A-Cumyl cations

The substituent effect on the stability of a-cumyl cations [2C ] based on the proton transfer equilibria of a-methylstyrenes [33] (Mishima et al., 1989d) can be correlated directly with the ordinary set of solution phase stabilities of substituted [2C ] cations (Mashima et al., 1989c). 

Fig. 24 The plots of gas-phase stabilities of substituted a-cumyl cations against Brown s cr in solution. Reproduced with permission from Tsuno and Fujio (1996). Copyright 1996 The Royal Society of Chemistry.

A living cationic polymeriza tion of isobutylene and copolymeriza tion of isobutylene and isoprene has been demonstrated (22,23). Living copolymerizations, which proceed in the absence of chain transfer and termination reactions, yield the random copolymer with narrow mol wt distribution and well-defined stmcture, and possibly at a higher polymerization temperature than the current commercial process. The isobutylene—isoprene copolymers are prepared by using cumyl acetate BCl complex in CH Cl or CH2CI2 at —30 C. The copolymer contains 1 8 mol % trans 1,4-isoprene... 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

The study of the stabilizing effect of the nitrone group in cumyl radical (205) (Scheme 2.72) (362) and cumyl cation (206) (Scheme 2.73) (363) has shown that the nitrone group appears to be a super radical stabilizer and, at the same time, a weak cation stabilizer. ... 

The ct" " substituent constant scale was developed based upon the solvolysis of substituted cumyl chlorides, where cumyl cations are formed in the rate-determining ionization. For the carbocation-forming reaction of Eq. 20, there is a good correlation of log versus the sum of the constants for the substituents directly on the carbon. Such a correlation clearly does not exist for the reaction where the... 

The cumyl cation (4) has been the subject of an X-ray crystallographic study, as its hexafluoroantimonate salt at —124 °C.31 It is nearly planar (8 ° twist), with a short bond between the C+ and the ring (1.41 A), consistent with benzylic delocalization. The Me—C+ bonds are also shortened, indicative of hyperconjugative interaction.31 However, calculations are taken to show that hyperconjugation is not important in isolated benzyl cations e.g. structures such as (6) are not important contributors to the overall structure of (5).32 The stabilization provided by alkyl groups would thus be because of their polarizability, and the Baker-Nathan effect would be due to steric hindrance to solvation.32 The heats of formation of some a-mcthylbcnzyl cations indicate that the primary stabilization in these species comes from the a-substitucnts, and that the stabilization provided by the aromatic ring is secondary.33... 

Several types of carbocation have proved to be stable when entrapped in a zeolite cage. For instance, (64) and related dimeric cations can be prepared from indene,105 and (65) and several other cations can be prepared from 4-vinylanisole, some of the latter being stable for several weeks.106 Absolute lifetimes of cumyl cations (66) have been measured in non-acidic zeolites, and their reactivities with co-absorbed alcohols have been studied.107 Triarylmethyl cations can be prepared by ship-in-a-bottle syntheses in large-pore zeolites.108... 

Richard and Jencks combined the above method with use of the azide clock to determine values of pA R for a-phenethyl carbocations bearing electron-donating substituents in the benzene ring and for the cumyl cation for a wider range of substituents.22,89 They inferred values for the parent... 

The nitrosonium ion does not react toward aromatics except in activated systems. It forms a Jt-complex with aromatics with deep color.533,534 However, it is a powerful hydride-abstracting agent in the case of activated benzylic or allylic positions. Olah and Friedman535 have demonstrated that isopropylbenzenes undergo hydride abstraction to cumyl cations 222 [Eq. (4.151)] which further reacts to give various condensation products. The reaction has been employed to prepare a variety of stable carbocations.536... 

There are a number of other aryl-substituted carbodicationic systems that can be properly described as distonic superelectrophiles. For example, dication 45 has been generated from 2,2 -p-phenylenedi-2-propanol in SbFs at —78°C.4 When compared to the dimethyl(phenyl)carbenium ion (cumyl cation) 46, NMR data indicate that the positive charges are dispersed to a considerable extent into the neighboring methyl groups in the dication 45. 

Fig. 2.20. C,C bond-length reductions and elongations in the cumyl cation (compared to cunene) confirming the stabilization of the carbenium ion center through conjugation, and H3d -C bond-length reduction (compared to a-methylstyrene) due to the additional stabilization of the carbenium ion center caused by hyperconjugation (cf. Figure 2.19).

In 1986 Faust and Kennedy reported the first example of controlled/living cationic polymerization of isobutene, which was initiated by a cumyl ace-... 

A net chemical reaction upon charge-transfer irradiation of [arene, TiCU] complexes is obtained if rapid fragmentation of the arene cation radical competes with the back electron transfer in Eq. 18 [116], Thus, dianthracene (Ant2) cleaves into two anthracene moieties (Eqs 19-21) and bicumene cation radical fragments into cumyl cation and cumyl radical (Eqs. 22, 23), which ultimately form cumene and 1-phenyl-1,3,3-trimethylindane, respectively, as final products [116],... 

The nitrosonium ion (NO ), the electrophilic species formed in nitrous acid media, is also a particularly effective hydride abstracting agent. Cumene reacts with NO+ to give various condensation products that involve intermediate formation of the cumyl cation. °The formation of the cumyl cation in a nonlinear hydride transfer reaction involves a pentacoordinate carbocation [Eq. (6.45)]. 

We report in Table 4 the experimental AjG°(25) for this ion as given in Refs. 51 and 52. Also reported are values for 4-methoxy and 3-nitro cumyl cations, taken from the same references. These are extreme cases of substituent effects in these systems and span a range of stabilities somewhat smaller than that of the homologous benzyl derivatives (19.9 versus 24.3 kcal moP ). Still, this yields a stability for the 4-methoxy derivative quite close to that of trityl cation. 

For comparison purposes it seems useful to take the cumyl cation 48 as a reference... 

Figure 8.1 The first example of a living cationic polymerization of isobutene using cumyl acetate as initiator and BCI3 as activator in dichloromethane at —30°C. Number-average molar mass, polydispersity index (numbers in the plot), and number of polymer chains (inset) are reported as a function of the mass of PIB obtained. Source Reprinted with permission from Faust R, Kennedy JP. J Polym Sci A Polym Chem 1987 25 1847 [28]. Copyright 1987 John Wiley and Sons, Inc.

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