Pentamethylbenzenium cation

Theoretical and Experimental NMR techniques provide powerful tools for the investigation of heterogeneous catalysis. Recent advances in in situ NMR techniques are summarized, as are advances in theoretical methods. The utility of our combined theoretical/experimental approach is illustrated by studies of the pentamethylbenzenium cation and the 1,3-dimethylcyclopentenyl cation in zeolite HZSM-5, acetylene adsorption on MgO, and the isopropyl cation on frozen SbF5. We also discuss the role of the basicity of adsorbates in the formation of stable carbenium ions on zeolites. 

Figure 1. The Cs geometry of the pentamethylbenzenium cation with carbons labeled as referred to in the text. Selected bond lengths in A are also shown. (Reproduced from reference 13. Copyright 1995 American Chemical Society.)...

Theory helps the experimentalists in many ways this volume is on chemical shift calculations, but the other ways in which theoretical chemistry guides NMR studies of catalysis should not be overlooked. Indeed, further theoretical work on two of the cations discussed above has helped us understand why some carbenium ions persist indefinitely in zeolite solid acids as stable species at 298 K, and others do not (25). The three classes of carbenium ions we were most concerned with, the indanyl cation, the dimethylcyclopentenyl cation, and the pentamethylbenzenium cation (Scheme 1), could all be formally generated by protonation of an olefin. We actually synthesized them in the zeolites by other routes, but we suspected that the simplest parent olefins" of these cations must be very basic hydrocarbons, otherwise the carbenium ions might just transfer protons back to the conjugate base site on the zeolite. Experimental values were not available for any of the parent olefins shown below, so we calculated the proton affinities (enthalpies) by first determining the... 

A valence isomerization reaction similar to that encountered with the tropylium ion has been observed when a variety of substituted benzenes are photolyzed in strong acid. The pentamethylbenzenium ion (18) photolyzed at —78° to give a single product, the pentamethylbicyclo-[3,l,0]hexenyl cation (19), in excess of 80% conversion (Childs et cd., 1968). 

Using the pulse flow catalytic reactor mentioned earlier, we were able to create a pentamethylbenzenium (Figure 1) in zeolite HZSM-5 (16). The cation was synthesized in the channel intersections or pore mouths by alkylating benzene or toluene with methanol at 523 K. This cation cannot be prepared in detectable concentration when the reaction is carried out in a sealed rotor, as is commonly done in in situ NMR studies. In contrast, when alkylation is carried out in a flow reactor, the co-product—water—is removed, and the cation accumulates as a significant product in the zeolite. 

The 4-hydroxy-l-X-l,2,3,5,6-pentamethylbenzenium ions (57) with electronegative groups X, with rising temperature of acid solutions, are converted into the 4-hydroxy-2,3,5,6-tetramethylbenzyl cation (725)The substituents X allow the completed in HSOjF within 5-10 min ... 

Besides, the conversion of 4-hydroxy-l-X-l,2,3,5,6-pentamethylbenzenium ions into 4-hydroxybenzyl cations could Be explained by the quinobenzylk rearrangement... 

However, 4-hydroxy-1-nitro-1,2,3,5,6-pentamethylbenzenium ion is converted into a benzyl cation at a temperature at which the corresponding dienone undergoes no rearrangement into a benzyl derivative... 

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