Rearrangements isopropyl cation

More recent work93,127-129 has shown that /1-hydrogen transfer takes place only if the cation corresponding to the N—R substituent is stable. Thus, CH2=N+(CH3)CH(CD3)2 and related species expel C3HD5, via specific D-transfer128 129, because isopropyl cation is stable with respect to a 1,2-H shift. In contrast, if the associated cation readily rearranges to a more stable isomer, there is actually a discrimination against... 

Some systems show NMR coalescence consistent with a rearrangement to an intermediate cation that is less stable. The NMR of the isopropyl cation 9, for example, shows a coalescence of the two proton signals between 0 and 40 °C. The activation barrier is 16 kcal/mol, close to the difference between a secondary and a primary cation. Calculations show that a comer protonated cyclopropane (11) is the only other minimum on the C3H energy surface. " The 1-propyl cation 10 is a transition state. 

The isopropyl cation formed by rearrangement undergoes substitution and elimination like the propyl cation. About half of the products arise from isopropyl cations. 

Occasionally rearrangements from more stable to less stable carbocations occur, but only if (1) the energy difference between them is not too large or (2) the carbocation that rearranges has no other possible rapid reactions open to it.9 For example, in superacid medium, in the temperature range 0-40°C, the proton nmr spectrum of isopropyl cation indicates that the two types of protons are exchanging rapidly. The activation energy for the process was found to be 16 kcal mole-1. In addition to other processes, the equilibrium shown in Equation 6.7 apparently occurs.10 In the superacid medium, no Lewis base is available... 

The temperature-dependent 1H NMR spectrum of isopropyl cation 2 (prepared from isopropyl chloride in SbFs-SC ClF solution) demonstrated96 rapid interchange of two types of protons. Line-shape analysis showed the reaction to be intramolecular, with an activation energy barrier of lb iO kcalmoP1. Based on these observations, Saunders and Hagen96 suggested that the rearrangement involves n-propyl... 

Ionization with rearrangement gives isopropyl cation... 

In 1979, Olah and Meidar found that alkylation of toluene offered solely a mixture of isopropylto-luenes. No n-propyltoluenes were detected. This was explained as proceeding by the initial formation of the Cl-protonated n-propyl chloride (2 Scheme 2), which rapidly rearranges to the isopropyl cation, followed by reaction with the aromatic ring. 

For the norbomyl cation (22), on the other hand, the average 13C chemical shift for C—1,2 was found at 126.1 ppm (at -150°C where all other rearrangements except for the degenerate alkyl shift are frozen out )55)- Thus, C-1,2 absorb by ca. 80 ppm further upfield that the corresponding carbons of the equilibrating ion (25). A similar discrepancy (85 ppm) is estimated on the basis of isopropyl cation and C—1 of norbomane as a model. Some doubt, however, remains whether these acyclic ions are good approximations of the hypothetical 2-norbornyl cation. 

El elimination, 196-198, 201 Friedel-Crafts alkylation, 451 53, 479 SnI nucleophilic substitution, 143-146, 315-320,331 isopropyl cation, 141, 224 methyl cation, 141 iert-pentyl cation, 929 rearrangements, 187-189, 201, 219-220, 319-320,331,452,479 structure, bonding, and stability, 140-143, 162... 

Possible mechanismfor rearrangement of isopropyl cation. (Adapted from reference 281.)... 

M. Saunders and E. L. Hagen, Rearrangement reactions of secondary carbonium ions. Isopropyl cation, J. Am. Chem. Soc. 90, 6881-82 (1968). 

The starting haloalkane rearranges by a hydride shift to the thermodynamically favored 1-methylethyl (isopropyl) cation in the presence of the Lewis acid. 

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