Delocalisation carbocations

It seems to the author that consideration of least motion effects in this fine detail with respect to the substrate is unlikely to give worthwhile insight into reactions in solution, particularly in solution in highly polar, and hence highly structured solvents. It is precisely in these solvents that many of the heterolytic reactions whose stereochemical outcome is held to support ALPH have been carried out. Most of these reactions are formally either the reaction of a delocalised carbocation with a nucleophile, or its microscopic reverse. The reactions of delocalized carbocations with nucleophiles have been studied extensively by Ritchie and co-workers, and the main conclusions of their work are particularly germane to considerations of least nuclear motion. 

Recently, during studies on the reaction of [3-carotene (1) with iodine, we succeeded in complete NMR characterisation of the monocation (52) [150], Chemical shifts and coupling constants were determined at -20°C in CDCI3 by NMR (600 MHz) including COSY, HSQC, HMBC and ROESY techniques, leading to complete H and l3C assignments. Considerable downfield H shifts of olefmic protons of 52 (6.4-8.1 ppm) relative to [3-carotene (1) (6.1-6.7 ppm) are compatible with delocalised carbocations with maximum positive charge at C-6. 

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 ... 

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. 

This reflects the greater stability of a secondary rather than a primary carbocation shifts in the reverse direction can, however, take place where this makes available the greater delocalisation possibilities of the ir orbital system of a benzene ring (i.e. tertiary — secondary) ... 

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 ... 

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. 

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... 

In (38 X, Z = MeO) this conjugative stabilisation results in easy formation of the carbocation (41), i.e. to a rapid step but the consequent delocalisation of +ve charge, away from the reaction centre (41a 41b), clearly makes (41) a less effective electrophile,... 

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