Carbocations stabilisation

Similar carbocation stabilisation can also occur in the hydrolysis of allyl halides, e.g. 3-chloropropene ... 

A particularly interesting case of carbocation stabilisation occurs with Hiickel 4n + 2 systems when n = 0, i.e. cyclic systems with 2-7re (p. 18). Thus derivatives of 1,2,3-tripropylcyclopropene (8) are found to yield ion pairs containing the corresponding cyclopropenyl cation (9) extremely readily,... 

The conclusion of Shibuya et al. (1990) from these experiments was that isoprenylation of L-tryptophan involved displacement of the pyrophosphate moiety of DMAPP by the indole residue with inversion at C-1 of the isoprene moiety. In the ternary enzyme substrate complex, dissociation of the C-1/ pyrophosphate bond of DMAPP would yield an ion pair or an allylic carbocation stabilised by the countercharge of the enzyme-bound pyrophosphate as shown in Figure 7. From C-4 of enzyme-bound L-tryptophan on the face opposite to the pyrophosphate, attack would create DMAT with inversion at C-1 and retention of double bond geometry. If the scrambling between allylic hydrogens occurred during these event, this can be readily accounted for by Figure 7... 

For S vl attack, considerable charge separation has taken place in the T.S. (cf. p. 81), and the ion pair intermediate to which it gives rise is therefore often taken as a model for it. As the above halide series is traversed, there is increasing stabilisation of the carbocation moiety of the ion pair, i.e. increasing rate of formation of the T.S. This increasing stabilisation arises from the operation of both an inductive effect,... 

Thus for hydrolysis in 50% aqueous acetone, a mixed second and first order rate equation is observed for phenylchloromethane (benzyl chloride, 10)—moving over almost completely to the SV1 mode in water alone. Diphenylchloromethane (11) is found to follow a first order rate equation, with a very large increase in total rate, while with triphenylchloromethane (trityl chloride, 12) the ionisation is so pronounced that the compound exhibits electrical conductivity when dissolved in liquid S02. The main reason for the greater promotion of ionisation—with consequent earlier changeover to the SW1 pathway in this series—is the considerable stabilisation of the carbocation, by delocalisation of its positive charge, that is now possible ... 

Thus solvolysis of (+)C6HsCHMeCl, which can form a stabilised benzyl type carbocation (cf. p. 84), leads to 98% racemisation while (+)C6H13CHMeCl, where no comparable stabilisation can occur, leads to only 34% racemisation. Solvolysis of ( + )C6H5CHMeCl in 80 % acetone/20 % water leads to 98 % racemisation (above), but in the more nucleophilic water alone to only 80% racemisation. The same general considerations apply to nucleophilic displacement reactions by Nu as to solvolysis, except that R may persist a little further along the sequence because part at least of the solvent envelope has to be stripped away before Nu can get at R . It is important to notice that racemisation is clearly very much less of a stereochemical requirement for S l reactions than inversion was for SN2. 

An essential requirement for such stabilisation is that the carbocation should be planar, for it is only in this configuration that effective delocalisation can occur. Quantum mechanical calculations for simple alkyl cations do indeed suggest that the planar (sp2) configuration is more stable than the pyramidal (sp3) by = 84 kJ (20 kcal) mol-1. As planarity is departed from, or its attainment inhibited, instability of the cation and consequent difficulty in its formation increases very rapidly. This has already been seen in the extreme inertness of 1-bromotriptycene (p. 87) to SN1 attack, due to inability to assume the planar configuration preventing formation of the carbocation. The expected planar structure of even simple cations has been confirmed by analysis of the n.m.r. and i.r. spectra of species such as Me3C SbF6e they thus parallel the trialkyl borons, R3B, with which they are isoelectronic. 

The possible formation of a delocalised benzyl type carbocation (16) results in much lower (70%) ANTI stereoselectivity than with trans 2-butene (5 =100% ANTI stereoselectivity, p. 180), where no such delocalisation is possible. It is also found that increasing the polarity, and ion-solvating ability, of the solvent also stabilises the carbocation, relative to the bromium ion, intermediate with consequent decrease in ANTI stereoselectivity. Thus addition of bromine to 1,2-diphenylethene (stilbene) was found to proceed 90-100% ANTI in solvents of low dielectric constant, but =50% ANTI only in a solvent with e = 35. 

Initial attack will always take place on a terminal carbon atom of the conjugated system, otherwise the carbocationic intermediate (64), that is stabilised by delocalisation, would not be obtained. It is because of this stabilisation that a carbocation intermediate is formed rather than a cyclic trromonium ion (cf. 66). Completion of overall addition by nucleophilic attack of Bre on (64) can then take place at C2 [1,2-addition, (a) ->(68)] or C4 [1,4-addition, (b) — (69) J ... 

The orientation of addition of an unsymmetrical adduct, HY or XY, to an unsymmetrically substituted alkene will be defined by the preferential formation of the more stabilised carbanion, as seen above (cf. preferential formation of the more stabilised carbocation in electrophilic addition, p. 184). There is little evidence available about stereoselectivity in such nucleophilic additions to acyclic alkenes. Nucleophilic addition also occurs with suitable alkynes, generally more readily than with the corresponding alkenes. 

This all suggests slow, rate-limiting breaking of the C—H bond to form the stabilised carbanion intermediate (54), followed by fast uptake of D from the solvent D20. Loss of optical activity occurs at each C—H bond breakage, as the bonds to the carbanion carbon atom will need to assume a planar configuration if stabilisation by delocalisation over the adjacent C=0 is to occur. Subsequent addition of D is then statistically equally likely to occur from either side. This slow, rate-limiting formation of a carbanion intermediate, followed by rapid electrophilic attack to complete the overall substitution, is formally similar to rate-limiting carbocation formation in the SNi pathway it is therefore referred to as the SE1 pathway. 

This reflects the relative ease with which the C—H bond in the alkane precursor will undergo homolytic fission, and more particularly, decreasing stabilisation, by hyperconjugation or other means, as the series is traversed. There will also be decreasing relief of strain (when R is large) on going from sp3 hybridised precursor to essentially sp2 hybridised radical, as the series is traversed. The relative difference in stability is, however, very much less than with the corresponding carbocations. 

It is significant that the substituents involved at the far left-hand side of the plot (38 X, Z = MeO) are powerfully electron-donating, and thus capable of stabilising the carbocation (41a ++ 41b), developing in step , by delocalisation of its +ve charge. It is indeed... 

But where the addendum is not an iodine atom and the classical carbocation is stabilised by resonance, then cis addition takes up which may later on by rearrangement give the trans isomer. It has also been found that the nature of the solvent also affects the amounts of cis and trans products. 

In some of their publications Higashimura s group, and others using the same terminology, are close to our view when they write about the modifiers reducing the reactivity of the carbocation . However, since in our view there is no carbenium ion to be stabilised, we see these donors as reducing the polarity of the ester bond and the reactivity of the 0-protons, and they obstruct physically the transfer of a P-proton to the monomer or to any other base. 

---