Hyperconjugation 3-silicon

The crucial influence on the reactivity pattern in both cases is the very high stabilization that silicon provides for carbocationic character at the p-carbon atom. This stabilization is attributed primarily to hyperconjugation with the C-Si bond (see Part A, Section 3.4.1).85... 

As discussed in Section II. A, theoretical studies predicted that phospha-silenes with silyl substituents attached to phosphorus should have planar, trigonally coordinated silicon, with the Si—P w-bonds strengthened by the hyperconjugative influence of the silyl group.16 Recently, this was proved by a single-crystal X-ray structure determination of the derivative... 

A pjr-djr effect was found also in the silicon derivative in the series p-CICgEEMMes (M = C, Si, Ge, Sn)74. In this case the hyperconjugation minimizes the difference in orbital energy 712-713, where 713 is the phenyl orbital affected by the conjugation, which is stabilizing for the chlorine 3p orbitals, destabilizing for the M empty d orbitals. 

The destabilizing effect of a silyl group compared with an alkyl group in trivalent carbocations was explained by the weaker hyperconjugation of the Si-R a-bond (R = alkyl) relative to a C-R cr-bond (R = H or alkyl) and by electrostatic repulsion between the adjacent positively charged cationic carbon and the electropositive silicon (10). 

The P-effect, the interaction of a P-carbon silicon a-bond with the vacant p-orbital at the C+ carbon, is called hyperconjugation i.e. the conjugation of the... 

The 29Si-NMR chemical shift of 12 (8 = 66.34 ppm) is considerably deshielded as compared to the progenitor alkene 13 (8 = 0.14 ppm), which is in accord with substantial P-Si-C-bond hyperconjugative delocalization of positive charge to silicon. 

Comparison of all reported cleavages gives the apparent ordering H > Ph (or aryl) > alkyl in terms of tendency for groups to be cleaved from silicon, with the exception that at least one aryl substituent is retained on silicon in the anion VI. It is clear that reduction of aryl silanes leads to significant weakening of Si—R bonds that have appropriate symmetry for hyperconjugative interactions with the reduced n systems, a point that has been cited as evidence for the importance of n — type interactions in unsaturated silanes (86). 

On the basis of complete neglect of differential overlap (CNDO/2) calculations which include silicon p and d orbitals in the basis set, the effect of the trimethylsilyl substituent is due to electron withdrawal by the interaction of both the silicon p and d orbitals with the aromatic ir-system (85). In as much as the silicon p orbitals are primarily involved in the cr and cr orbitals of the trimethylsilyl group, interaction of the silicon p orbitals with the ir-system amounts to a hyperconjugative, v — cr, electron withdrawl (85). 

Replacement of C(6) of 93 by various heteroatoms was found by Cremer and coworkers to decrease the homoaromatic resonance energy with increase in the electronegativity of the heteroatom (24 kcalmol"1 whenX = BH to 4 kcalmol"1 whenX = 0)l9°. Silicon atoms in positions 1,3 and 5 were found to give homoaromatic analogues of 93 however, it was found difficult to segregate homoaromatic stabilization and Si—C—C+ hyperconjugation effects. 

The electronic effects of an R3 S i group can be divided into four components (i) inductive effects, (ii) field effects, (iii) (p-d) jr bonding and (iv) hyperconjugative effects. The total electronic effect of an R3 S i group in a molecule or intermediate will be a combination of these effects, and much effort has been made in the last ten years to quantify the contributions of the various effects to certain properties of silicon compounds, particularly as regards the importance of (p-d) jr bonding and hyperconjugative effects. 

Solvolysis of 15 in 97% trifluoroethanol gave a secondary isotope effect of 1.17, which indicates a vertically stabilized transition state. Thus the highly unsymmetrical dihedral dependence of silicon participation can almost entirely be attributed to the hyperconjugation model with little non-vertical involvement of the silicon nucleophile. 

In the absence of hyperconjugative and internal participation, there is essentially no silicon /1-effect. The inductive effect of the Me3Si group is essentially zero. This agrees with the calculations of Ibrahim and lorgensen39, which predict no inductive stabilization for secondary systems. 

The /-substituent data in Tables 5 and 6 show that phenyl and alkoxy substituents at silicon are less effective than methyl at stabilizing the partial positive charge build-up on /1-silicon by hyperconjugation. However, the /-trimethylsilyl group does stabilize this build-up of positive charge. 

In addition to the stabilization of /1-carbocations, the /1-effect of silicon can also be observed in the ground states of neutral molecules. Lambert and Singer58 studied compounds of the type 42 where hyperconjugation should be enhanced by increasing the electron-accepting properties of the substituent X (MeO < Me < H < CN). o-tt overlap in this system gives the resonance structure 43 shown in equation 19. 

Hyperconjugation should raise the bond order between the ipso and benzylic carbons, and lower the bond order between the benzylic carbon and the silicon atom. As the ipso-benzylic bond length decreases, the 13C-13C coupling constant should increase, and as the benzylic carbon-silicon bond length increases, the 13C-29Si coupling constant should... 

Ground state /-effects of silicon may be responsible for the elongated C(alkyl)-O(ester) bond in n.s-3-trimelhylsilylcyclohexyl p-nitrobenzoate 59 relative to the silicon-free derivative. It is suggested that the ground state /-effect could be due either to homohyperconjugation, 60, or to inductively enhanced C—C hyperconjugation where the trimethylsilyl substituent increases the importance of the resonance form 61 relative to the silicon-free derivative. 

The stabilization of a positive charge by 5-silicon is believed to be through double hyperconjugation, represented by the resonance structures shown in equation 22. 

Provided that the silicon-carbon bond can be coplanar with the vacant p orbital, the /J-silyl substituted carbocation should be stabilized by hyperconjugation, and this has been demonstrated by Kresge and coworkers47,49. 

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