Nucleophilic displacements five-coordinate intermediates

Role of a Five-Coordinate Intermediate in Nucleophilic Displacement... 

Role of a five-coordinate intermediate in nucleophilic displacement 332... 

Under basic conditions (pH > 2.5) it is likely that water dissociates to produce nucleophilic hydroxyl anions in a rapid first step. The hydroxyl anion then attacks the silicon atom. Her [124] and Keefer [125] propose a mechanism in which hydroxyl anion displaces OR— with inversion of the silicon tetrahedron (Figure 24.14), while Pohl and Osterholz [126] favor a mechanism involving a stable five-coordinated intermediate which decays through second transition state in which any of the surrounding ligands can acquire a partial negative charge. 

In gas-phase reactions followed using the flowing afterglow technique, MesSiCl and Mel were found to react with anions (H2N , OH , F", HS, NC ) at each encounter, providing the anions were sufficiently basic. As the basicity of the nucleophile decreased, so did the rate of its reaction with Mel. In contrast, Me3SiCl reacted at each encounter with these nucleophiles until the net reaction ceased, when the displacement became endothermic. In the thermoneutral reaction between and Me3SiCl, the displacement of unlabeled Cl" was rapid with the proposed five-coordinate intermediate collisionally stabilized by the carrier gas and having a lifetime >10" s. 

Reversibility also holds for RE of alkyl halides where an Sn2 pathway (Fig. 6.2) applies for the OA direction. Iodide attacks the coordinated methyl trans to the open site and nucleophilically displaces the Pt(II) complex, a good leaving group. The reactive five-coordinate intermediate, isolable in some cases, can also undergo concerted reductive elimination of ethane if the 1 concentration is low. " ... 

In conclusion, all these observations suggest the presence of a five-coordinate silicon intermediate both in nucleophilic displacements at silicon and in the racemization of halosilanes as previously suggested (5, 59). The ability to form a pentacoordinate silicon adduct is not controlled by the electronegativity of the substituents at silicon, but by the tendency of the Si—X bond to be stretched under the influence of a nucleophile ... 

The stereochemistry and mechanistic implication for nucleophilic substitution at silicon continue to receive attention, and have been the subject of a comprehensive reviewThe nucleophiles HMPA, DMSO, DMF, and PhaPO catalyze the hydrolysis of triorganochlorosilanes and also their racemization. Stable 1 1 ionic adducts of HMPA with MeaSiBr have been isolated in the past and suggested to be possible intermediates in the racemization process. However, it has now been shown that such ionic 1 1 adducts are not usually formed and that the reported example was a special case which should not be generalized into a mechanism involving displacement of halides by the nucleophile. The previously accepted mechanism involving the formation of five- and six-coordinate species still seems more feasible. 

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