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Allylstannanes electrophilic substitution

There is an extensive review of the electrophilic substitution reactions of allylsilanes and vinylsilanes in Organic Reactions and the reactions of allylstannanes have been reviewed briefly. The major books on synthetic organosilicon and organotin chemistry also cover the types of reaction discussed here. [Pg.564]

In the absence of Lewis acids, stereoselectivity is strongly dependent upon the geometry of the double bond in the allylstannane, both in simple thermal reactions (Scheme 28) and in pressure-induced reactions, as expected for a cyclic transition state. As well as being selective for the formation of the anti arrangement of substituents on the carbon chain, the overall electrophilic substitution in the thermal reaction of an ( )-allylstannane, is stereospecifically syn (Scheme 29), with a cyclic transition state (51),... [Pg.574]

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],... [Pg.117]

Because of the high nucleophilicity of the donor-substituted double bonds, rate constants for the attack of electrophiles at silyl enol ethers have been determined with the weak electrophile (p-Me2N-C6H4)2CH+ (Scheme 46) [206]. One can extract from these data that the nucleophilicity of silyl enol ethers is in between that of analogously substituted allylsilanes and allylstannanes (Scheme 47). [Pg.117]

The P-effect similarly enhances the reactivity of alkynyl- (Section 8.2.2), alkenyl-(Section 8.1.2), allyl- (Section 9.1.3.2), and aryl- (Section 7.1) stannanes in their reactions with electrophiles (equations 3-33-3-36), and its effect can be recognised in other contexts such as the ene reactions of allylstannanes (Section 9.1.3.4), and the charge-transfer reactions and ring-substitution reactions of benzylstannanes.22... [Pg.37]

Seyferth showed in 1959 that allyllithium, which is not available from allyl halides because of competing substitution reactions, can be prepared from the reaction of allylstannanes with organolithium reagents (XM = R Li Section 22.1). These reactions involve nucleophilic attack of R upon tin to form a pentacoordinate intermediate. Lewis acids, MX, on the other hand, react by electrophilic attack at the y-position. [Pg.136]

When skeletal bonds to groups R and R are to be made in the vicinity of a carbon-carbon double bond, substitution of allylic acetates by organocuprates or by malonates (Pd(O)-catalyzed) comes to mind. Alternatively, when R and R are to be introduced as electrophiles (a-synthons), one considers the Lewis acid catalyzed substitution of allylsilanes or allylstannanes. The incorporation of a branched methallyl or isoprene moiety in this manner represents a standard example of a building block oriented strategy in synthesis [28] (Scheme 3.18). [Pg.56]

Many carbon nucleophiles tend to he too basic for successful addition to the carbocationic center provided by DMTSF activation of dithioacetals and usually result in vinyl sulfide formation. In addition, the Mc2S generated competes for the electrophilic center. Allylstannanes overcome these constraints and successfully generate new carbon-carbon bonds (eq 12). DMTSF activation of a thioacetal resulted in an electrophilic cyclization at the 3-position of a 3-substituted indole. ... [Pg.220]

Allyltributylstannanes are common reagents in both radical and Lewis acid-mediated reactions, although they are unknown to react with enones. However, there has been the first report of a reaction of an electrophilic allylstannane with an enone (Scheme 1). While the parent allylstannane does not react with enones, those substituted with an ester substitute smoothly at the y3-position of enones. The authors claim this to be the first non-basic and non-nucleophilic alternative to the Michael reaction. [Pg.166]


See other pages where Allylstannanes electrophilic substitution is mentioned: [Pg.832]    [Pg.515]    [Pg.745]    [Pg.832]    [Pg.954]    [Pg.954]    [Pg.374]    [Pg.375]    [Pg.234]    [Pg.511]   
See also in sourсe #XX -- [ Pg.161 ]




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