Crystals of 6 suitable for X-ray crystallography were grown from DCM/heptane (Figure ). Interestingly, the acid OH was intact since it was not clear if an internal salt, a hydrogen bond to the amine N or hydrogen bond to neighboring molecules is formed from the acid. This is particularly welcome since an internal salt could be too stable to act as an proton donor, thus probably would have no use in catalysis. Hydrogen bonds, on the other hand, open up the possibility of acidity that is strong enough for catalysis. In Figure one can clearly see the acid OH pointing towards the amine N forming an internal hydrogen bond. The position of the hydrogen could be clearly established due to excess electron density between the carboxylic acid group and the amine nitrogen (Figure ). Next we focused on introducing benzyl(thio)ureas. Thioureas have been used as asymmetric organocatalysts in a large variety of different reactions such as Michael reactions , Mannich reactions , Friedel-Crafts reactions , Strecker reactions , Morita-Baylis-Hillman reactions , Pictet-Spengler reactions , halohydrin reactions and hydrophosphonylations . Beyond synthetic chemistry, thioureas have also found widespread use in medicinal applications . Due to the rather mild pKa value of thiourea, it is often characterized as a hydrogen-bond donor catalyst rather than a classic Brønsted acid. The N-attached hydrogen atoms in thiourea derivates function as hydrogen bond donors in the catalytic process . Thioureas are often used in conjunction with Brønsted or Lewis bases, usually tertiary amines, so the resulting organocatalyst can activate both electrophiles and nucleophiles .