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01:17Thermal Electrocyclic Reactions: Stereochemistry
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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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01:33Radical Halogenation: Stereochemistry
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Stereochemistry is the study of the different spatial arrangements of atoms in a given molecule. The stereochemistry of radical halogenations can be understood from three different situations:
Halogenation to form a new chiral center:
Halogenation to form a new chiral center:
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01:12Stereochemical Effects of Enolization
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The chiral α-carbon of the carbonyl compound is the stereocenter of the molecule. As shown in the figure below, when such a carbonyl compound undergoes racemization under an acidic or basic condition, an achiral enol is formed.
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02:36Regioselectivity and Stereochemistry of Hydroboration
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A significant aspect of hydroboration–oxidation is the regio- and stereochemical outcome of the reaction.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
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01:26Photochemical Electrocyclic Reactions: Stereochemistry
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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
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02:52Stereoisomerism
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Isomerism in Complexes
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis...
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis...
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Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers
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Practical Stereochemistry.
1Syncom BV , Kadijk 3, Groningen 9747 AT, The Netherlands.
Accounts of Chemical Research
|March 2, 2017
Summary
Fundamental research in stereochemistry and chirality has unexpectedly led to practical applications. Discoveries in orbital symmetry, crown ethers, and nucleation have yielded new synthetic methods and commercially valuable enantiopure compounds.
Area of Science:
- Organic Chemistry
- Stereochemistry
- Chirality
- Physical Chemistry
Background:
- The relationship between fundamental and applied scientific research is often viewed as contentious, with a perception that fundamental research is a financial burden.
- This perspective overlooks the long-term, often unpredictable, pathways through which foundational discoveries can lead to significant practical applications.
- The author's 40+ years of research in stereochemistry and chirality provide a case study illustrating this connection.
Purpose of the Study:
- To demonstrate how fundamental research in stereochemistry and chirality has led to diverse practical applications.
- To trace a personal research journey from orbital symmetry and crown ethers to deracemization and nucleation phenomena.
- To highlight the interdependence of fundamental and applied science, showcasing real-world examples.
Main Methods:
- Exploration of orbital symmetry principles (Woodward-Hoffmann rules) leading to hindered alkene synthesis.
- Investigation of crown ethers and their impact on nucleophilic substitutions, particularly with cesium salts.
- Development of diastereomeric resolution techniques, including mechanistic studies and the 'Dutch resolution' approach.
- Study of nucleation phenomena in relation to chirality and its application in attrition-driven deracemizations.
- Scanning Tunneling Microscopy (STM) analysis of helicene nucleation on metal surfaces.
Main Results:
- Orbital symmetry research yielded a novel synthesis of hindered alkenes, applicable to molecular motor production.
- Crown ether research revealed the utility of cesium salts for racemization-sensitive nucleophilic substitutions.
- Diastereomeric resolution studies uncovered the complex interplay between nucleation and chirality, leading to attrition-driven deracemizations.
- Attrition-driven deracemization technology enables quantitative conversion of racemates to single enantiomers, with applications in producing naproxen and clopidogrel.
- STM investigation provided insights into helicene nucleation behavior on metal surfaces.
Conclusions:
- Fundamental research in stereochemistry and chirality has demonstrably led to significant practical and commercial applications.
- The described research path illustrates the often-unforeseen but crucial links between basic science and technological innovation.
- The interdependence of fundamental and applied science is essential for continued progress in chemistry and related fields.