Fused Polycyclic Hydrocarbons Through Superacid-Induced Cyclialkylation of Aromatics
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Fused Polycyclic Hydrocarbons Through Superacid-Induced Cyclialkylation of Aromatics ´ rpa´d Molna´r Istva´n Ledneczki Æ Peter Forgo Æ A
Received: 25 June 2007 / Accepted: 30 July 2007 / Published online: 7 September 2007 Springer Science+Business Media, LLC 2007
Abstract Benzene and substituted derivatives (toluene, ortho-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, anisole), when applied in large excess, react with 1,4 diols (pentan-1,4-diol, hexan-2,5-diol, and 2,5-dimethylhexan-2,5-diol) or an oxolane (2,2,5,5-tetramethyltetra hydrofuran) in the presence of the Brønsted superacid trifluoromethanesulfonic acid (triflic acid, TFSA) to afford substituted tetralins in excellent yields with high selectivity. Reacting benzene with a small excess of alkylating agents yields octahydroanthracenes. The transformation of naphthalene with oxolane leads to a partially saturated octamethyloctahydrotetracene under similar conditions. Product formation is interpreted by intermolecular FriedelCrafts alkylation followed by cyclialkylative ring closure. Keywords Alkylation Cyclialkylation Arenes Carbocations Superacid
1 Introduction Superacid-catalyzed transformations of organic compounds are still attracting considerable interest. Although basic knowledge of superacid chemistry was established in the 1960s and 1970s [1], numerous new observations with respect to both mechanistic studies and synthetic applications are reported. Superacidic trifluoromethanesulfonic acid (triflic acid), in particular, has been used very often. This is primarily due to its particularly useful and attractive properties, namely, its considerably high acidity (H0 =
–14.1) and the lack of any side-reaction such as sulfonation and oxidation which may be observed using certain other superacids. This high and renewed interest is also attributed to the fact that the involvement of superelectrophilic species [2] in triflic acid-mediated transformations has been often implicated and, in fact, superelectrophiles have been detected by 1H and 13C NMR spectroscopy [3–5]. Triflic acid has been proved to be particularly useful in various alkylation processes [6–10] and in varied cyclization reactions including the synthesis of carbocyclic [9, 11–14] and heterocyclic multiple ring systems [3, 10, 15, 16]. These results have considerably widened the scope of the use of triflic acid in synthetic processes and resulted in a better understanding of the mechanistic background of the catalyzed transformations. We have explored the use of diols and cyclic ethers as reagents in Friedel-Crafts alkylation focusing on the use of benzene in two previous papers [17, 18]. With respect to the topic of the present article, we found that 1,4-diols and oxolanes, particularly 2,2,5,5-tetramethyltetrahydrofuran, proved to be excellent reagents to form bicyclic and tricyclic partially saturated naphthalene and anthracene derivatives. On the basis of these observations and in continuation of our interest in electrophilic catalysis we have performed a detailed study
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