Retention and inversion of configuration

Stereochemistry The SnI reaction involves a flat carbocation intermediate that can be attacked from either face. Therefore, the SN1 usually gives a mixture of inversion and retention of configuration. 

Predicting the outcome of substitutions and eliminations is only straightforward in certain cases. For primary halides, S 2 and E2 reactions are predicted. For tertiary halides, SnI, E2 andEl(to a certain extent) are the choices. The possibilities for secondary halides are more complicated. In addition, many reactions yield both substitution and elimination products, and both inversion and retention of configuration may occur in the same reaction. 

Summary The rich variety of the coordination chemistry of silicon is discussed and some theoretical issues are raised. In an attempt to understand further the underlying chemistry, some thermodynamic and kinetic parameters for the formation and substitution of pentacoordinate silicon compounds have been measured by NMR methods. Values of -31 3 kJ mol for SHand -100 10 J K mor for A5-were measured for the intramolecular coordination of a pyridine ligand to a chlorosilane moiety. A detailed kinetic analysis of a nucleophilic substitution at pentacoordinate silicon in a chelated complex revealed that substitution both with inversion and retention of configuration at silicon are taking place on the NMR time-scale. The substitution with inversion of configuration is zero order in nucleophile but a retentive route is zero order in nucleophile at low temperature but shows an increasing dependence on nucleophile at higher temperatures. These results are analysed and mechanistic hypotheses are proposed. Some tentative conclusions are drawn about the nature of reactivity in pentacoordinate silicon compounds. 

Halogermane reductions by complex hydrides are efficient, preferred methods for germane synthesis " . The complex hydrides used are MBH (M = Li, Na, K) and LiAlH.,. Reduction of Ge—X bonds by this method can be used for any molecule that otherwise is unsusceptible to complex hydride reduction or reaction. Lithium tetrahydroaluminate reduction of chiral halogermanes and alkoxygermanes results in inversion and retention of configuration, respectively. The LiBH., and LiAlH reactions require aprotic solvents, such as EtjO, THF, n-Bu O or glyme ethers. Sodium and K... 

When the reaction step R—X —> R takes place at a chiral carbon, racemization is almost always observed because free radicals do not retain configuration. Exceptions to this rule are found at cyclopropyl substrates, where both inversion and retention of configuration have been reported, and in the reactions mentioned on p. 942. Enantioselective radical processes have been reviewed. ... 

Both inversion and retention of configuration occur equally. A pair of enantiomers is the result. This is an optically inactive racemic mixture. 

Replacement of the hydroxy groups by fluorine in dimethyl tartrates with sulfur tetrafluoride takes place with inversion and retention of configuration at the two vicinal carbon atoms37. 

The enol ether products are essentially only formed with inversion of configuration and are concluded to be produced via an SnV2 reaction, and not via an intermediate primary vinyl cation. Both the in-plane S Ycr and the out-of-plane SnV77 route are theoretically feasible as SnV2 pathways, leading to inversion and retention of configuration, respectively (see Scheme 1), and the major substitution reaction of 11 follows the first route, and the minor reaction the second route. 

Inversion and retention of configuration are described by changes in R and S configurations at a given reaction center. 

FIGURE 2.3 Example reactions showing inversion and retention of configuration. 

The modeling of monomeric (,R,S)-2 [B3LYP/6-31+G(d)] indicates that the highest occupied molecular orbital (HOMO) is chiefly located at the metalated carbon center and the aromatic ring system (Fig. 5). It can be deduced from the calculated orbital coefficients that both inversion and retention of configuration are almost equally likely to result from electrophilic attack. Only the fact that the site opposite the lithium center is sterically accessible to attack by electrophiles (the coordination polymer of the solid-state structure should be broken up in solution) makes it possible for (R,S)-2 to react selectively with inversion of configuration at C(3) under kinetic control in nonpolar solvents. 

Transition states for inversion and retention of configuration channels in the reactions of alkyl and silyl fluorides with a water molecule " ... 

Scheme 28 Inversion and retention of configuration during monomer insertion into mononuclear active species operating via a dissociative and a nondissociative mechanism.

---