Carbocations symmetrically stabilized

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion stracture is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case, racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

Concerning the mechanism of Cr(VI) oxidations, initial attack of Cr03 to form a symmetric intermediate was proposed.677 Hydrogen atom or hydride abstraction from the allylic position leads to resonance-stabilized allylic radical or carbocation, respectively, which is eventually converted to the unsaturated carbonyl compound. 

Further support comes from calculation of 1 Jch spin-spin coupling constants at IGLO-DFPT/BIII level. The data were interpreted as giving strong evidence for intramolecularly TT-stabilized silanorbomyl cation structure lacking coordination to solvent or counterion. The species can also be regarded as an almost symmetrically bridged /J-silyl carbocation with siliconium ion character. 

MP2-FC/6-31G calculations reveal the symmetrically bridged 6-sila-2-norbomyl cation 1 not only to be a local minimum (Fig. 18), but also to be 17.2 kcal mof more stable than the 2-norbomyl cation (Eq. 3) at MP2-FC/6-31G + AZPE(SCF/6-31G 0.89). However, the inherently greater stability of silyl cations contributes to this difference. The Si NMR chemical shift of the bridging silicon atom, ca 1 ppm vs TMS (IGL0/H //MP2-FC/6-31G ), is very strongly shielded in comparison with ca 300 ppm expected for a free RSiH2 species [38]. Thus, the sila congener of the 2-norbomyl carbocation also possesses a nonclassical stracture which is reflected by its stracture as well as its NMR properties. 

On the basis of theoretical and experimental results a symmetrical mercurinium ion, with most of the positive charge on mercury, has therefore been proposed in reactions of symmetrically substituted alkenes , while asymmetrical mercurinium ions or weakly bridged mercury-substituted carbocations have been proposed when there is a substituent, such as an aryl group, on the double bond -. Finally, with substituents highly capable of stabilizing carbocations, fully open intermediates have been proposed . ... 

In summary, the silanorbomyl cations 5 and 25 are stable compounds in arene solvents at ambient temperature, lacking direct coordination to solvent or counterion. They are stabilized by intramolecular Jt-interaction and can be therefore regarded as an almost symmetrically bridged p-silyl-substituted carbocation with siliconium ion character. 

But this compound is symmetrical, with Cl being equivalent to C4, and C2 being equivalent to C3. Therefore, there are only two distinct possibilities for the protonation step at Cl or at C2. The resonance-stabilized, allylic carbocation is only formed via protonation at Cl ... 

There are other ways to stabilize a diradical. Solvolysis of 11.53 or protonation of I 1.54 initially generates the carbocation I 1.55 [41]. The central C—C bond in the bicyclobutane portion of the molecule is again weakened by strain and so bond-stretch isomerization of I 1.55 to I 1.56 should be possible. Now the antisymmetric combination of the two p orbitals which form the diradical is markedly stabilized by a through-space interaction with the empty p orbital on the carbocation. This is illustrated in 11.57. The symmetric combination, 11.58,... 

The structural analogy between carbenes and their isoelec-tronic carbocations is also observed for singlet cyclopentylidene (8), which has a C2 symmetric structure (Figure 8) similar to the cyclopentyl cation. " Similarly to 2, two CH bonds stabilize the carbene center in 8. As the system is incorporated into a five-membered ring, the CCC angle at the carbene center is smaller than in 2 (104° vs. 112°). Thus, the. singlet. state... 

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