A Morita-Baylis-Hillman (MBH) reaction catalyzed by thiourea was monitored by ESI-MS (/MS) and key intermediates were intercepted and characterized. These intermediates suggest that thiourea acts as an organocatalyst in all steps of the MBH reaction cycle, including the rate-limiting proton-transfer step. DFT calculations, performed for a model MBH reaction between formaldehyde and acrolein with trimethylamine as base and in the presence or the absence of thiourea, suggest that thiourea accelerates MBH reactions by decreasing the transitionstate (TS) energies through bidentate hydrogen bonding throughout the whole catalytic cycle. In the rate-limiting proton-transfer step, the thiourea acts not as a proton shuttle, but as a Brønsted acid stabilizing the basic oxygen center that is formed in the TS. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.
|Journal||Chemistry - A European Journal|
|Publication status||Published - 1 Jan 2009|
- Density functional calculations
- ESI mass spectrometry
- Morita-baylis-hiliman reaction
- Reaction mechanisms