W. Bruce Kover

Rearrangement, Nucleophilic Substitution, and Halogen Switch Reactions of Alkyl Halides over NaY Zeolite : Formation of the Bicyclobutonium Cation Inside the Zeolite Cavity

Rearrangement and nucleophilic substitution of cyclopropylcarbinyl bromide over NaY and NaY impregnated with NaCl was observed at room temperature. The first-order kinetics are consistent with ionization to the bicyclobutonium cation, followed by internal return of the bromide anion or nucleophilic attack by impregnated NaCl to form cyclopropylcarbinyl, cyclobutyl, and allylcarbinyl chlorides. The product distribution analysis revealed that neither a purely kinetic distribution, similar to what is found in solution, nor the thermodynamic ratio, which favors the allylcarbinyl halide, was observed. Calculations showed that bicyclobutonium and cyclopropylcarbinyl carbocations are minimal over the zeolite structure, and stabilized by hydrogen bonding with the framework structure. A new process of nucleophilic substitution is reported, namely halogen switch, involving alkyl chlorides and bromides of different structures. The reaction occurs inside the zeolite pores, due to the confinement effects and is an additional proof of carbocation formation on zeolites. The results support the idea that zeolites act as solid solvents, permitting ionization and solvation of ionic species.

Mechanistic studies of isopentane activation over solid acid catalysts. H-D exchange with USY zeolite, amorphous silica-alumina and Amberlyst-15

Abstract The mechanism of activation of isopentane on USY zeolite, amorphous silica-alumina and Amberlyst-15 catalysts was studied by H-D exchange. Only the zeolite showed exchange with the isopentane at 373 K. The addition of 10% of 2-methyl-1-butene to the isopentane accelerates the exchange with USY zeolite and promotes a small exchange with amorphous silica-alumina and Amberlyst-15. Nevertheless, the selectivity was different. With USY, no olefin was detected in the products, suggesting that they were protonated (deuterated) to give the tertiary carbenium ion, which, after hydride transfer, desorbs as a deuterated isopentane molecule. CP/MAS 13 C NMR analysis of the zeolite after exchange showed a significant amount of adsorbed olefinic material. On the other hand, the main reaction on Amberlyst-15 and silica-alumina is the apparent isomerization of 2-methyl-1-butene to 2-methyl-2-butene. The reactivity of the zeolite for the reaction with isopentane is probably due to steric effects, associated with the distortion of the crystalline structure. These effects are absents in Amberlyst-15 and amorphous silica-alumina catalysts.

Surface-mediated chemospecific hydrobromination of limonene: stereoselective transformation of the enantiomers of limonene into α-terpinyl bromide

(R) and (S)-α–terpinyl bromide can easily be prepared in excellent yields by reaction of (R) and (S)-limonene, respectively, with PBr3 / SiO2 in dichloromethane.

Reactivity of zeolite hydroxy groups toward σ-donor bases. H–D exchange with 3-methylpentane

The H–D exchange of deuteriated Y zeolites, silica–alumina and γ-alumina with 3-methylpentane has been studied by IR spectroscopy. At 373 K exchange ocurred only in the zeolites. Silica–alumina required higher temperatures (523–623 K) and exchange did not occur in γ-alumina in the range of temperature studied. Ultrastable Y zeolite with non-framework alumina (NFA), showed a higher reactivity for H–D exchange and hexane cracking than Y zeolite dealuminated by (NH4)2SiF6. These findings were explained by the difference in acid strength between the zeolites, probably due to the presence of NFA as proposed in the literature, and stress the dependence of H–D exchange on acid strength. The IR study of H–D exchange between 3-methylpentane and Y zeolites having pyridine pre-adsorbed, neutralizing some of the hydroxy groups, showed that the high-frequency (hf) hydroxy group is responsible for the exchange. The low-frequency (If) and NFA hydroxy groups appear to exchange via hydrogen bridges with the hf hydroxy group, rather than by direct interaction with the hydrocarbon. The H–D exchange between solid acids and alkanes could be a qualitative test for acid strength, since only very strong acids would attack σ bonds.

Regiospecific Baeyer-Villiger Oxidation of α,α-Dichlorocyclobutanones: A Norbisabolide γ-lactone Analogue1

Abstract Details and methodological comparisons are presented on the preparation of a norbisabolide γ-lactone analogue (4) via dichloroketene cycloaddition to limonene followed by regioselective Baeyer-Villiger oxidation (CH3CO3H) of the α,α-dichlorocyclobutanones 3 and C-Cl reduction (Zn/HOAc). The sequence of reduction followed by oxidation (HOCI) applied to 3 produced 4 in much better yield.

Solvomercuration-demercuration of limonene with Hg(BF4)2; A chemo-and regiospecific route to 8-substituted p-menthenes

Abstract Solvomercuration-demercuration of limonene with equimolar Hg(BF 4 ) 2 and excess nucleophile (−20°C) functionalizes the acyclic double bond and provides specifically 8-substituted p-menthenes.

Methods of cyclization of some δ-diketones

Abstract Three methods of cyclizing δ-diketones where the β- and γ-carbons form part of a ring system were studied: 1) strong base in protic media; 2) strong aqueous acid biphasic with apolar solvent; 3) alumina with saturated hydrocarbons. General limits were established for each method and a system validated for preparative scale cyclization without isomerization of an external double bond.

Synthesis of the Four Epimeric Tosylates of (5R)-2, 3-Epoxy-5-isopropenyl-cyclohexanol

Abstract Described is the synthesis of the four optically pure epoxy-tosylates 5, 6, 9 and 12, each with four chiral centers determined, from a single starting material, (R)(-) carvone.