Charge distribution, allylic cation

The positive charge of allyl cations essentially distributes itself among C-1 and C-3 of the allyl skeleton. Terminal alkylation concentrates the positive charge at the alkylated carbon, as shown for the 1-methylcyclopentenyl cation [494],... 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

The ratio of isomeric ethers is strongly affected by polar substituents which induce an asymmetric distribution of charge in allylic cations. Photolysis of methyl 2-diazo-4-phenyl-3-butenoate (20) in methanol produced 24 in large excess over 25 as the positive charge of 22 resides mainly a to phenyl (Scheme 8).19 As would be expected, proton transfer to the electron-poor carbene 21 proceeds reluctantly intramolecular addition with formation of the cyclopropene... 

Information on electron delocalization in the bicyclo[3.1.0]hexenyl cations is available from their reported NMR spectra Data obtained with a variety of systems point to a completely different charge delocalization pattern to that found with the homotropenylium ions. For example, Olah and colleagues have obtained the NMR spectrum of the parent ion"", 61, and compared this with those of 42 and 11. As can be seen from the data summarized in Scheme 18, the chemical shifts of the five-membered ring carbons of 61 resemble those of the cyclopentenyl cation. There is a considerable difference in chemical shifts, and hence charge distribution, at C(2), C(4) and C(3) of 61. There is no evidence for the fairly even charge distribution as is found for the homotropenylium and homocyclopropenium ions (see previous Sections III. A and III. B). It was also noted by Olah that the chemical shift of C(6) is consistent with large delocalization to this position, i.e. to conjugation of the allyl system of 61 with the external cyclopropyl bonds. 

Fig. 2-22 Total 7t-electron population (and excess-charge distribution in brackets) in the allyl anion and the allyl cation...

These resonance structures are the nitrogen and oxygen analogs of the allyl cation. The effect of this tt delocalization is to attenuate the polar destabilization by these substituents. These interactions are reflected in MO energies, bond lengths, and charge distributions calculated for such cations (review Section 3.4.1). 

Compute the charge distributions for allyl cation using the following methods ... 

The carbocation of Figure 5.19 can be written either as one of the two canonical forms (5.26) and (5.27) or using an electron smear (5.28) as shown in Figure 5.20. The smear is closer to reality since the molecular orbitals of the molecule will be distributed across the three carbons of the allylic cation. However, it will be able to react as if the positive charge were localised at either end. For each individual reaction, the nature of the other reactive species, the reaction conditions and the nature of the product will determine which way round the system will react. 

In contrast to the symmetrical charge distribution in the parent allyl cation, unequal charge distributions can result when substituents are present. Consider the methyl-substituted allylic ion represented by the following two resonance structures. 

The electrostatic potential map (bottom) shows the unequal charge distribution in a methyl-substituted allyl cation. 

Recall that in general, a distributed charge in a molecule is more stabilizing than a more localized charge. It has been determined experimentally that the double bond of one adjacent vinyl group provides approximately as much stabilization as two alkyl groups. Thus, the allyl cation and 2° isopropyl cation are of comparable stability. 

In ions, delocalization of electrons (distributing them over as many atoms as possible) is especially important. Delocalization of electrons is a process that allows more than one atom to share electrons and it is almost always stabiliang. Electrons are like water, if allowed to spread out, they wiU. The allyl cation is a good example—the two end carbons each bear one-half of the positive charge (Rg. 1.32). 

The microstructure of the polybutadiene and polyisoprene produced by anionic polymerization is correlated to the structure of the allylic anion of the active chain end. The charge distribution over this anion is affected greatly by changing counterions, the use of solvating solvents, and the presence of cation chelating additives. 

Fig. 2. Charge distributions and orbital energies for cyclopropyl and allyl cations.

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