Lewis Base Centers Research Articles

Tungsten pentacarbonyl as a potential protecting group for soft lewis base centres in alkylation of multifunctional molecules

Reaction of W(CO) 6 or (PhNH 2)W(CO) 5 with the molecules which possess both diphenylphosphanyl- and dialkylamino group or nitrogen — containing hetcrocyclic ring proceeds selectively through “soft” Lewis base groups and can be used for the protection of the groups from alkylalion. As a rule, the diphenylphosphanyl group participates in the co-ordination, but in some cases an sp 2-hybridized nitrogen was shown to be the preferable co-ordination site. The new synthesized compounds have been characterized by NMR, IR spcctroscopy, and elemental analysis.

Reactions of methylbutynol on alkali-exchanged zeolites. A Lewis acid-base selectivity study

Alkali-exchanged zeolites have both Lewis acid and Lewis base centres. They provide an easy way to tune up the relative strengths of these two sites by controlling Si/Al ratio and electropositivity of the counter cation. These effects can be monitored both byFTIR and N1s XPS of chemisorbed pyrrole. A systematic study of the relationship between acid-base properties and the reactions of methylbutynol (MBOH) was performed over these alkali-exchanged zeolites. The results show that the Lewis base centres catalyze the cleavage reaction ofMBOH, which produces acetone and acetylene, while the strong Lewis acid centres catalyze the dehydration ofMBOH to 3-methyl-3-buten-1-yne (MBYNE). The strong basicity of alkaliexchangedX zeolites produces almost 100% selectivity to acetone and acetylene, whereas the strong Lewis acidity of alkali-exchangedZSM-5 zeolites results in a high selectivity toMBYNE. Compared with the catalytic results overH-ZSM-5 zeolites, it is further concluded that the catalytic conversion ofMBOH to 3-methyl-3-butene-2-one (MIPK) requires the presence of not Lewis acid but strong Bronsted acid sites. The reactions ofMBOH over acid-base catalysts are considered as standard reactions for the characterization of acid and base centres. A correlation between these reactions and theFTIR, andXPS of pyrrole data will be discussed. The deactivation mechanism duringMBOH conversion over alkali-exchanged zeolites is further discussed.

RING MOLECULES BUILT FROM GAS-UNITS

Abstract Different synthetic routes are available for the formation of dihalogeno-(organylthio)gallanes. Some of the most important reactions leading to these compounds are reported.1–4 Trivalent compounds of gallium are naturally Lewis acids; moreover, in organylthiogallanes a Lewis-base center exists on the sulfur atom. Therefore these molecules have the possibility of both intermolecular and intramolecular Lewis-acid-base interactions, which in the case of intermolecular interactions lead to Lewis-acid-base adducts. The thus formed adducts can be chains or rings of different sizes. Diiodo(organylthio)gallanes are able to form monomers, dimers, and even trimers of different shapes. These differences in the structure are not only due to the group R on the sulfur atom, because investigations on these compounds in solution (molecular weight determinations, and 1H-n.m.r. spectroscopy) and in the solid state (X-ray-structure determinations) have shown differences in the ring sizes for one and the same molecu...