Displacements steric course

The most frequently encountered reactions in organic sulfur chemistry are nucleophilic displacement reactions. The mechanism and steric course of reactions have been the main points of interest of research groups all over the world, in particular, Andersen, Cram, Johnson, and Mislow in the United States Kobayashi and Oae in Japan Kjaer in Denmark and Fava and Montanari in Italy. The results of these investigators have been discussed exhaustively in many reviews on sulfur stereochemistry. In a recent report on nucleophilic substitution at tricoordinate sulfur, the literature was covered by Tillett (10) to the end of 1975. Therefore only some representative examples of nucleophilic substitution reactions at chiral sulfur are discussed here. However, recent results obtained in the authors laboratory are included. 

Oae and co-workers (288) were the first to show that nucleophilic displacement at sulfur is accompanied by retention of configuration. They found that chiral 0-labeled alkyl aryl sulfoxides exchange oxygen with dimethylsulfoxide at about 150°C, almost without racemization. To explain the steric course (retention) of this reaction, the formation of a trigonal-bipyramidal intermediate 246 was postulated in which the entering and departing oxygen atoms occupy apical and equatorial positions, respectively. 

The palladium-mediated substitution of allylic substrates proceeds in two independent steps. For stabilized carbanions both oxidative addition and the nucleophilic displacement occur with inversion of configuration. Thus, overall retention results, in contrast to the corresponding reactions of nonstabilized carbanions as nucleophiles (see Section D.l. 5.6.3.). The steric course of the reaction is proved by the absence of racemization in Lhe conversion of chiral substrates into chiral alkylated products. Furthermore, chiral n-allylpalladium complexes formed with inversion from stoichiometric reactions of palladium(O) with allyl substrates have been isolated. Coupling of these stereodefined complexes with soft carbanions yields the chiral alkylated products, again with inversion of configuration. 

Secondly, the factors controlling nucleophilic displacements of sulfonic esters had already been determined to a considerable extent, and have been described in reviews in this Series103,104 and, in addition, the steric and polar factors governing such displacements have been summarized qualitatively,101 and will therefore not be discussed here in any detail. This accumulated knowledge made possible the prediction of the course of such fluoride displacements. 

The MesSi group is, of course, large. But the C-Si bond is long and the Me3Si group has a smaller steric effect than the Me3C(t-butyl) group. For evidence, lookatthis last sequence nucleophilic displacement at a carbon atom... 

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