Superacids, natural product

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

A book containing authoritative reviews of many aspects of carbocation chemistry has been published.1 These reviews are up-to-date to the end of 1991, as the book is the result of a symposium held in honour of George Olah in 1992 it features many of the world s leading carbocation chemists. At least one book review of it has also appeared.2 Olah has an introductory chapter concerning his decades-long search for stable long-lived carbocations in superacid media,3 and other review chapters include ones on carbocations at surfaces and interfaces,4 on the X-ray structural analyses that have been performed on many carbocation salts and related compounds in recent years,5 and on natural product chemistry in superacids.6 Other review chapters will be referred to below, as appropriate. 

In recent years, oxyfunctionalization of various natural products (steroids, alkaloids) under superacidic conditions have also been explored. In addition, Nafion resins in combination with various oxidizing agents have also been used in the oxygenations. 

Rearrangement of phenols to dienones occurs readily in superacids. Some simple bicyclic phenols and their ethers have been investigated as model compounds by use of HF-SbFs [73]. This method is also applicable to natural products [74]. For example, treatment of estrone derivatives in HF-SbFs, then aqueous bicarbonate work-up led to estra-4,9-diene-3,17-dione (Eq. 30). The opposite rearrangement, i.e. that of dienones to phenols, can also be achieved in an HF-SbFs medium (Eq. 31) [75]. 

The reversible conversion of phenols to dienone intermediates is an important transformation in the synthesis of natural products. This rearrangement occurs efficiently in superacid solutions The corresponding version for the halophenols to give halodienones has been reviewed in an earlier volume of this series. 4-Bromo-2,4,6-trialkylcyclohexa-2,5-dienones have recently been synthesized by electrophilic bromination of the corresponding phenols. 

This isomerization is of substantial importance in natural product syntheses, usually catalyzed by a strong base. The reaction occurs with good yields in polycyclic systems under superacidic conditions, as shown by Gesson and Jacquesay. 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

In conclusion, more efficient and clean solid (acid and superacid) catalysts will be used in the coming years to reduce not only the emission of environmentally harmful products but also the use of noxious catalysts. The optimal catalytic systems will be determined from the nature of acid strength of its active sites, the nature of the reaction, and the reaction conditions. 

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