Carbocation structure

FIGURE 5 8 Methyl migration in 1 2 2 tnmethylpropyl cation Structure (a) is the initial second ary carbocation structure (b) is the transition state for methyl migration and structure (c) is the final tertiary carbocation... 

It is believed that this process involves migration through a pentacoordinate protonated cyclopropane in which an alkyl group acts as a bridge in an electron-deficient carbocation structure. The cyclohexyl- methylcyclopentyl rearrangement is postulated to occur by rearrangement between two such structures. 

The C-NMR chemical shift of the trivalent carbon is a sensitive indicator of carbocation structure. Given below are the data for three carbocations with varying aryl substituents. Generally, the larger the chemical shift, the lower is the electron density at the carbon atom. 

An important tool for the investigation of carbocation structure is measurement of... 

In this chapter we review published results of studies of the kinetics and products of stepwise nucleophilic substitution and elimination reactions of alkyl derivatives, and we present a small amount of unpublished data from our laboratory. Our review of the literature is selective rather than comprehensive, and focuses on work that provides interesting insight into the factors that control the rate constant ratio ks/kp for partitioning of carbocations, and that provides an understanding of how the absolute rate constants ks and kp that constitute this ratio change with changing carbocation structure. 

In summary, there now exists a body of data for the reactions of carbocations where the values of kjkp span a range of > 106-fold (Table 1). This requires that variations in the substituents at a cationic center result in a >8 kcal mol-1 differential stabilization of the transition states for nucleophile addition and proton transfer which have not yet been fully rationalized. We discuss in this review the explanations for the large changes in the rate constant ratio for partitioning of carbocations between reaction with Bronsted and Lewis bases that sometimes result from apparently small changes in carbocation structure. 

There are at least two other studies of competitive reactions to form the products of solvolysis and elimination reactions that may provide insight into the relationships between carbocation structure and reactivity toward nucleophile addition and deprotonation. 

The 2-methyl-2-butyl cation (12) is the smallest tertiary carbocation structurally suitable for stabilization through C-C hyperconjugation. 

These examples show that the quantum chemical calculation of nuclear spin-spin coupling constants which has been available for the general scientific community only quite recently is an additional valuable and reliable tool for the interpretation and assignment of NMR spectra of carbocations. Taken together quantum chemical ab initio calculation of chemical shifts and nuclear spin-spin coupling constants opens the possibility for a complete simulation of NMR spectra solely based on calculated carbocation structures. 

The unusual carbocation structure 53, a linear 5-center 4-electron C-H-C-H-C array can be derived from three anthracenes joined up around a C-H-C-H-C core.73 The GIAO-DFT computed H and 13C NMR chemical shifts for the bridging hydrogens (2.9 ppm) and carbons (112 and 182 ppm) of the 5-center array differ considerably from those found in typical 3-center 2-electron systems. 

One of the most stable carbocation structures is the employing all three rings. Trityl chloride ionizes read-triphenylmethyl cation (trityl cation). In this struc- ily, and can capture an available nucleophile, ture, the positive charge is stabilized by resonance... 

An important tool for the investigation of carbocation structure is measurement of the C nmr chemical shift of the carbon atom bearing the positive charge.66 This shift approximately correlates with electron density on the carbon. 13C chemical shifts for a number of ions are given in Table 5.2.67 As shown in the table, the substitution of an ethyl for a methyl or a methyl for a hydrogen causes a downfield shift, indicating that the central carbon... 

Both compounds react by an SN1 mechanism, and their relative rates reflect their activation energies for carbocation formation. Because the allylic chloride is more reactive, we reason that it ionizes more rapidly because it forms a more stable carbocation. Structurally, the two carbocations differ in that the allylic carbocation has a vinyl substituent on its positively charged carbon in place of one of the methyl groups of tert-butyl cation. 

Two resonance-contributing structures (3a and 3b), in the formalism of ylide structures, can be used to describe metal carbene intermediates. The highly electrophilic character of those derived from Cu and Rh catalysts suggests that the contribution from the metal-stabilized carbocation 3b is important in the overall evaluation of the reactivities and selectivities of these metal carbene intermediates. Emphasis on the metal carbene structure 3a has led to the subsequently discounted proposal that cyclopropane formation from reactions with alkenes occurs through the intervention of a metallocyclobutane intermediate [18]. The metal-stabilized carbocation structure 3b is consistent with the cyclopropanation mechanism in which LnM dissociates from the carbene as bond-formation occurs between the carbene and the reacting alkene (Eq. 5.4) [7,15]. 

The chemistry of highly fluorinated carbocations, preparations, and reactions is die subject of an extensive review.82 Also available is a discussion on die use of intrinsic 19F NMR isotope shifts for die determination of carbocation structures.83... 

This correlation is shown for a range of carbocation structures in Fig. 1, with the arenonium ions shown as filled circles. The correlation includes protonated mesitylene, studied by Marziano,107 and earlier measurements of the protonation of methoxy-substituted benzenes108 and azulenes.109 It also... 

Alkylcarboxonium (Alkoxycarbenium) Cations. The ionic structure of pyrylium salts was clearly stated by Hantzsch592 as early as 1922. In pyrylium salts 297, there is a contribution from carbocation structures, a fact apparent in the behavior toward strong nucleophiles leading to phenols. 

In a search fora pure y-deuterium equilibrium isotope effect, Kates (1978) studied [67] using C-nmr spectroscopy. The results showed equilibrating carbocation structures, and from the isotopic splitting of shifts of methyl groups equilibrium constants were determined. 

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