P-silicon effect

The presence of group 4 metals P to a carbenium ion has a dramatic effect on the stability of the carbenium ion, a remarkable phenomenon known as the P-effect.12 29, 30 The silicon P-effect was discovered in 1937 by Ushakov and Itenberg,31 who noted the exceptional reactivity of P-silyl-substituted halides R3SiCH2CH2X towards elimination (Scheme 1). The effect has since been the subject of many mechanistic32-36 and theoretical studies37-39 and several reviews.10-12,29... 

Electrophilic substitution of vinylsilanes may be combined with the fazarov cyclization to build five-membered rings, as depicted in Figures i5.11 and Si5.12 (LA = Lewis acid). Note that the reaction depicted in igure Si5.12 is anomalous because the vinylsilane is attacked by the lectrophile at the end of the double bond remote from silicon. This is ecause the sulphur lone pairs stabilize a carbocation more effectively than le silicon P effect. 

The second Prins reaction goes through the oxonium ion 223 to give the final product. Again the nucleophile is an allyl silane 223 and the second intermediate is a cation stabilised by the silicon p-effect. The relative stereochemistry is decided in the second reaction and 220 has the favoured diequatorial conformation. 

Attacking the problem of cross-metathesis selectivity from a different angle, Crowe and co-workers explored the reactivity of a more nucleophilic partner for the Ji-substituted alkenes. They chose to use allyltrimethylsilane since they proposed that the CH2SiMe3 substituent should have a negligible effect on alkyli-dene stability, but enhance the nucleophilicity of the alkene via the silicon P-ef-fect (Fig. 1). 

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... 

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). 

The trimethylsilyl group is exchanged for a hydrogen atom using PfSA in acetonitrile, a reaction referred to as a protodesilylation Protonation of vinylsilane 7 leads in the process first to cation 25, which is stabilized by the p-effect of silicon.10 A nucleophile subsequently abstracts the TMS group, producing olefin 8. Vinylsi-lanes can be transformed into olefins not onl> with acid.10 but also with fluoride1011 or with catalytic amounts of iodine m the presence of water... 

Overman15 discusses two conceivable mechanisms for the cyclization. One possibility assumes a direct cyclization of iminium ion 9 via p-silyl cation intermediate 24 to the indolizidinone 10. Cation 24 is stabilized by a p-effect of the silicon atom. Alternatively, iminium ion 9 might first undergo a charge-accelerated cationic aza-Co/fe rearrangement to allylsilaniminium ion 25, which would then cyclize to 10 with loss of a silyl cation. 

FIGURE 2. The dihedral angle dependence of the p effect of silicon... 

Silicon also stabilizes an anion at the a position and a cation at the p position. The a effect is explained by the dilution of the negative charge into the interaction between the carbon-silicon cr-bond and the empty p orbital (Figure 13.2) [1, 2]. 

Another approach uses silicon s ability to stabilize P-carbocations (P-effect). In addition, the TMS group behaves like a proton and is removed after nucleophilic activation ... 

Figure 4. The dihedral angle dependence of the p-effect of silicon. Reprinted from J. B. Lambert, X. Zhao, J. Organomet. Chem. 1996,521,203, with permission from Elsevier Science, Kidlington, UK.

The so-called p-effect has been ascribed to overlap between the vacant p orbital on the P carbon atom and the a orbital between the silicon atom and the a carbon atom (Figure Si 1.1). (Due to the electronegativities of the atoms involved, the carbon-silicon o-orbital has a higher coefficient on carbon.)... 

PhjPO solutions of various halosilanes (e.g., SiCl, MeSiClj, Me SiClj, MCjSiCl) show significant downfield P shifts relative to the free base. The coordination shift of approximately 8-9 ppm indicates bonding interaction between phosphoryl oxygen and silicon. An effect of added MeCN on the chemical shift of PhjPO— McjSiCl solutions has been interpreted in terms of a coordination interaction between the nitrile group and silicon 23,324)... 

Very few pericyclic reactions of carbene complexes have been studied that are not in the cycloaddition class. The two examples that are known involve ene reactions and Claisen rearrangements. Both of these reactions have been recently studied and thus future developments in this area are anticipated. Ene reactions have been observed in the the reactions of alkynyl carbene complexes and enol ethers, where a competition can exist with [2 + 2] cycloadditions. Ene products are the major components firom the reaction of silyl enol ethers and [2 + 2] cycloadducts are normally the exclusive products with alkyl enol ethers (Section 9.2.2.1). As indicated in equation (7), methyl cyclohexenyl ether gives the [2 -t- 2] adduct (84a) as the major product along with a minor amount of the ene product (83a). The t-butyldimethylsilyl enol ether of cyclohexanone gives the ene product 9 1 over the [2 + 2] cycloadduct. The reason for this effect of silicon is not known at this time but if the reaction is stepwise, this result is one that would be expected on the basis of the silicon-stabilizing effect on the P-oxonium ion. 

Regioselectivity in hydroformylation is influenced by electronic and steric effects [4, 5]. Thus the formation of the C a-Rh bond is favored over that of the C P-Rh bond by the well known P-silicon effect (Fig. 3), which stabilizes a positive charge on the p-C atom. From the resulting intermediate la the /50-product should form predominantly. On the other hand, steric effects induced by bulky substituents on silicon or rhodium would favor the sterically less hindered normal alkyl rhodium complex with the C P Rh intermediate Ila as the precusor to the -aldehyde. The observed //so-ratios very close to 1 1 for the Rh-catalyzed hydroformylation of vinyltrimethylsilane indicate that the electronic P-effect obviously is canceled out by the steric demand of the MesSi-groups. Since addition of PPha will favor an active complex with a larger number of bulky phosphine ligands (L = PPhs in Fig. 2), the formation of the linear alkylrhodium complex intermediate Ila to lid is prefered [6]. 

S. McGinnis, P. Sines, and B. Das, Pulsed current anodization An effective method for fabricating patterned porous silicon p-njunction light emitting diodes, Electrochem. Solid-State Lett. 2, 468, 1999. 

By intermolecular competition of an allylsilane and a homoallylstannane for reaction with an electrophile, the P-effect of silicon has been shown to be more effective than the y-effect of tin in activating the double bond towards addition.40 41... 

Guo, L.J. Krauss, P.R. Chou, S.Y. Nanoscale silicon field effect transistors fabricated using imprint lithography. Appl. Phys. Lett. 1997, 71 (13), 1881-1883. 

On the other hand, the organosilyl group is capable of stabilizing the carbocation (3 to silicon. This effect is believed to be attributable to the electron-donating property of the a-bond between silicon and a-carbon atoms ((a-p)7i conjugation) (Scheme IB). The reactions of allylsilanes or vinylsilanes with electrophiles are facilitated with this stabilization of intermediate carbocations (eq (10)). 

Fig. 8.21. Absorption at LHeT of lines 1- 4A and 2p of B in silicon (p is 1.1 x 101B cm-3 at RT) under a <111> uniaxial stress. The encircled numbers together with the vertical arrows indicate the positions of the lines at zero stress. The two insets show the effect of a larger stress [115]. The estimated stress is 27 MPa, and 44 MPa for the insets. Copyright 1967 by the American Physical Society...

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