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Related Concept Videos

Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

2.2K
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
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Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

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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.
2.5K
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.9K
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.
2.9K
Carbocations02:10

Carbocations

13.3K
Carbocations are one of the reaction intermediates formed during several nucleophilic substitutions or elimination reactions. A carbocation is an electron-deficient species with the central carbon atom having six electrons and three bonded atoms. The central carbon in a carbocation is sp2 hybridized with trigonal planar geometry. It has an empty p orbital perpendicular to the plane of the structure that can accept electrons. Thus, carbocations act as strong electrophiles and may react with any...
13.3K
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.5K
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.
2.5K
Aldehydes and Ketones with HCN: Cyanohydrin Formation Overview01:32

Aldehydes and Ketones with HCN: Cyanohydrin Formation Overview

3.8K
Cyanohydrins are compounds that contain –CN and –OH groups on the same carbon atom. They are formed by the nucleophilic addition of the cyanide ions to the carbonyl group. Cyanide ions are highly basic and nucleophilic and can be generated from HCN under aqueous conditions. However, since HCN is a weak acid, the number of cyanide ions generated is very small. Hence, a small amount of base or KCN/NaCN is added to HCN to increase the concentration of the cyanide ions in the reaction...
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Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization
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Deciphering Carbene Reactivity: Ultrafast Spectroscopy and Computational Insights into Photochemical Heterocycle

Yinjiao Zhao1, Yang Cheng1, Shu-Lin Zhang1

  • 1Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, Shaanxi Provincial Key Laboratory of New Concept Sensors and Molecular Materials, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710119, China.

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Researchers explored how solvents and molecular structure control carbene reactivity in photochemical synthesis. They found that solvent choice and substituent variations dictate reaction pathways, enabling precise synthesis of heterocycles.

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Area of Science:

  • Organic Chemistry
  • Photochemistry
  • Reaction Mechanisms

Background:

  • Carbenes are crucial intermediates in synthesizing heterocyclic compounds.
  • Previous methods using diazo compounds and acylsilanes for carbene generation lacked mechanistic clarity regarding solvent and substituent effects.

Purpose of the Study:

  • To elucidate the excited-state dynamics and reaction mechanisms of carbene generation from diazo compounds and acylsilanes.
  • To understand how solvent and structural modifications influence carbene reactivity and product distribution in photochemical synthesis.

Main Methods:

  • Femtosecond transient absorption spectroscopy to probe excited-state dynamics.
  • Density functional theory (DFT) calculations to model reaction pathways and intermediates.
  • Synthesis and photochemical study of a diazo compound (C1) and two acylsilanes (C2, C3).

Main Results:

  • The diazo compound (C1) exhibited spin-state and solvent-dependent reactivity: triplet cyclization in acetonitrile and Wolff rearrangement in ethanol.
  • Acylsilane reactivity was tuned by side-chain variations: C2 yielded a cyclopropane derivative, while C3 produced pyran products via a cascade reaction.
  • Demonstrated control over carbene reaction pathways through solvent selection and structural design.

Conclusions:

  • Solvent and substituent effects are critical levers for controlling carbene reactivity in photochemical heterocycle synthesis.
  • Provides general principles for designing efficient, catalyst-free, and green synthetic routes to valuable heterocyclic scaffolds.