Related Concept Videos
02:36Regioselectivity and Stereochemistry of Hydroboration
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.
01:17Thermal Electrocyclic Reactions: Stereochemistry
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.
02:43E2 Reaction: Stereochemistry and Regiochemistry
Elimination reactions of alkyl halides can yield one or more alkenes depending on the specific regiochemical and stereochemical considerations. While the regiochemistry of the reaction governs the location of the double bond in the product, the stereochemical requirements often influence the geometry.
When a substrate with two different β hydrogens undergoes an E2 elimination, the presence of a strong base can yield two regioisomeric alkenes. The more-substituted alkene is the major product and...
When a substrate with two different β hydrogens undergoes an E2 elimination, the presence of a strong base can yield two regioisomeric alkenes. The more-substituted alkene is the major product and...
02:23SN2 Reaction: Stereochemistry
In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.
02:35Regioselectivity of Electrophilic Additions-Peroxide Effect
In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.
01:12Stereochemical Effects of Enolization
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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Orbital phase environments and stereoselectivities.
1Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113, Japan, ohwada@mol.f.u-tokyo.ac.jp.
Topics in Current Chemistry
|February 1, 2011
Summary
A new theory, orbital phase environment, explains stereoselectivity in organic reactions. It details how orbital interactions dictate reaction outcomes for nucleophilic additions, electrophilic additions, and Diels-Alder reactions.
Area of Science:
- Organic Chemistry
- Stereochemistry
- Quantum Chemistry
Background:
- Stereoselectivity in organic reactions is crucial for synthesis.
- Existing theories often focus on steric or electronic factors.
- Secondary orbital interactions play a role but lack a unified framework.
Purpose of the Study:
- To propose a new theory, orbital phase environment, for understanding stereoselectivity.
- To generalize the concept of secondary orbital interactions.
- To explain stereochemical outcomes in various organic reactions.
Main Methods:
- Review of facial selectivity in organic reactions.
- Conceptual framework development based on orbital overlap.
- Application of the theory to carbonyl additions, olefin additions, and Diels-Alder reactions.
Main Results:
- The orbital phase environment theory successfully predicts stereoselectivity.
- Nucleophilic additions to carbonyls favor faces opposite to electron-donating β-orbitals.
- Electrophilic additions to olefins and Diels-Alder reactions also show predictable outcomes based on this theory.
Conclusions:
- The orbital phase environment provides a generalized explanation for stereoselectivity.
- This theory unifies the understanding of orbital interactions in diverse organic reactions.
- It offers a new perspective for predicting and controlling reaction outcomes.