Allylsilanes 3-silicon effect

Closer examination of tetrahydropyrans 173 clearly reveals that two molecules of aldehyde 174 have been appended onto allylsilane 171 via a novel three-component coupling reaction. Marko et al. proposed the mechanism depicted in Scheme 13.61 [65], Formation of heterocycles 173 is described as a sequence of two processes an initial ene-type reaction [80] which leads to alcohol 177 via the chair-like transition state 176, in which both the aldehydic R-group and the OTMS substituent assume an equatorial position. The high regio- and stereoselectivity observed in this ene-reaction can be nicely explained by considering the stabilizing /(-silicon effect and the repulsive 1,3-diaxial interactions. Transition state 176 contains no 1,3-diaxial interactions and benefits fully from the stabilizing /(-silicon effect [81, 82] (for more detailed transition-state discussion see ref. [63]). 

Figure 6,46 (a) Positive hyperconjugative stabilization of carbocations in the silicon effect and relative stabilization energies provided by C-H, C-C, and C-Si donors, (b) Selectivity observed in vinyl- and allylsilane electrophilic... 

The / -silicon effect on carbocation stability has been widely exploited in organic synthesis, perhaps most notably in the reactions of allylsilanes toward electrophiles. In this, allylsilanes behave remarkably like silyl enol ethers, as shown by the following generic transformations ... 

In the intramolecular amidoalkylation of certain (Z)-allylsilanes, piperidine derivatives (n = 2) are formed less stereoselectively than the corresponding pyrrolidines (n = 1). The complete regiocontrol is induced by the /3-effect of the silicon atom. 

The substituent effect of vinylsilanes is similar to that of allylsilanes. The reactivity of vinylsilanes increased as the number of chlorine atoms on the silicon increased, but decreased as the number of methyl groups increased. However, vinyltrimethylsilane does not react with benzene to give alkylated products. " In the aluminum chloride-catalyzed alkylation of arenes with allylsilanes or vinylsilanes, one or more chlorine substituents on the silicon atom of silanes are required. 

The addition reaction of allylsilane to acetaldehyde with BF3 as the Lewis acid has been modeled computationally.95 The lowest-energy TSs found, which are shown in Figure 9.2, were of the synclinal type, with dihedral angles near 60°. Although the structures are acyclic, there is an apparent electrostatic attraction between the fluorine and the silicon that imparts some cyclic character to the TS. Both anti and syn structures were of comparable energy for the model. However, steric effects that arise by replacement of hydrogen on silicon with methyl are likely to favor the anti TS. 

If a substituent is present that can stabilize the cationic intermediate more effectively than the silicon, this may alter the course of the reaction. For example, deuterionation of the allylsilane 113 leads to a mixture of the expected product 114 and an anomalous product 115 (equation 65)135. 

As with vinylsilanes and alkynylsilanes, substitution is favoured over addition for allylsilanes. However, this can be affected by the steric and electronic effects of the silicon substituents. Mayr and Hagen have studied the reactivities of allylsilanes towards the p-methoxy substituted diphenylcarbocation (Scheme 7)136. 37 Relative rate data and observed products are summarized in Table 11 for allylsilanes of the structures 119-122, with various silicon substituents. 

The allylation of a-iV-tosylimino esters, highly activated imines, proceeds by catalytic use of a Lewis acid. Tol-BINAP-Cu(l) complexes are effective catalysts of asymmetric allylation of these imines with allylsilanes (Equation (28)).124 125 The presence of an aryl group at the position j3 to silicon improves the enantioselectivity. 

Methyl groups at the position of electrophilic attack exert exactly the same enthalpic and entropic effects as in the alkene series (Table 4), and one can summarize that the attack of carbocations at alkenes and at allylsi-lanes, allylgermanes, and allylstannanes follows the same mechanism. The differences between these classes of nucleophiles are encountered after the rate-determining step While ordinary carbocations (produced from alkenes) usually accept a chloride ion to give addition products, the /3-metal-substituted carbocations are generally demetalated to yield the Se2 products. It has been reported, however, that j8-silyl-substituted carbe-nium ions with bulky substituents at silicon may also act as chloride acceptors with the consequence that in these cases allylsilanes yield addition products in the same way as ordinary alkenes do [159],... 

The second proposal by Speckamp and co-woikers used an allylsilane as an excellent nucleophile for an A -acyliminium cyclization reaction (Esch et al. 1987) (Scheme 7.13). The 3-effect of the silicon atom is a powerM determinant of the regiochemistry of allylsilane reactions with electrophiles, so the new carbon-carbon bond is formed at the vinyl caibon distal to silicon, that is, at the y-position (Hiemstra et al. 1984,1985). 

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... 

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)). 

All things being equal, allylsilanes undergo electrophilic addition or substitution via an intermediate /J-silylcarbonium ion. This silicon directing effect is usually observable unless the molecule contains a substituent that can stabilize the cationic intermediate to a larger extent than the silicon (equation 63). For example, 36 (see equation 64) which is... 

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