3.3- Rearrangements cationic variations

Static forms of cations such as 7 and 8 have been observed by NMR in solid SbFs matrices, with chemical shifts for the static ions matching computed values. The signals for the static ions could be observed even at temperatures above the solution-phase limits, which suggests there is a variation in the lattice sites in the solid, and the cations find themselves in different environments. A broad distribution of rearrangement barriers results. 

There are many variations on the Pummerer rearrangement but they all involve the same steps a leaving group is lost from the sulfur atom of a sulfonium ylid to create a cationic intermediate that captures a nucleophile at the a carbon atom. Often the starting material is a sulfoxide. 

The yield and ratio of the two Stevens rearrangement products are dependent on both the base and the solvent however, the ortho-rearrangement product was only detected when butyllithium was used in hexane. Variation of the halide ion had only a small effect on the ratio of rearrangement products. Several mechanisms were considered 93) as routes to the formation of the rearrangement products, 1) ion-pairs, 2) cationic rearrangement, 3) carbenoid, 4) dimetallated intermediates, 5) free-radical, 6) predissociation of the ammonium salt and, 7) sigmatropic shift. 

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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