T
Threo-
Definition: A relative stereochemical descriptor for diastereomers with two adjacent chiral centers having substituents on opposite sides in a Fischer projection.
Context: The opposite of erythro-; used in carbohydrate and amino acid stereochemistry.
Example: Threo-2,3-butanediol has hydroxyl groups on opposite sides.
Related Terms: Erythro-; Diastereomer; Relative Configuration.
Reference: Eliel & Wilen (1994).
Topicity
Definition: Relationship of identical groups/sites: homotopic, enantiotopic, diastereotopic.
Context: Guides selectivity predictions in synthesis and NMR.
Example: Diastereotopic methylene hydrogens in chiral environments.
Related Terms: Enantiotopic, Diastereotopic.
Reference: Eliel & Wilen (1994).
Topological Chirality
Definition: Chirality arising from molecular topology (e.g., knots, catenanes) rather than stereocenters.
Context: Inspires novel drug-like architectures and materials.
Example: Molecular trefoil knots.
Related Terms: Axial Chirality, Helicity.
Reference: IUPAC Gold Book.
Tropos Ligand
Paired Concept:
Atropos Ligand
Definition: A chiral ligand that possesses configurational flexibility, allowing interconversion between different atropisomeric conformations through rotation about a hindered bond under normal reaction conditions. Unlike atropisomerically stable ligands, tropos ligands do not maintain a permanently fixed chiral axis.
Context: The term tropos (from the Greek tropos, meaning "turn" or "change") was introduced to distinguish conformationally mobile atropisomeric ligands from rigid, configurationally stable atropisomeric ligands (atropos ligands). Tropos ligands can become effectively chiral upon coordination to a metal center or through interaction with a chiral auxiliary, making them valuable in asymmetric catalysis. Their dynamic nature allows adaptation to different catalytic environments, sometimes leading to high enantioselectivities.
Example: Certain biphenyl diphosphine ligands with relatively low rotational barriers around the biaryl axis behave as tropos ligands, rapidly interconverting between axial conformations in solution. Upon complexation with a metal catalyst, one conformation may be preferentially stabilized, generating an active chiral catalyst.
Related Terms: Atropos Ligand, Atropisomerism, Axial Chirality, Dynamic Chirality, Chiral Ligand, Asymmetric Catalysis.
Reference: Mikami, K., Aikawa, K., Yusa, Y., J. J. Jodry, & Yamanaka, M. (2002). "New Chiral Ligands and Catalysts Based on Dynamic Chirality." Synlett, 2002(10), 1561-1578.
Turbo Chirality
Definition: A higher-order chirality phenomenon in amino acid and peptide derivatives in which planar amide and carboxyl groups arrange as propeller-like blades around an α-carbon, amplifying chiral expression beyond classical point chirality.
Context: Conventional descriptors such as R/S or L/D do not fully capture this higher-order stereochemical behavior in certain amino acid and peptide derivatives. Analysis of X-ray structures of N-acetyl amino acids and the peptide biphalin suggests that surrounding functional groups organize into a directional, propeller-like motif, which they present as a distinct chirality phenomenon referred to as turbo chirality.
Example: An amino acid derivative in which the amide and carboxylic acid substituents around the α-carbon are not merely attached to a chiral center, but are spatially arranged as coordinated "propeller blades," producing a stronger, more distributed chiral signature than point chirality alone. This behavior is demonstrated by the opioid peptide biphalin.
Related Terms: Absolute Configuration; Homochirality; Peptide Chirality; Stereogenic Center; Chiral Fidelity; Conformational Chirality.
Reference: Yuan, Q.; Pandey, A.; Liu, H.; Bouley, B.; Li, Z.; Zhu, H.; Liang, R.; Li, G. A New Chirality Phenomenon in Amino Acid and Peptide Derivatives. ChemRxiv (2026). DOI: 10.26434/chemrxiv-2026-kncjf.
Twist-Boat Conformation
Definition: A less strained intermediate form of cyclohexane between the boat and chair conformations.
Context: Has lower energy than pure boat form; relevant in conformational equilibria.
Example: Cyclohexane adopts twist-boat at high energy transition states.
Related Terms: Boat Conformation; Chair Conformation; Conformational Isomer.
Reference: Eliel & Wilen (1994).
