Tertiary Alkyl Aryl Sulfonium Ions

Tertiary Alkyl(Aryl)Sulfonium Ions. In contrast to tertiary oxonium ions, the tertiary sulfonium ions are stable and are prepared rather readily.167-169 They are even stable in aqueous solutions. Trialkylsulfonium ions are obtained by alkylation of dialkyl sulfides with alkyl halides. Trialkyloxonium and dialkylhalonium ions readily transalkylate basic dialkylsulfides. 

In contrast, alkylation of diaryl sulfides and thiophene requires rather drastic conditions due to poor nucleophilicity of sulfur. The alkylations have been achieved using alkyl triflates170 [Eq. (4.42)] or with alkyl halide and silver tetrafluoroborate171 [Eq. (4.43)]. 

Another method is the use of esters as alkylating agents in triflic acid172 [Eq. (4.44)]. This method has been used to synthesize. S -methylated phenylene sulfide oligomers (methyl triflate, triflic acid, 25°C, 10 h) and to solubilize poly(phenylenesulfide) by transforming it to poly(arylenesulfonium) salts 62 (methyl triflate, methyl or ethyl formate, triflic acid, 110°C, lOh). In a similar way, alkylation of thianthrene to yield the corresponding sulfonium salts (63) has been achieved using alkyl formates.173 

Cyclizations shown in Eq. (4.45) have been used to obtain. S -peril uoroalkyl-benzothiophenium ion 64 and ring-substituted derivatives.174,175 Salts 64 and the. S -(difI uoromethyl)- and. S -(monofluoromethyl)-diarylthiophenium salts 65 reported recently by Prakash, Olah, and co-workers176,177 are used as fluoromethylating agents. 

The absolute configuration of ethylmethylpropylsulfonium ion, the simplest chiral sulfonium ion has been determined.178 

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