We present the synthesis and the photochemical and catalytic switching properties of an azopyridine as a photoswitchable ligand, covalently attached to a Ni(II)-porphyrin. Upon irradiation with 530 nm (green light), the azopyridine switches to the cis configuration and coordinates with the Ni2+ ion. Light of 435 nm (violet) isomerizes the ligand back to the trans configuration, which decoordinates for steric reasons. This so-called record player design has been used previously to switch the spin state of Ni2+ between singlet and triplet. We now use the coordination/decoordination process to switch the catalytic activity of the dimethylaminopyridine (DMAP) unit. DMAP is a known catalyst in the nitroaldol (Henry) reaction. Upon coordination to the Ni2+ ion, the basicity of the pyridine lone pair is attenuated and hence the catalytic activity is reduced. Decoordination restores the catalytic activity. The rate constants in the two switching states differ by a factor of 2.2, and the catalytic switching is reversible.
Pyridinyl amide ion pairs carrying various electron-withdrawing substituents were synthesized with selected ammonium or phosphonium counterions. Compared to neutral pyridine-based organocatalysts, these new ion pair Lewis bases display superior catalytic reactivity in the reaction of isocyanates with alcohols and the aza-Morita−Baylis−Hillman reaction of hindered electrophiles. The high catalytic activity of ion pair catalysts appears to be due to their high Lewis basicities toward neutral electrophiles as quantified through quantum chemically calculated affinity data.
The reaction of carboxylic acid chlorides with secondary alcohols carrying either flexible alkyl or rigid aryl substituents was studied through a series of competition experiments. Aliphatic acid chlorides react preferentially with the aryl-substituted alcohols, while acid chlorides derived from aromatic carboxylic acids react with very low selectivity. Catalysis by 9-azajulolidine (TCAP) increases the selectivity strongly, while solvent and temperature effects are only moderate. The size of the alcohol substituents seems to impact selectivities only for rigid aryl substituents, and highest selectivities have been found for 1-(1-pyrenyl)ethanol.
ZUSAMMENFASSUNG:Durch Reaktivierung der kurzlebigen Vanadiumkontakte mit Hexachlorcyclopentadien wird eine bessere Katalysatornutzung und damit eine groI3ere Wirtschaftlichkeit bei der Herstellung von hithylen/Propylen-Copolymerisaten erzielt.Hexachlorcyclopentadien wirkt dadurch reaktivierend, daB es das bei der Umsetzung von hoherwertigen Vanadium-und Al-organischen Verbindungen entstandene inaktive V(I1) zum V(II1) oxydiert. V(II1) kann dann erneut durch Reaktion mit Al-organischen Verbindungen zu einem aktiven Katalysator werden.
SUMMARY:Reactivation of the short living vanadium catalyst with hexachloro cyclopentadiene yields a higher catalyst mileage in the ethylene/propylene copolymerization.Hexachloro cyclopentadiene oxidizes the inactive &dent vanadium component of the catalyst to a trivalent vanadium compound, the former resulting from the reaction of higher valent vanadium compounds with aluminum organic compounds. The trivalent vanadium compound may again react with the aluminum organbeompound to yield an active catalyst.
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