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非対称なコントラニオン誘導フォトレドックス触媒

  • 0Max-Planck-Institut für Kohlenforschung, D-45470 Mülheim an der Ruhr, Germany.
Clinical Neuroscience (new York, N.y.) +

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2023年1月19日

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Reduction of Alkenes: Asymmetric Catalytic Hydrogenation 02:17

3.4K

Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...

Photochemical Electrocyclic Reactions: Stereochemistry 01:26

1.9K

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

Cycloaddition Reactions: MO Requirements for Photochemical Activation 01:12

2.2K

Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.

Thermally-induced [2 + 2] cycloadditions are symmetry forbidden. This is because the ground state HOMO of one ethylene molecule and the LUMO of the other ethylene are out of phase, preventing a concerted suprafacial-suprafacial overlap.
Absorption...

Thermal and Photochemical Electrocyclic Reactions: Overview 01:26

2.4K

Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.

Electrocyclic reactions are highly stereospecific. For a substituted polyene, the stereochemical outcome...

Woodward–Hoffmann Selection Rules and Microscopic Reversibility 01:34

3.2K

Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...

Oxidation of Alkenes: Syn Dihydroxylation with Osmium Tetraoxide 02:44

10.6K

Alkenes are converted to 1,2-diols or glycols through a process called dihydroxylation. It involves the addition of two hydroxyl groups across the double bond with two different stereochemical approaches, namely anti and syn. Dihydroxylation using osmium tetroxide progresses with syn stereochemistry.

Syn Dihydroxylation Mechanism
The reaction comprises a two-step mechanism. It begins with the addition of osmium tetroxide across the alkene double bond in a concerted manner forming a...