title: A new charge-tagged proline-based organocatalyst for mechanistic studies using electrospray mass spectrometry.
abstract: 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.
authors.affiliation: University of Bonn, Kekulé-Institute of Organic Chemistry and Biochemistry, Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany.
authors.forename: J Alexander
authors.initials: JA
authors.lastname: Willms
authors.affiliation: University of Bonn, Kekulé-Institute of Organic Chemistry and Biochemistry, Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany.
authors.forename: Rita
authors.initials: R
authors.lastname: Beel
authors.affiliation: University of Bonn, Kekulé-Institute of Organic Chemistry and Biochemistry, Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany.
authors.forename: Martin L
authors.initials: ML
authors.lastname: Schmidt
authors.affiliation: University of Bonn, Kekulé-Institute of Organic Chemistry and Biochemistry, Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Mundt
authors.affiliation: University of Bonn, Kekulé-Institute of Organic Chemistry and Biochemistry, Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany.
authors.forename: Marianne
authors.initials: M
authors.lastname: Engeser
keywords: charge tag; electrospray; mass spectrometry; organocatalysis; proline; template