Electrophilic attack insertion reactions with sulfur

In the first step an S03 molecule is inserted into the ester binding and a mixed anhydride of the sulfuric acid (I) is formed. The anhydride is in a very fast equilibrium with its cyclic enol form (II), whose double bonding is attacked by a second molecule of sulfur trioxide in a fast electrophilic addition (III and IV). In the second slower step, the a-sulfonated anhydride is rearranged into the ester sulfonate and releases one molecule of S03, which in turn sulfonates a new molecule of the fatty acid ester. The real sulfonation agent of the acid ester is not the sulfur trioxide but the initially formed sulfonated anhydride. In their detailed analysis of the different steps and intermediates of the sulfonation reaction, Schmid et al. showed that the mechanism presented by Smith and Stirton [31] is the correct one. 

Reactions involving a metal complex and an electron-deficient species (the electrophile, E+) are common in organometallic chemistry and often useful synthetically. There are several possibilities for interaction of a metal complex and E+, some of which we have already encountered. For example, the insertion of S02 (equation 8.22), electron deficient at sulfur, begins with attack by the electrophile at the carbon attached to the metal. 

These and other results are best explained, at least when the reaction is carried out in liquid SOj, by the mechanism shown in Figure 12.2. Initial 5 2 attack by the electrophilic sulfur of SOj inverts the configuration at carbon and forms an ion pair. Collapse of the ion pair to form an 0-bound sulfinate occurs reversibly, and the more stable S-bound sulfinate is eventually formed. The cation [CpM(L)(L )] is slow to invert, leading to retention of stereochemistry at the metal. Sulfur electrophiles that are similar to SOj, such as N-sulfinyl-sulfonamides (RS02)NS0 and sulfur bis(sulfonylimide)s (RSOjN)jS, also insert into transition metal-carbon a-bonds with inversion of stereochemistry at carbon, - probably by a mechanism analogous to that shown in Figure 12.2. 

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