Cations sulfur-stabilized

Radical cations of 2-alkylidene-l,3-dithianes can be generated electrochemically by anodic oxidation using a reticulated vitreous carbon (RVC) anode <2002TL7159>. These intermediates readily react with nucleophiles at C-1. Upon removal of the second electron, the sulfur-stabilized cations were trapped by nucleophilic solvents, such as MeOH, to furnish the final cycloaddition products. Hydroxy groups <20010L1729> and secondary amides <2005OL3553> were employed as O-nucleophiles and enol ethers as C-nucleophiles (Scheme 50) <2002JA10101>. 

What is the point of this Silyl enol ethers can be alkylated only by compounds that give carboca-tions in the presence of Lewis acids. The mechanism for the alkylation therefore involves the formation of a sulfur-stabilized cation. 

When heteroatoms are attached to the positive carbon, the cation gains added stability via donation of electrons (back donation) to the electron deficient center, which leads to greater stability, often by resonance delocalization of the charge on the heteroatom. Two common examples of this are oxo-stabilized cations such as 106 and sulfur-stabilized cations such as 107. In both cases, back donation from the heteroatom leads to resonance stabilized cations. Both 106 and 107 are more stable than a simple tertiary cation bearing three... 

Another variation of this ring expansion used sulfur stabilized cations. When aldehyde 65 was treated with trimethylsilyl methyl sulfide and trimethylsilyl triflate, cation 66 was formed and rearranged to the more stable tertiary cation 67. Elimination of the silyl group gave a 59% yield of the ejco-methylene derivative (68). 9... 

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