Asymmetric Aldol Reaction Catalyzed by l‐Proline and Achiral Thiourea Fluoroboric Acid Salt

Isothiouronium Organocatalysts Through Hydrogen Bonding

“…In addition, the chemical modification of isothiouroniums is readily varied using synthetic methods to make several types of functional molecular systems. Consequently, isothiouronium-derived catalysts have been explored as a new field in hydrogen-bonding organocatalysts (Figure 2) [18][19][20][21][22][23].…”

“…Reduced catalyst loading was also tolerated (from 15 to 5%). [22,23] With a keen interest in organocatalyst systems based on isothiouronium derivatives, Kim et al introduced isothiouronium iodide salt as an efficient cocatalyst with L-proline in the direct asymmetric aldol reactions between cyclohexanone and aromatic aldehydes [22]. This method produced good-to-excellent yields (up to 93%) with good stereoselectivities (up to 93:7 dr and 99% ee).…”

“…This aldol protocol includes a solvent-free catalytic system inside a refrigerator without stirring in the hunt for an inexpensive and green process ( Table 1, Scheme 8, and Figure 6). When exploring an asymmetric aldol reaction, the authors also introduced the mimic system of isothiouronium salts which were prepared indirectly by adding acids to thiourea ( Table 2) [23]. The isothiouronium core of salts from thiourea and acids could form a network of Hbonding interactions with the carboxylate of proline as well as with the carbonyl moieties of cyclohexanone and aromatic aldehyde, thus enhancing their electrophilicity (Figure 7).…”

Defect-Engineered Chiral Metal–Organic Frameworks for Efficient Asymmetric Aldol Reaction

“… a The asymmetric aldol reaction was carried out with 4-nitrobenzaldehyde (0.66 mmol) and acetone (5 mL) as solvents, a catalyst (25 mol % chiral ligand), and trifluoroacetic acid (additive, 6 μL), at 25 °C for 2 days. b Isolated yield of the purified material. c The enantiopurity was determined by high-performance liquid chromatography. d The catalyst was DC-CuBTC-0.6. e The products are in the S configuration. …”

“… a The asymmetric aldol reaction was carried out with 4-nitrobenzaldehyde (0.66 mmol) and acetone (5 mL) as solvents, a catalyst (25 mol % chiral ligand), and trifluoroacetic acid (additive, 6 μL), at 25 °C for 2 days. …”