Atropos Ligand
Paired Concept:
Tropos Ligand
Definition: A chiral ligand that possesses a configurationally stable stereogenic axis, such that its atropisomeric forms do not readily interconvert under normal reaction conditions. The ligand's axial chirality is effectively "locked," allowing the individual atropisomers to be isolated and used as distinct chiral entities.
Context: The term atropos (from the Greek Atropos, meaning "inflexible" or "unchangeable") describes atropisomeric ligands with sufficiently high rotational barriers around a bond-typically a biaryl axis-to prevent racemization at ambient temperatures. Atropos ligands play a central role in asymmetric catalysis because their well-defined and persistent chirality can be efficiently transferred to substrates, often resulting in high enantioselectivities. Many of the most successful chiral ligands used in industry belong to this category.
Example: BINAP is a classic atropos ligand. Its biaryl axis is configurationally stable, and the two atropisomers, (R)-BINAP and (S)-BINAP, can be isolated and used independently in asymmetric catalytic reactions.
Related Terms: Tropos Ligand, Atropisomerism, Axial Chirality, Chiral Ligand, Configurational Stability, Asymmetric Catalysis.
Reference: Noyori, R. (2002). Asymmetric Catalysis: Science and Opportunities (Nobel Lecture). Angewandte Chemie International Edition, 41, 2008-2022; Bringmann, G., Mortimer, A. J. P., Keller, P. A., Gresser, M. J., Garner, J., & Breuning, M. (2005). "Atroposelective Synthesis of Axially Chiral Biaryl Compounds." Angewandte Chemie International Edition, 44, 5384-5427; Noyori, R. (1994). Asymmetric Catalysis in Organic Synthesis. John Wiley & Sons. ISBN: 978-0471351970. International Union of Pure and Applied Chemistry. Compendium of Chemical Terminology (Gold Book) - Entries related to atropisomerism and axial chirality
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.
