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的三组分光化学1,2,5-三功能化到密集功能化的钉

  • 0Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstraße 36, 48149 Münster, Germany.

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相关概念视频

Photochemical Electrocyclic Reactions: Stereochemistry 01:26

1.8K

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

2.1K

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...

Aldehydes and Ketones to Alkenes: Wittig Reaction Mechanism 01:14

3.5K

The Wittig reaction, which converts aldehydes or ketones to alkenes using phosphorus ylides, proceeds through a nucleophilic addition‒elimination process.
The reaction begins with the nucleophilic addition between a phosphorus ylide and the carbonyl compound. Due to its carbanionic character,  phosphorus ylide acts as a strong nucleophile and attacks the electrophilic carbonyl group. This generates a charge-separated dipolar intermediate called betaine. The negatively charged oxygen atom and...

Alkenes via Reductive Coupling of Aldehydes or Ketones: McMurry Reaction 01:22

1.9K

The radical dimerization of ketones or aldehydes gives vicinal diols through a pinacol coupling reaction. However, the behavior of titanium metals used for the reaction as a source of electrons is unusual. When the reaction is carried out in the presence of titanium, diols can be isolated at low temperatures. Else titanium further reacts with diols, forming alkenes through the McMurry reaction.

The reaction is a two-step process. The mechanism is still under study, but for some reagent...

Thermal and Photochemical Electrocyclic Reactions: Overview 01:26

2.3K

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...

Aldehydes and Ketones to Alkenes: Wittig Reaction Overview 01:19

7.8K

The Wittig reaction is the conversion of carbonyl compounds-aldehydes and ketones-to alkenes using phosphorus ylides, or the Wittig reagent. The reaction was pioneered by Prof. Georg Wittig, for which he was awarded the Nobel Prize in Chemistry.

Phosphorus ylide is a neutral molecule containing a negatively charged carbon directly bonded to a positively charged phosphorus atom. The molecule is stabilized by resonance.

The Wittig reagents are synthesized from unhindered alkyl halides in two...