Morita–Baylis–Hillman Reaction: ESI-MS(/MS) Investigation with Charge Tags and Ionic Liquid Effect Origin Revealed by DFT Calculations

Abstract

The use of a charge-tagged acrylate derivative bearing an imidazolium tag to study the Morita-Baylis-Hillman reaction via ESI-MS(/MS) monitoring and the effect of such tag (imidazolium cations and ion pairs) over TSs is described. The ionic nature of the substrate was meant to facilitate ESI transfer to the gas phase for direct mass spectrometric analysis. The detection and characterization of charged intermediates has suggested major reaction pathways. DFT calculations considering the effect of a polar and protic solvent (methanol), of a polar and aprotic solvent (acetonitrile), and of no solvent (gas phase) were used to predict possible TSs through a common accepted intermediate. The controversial proton transfer step, which may proceed via Aggarwal's or McQuade's proposals, was evaluated. Calculations predicted the formation of electrostatic intermediate complexes with both the cation and anion when charge-tagged reagents are used. These complexes contribute to the positive ionic liquid effect, and based on the formation of these unique complexes, a rationale for the ionic liquid effect is proposed. These complexes also pointed to a plausible explanation for the positive ionic liquid effect observed in several reactions that are difficult to be carried out in organic solvents but have shown a beneficial effect when performed in ionic liquids.

Styrene polymerization efficiently catalyzed by iron-containing imidazolium-based ionic liquids: Reaction mechanism and enhanced ionic liquid effect

“…Fig. 4 indicates therefore that styrene polymerization starts with the formation of a key styrene chloronium cation which [30][31][32] and, more recently, by us [33]. Interestingly, the Schulz-Flory distributions for polystyrene produced with BMI.Fe 2 Cl 7 as the catalyst pointed to two or three active catalytic species (Table 4), hence the ESI-MS(/MS) data was in accordance with this prediction.…”

Elusive Reaction Intermediates in Solution Explored by ESI‐MS: Reverse Periscope for Mechanistic Investigations

“…Reaction scheme for the Morita-Baylis-Hillman reaction. The intermediates 2, 3, 4,and 5 have been identified by ESI-MS. [36] Angewandte Chemie by ESI-MS they detected several of the expected intermediates.I np articular,i no ffline nano-electrospray experiments with UV and NMR quantification they demonstrated that the reaction rate is improved in the spray droplets.…”

“…Recently,N eto and co-workers published ar eport on the MBH reaction, and they described the use of an imidazolium-based charged acrylate derivative as ar eactant. [36] This approach, which was aimed at improving ESI-MS detection of the intermediates,a llowed characterization of transient zwitterionic species and noncovalent adducts.I np articular they identified thiourea-containing derivatives which are responsible for the enhanced reaction rate observed when using thiourea ( Figure 5). Thet wo hydrogen-bonding interactions of thiourea with the acrylate derivative are responsible for the noncovalent organocatalysis.…”

Elektrospray‐Massenspektrometrie: ein umgekehrtes Periskop zur Erforschung von Reaktionsmechanismen in Lösung

“…Neben diesen MBH‐Studien, in denen erstmals entscheidende Zwischenstufen abgefangen und charakterisiert wurden, befasste sich eine Reihe von weiteren ESI‐MS‐Untersuchungen mit noch offenen mechanistischen Fragen bei der MBH‐Umsetzung. Hierzu verwendeten Neto und Mitarbeiter in einer neueren Studie ein geladenes Imidazolium‐Acrylsäurederivat als Reaktionspartner . Während die Autoren ursprünglich den Nachweis von Intermediaten durch ESI‐MS verbessern wollten, charakterisierten sie transiente zwitterionische Spezies und nichtkovalente Addukte.…”