P-H-bridged carbocations

Although these types of bridged phenonium ions are accepted intermediates in a number of reactions, they do not appear to be involved in the pinacol rearrangement (see Schubert, W. M. LeFevre, P. H. J. Am. Chem. Soc. 1972, 94, 1639). Stabilization of the carbocation 4-10 by resonance with the ojgrgen substituent may be a factor in determining the preference for phenyl migration over phenonium ion formation in the pinacol rearrangement. 

A value of ku/kD = 1.07 per /3-D was observed when the deuteriums were on the bridge at C-12 [5] and the dihedral angle between the p-orbital of the carbocation and the Cp—H bonds was approximately 30° and hyperconjuga-tion could occur. When [6] was used, the dihedral angle was 90° and there was no overlap between the empty p-orbital of the carbocation and the Cp—H bonds i.e. no hyperconjugation could occur and only a small inverse, inductive KIE, kH/fcD = 0.99, was observed. This study and other studies by Shiner and co-workers (Shiner, 1970b) have established that the maximum secondary /3-deuterium KIE in any system is observed when the dihedral angle is either 0° or 180°, i.e. where the overlap between the Cp—H and the p-orbital on the... 

In a recent extensive computational study [BLYP/6-31G(d,p) level, atoms in molecules (AIM) and NBO theories], DuPre has found charge distribution in cation 86, which prevents the development of unstable bridgehead carbocation. Electron delocalization results in nearly neutral atoms across the C—H—C bonding with the bridging hydrogen essentially in the Is electron configuration, and thus the proton is highly shielded in the NMR spectrum. 

Among 2° carbocations, only the isopropyl, sec-butyl, and cyclopentyl carbocations have been observed in solution. These three structures nevertheless illustrate the kinds of delocalization effects that may be expected in other 2° carbocations. The cyclic structure of the cyclopentyl cation prevents significant C-C-C bridging. The calculated geometry (Figure 5.43) shows a twisted C2 structure with the C -H bonds aligned parallel with the empty p orbital for maximum hyperconjugative overlap. ... 

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