What is the mechanism of amine conjugate additions to pyrazole crotonate catalyzed by thiourea catalysts?

Aminoindanol-derived thioureas catalyze proton transfer and do not stabilise anions in an asymmetric conjugate addition to pyrazole crotonate. Calculations show that the urea H-bonds play different roles in the preferred transition state and in the one leading to the minor enantiomer in the mechanism of hydroxy-thiourea catalyzed conjugate additions to pyrazole crotonates.