Degenerate rearrangements carbocations

Carbocations that rearrange to give products of identical structure (e.g., 5 5, 6 6 ) are called degenerate carbocations and such rearrangements are degenerate rearrangements. Many examples are known. ... 

The cyclopentyl cation (39) undergoes a rapid degenerate rearrangement which can be frozen out at cryogenic temperatures as shown by solid state CPMAS 13C NMR spectra.57 MP2/6-31G(d,p) calculations show that cyclopentyl cation has a twisted conformation 4058 in which the axial hydrogens are bend toward the carbocation center. This is due to the pronounced geometrical distortion caused by the hyper-conjugative interaction of the /i-cr-C-H-bond with the formally vacant 2pz-orbital at the C+ carbon of this secondary carbocation. 

It has been found by NMR that carbocations la-d and 3 are prone to undergo degenerate rearrangements [automerization reactions (26)] via 1,2-shifts of the vinyl and the methyl-substituted vinyl groups (Schemes 5, 6), with... 

This conclusion is in accord with the results of MINDO/3 quantum-chemical calculations. Cation 7 is also prone to undergo a rather slow degenerate rearrangement by 1,2-bridge shift (AG is about 60 kJ/mol) (Scheme 8). This rearrangement was probed via 2H NMR spectroscopy, when the 9-CD3-analog of carbinol 5 was used as precursor for carbocation generation. At -50 °C cations 6-8 transform firstly into l,l,2,3,3a,4,8b-heptamethyl-l,3a,8b-trihydrobenzopentalenium ion (9) and then (partially) into 1,1,2,3,4,4,8b-heptamethyl-l,4,8b-trihydrobenzopentalenium ion (10) (Scheme 9). 

Timberlake and coworkers have studied the degenerate rearrangement of pentacyclo-propylethyl cation 56 (involving 1,2-cyclopropyl shifts) under long-lived stable ion conditions81 82 (equation 39a). The rearrangement could not be frozen even at -80 °C. However, additivity of 3C NMR chemical shift analysis7 indicates the classical trivalent nature of the carbocation. 

Degenerate rearrangements of (CH)J carbocations 334 CjHs ions the cyclopropenyl cation 335 336 340... 

The present review deals with the structures, reactions and mechanisms of carbocations undergoing degenerate rearrangements. For convenience rearrangements which interconvert enantiomers have also been included. The study of such rea ctions is of fundamental importance in carbocation chemistry... 

The success of a carbocation preparation in superacid is frequently very technique dependent. Despite this fact, rather few detailed accounts have been published of the special techniques that have been developed. To be able directly to observe reactive (unstable) ions by nmr, for example, the ions not only have to be studied at low temperatures (where non-degenerate rearrangements are slow) they also have to be prepared at low temperatures for the very same reason. Furthermore, side reactions have to be suppressed. Common reactions of this type are dimerizations and polymerizations water and oxygen also have to be excluded. 

The bicyclo[3.1.0]hex-3-en-2-y[ cation [167] and related ions constitute another group of carbocations capable of undergoing degenerate rearrange-... 

A study of the rate of degenerate rearrangements of arenium ions as a function of the C-chemical shifts (electron deficiency) of their carbocation centre has been made (Borodkin et al., 1974b). Linear correlations [e.g. (21 la)] oflogA for 1,2-methide shifts in some 9,9,10-trimethyl phenanthrenium ions, [307], [322 R = CH3, Br and CF3], [321] and [279 R = CH3] and 8i3c+ were found at different temperatures. A linear correlation (211b) was... 

This carbocation has to have a monocyclic structure and is the simplest potentially degenerate (CH) carbocation. It can either be a degenerate cyclopropenyl cation [339] rearranging as in (221) or a static aromatic 2 t-electron structure [340], i.e. the simplest aromatic system (222). The ion... 

Various spectroscopic techniques have been applied to the characterization of carbocations as stable ions in solution and in the solid state. The techniques that have found most application in the study of hypercoordinate carbocations were developed primarily to be capable of distinguishing trivalent cations undergoing rapid degenerate rearrangements from bona fide hypercoordinate ions. In recent years, theoretical studies have become also very useful in the study of hypercoordinate carbocations. Furthermore, many properties such as chemical shifts ( H, etc.) and vibrational spectra can be accurately computed. 

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