Allylic cations stability

The first step is the migration of one of the bonds (shown in green) of the three-membered ring to displace the pyrophosphate leaving group, expand the ring to four-membered, and release some strain. Now the cyclobutyl cation breaks down to give an open-chain allylic cation stabilized by one of the alkenes. This is the ca tion that is reduced by NADPH. 

Now let s ask if the molecular orbital description can predict both products. Of course it can, or it would hardly be very useful. What happens as the chloride ion, with its filled nonbonding orbitals, begins to interact with the allylic cation Stabilizing interactions occur between filled orbitals and empty orbitals. Chloride bears the filled orbital therefore we must look for the lowest unoccupied molecular orbital (LUMO) of allyl. Figure 9.15 shows it, O2. There are two points at which chloride can add to O2, and they lead to the two observed products. Note that the middle carbon, through which the node passes, cannot be attacked. So the molecular orbital description also explains the regiochemistry of the addition. 

Finally, the l,3-pentadien-5-yl cation can be regarded as an allyl cation stabilized by one vinyl group. The stabilization energy, stab(vinyl), for this variation can be calculated from equation (12) ... 

As shown in Figure 27, an in-phase combination of type-V structures leads to another A] symmetry structures (type-VI), which is expected to be stabilized by allyl cation-type resonance. However, calculation shows that the two shuctures are isoenergetic. The electronic wave function preserves its phase when tr ansported through a complete loop around the degeneracy shown in Figure 25, so that no conical intersection (or an even number of conical intersections) should be enclosed in it. This is obviously in contrast with the Jahn-Teller theorem, that predicts splitting into A and states. 

A rule of thumb is that a C=C substituent stabilizes a carbocation about as well as two methyl groups Al though allyl cation (H2C=CHCH2 ) is a primary carbocation it is about as stable as a typical secondary carbocation such as isopropyl cation (CH3)2CH-"... 

Just as allyl cation is stabilized by electron delocalization so is allyl radical... 

The carbocation is stabilized by delocalization of the tt electrons of the double bond and the positive charge is shared by the two CH2 groups Substituted analogs of allyl cation are called allylic carbocations Allyl group (Sections 5 1 10 1) The group... 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

Any structural effect which reduces the electron deficiency at the tricoordinate carbon will have flie effect of stabilizing the caibocation. Allyl cations are stabilized by delocalization involving the adjacent double bond. 

The addition of hydrogen halides to dienes can result in either 1,2- or 1,4-addition. The extra stability of the allylic cation formed by proton transfer to a diene makes the ion-... 

The stabilized fluorinated allylic cation, generated from cis- or trans-l-(p-methoxyphenyl)pentafluoropropene and antimony pentafluoride in sulfur dioxide, is solvolyzed by methanol to methyl 2-(p-methoxyphenyl)difluoroacrylate [36] (equation 37)... 

How can we account for the formation of 1,4-addition products The answer is that allylic carbocations are involved as intermediates (recall that allylic means "next to a double bond"). When 1,3-butadiene reacts with an electrophile such as H+, two carbocation intermediates are possible a primary nonal-lylic carbocation and a secondary allylic cation. Because an allylic cation is stabilized by resonance between two forms (Section 11.5), it is more stable and forms faster than a nonallylic carbocation. 

It has been contended that here too, as with the benzene ring (Ref 6), the geometry is forced upon allylic systems by the a framework, and not the 7t system Shaik, S.S. Hiberty, P.C. Ohanessian, G. Lefour, J. Nouv. J. Chim., 1985, 9, 385. It has also been suggested, on the basis of ab initio calculations, that while the allyl cation has significant resonance stabilization, the allyl anion has little stabilization Wiberg, K.B. Breneman, C.M. LePage, T.J. J. Am. Chem. Soc., 1990, 112, 61. 

Allylic silanes show a pronounced tendency to react at the a-carbon in the presence of bidentate ligands.148 This regiochemistry is attributed to the preferential stabilization of cationic character by the silyl substituent. The bidentate ligands enhance the electrophilic character of the TS, and the cation stabilization of the silyl group becomes the controlling factor. 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Structures 4 and 5 resembles 1° carbocations and yet the allyl cation is more stable than a 2° carbocation => resonance stabilization. 

Addition can be attributed directly to the stability and delocalized nature of allylic cation. 

In step 1, a proton adds to one of the terminal carbon atoms of 1,3-butadiene to form the more stable carbocation => a resonance stabilized allylic cation, i) Addition to one of the inner carbon atoms would have produced a much less 1 ° cation, one that could not be stabilized by resonance. 

The allyl-resonance stabilized E- and Z-pent-l,3-dienyl-2-cations (22 and 23) are the smallest member of vinyl cations observed as persistent species in superacid solution 49 These are difficult to generate experimentally50 but structures with only five heavy atoms are suitable candidates for coupled cluster model calculations. A challenging task of quantum chemistry was to assign the 13C NMR spectrum of the mixture of isomers (Fig. 3), which exhibits pairs of signals of 22 and 23 which differ only by a few ppm, to the chemical shifts for the specific carbon atoms of the E- and Z-isomers, respectively. 

Other factors which affect the case of electrocyclic ring opening include the nature of substituents which can stabilize or destabilize the development of possible charge and the release of strain in small cyclic systems. Thus different stereochemistries have been observed in the ring opening of cyclopropyl derivatives. All cis derivatives generate an all-cis allyl cation but the anti derivatives will form the all trans cation. 

Some of these modules also show fairly low (17.7 to 21.9 kcal/mol) barriers to rotation about the N=C bond. This can be ascribed to the stabilized allylic cation appearing in the transition state. Although no further splitting occurs above... 

One possible mechanism is electrophilic attack of the complexed carbene carbon atom at the terminal carbon of the diene. The resulting zwitterionic intermediate can now eliminate the metallic group (CO)5M directly, or, alternatively, the metallic group can migrate to yield a new, more stable zwitterion (stabilization of the allyl cation by the heteroatom X). 

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