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Transition Metal-Catalyzed Dual C-H Activation/Annulation Reactions Involving Internal Alkynes
Fatemeh Doraghi1, Mohammad Sadegh Karimtabar1,2, Mehran Ghasemi3
1Endocrinology and Metabolism Research Center, Endocrinology and Metabolism Clinical Sciences Institute, Tehran University of Medical Sciences, Tehran, Iran.
Transition metal catalysis enables efficient synthesis of complex polycyclic aromatic compounds using internal alkynes. These dual C-H activation/annulation reactions offer a powerful, economical strategy for creating valuable molecules.
Area of Science:
- Organic Chemistry
- Catalysis
- Synthetic Methodology
Background:
- Transition metal-catalyzed reactions are crucial for synthesizing complex organic molecules.
- Ortho-C-H bond activation/annulation offers efficient routes to polycyclic aromatic scaffolds.
- Internal alkynes are versatile building blocks in organic synthesis.
Purpose of the Study:
- To review recent advancements in dual C-H bond activation/annulation reactions.
- To highlight the synthesis of highly substituted polycyclic aromatic scaffolds.
- To discuss functionalization reactions involving diaryl/alkyl alkynes.
Main Methods:
- Focus on transition metal-catalyzed reactions.
- Utilizing dual C-H bond activation strategies.
- Employing internal alkyne coupling partners.
Main Results:
- Demonstration of efficient synthesis of polycyclic aromatic scaffolds.
- Highlighting atom and step-economical synthetic approaches.
- Showcasing the versatility of diaryl/alkyl alkynes in annulation reactions.
Conclusions:
- Dual C-H activation/annulation represents a powerful strategy for molecular assembly.
- These methods provide access to multifunctional bioactive molecules.
- Continued development in this area promises further synthetic innovations.
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02:24Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
02:47Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
02:40Alkynes to Aldehydes and Ketones: Acid-Catalyzed Hydration
Analogous to alkenes, alkynes also undergo acid-catalyzed hydration. While the addition of water to an alkene gives an alcohol, hydration of alkynes produces different products such as aldehydes and ketones.
02:38Electrophilic Addition to Alkynes: Halogenation
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
02:27Preparation of Alkynes: Alkylation Reaction
Alkylation of terminal alkynes with primary alkyl halides in the presence of a strong base like sodium amide is one of the common methods for the synthesis of longer carbon-chain alkynes. For example, treatment of 1-propyne with sodium amide followed by reaction with ethyl bromide yields 2-pentyne.
02:34Preparation of Alkynes: Dehydrohalogenation
Alkynes can be prepared by dehydrohalogenation of vicinal or geminal dihalides in the presence of a strong base like sodium amide in liquid ammonia. The reaction proceeds with the loss of two equivalents of hydrogen halide (HX) via two successive E2 elimination reactions.