Th. M. Wortel

Selective bromination of halobenzenes using zeolite catalysts

The substitution of halobenzenes by bromine at 25 °C is catalyzed by Y-type zeolites, partially exchanged with di- and trivalent cations. Catalyst activity and para/ortho ratio depend upon the type of cation, the extent of cation exchange, the activation temperature, the solvent, and the amount of catalyst used. In all cases the para/ortho ratio was substantially higher than with conventional procedures. Catalyst deactivation by hydrogen bromide, liberated during the reaction, was suppressed by adding sodium hydrogen carbonate and zeolite KA. The highest para/ortho ratios were obtained in a solvent-free procedure. The acidic Bronsted sites of the zeolite are considered to be the catalytically active sites.

Zeolite ZSM-5 and related materials as catalyst in benzene chlorination

Abstract ZSM-5 type zeolites and related high siliceous materials induce addition of chlorine to benzene yielding hexachlorocyclohexanes, whereas Y-type zeolites favour substitution to yield chlorobenzenes.

Conformational analysis of 7-alkyl-3-oxabicyclo-[3.3.1]nonanes and complexes with lanthanide shift reagents

Abstract The conformation of 3-oxabicyclo[3,3.1]nonane and of some of its 7α- and 7β-alkyl substituted derivatives has been studied with the use of 13C and 1H NMR spectroscopy. A comparison is made with the carbocyclic analogues; it turns out that the replacement of the 3-methylene group by oxygen has no substantial influence on the conformational preferences. With the aid of 3JHH coupling constants it is shown that the geometry of the cyclohexane rings is about the same as in the corresponding carbocyclic compounds. The results of calculations on the lanthanide induced shifts indicate that the tetrahydropyran ring is not flattened but probably somewhat puckered. The calculated location of Eu(III) in complexes of Eu(dpm)3 with the 3-oxabicyclo [3.3.1]nonanes is compared with that in the complexes of the related compounds 2-oxaadamantane and 4-methyltetrahydropyran. The data indicate that the lanthanide ion coordinates “axially” to the latter compound.

The electron impact induced fragmentations of some 7-alkyl-3-oxabicyclo[3.3.1]nonanes

Abstract The mass spectra of a series of 7-alkyl substituted 3-oxabicyclo[3.3.1]nonanes are recorded. The fragmentation has been studied by the use of the metastable DADI and defocussing techniques. The character of the alkyl group is found to influence the fragmentation pattern. Stereoselective fragmentations for the 7-t-butyl derivatives are observed. In the endo -isomer, in contrast to the exo -isomer, a transannular hydrogen transfer plays a role.

Synthesis and conformation of some 2,4-dioxa- and 2,4-dioxa-3-silabicyclo[3.3.1]nonanes

Abstract The conformation of the title compounds established by 13 C and 1 H NMR spectroscopy shows that replacement of the 2- and 4-methylene groups in bicyclo[3.3.1]nonane by ether oxygen atoms strongly destablizes the cc conformation: in 2-oxabicyclo[3.3.1]-nonane the cc and the bc conformers are about equally populated, whereas in 2,4-dioxabicyclo[3.3.1]nonane the bc conformation predominates. The 2,4-dioxa-3-silabicyclo[3.3.1]nonanes also occur predominantly inthe bc conformation. As in the carbocyclic bicyclo[3.3.1]nonanes both wings are strongly flattened. A stereoselective synthesis of 3α-methylbicyclo[3.3.1]nonane, an important model compound in this study, is described.