Group silicon effect, allylsilanes

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]). 

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 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],... 

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)). 

Gouvemeur, whereby treatment of a broad range of allylsilanes in the presence of Togni or Umemoto reagents and 10 mol% of a copper(i) halide catalyst effected the desired transformation. Gouvemeur demonstrated that in the absence of the silicon group the reaction was low yielding and non-selective. 

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