Nucleophiles electronic character

These reactions clearly indicate that the exocyclic carbon of the complexed QM in these systems is nucleophilic in character, in contrast to its electrophilic nature in free o-quinone methides. The Cp Ir metal center stabilizes the mesomeric form in which the exocyclic carbon experiences high electron density (Scheme 3.18).29... 

Thus examples such as 16 that have a phosphino substituent on the N atom are more nucleophilic in character and react readily with electron-poor alkenes and alkynes (see above) but fail to react with those that are electron rich. In contrast, the ones with a phosphonio substituent (e.g., 24) were found to react readily with... 

In the case of methyl radical addition to double bonds, L. Herk, A. Stefani, and M. Szwarc [J. Am. Chem. Soc., 83, 3008 (1961)] have drawn attention to the importance of the electron-withdrawing power of the conjugated substituent in determining the reactivity of olefins. More recently, to explain a somewhat similar phenomenon, F. Minisci and R. Galli (Tetrahedron Letters, 1962, 533) have invoked the concept that CHS is nucleophilic in character. 

Displacement reactions must proceed either with retention or inversion of configuration at silicon. The change in stereochemistry is mainly a function of the nature of the leaving group and of the electronic character of the nucleophile. 

An increase of the solvating power of the solvent (Et20 < THF < DME) leads to a shift of the stereochemistry toward retention. Striking data are reported in Table VIII for 1-NpPhViSi—F (7a). The solvating power can modify the electronic character of the nucleophile. A Grignard reagent is softer in ether than in THF or DME (Scheme 5). 

In concluding this section on the influence of the nature of the nucleophile, it is important to stress the dominant influence of the nucleophile on the stereochemistry at silicon. This effect cannot be interpreted in terms of the stability of the intermediate on the basis of the apicophilic-ity rule as stated in phosphorus chemistry. It fails to explain the retention of configuration as stereochemical outcome. No better explanation can be extracted from the quasicyclic SNi-Si mechanism (/. 2). On the other hand, data obtained with various nucleophiles show clearly that the stereochemistry is controlled by the electronic character of the nucleophile. In other words, this factor at first determines the geometry of attack of the nucleophile at silicon, which leads in a first determinant step to the formation of a pentacoordinate intermediate (55). We proposed the following ... 

In the foregoing discussion (Section II,B), we stressed the dominant influence of the electronic character of the nucleophile on the stereochemistry at silicon. We wish now to propose the following concerning this effect. 

Studies in the stereochemistry of nucleophilic displacements at silicon over the past few years have been numerous. Significant progress has been achieved in establishing the controlling factors of the stereochemistry (i) the lability of the leaving group, (ii) the electronic character of the nucleophile, and (iii) the bond angles at silicon. 

The stereochemical data reported in the case of germanium compounds by Eaborn et al. 90), Brook and Peddle 91), or Carre and Corriu (92) are quite parallel to those discussed here. The nature of the leaving group and the electronic character of the nucleophile, rationalized in terms of hard and soft reagents, are also the dominant factors that govern the stereochemistry. 

Similarly, electrophilic CHj radicals add faster to ethylene than to tetrafluo-rethylene, in which the inductive effects of the fluorine substituents make the double bond somewhat electron deficient. The reverse selectivity is shown by CH3 radicals, which are nucleophilic in character [24]. This parallels the observations in organic chemistry thatcarbonium ion formation is facilitated by replacement of hydrogens by alkyl groups and hindered by CF3 groups. There is a polarization of a electrons towards the trifluoromethyl group, whereas these electrons can be more effectively released by alkyl substituents. 

Studies primarily of effects of substituents on reactions on phenyl rings permit the assignment of nucleophilic (electron-releasing) character to various groups. These include alkyls, vinyl, hydroxyl, ether, phenyl, and... 

The metal hydrides exhibit different reducing properties. The complex metal hydrides (LiAlH4, NaBH4) are nucleophilic in character hydride is transported from the complex anion to the electron-deficient centers of the functional groups. Another group of metal hydrides (boranes, alanes) are strong Lewis acids they interact with centers that are relatively richer in electrons. The selectivity of the reduction can be improved and the scope of its application can be extended by the joint use of these two types (mixed hydrides). 

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