Organosilanes, nucleophilic reactivity

To get a broader concept of the nucleophilic reactivity of phosphorus acid derivatives towards organosilanes we studied mechanism of reactions of triorganosilyl halides with esters of tricoordinate and tetracoordinate phosphorus having the general formulae ... 

Alkylsilanes are not very nucleophilic because there are no high-energy electrons in the sp3-sp3 carbon-silicon bond. Most of the valuable synthetic procedures based on organosilanes involve either alkenyl or allylic silicon substituents. The dominant reactivity pattern involves attack by an electrophilic carbon intermediate at the double bond that is followed by desilylation. Attack on alkenylsilanes takes place at the a-carbon and results in overall replacement of the silicon substituent by the electrophile. Attack on allylic groups is at the y-carbon and results in loss of the silicon substituent and an allylic shift of the double bond. 

The second step is now the study of the reactivity of these hexacoordinated silicon species towards nucleophiles in order to obtain Si-C or SiH bond formation. The nucleophilic attack takes place at silicon with displacement of catechol and formation of organosilanes. 

A comparison of steric influence resulting from more than 80 different silyl groups is summarized in Table 1. The trend is established on the basis of reaction yields, rates and selectivity, as well as on reactivity and physical properties of organosilanes. Two different effects may be involved (a) the effect of a silyl group on reactions taking place at the neighboring centers, and (b) the effect of the groups attached to silicon on the nucleophilic attack at the silicon atom. 

Evidence that the 1Sn2 reaction at silicon does indeed go through a pentacovalent intermediate comes from the silicon analogue of the migration step in hydroboration-oxidation. Treatment of reactive organosilanes (that is, those with at least one heteroatom—F, OR, NR2—attached to silicon to encourage nucleophilic attack of hydroperoxide at silicon)... 

Organosilanes bearing an electronegative heteroatom(s) at the a-carbon are susceptible to nucleophihc activation leading to silicon-carbon bond-cleavage, because of the electronic effect of the heteroatom. a-Heteroatom-substituted organosilanes are therefore quite valuable as protected carbon nucleophiles. The silicon-carbon bond is also readily activated by a transition metal complex. The reactivity is successfully utilized for catalytic carbon-carbon bond formation. 

Nucleophilic Substitution. Eabom reported that fluorosi-lanes are more reactive toward organometallic reagents than the corresponding chlorosilanes, and therefore more suited for preparation of sterically hindered organosilanes. Two examples of this enhanced reactivity are shown for the Tsi anion (eq 1) and a dianion (eq 2). The use of less-hindered reagents in stoichiometric amounts still allow for the sequential introduction of nucleophiles (eq 3).l ... 

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