to trimethyloxonium tetrafluoroborate (Me30 BF4 ) and triethyloxonium tetrafluoroborate (Et30 F4 ). Named after the inventor Flans Meerwein, these trialkyloxonium salts are powerful alkylating agents. 

In addition to diazomethane and trialkyloxonium salts, methyl fluorosulfonate, Me0S02F, may be used as an alkylating agent. Similarly, instead of LiR, many other nucleophilic lithium compounds may be utilized. 

Alkoxy-sulphonlum Salts.— Alkylation of sulphoxides by a trialkyloxonium tetrafluoroborate gives these salts, which are relatively stable to air and moisture. The compounds, which are intermediates in one form of the Pummerer rearrangement, are efficient alkylating agents/ The role of methyl phenyl sulphide as the catalytic electron-carrier in an electrochemical oxidation of alcohols is proposed to involve alkoxysulphonium salt formation, followed by a hydride shift/ The fact that some sulphoxide is also formed in this reaction appears to have encouraged the authors in their proposal PhSMe — 2e PhSMe Ph MeOCHR R Ph (CH2X>CHR R2 - PhSMe + R COR/ However, there are inconsistencies in the adoption of this mechanism, which has been established for dimethyl sulphoxide in the presence of a proton acceptor, particularly since alcohols are readily oxidized in the same electrochemical system in the absence of a... 

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