Silyl and Stannyl Substituents

Deuteration studies with a silylbutatriene with KOtBu in tBuOD provided evidence for a selective a-deprotonation [340]. 

Most of the allenes with silyl or stannyl substituents are prepared by the routes discussed in Chapter 9. 

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The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they act as electron-releasing groups promoting addition and also control the regiochemistry. A silyl or starmyl substituent strongly stabilizes carboca-tion character at the /3-catbon atom and thus directs the electrophile to the a-carbon. The reaction is then completed by the limination step, in which the carbon-sihcon or carbon-tin bond is broken. 

Table 24 Synthesis of phospholes with silyl and stannyl substituents...

Fluorodemetallation. Replacement of silyl and stannyl substituents from aryl-silanes and )3-trialkylstannyl enones is rapid. The latter reaction is effected in the presence of AgOTf as catalyst. 

The ability to promote j8 elimination and the electron donor capacity of the jS-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they promote addition and strongly control the regiochemistry. A silyl or stannyl substituent strongly stabilizes carbocation character at the j3-carbon atom and thus directs the electrophile to the a-carbon. Molecular orbital calculations indicate a stabilization of 38 kcal/mol, which is about the same as the value calculated for an a-methyl group. The reaction is then completed by the elimination step, in which the carbon-silicon or carbon-tin bond is broken. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

FIGURE 70. Calculated transition states at MP2 of the 1,2-migration of silyl, germyl and stannyl substituents of radicals H3E-CH2X to H3EX-CH2 (X = CH2, NH, O E = Si, Ge, Sn). Bond distances are in A, angles in deg. The calculated activation energies (kcal mol 1) at QCISD//MP2 refer to the forward reaction ( A E1 i ) and the reverse reaction (A E ), respectively. Reproduced by permission of The Royal Society of Chemistry from Reference 190... 

The degree of regioselectivity is controlled in large part by the orbital coefficients in the HOMO of the diene and in the LUMO of the dienophile. One can exploit this aspect of the [4+2]-cycloadditions to enhance regioselectivity [12]. For instance, an auxiliary silyl or stannyl substituent (cf. compound 41) may serve to improve an otherwise unsatisfactory regioselectivity during a Diels-Alder cycloaddition [13]. A boryl substituent, such as in diene 42, defines a position in the Diels-Alder product at which other (alkyl or aryl) substituents can be introduced in follow-up steps unfortunately, diene 42 confers poor regioselectivity. 

As a result of these facile elimination reactions, silicon and tin compounds are becoming very useful in synthetic chemistry. Vinylsilanes and vinylstannanes, when attacked by electrophilic species, often undergo elimination of the silyl or stannyl substituent ... 

Oxidation of (3-silyl and (3-stannyl acids leads to loss of the substituent and alkene formation.283... 

Further evidence for the above-mentioned mechanism of HOMO elevation by group 14 elements is provided by studies of thioethers. The decrease in oxidation potential of silyl ethers as compared to ethers is not realized in the case of a-silylthioethers whereas a-stannyl substituents in thioethers cause a considerable cathodic shift in oxidation potential. Moreover, the effect is geometry-dependent. Values for substituted cyclic dithianes 15 are summarized in Table 21. The difference between Si and Sn in this case is illustrative. The lone nonbonding pair in the 3p orbital of sulfur is much too low in energy compared to... 

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