Persistent carbocations

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. 

The chemistry of stable, long-lived (or persistent) carbocations, as they became known, thus began and its progress was fast and widespread. Publication of research done in an industrial laboratory is not always easy. 1 would therefore like to thank again the Dow Chemical Company for allowing me not only to carry out the work but eventually also to publish the results. 

On the basis of my extensive study of stable, persistent carbocations, reported in more than 300 publications, I was able to develop the general concept of carbocations referred to in Chapter 9. Accordingly, in higher-coordinate (hypercoordinate) carbonium ions, of which pro-... 

As mentioned above, persistent carbocation 9 underwent rearrangement into cation 10 which rearranged further into cation 11. To reveal general relations/factors governing cationic rearrangements in benzopentalene derivatives, the behavior of 5,5,10,10-tetramethyl-5,10-dihydroindeno[2,l-fl]indene (12) in superacids was studied (52). It had been expected that hydrocarbon 12 would transform into the long-lived 5,5,10,10-tetramethyl-4b,5,9b,10-tetrahydroindeno[2,l-a]inden-4b-yl cation (13). However, H and 13C NMR data showed that hydrocarbon 12 transformed firstly into isomeric ion 14 which transformed further into cation 15 (Scheme 11). 

The heterolytic cleavage of a C(sp )-X bond (X good leaving group e.g. halide, sulfonates and perchlorate) is the first step in a typical SnI reaction and it is the most popular synthetic approach to persistent carbocations. The high bond energy... 

In contrast, a persistent carbocation could not be generated from 9,10-dihydro-BaP introduction of bulky substituents at C-6 prevented side reactions, and the initially formed carbocation underwent rearrangement to the corresponding bay-region carbocation. Introduction of methoxy substituents into the 1- or 3- positions of 9,10-dihydro-BaP-7(8//)-one increased its electrophilic reactivity to the extent that stable carboxonium-arenium dications were produced in FSO3H/SO2CIF (Fig. 9). 

In a later study, persistent carbocations were generated from five A-ring mono-and disubstituted phenanthrenes [3-OMe, 4-OMe, 1,3-bis(OMe), 2,4-bis(OMe), and... 

Two chapters in this volume describe the generation of carbocations and the characterization of their structure and reactivity in strikingly different milieu. The study of the reactions in water of persistent carbocations generated from aromatic and heteroaromatic compounds has long provided useful models for the reactions of DNA with reactive electrophiles. The chapter by Laali and Borosky on the formation of stable carbocations and onium ions in water describes correlations between structure-reactivity relationships, obtained from wholly chemical studies on these carbocations, and the carcinogenic potency of these carbocations. The landmark studies to characterize reactive carbocations under stable superacidic conditions led to the award of the 1994 Nobel Prize in Chemistry to George Olah. The chapter by Reddy and Prakash describes the creative extension of this earlier work to the study of extremely unstable carbodications under conditions where they show long lifetimes. The chapter provides a lucid description of modern experimental methods to characterize these unusual reactive intermediates and of ab initio calculations to model the results of experimental work. 

The silyl fragmentation in superacids initiated by a controlled temperature increase is a method to generate persistent carbocations, such as the vinyl cation 378, which are not accessible by direct protonation of unsaturated hydrocarbons because of excessive formation of oligomeric and polymeric products. 

NMR of Persistent Carbocations from Polycyclic Aromatic Hydrocarbons (PAHs)... 

The available NMR data for persistent carbocations derived from various classes of PAHs are compiled and their key features are highlighted and compared. The review covers PAH arenium ions, PAH carboxonium ions and a-PAH-substituted carbocations. Charge delocalization mode and substituent effects are evaluated on the basis of the A8,iC values. 

The available NMR data for persistent carbocations derived from the benz[a]anthracene (BA) skeleton and for a series of secondary a-substituted carbocations (and related model carbocations) and a-carboxonium ions are gathered in Figs 37 and 38.32 The me so positions (C-7/C-12) in BA are most reactive. The conformation(s) of the carboxonium group was deduced on the basis of NOED experiments. The A513C values (Fig. 38) indicate the strong anthrecenium ion character in benz[a]anthrenium and their a-carbocations. 

Recent protonation studies on methylene-bridged PAHs led to the generation and complete NMR characterization of a series of persistent carbocations.17 33 Figure 51 summarizes the proton data for these non-altemant arenium ions, whereas carbon NMR data and A5 values are gathered in Fig. 52. The protonation sites... 

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