Imre Bucsi

Transformation of diols in the presence of heteropoly acids under homogeneous and heterogeneous conditions

1,2-Diols (1,2-propanediol, isomeric 2,3-butanediols and 2,3-dimethyl-2,3-butanediol) readily undergo the pinacol rearrangement to yield the corresponding carbonyl compounds (propanal, 2-butanone and 3,3-dimethyl-2-butanone, respectively) in the presence of heteropoly acids (H4[SiMo12O40], H3[PMo12O40], H4[SiW12O40] and H3[PW12O40]). The selectivity depends on the acid strength and the form of the heteropoly acid catalyst (transformation under homogeneous conditions or in the presence of heteropoly acid supported on silica). Heat treatment of supported H3[PW12O40] results in a highly stable catalyst, due to the strong interaction between the acid and the support. A similar stabilization of H4[SiMo12O40] could not be achieved. The highly selective formation of cyclic ethers, characteristic of the dehydration of higher diol homologues (1,4-butanediol, isomeric 2,5-hexanediols, 2,5-dimethyl-2,5-hexanediol, 1,5-pentanediol and 1,6-hexanediol) is also catalyzed by heteropoly acids. This cyclodehydration occurs by a stereospecific intramolecular SN2 mechanism.

Transformation of 1,2-diols over perfluorinated resinsulfonic acids (Nafion-H)

Abstract The transformations of 1,2-diols over perfluorinated resinsulfonic acids (Nafion-H) were studied, and correlation were examined between the structures of the investigated diols, the possible dehydration routes and the catalytic properties of Nafion-H. Comparisons were also made between the catalytic properties of Nafion-H and NaHX zeolite. Because of its stronger acidity, Nafion-H functions at temperatures considerably lower than those for the usual dehydrating catalysts, e.g. the zeolites. As is well established for other solid electrophilic catalysts, the dehydration of 1,2-diols mainly proceeds via the pinacol rearrangement. The lower temperatures and the stronger acidity of Nafion-H strongly favour the pinacol rearrangement versus 1,2-elimination. The reaction conditions are also advantageous for the formation of substituted 1,3-dioxolanes in a secondary condenstion between the unreacted diol and the primarily formed carbonyl compounds. Nafion-H gradually deactivates during long use, but it can be partially reactivated by washing with acetone.

Transformation of 1,3-, 1,4- and 1,5-diols over perfluorinated resinsulfonic acids (Nafion-H)

Abstract The transformations of 1,3-, 1,4- and 1,5-diols over perfluorinated resinsulfonic acids (Nafion-H) were studied and correlations were examined between the structure of the investigated diols, the possible transformation directions and the catalytic properties of Nafion-H. Comparisons were also made between the catalytic properties of Nafion-H and zeolites. The characteristic transformations of 1,3-diols depend on their structure. 1,3-Propanediol undergoes dehydration via 1,2-elimination and yields oligomers via intermolecular dehydration. 1,3-Diols with an alkyl substituent on the carbon between those bearing the OH groups undergo 1,2-elimination yielding unsaturated alcohols and dienes, and give carbonyl compounds via the loss of water and hydride shifts analogous to the pinacol rearrangement. The strong acidity of Nafion-H and the lack of strong basic sites are advantageous for the latter reaction. 1,3-Diols with two substituents at this position mainly yield fragmentation products. Stereoselective cyclodehydration to the corresponding oxacycloalkanes is the characteristic transformation of 1,4- and 1,5-diols over Nafion-H.

Friedel-Crafts Reactions of Buckminsterfullerene

Abstract Buckminsterfullerene reects with aromatic benzene derivatives in the presence of Lewis acids such as aluminum trichloride or aluminum tribromide to give Friedel-Crafts fullerenation products. Polyhalogenated fullerenes also react with aromatic compounds to afford fullerylated aromatic compound under the Friedel-Crafts reaction conditions.

Brønsted acid catalyzed formation of 1,3-dioxolanes from oxiranes and ketones

Abstract The catalytic effect of various acid catalysts (heteropoly acids, Nafion-H, K10 montmorillonite, trifluoroacetic acid, methanesulfonic acid) and the effect of the structure of the reactants were investigated on the formation of 1,3-dioxolanes from oxiranes and ketones. The yield of 1,3-dioxolanes strongly depends on competing reactions, mainly on the ratio of polymerization. K10 montmorillonite was found to be the best catalyst resulting in the formation of 1,3-dioxolanes in 50–80% yield under optimal reaction conditions.

Reactions of organosilicon compounds on metals: III. Selective poisoning by Et3SiH of catalytic hydrogenation and dehydrogenation☆

Abstract The transformation of Et3SiH occurring with CSi bond breaking was studied on CabOSilsupported Cu, Ni, Rh, Pd, and Pt catalysts by the pulse technique in H2 and He at 473–673 K. A serious poisoning of the CSi bond breaking capability of the metals was observed. Cu proved to be much less sensitive to poisoning, and, contrary to the other metals, could be partially reactivated. The poisoning effect of the surface silicon residues was found to be different for the hydrogenation of propene, cyclohexene, and benzene and for the dehydrogenation of 2-propanol and cyclohexene. This new selective poisoning effect may be a new tool for modifying certain metal catalysts.

Pinacol Rearrangement on Zeolites

Abstract The transformations of simple methyl-substituted 1,2-diols (ethanediol, 1,2-propanediol, (±)- and meso -2,3-butanediol, 2-methyl-2,3-butanediol and 2,3-di-methyl-2,3-butanediol) have been studied on NaX, NaY, NaHX and NaHY zeolites. The effects of the structure of the diol, the type of zeolite and the reaction conditions have been correlated with the two main dehydration reactions, the pinacol rearrangement ( N 60-90%) leading to the formation of carbonyl compounds and 1,2-elimination ( N 10-30%) yielding unsaturated alcohols and dienes. Factors affecting selectivities are discussed.

Reversal of the enantioselectivity in aldol addition over immobilized di- and tripeptides: studies under continuous flow conditions

Heterogeneous asymmetric direct aldol reactions between aldehydes (2-nitrobenzaldehyde, 2-methylpropanal) and acetone catalyzed by polystyrene resin (PS) supported di- and tripeptides H-Pro-Pro-, H-Pro-Pro-Pro-, H-Pro-Glu(OH)-, H-Pro-Pro-Glu(OH)-, H-Pro-Asp(OH)-, H-Pro-Pro-Asp(OH)-, H-Ser-Glu(OH)-, H-Ser-Ser-Glu(OH)-, H-Val-Glu(OH)-, H-Val-Val-Glu(OH)-MBHA-PS, were studied under identical experimental conditions at room temperature in a continuous-flow fixed-bed reactor (CFBR) system. In the asymmetric aldol reactions reversal of enantioselectivity was observed on H-Pro-Pro-Glu(OH)- and H-Pro-Pro-Asp(OH)-MBHA-PS-supported catalysts (ee 42–67% S) as compared to the H-Pro-Glu(OH)- and H-Pro-Asp(OH)-MBHA-PS-supported catalyst (ee 28–82% R). In the case of H-Pro-Pro- and H-Pro-Pro-Pro-MBHA-PS-supported catalysts reversed enantioselectivity was observed by using the benzoic acid additive (12% S) as compared to the H-Pro-MBHA-PS catalyst (25% R). The stability of the catalysts in the flow system was consistent with the heterogeneous character of the reaction, as was the linear behavior obtained using mixtures of L- and D-enantiomers of the supported H-Pro-MBHA-PS catalyst. The enamine character of the reaction intermediates was supported by ESI-MS measurements. Based on these and the computed structure of the peptides, the conformation of the intermediate adducts is held responsible for chiral induction, therefore for the enantioselectivity inversion observed in these reactions.

Selective Ring-Opening of Isomeric 2-Methyl-3-Phenyloxiranes on Oxide Catalysts

Summary The rearrangement of light and deuterium-labelled cis- and trans-2-methyl-3-phenyloxiranes (1, 2 and 1, 2) was studied on ZnO, A1 2 O 3 , and WO 3 , and in the presence of BF 3. Both in the gas phase (473–673 K) and in the liquid phase (298–413 K), l-phenyl-2-propanone (3) and 2-phenylpropanal (4) were formed with high selectivities (0-90% and 11-80%, respectively). Ring-opening was found to occur by selective fission of the benzyl C-0 bond. Mechanistic studies revealed the formation of an open carbenium ion or a double-bonded surface intermediate. The acidic (electrophilic) and basic characters of the oxides determine the product distributions by affecting the relative importance of the competing mechanisms.

Alkylation of benzene with cyclic ethers in superacidic media

Superacidic trifluoromethanesulfonic acid (triflic acid, TFSA) catalyzes the alkylation of benzene with various cyclic ethers. The characteristic products formed in the reaction of highly reactive oxiranes (methyloxirane, styrene oxide, and cyclohexene oxide) are phenyl-substituted compounds formed as a result of Friedel–Crafts-type mono- and dialkylation. Oxetanes (2-isopropyloxetane) and oxolanes (tetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,2,5,5-tetramethyltetrahydrofuran), in turn, undergo electrophilic cyclialkylative ring closure (cyclialkylation) to form mainly bicyclic compounds (tetralin, dihydronaphthalene, and naphthalene derivatives). In the majority of cases, alkylated products can be isolated in good to reasonable yields. Reactivity of cyclic ethers, product formation and product distributions are interpreted by taking into account ring strain, acidity of reaction mixtures and stability of carbocationic intermediates.