C-Sn Hyperconjugation

Addition reactions at the alkyne bonds are dealt with in the section on alkenylstannanes that are produced. The alkynyl-tin bond is more readily cleaved by both electrophiles and nucleophiles than is the alkenyl- or alkyl-tin bond. Strong electrophiles such as halogens or halogen acids attack at the z/Mzi-position of the triple bond to give a /3-stannyl cation that is stabilized by C-Sn hyperconjugation, but this is followed by cleavage of the C-Sn bond (Equation (83)). 

Studies on the distannylmethyleneborane 127109 using UB and 13C NMR spectroscopy provides evidence for strong C-Sn hyperconjugation with the boron... 

The temperature-dependence of the ESR spectrum of the 2-stannylethyl radical shows that it is most stable in the conformation 3-8, in which the C-Sn bond eclipses the axis of the singly occupied 2p orbital. The rotational barrier about the C -Cp bond, which is probably a reasonable measure of the C-Sn hyperconjugation, is ca. 8 kJ mol-1.12-14... 

C-Sn Hyperconjugation in an intermediate P-stannyl radical is also important in the conjugative homolytic substitution of an allylstannane (equation 3-30)15-16 (see Section 9.1.3.3), and in the ipso substitution of a vinylstannane (equation 3-31 see Section 8.1.2). 

The Wheland intermediates that are formed in reaction 3-36 are important as their pentadienyl cation LUMO (3-10) has the appropriate symmetry for (Whiffen) enhanced hyperconjugation, which is in large part responsible for the high ipso reactivity, and the relative reactivities of compounds C6H5MEt3, compared with benzene, towards electrophiles, as M varies, are approximately (benzene 1), Si, 5 x 104, Ge 106, Sn 5 x 1011, Pb 1020.25 The 4-stannylbenzyltin compounds should further benefit from both the Whiffen effect at the ipso position and the simple C-Sn hyperconjugation at the 4-position (3-11). 

Evidence for C-Sn hyperconjugation with a filled n-system also comes from the NMR spectra of ring-substituted benzyltin compounds (3-12).31-32... 

An interesting difference between the homoaryl and heteroaryl compounds is found in the reaction of 2-(tributylstannyl)-furan and -thiophene towards benzyhydryl cations.115 Whereas homoarylstannanes appear always to undergo ipso attack by electrophiles, the relative reactivities at the 2 (ipso) and 5 position in these two hetero-arenes are shown in Scheme 7-2 at both positions the reactivity is increased, but the effect is much larger at the 5-position. If C-Sn hyperconjugation is important in stabilising the Wheland intermediate, this unusual regioselectivity might be taken to imply that here there is an early transition state in which such an effect would be less important. 

Thermolysis of the syn and anti /J-acetoxy stannanes, obtained by addition of Bu3SnLi to tram- or cis- 1,2-dimethyloxirane and subsequent acetylation, led to (Z)- or (E)-2-butene, respectively, by a stereospecific anti process (equation 34).73. It is postulated that a hyperconjugative interaction of the C—Sn bond facilitates departure of the acetate group in these acyclic systems. The trimethylstannyl and triphenylstannyl analogues likewise undergo anti elimination. 

Part of the reason for this enhanced reactivity is the stabilization that accrues from hyperconjugation of the C-Sn bond with the LUMO of the pentadienyl cation, which is much more powerful than C-H hyperconjugation. This is illustrated above in formula 2 where it is presented in terms of the solvation of a stannonium ion by an arene.149... 

Hyperconjugation by a C-Sn o bond (and indeed by most carbon-metal a bonds) is much more effective than C-H hyperconjugation, and it is an important factor in determining the structure and stability of not only radicals and cations, but also of compounds with filled n systems such as allyl-, benzyl-, and cyclopentadienyl-stannanes. The importance of vinyl-, allyl-, and aryl-stannanes in organic synthesis owes much to the stabilisation of radical and cation intermediates by a stannyl substituent, and under suitable conditions this can accelerate a reaction by a factor of more than 1014. 

The hyperconjugative effect between a C-Sn bond and a P-positive charge is greater in a cation than in a radical, and there are many examples of the enhancement of the rate of both nucleophilic and electrophilic reactions resulting from the introduction of a stannyl... 

The stabilisation of a carbocation by hyperconjugation by a P-C-Sn bond is sometimes referred to simply as the P-tin effect. It is apparent in the abstraction of hydride by Ph3C+ BF4 from alkyltin compounds.19 Tetramethyltin is unreactive, but ethyltin compounds undergo elimination. 

However, when such factors are kept constant (via varying electron demand of a x-substituent X for the same hyperconjugative donor M at the benzylic position) and when the substituent and the probe are separated in space - so that local anisotropic contributions are avoided - the monotonic decrease in the direct M-C coupling constants has been observed with increasing electron withdrawal for C- Si and C-" Sn bonds (Figure 12.53). This change is consistent with the increased importance of hyperconjugation. 

Computational studies of the formation of ( )-5-stannyl homoallylic alcohols by an allene hydroboration-aldehyde allylboration sequence show that the kinetic allene hydroboration product is less stable and isomerizes to the more sterically congested a-stannylallylborane with the C—Sn to boron a—n hyperconjugation interaction sufficiently stabilizing to override the steric congestion (Scheme 77). " ... 

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. 

Sn-C hyperconjugation can similarly be invoked to account for the stability of the adducts between arenes and the stannonium ions, 2 (where the symmetry of the SOMO is shown), and of the 2-dialkylstannanorbornyl cation 3.151... 

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