title: Bifunctional Thioureas with α-Trifluoromethyl or Methyl Groups: Comparison of Catalytic Performance in Michael Additions.
abstract: Thioureas are an important scaffold in organocatalysis because of their ability to form hydrogen bonds that activate substrates and fix them in a defined position, which allows a given reaction to occur. Structures that enhance the acidity of the thiourea are usually used to increase the hydrogen-bonding properties, such as 3,5-bis(trifluoromethyl)phenyl and boronate ureas. Herein, we report the synthesis of bifunctional thioureas with a chiral moiety that include either a trifluoromethyl or methyl group. Their catalytic performance in representative Michael addition reactions was used in an effort to compare the electronic effects of the fluorination at the methyl group. The observed differences concerning yields and ee values cannot be attributed solely to the different steric environments; theoretical results indicate distinct interactions within the corresponding transition states. The calculated transition states show that the fluorinated catalysts have stronger N-H···O and C-H···F hydrogen bonds, while the nonfluorinated systems have C-H···π contacts. These results have shown that a variety of hydrogen-bonding interactions are important in determining the yield and selectivity of thiourea organocatalysis. These details can be further exploited in catalyst design.
authors.affiliation: Institute of Chemistry, National Autonomous University of Mexico , Circuito Exterior, Ciudad Universitaria, Del. Coyoacán, 04510 Cd. Mx., Mexico.
authors.forename: Eddy I
authors.initials: EI
authors.lastname: Jiménez
authors.affiliation: Institute of Chemistry, National Autonomous University of Mexico , Circuito Exterior, Ciudad Universitaria, Del. Coyoacán, 04510 Cd. Mx., Mexico.
authors.forename: Wilmer E
authors.initials: WE
authors.lastname: Vallejo Narváez
authors.affiliation: Institute of Chemistry, National Autonomous University of Mexico , Circuito Exterior, Ciudad Universitaria, Del. Coyoacán, 04510 Cd. Mx., Mexico.
authors.forename: Carlos A
authors.initials: CA
authors.lastname: Román-Chavarría
authors.affiliation: Institute of Chemistry, National Autonomous University of Mexico , Circuito Exterior, Ciudad Universitaria, Del. Coyoacán, 04510 Cd. Mx., Mexico.
authors.forename: Josue
authors.initials: J
authors.lastname: Vazquez-Chavez
authors.affiliation: Institute of Chemistry, National Autonomous University of Mexico , Circuito Exterior, Ciudad Universitaria, Del. Coyoacán, 04510 Cd. Mx., Mexico.
authors.forename: Tomás
authors.initials: T
authors.lastname: Rocha-Rinza
authors.affiliation: Institute of Chemistry, National Autonomous University of Mexico , Circuito Exterior, Ciudad Universitaria, Del. Coyoacán, 04510 Cd. Mx., Mexico.
authors.forename: Marcos
authors.initials: M
authors.lastname: Hernández-Rodríguez