Regioselective acylation of 2-methoxynaphthalene catalyzed by a C-H superacid and chemistry of mercaptoazulenes
University of Kansas
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Abstract This Thesis work consists of three Chapters. The first two chapters describe the synthesis and catalytic application of carbon-based superacids and attempts of their immobilization on inorganic supports. Pentafluorophenyl(bis)triflyl methane was synthesized in two steps from pentafluorobenzyl bromide. Due to the strong electron withdrawing nature of the trifluoromethylsulfonyl and pentafluorophenyl groups, the methyne hydrogen atom is exceptionally labile. The para-fluorine atom in the above superacid is labile enough to be substituted by oxygen nucleopiles (hydroxyl and propyloxy). Both the "parent" and 4-substituted acids catalyze highly regioselective acylation of 2-methoxynaphthalene (2-MON) by acetic anhydride. This acylation reaction affords highly valuable 6-methoxy-2-acetonaphthone (2,6-AMN) along with a small amount of 7-methoxy-1-acetonaphthone (1,7-AMN) as opposed to the classic Friedel-Crafts acylation that employs strong Lewis acids such as aluminum trichloride and produces mainly the ortho-acylation product 1,2-AMN. The mechanistic studies described herein suggest that the acylation reaction leading to the formation of the thermodynamically preferred 2,6-AMN is a two step process that involves acylation of 1-position of 2-MON followed by the acyl group transfer to the 6-position of the ring. All of the reaction products - 1,2-AMN, 2,6-AMN, 1,7-AMN and diacetyl-methoxynaphthalene - were isolated or independently synthesized and their structures were confirmed by spectroscopic and mass-spectral methods. The diacylation product was determined to be 1,7-diacetyl-2-methoxynaphthalene, not the 2-acetylaceto-6-methoxynaph-thalene that was previously proposed to be the diacylated species. The acylation of 2-MON was found to be sensitive to the concentration of the catalyst. Lower catalyst loadings (ca. 1 mol %) led to selective formation of 1,2-AMN, while a loading greater than 9 mol % afforded selectively 2,6-AMN and 1,7-AMN 8:1 molar ratio, respectively. The outcome of the reaction was also found to depend dramatically on the solvent medium. Indeed, employing nitromethane as a solvent produced 2,6- and 1,7-AMN, while running the reaction in hexanes resulted in selective formation of 1,2-AMN with a nearly 100% conversion. Only trace amounts of 2,6-AMN were detected when the reaction was conducted in ethyl acetate. Systematic theoretical studies have supported these observations - the relative energies of formation of the acylated products, as well as the relative energies of the corresponding -complex intermediates in nitromethane and hydrocarbons indicate the advantage of 2,6- and 1,7-AMN isomers over the kinetically preferred 1,2-AMN. However, the energy gap in hydrocarbons was too large for the transacylation to proceed. The pentafluoro(bis)triflyl methane and its para-hydroxy derivative were physisorbed on silicon oxide in attempts to immobilize the catalyst on an inorganic support. Both of the silicas have catalyzed acylation of 2-MON with high regioselectivity. The product distribution suggests that the acylation process occurs inside of the pores or on the surface of the silica. However, the catalyst leaches out of the support, making this, method not industrially feasible. The studies of the physisorbed CH acid's catalystic activity supports the hypothesis that once permanently tethered to an insoluble inorganic support, the CH acid would remain a highly active catalyst. A few unsuccessful attempts to chemically bind the catalyst and its derivatives to silicon oxide network have been made. The third Chapter of this Thesis describes synthesis and characterization of a series of mercaptoazulenes, their coordination chemistry and self assembly on Au(111) surfaces. 1,3-Diethoxycarbonyl-2-mercaptoazulene and 1,3-diethoxycarbonyl-6-mercaptoazulene were synthesized from the corresponding haloazulenes in good yields. 1,3-Diethoxycarbonyl-2-mercaptoazulene was decarboxylated to afford 2-mercaptoazulene and 1,3-dicyano-2-mercaptoazulene was synthesized to complete the series. Diethyl 2- and 6-mercaptoazulene-1,3-dicarboxylates react with the dinuclear complex [Au2(dcpm)]Cl2 (dcpm = bis(dicyclohexylphosphino) methane) to give the corresponding bis(thiolate)-digold complexes. Both gold complexes as well as two of the mercaptoazulenes were characterized by XRD. The di-gold motifs in the complexes of 2- and 6-azulenyl thiolates feature strong aurophilic interactions; although the relative orientation of the azulene rings in these species is quite different from the analogous isocyanoazulene complex. All of the mercaptoazulenes have formed air- and moisture stable monolayers on Au(111) films. Characterization of some azulenyl thiolate films was greatly facilitated by incorporation of ancillary nitrile "spectroscopic" handles, a novel approach in the surface chemistry of organic thiols, to the best of our knowledge.
- Chemistry Dissertations and Theses 
- Dissertations 
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