Mechanistic studies of an L-proline-catalyzed pyridazine formation involving a Diels-Alder reaction with inverse electron demand.

The mechanism of an L-proline-catalyzed pyridazine formation from acetone and aryl-substituted tetrazines via a Diels-Alder reaction with inverse electron demand has been studied with NMR and with electrospray ionization mass spectrometry. A catalytic cycle with three intermediates has been proposed. An enamine derived from L-proline and acetone acts as an electron-rich dienophile in a [4 + 2] cycloaddition with the electron-poor tetrazine forming a tetraazabicyclo[2.2.2]octadiene derivative which then eliminates N2 in a retro-Diels-Alder reaction to yield a 4,5-dihydropyridazine species. The reaction was studied in three variants: unmodified, with a charge-tagged substrate, and with a charge-tagged proline catalyst. The charge-tagging technique strongly increases the ESI response of the respective species and therefore enables to capture otherwise undetected reaction components. With the first two reaction variants, only small intensities of intermediates were found, but the temporal progress of reactants and products could be monitored very well. In experiments with the charge-tagged L-proline-derived catalyst, all three intermediates of the proposed catalytic cycle were detected and characterized by collision-induced dissociation (CID) experiments. Some of the CID pathways of intermediates mimic single steps of the proposed catalytic cycle in the gas phase. Thus, the charge-tagged catalyst proved one more time its superior effectiveness for the detection and study of reactive intermediates at low concentrations.

Probing the gas-phase structure of charge-tagged intermediates of a proline catalyzed aldol reaction - vibrational spectroscopy distinguishes oxazolidinone from enamine species.

An l-proline based catalyst with a charged phenyl-pyridium substituent (1) was used to analyze intermediates of an organocatalyzed aldol reaction by infrared multi-photon dissociation (IRMPD) mass spectrometry after transfer into the gas phase via electrospray ionization (ESI). IRMPD spectra were interpreted with the aid of density functional theory (DFT) computations. A structurally restricted enamine species was used as a reference molecule for the calculated vibrational frequencies. A close correlation between theory and experiment was found for the energetically most favoured oxazolidinone structures.

Investigations of the Copper-Catalyzed Oxidative Cross-Coupling of Tetrahydroisoquinolines with Diethylzinc by a Combination of Mass Spectrometric and Electrochemical Methods.

The aerobic oxidative cross-coupling of tetrahydroisoquinolines (THIQs) with diethylzinc catalyzed by CuCl2 has been examined by means of electrospray mass spectrometry (ESI-MS). Substrates, intermediates, and the product were readily detected. Particular emphasis has been placed on the role of CuCl2 . Formation of the intermediate iminium species has been investigated in more detail by ESI-MS, electrochemistry-coupled ESI mass spectrometry (EC-MS), and cyclic voltammetry (CV). Our experiments have consistently revealed strong influences of the N-substituent of the THIQ derivative and its oxidation stability with respect to CuCl2 . The results may help to expand the synthetic scope of the reaction, while also further establishing EC-MS as a valuable technique for linking mass spectrometry with cyclic voltammetry in mechanistic studies of organic redox reactions.

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.