A new charge-tagged proline-based organocatalyst for mechanistic studies using electrospray mass spectrometry.

A new 4-hydroxy-L-proline derivative with a charged 1-ethylpyridinium-4-phenoxy substituent has been synthesized with the aim of facilitating mechanistic studies of proline-catalyzed reactions by ESI mass spectrometry. The charged residue ensures a strongly enhanced ESI response compared to neutral unmodified proline. The connection by a rigid linker fixes the position of the charge tag far away from the catalytic center in order to avoid unwanted interactions. The use of a charged catalyst leads to significantly enhanced ESI signal abundances for every catalyst-derived species which are the ones of highest interest present in a reacting solution. The new charged proline catalyst has been tested in the direct asymmetric inverse aldol reaction between aldehydes and diethyl ketomalonate. Two intermediates in accordance with the List-Houk mechanism for enamine catalysis have been detected and characterized by gas-phase fragmentation. In addition, their temporal evolution has been followed using a microreactor continuous-flow technique.

Biotransformation of the UV-filter sulisobenzone: challenges for the identification of transformation products.

The UV-filter substance Sulisobenzone (BP-4) is widely employed in sunscreens and other personal care products. In the current study, its behavior during biological wastewater treatment was investigated. In contact with activated sludge BP-4 was degraded in aerobic batch experiments forming at least nine transformation products (TPs). The mass balance in the batch experiments was closed, as measurements with LC-UV underlined that the quantity of the TPs was comparable to the BP-4 quantity transformed. The chemical structures of the nine TPs could be proposed based on accurate mass measurements by high resolution mass spectrometry (LTQ-Orbitrap-MS), several fragmentation experiments up to MS(6) and synthesis of one TP. NMR analyses of the main TP confirmed its proposed chemical structure. At the beginning of the biotransformation of BP-4, a benzhydrol analogue was formed due to the reduction of the keto moiety. Further reactions (e.g., oxidation, demethylation, decarboxylation) led to the formation of extremely polar TPs. A biodegradation pathway was proposed based on the TP structures identified and the sequence of the TP formation. The isolated TPs exhibited higher toxic effects on Vibrio fischeri than BP-4. The results contribute to a better general understanding and prediction of the biotransformation of aromatic sulfonic acids in WWTPs.

Direct experimental evidence for an enamine radical cation in SOMO catalysis.

SOMO catalysis has lately obtained large interest as a new and powerful version of enantioselective organocatalysis which includes radical steps initiated by a one-electron oxidation. The intermediate enamine radical cation has been postulated, but has not been observed directly so far. This communication now reports the direct detection of this key intermediate.