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Published on: May 21, 2019
Phosphine-Promoted Photoredox [3+2] Cycloaddition of Alkynes and Isocyanides
Yihong Li1, Yutao Shi1, Xiaochen Ji1
1Key Laboratory of Environmentally Friendly Chemistry and Application of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China.
Abstract:
While the [3+2] cycloaddition of alkynes and isocyanides has been established on the basis of transition metal catalysis, a photochemical protocol is alternatively disclosed in the present work. This novel phosphine-promoted photoredox catalysis enables the regioselective formation of 2,4-disubstituted pyrroles with broad functional group tolerance under mild conditions. Mechanistically, the ready addition of the phosphine radical cation formed via photoredox catalysis to alkynes is the key to initiating the cycloaddition with isocyanides.
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Cycloaddition Reactions: Overview
Reduction 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.
Cycloaddition Reactions: MO Requirements for Photochemical Activation
Electrophilic 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.
Alkynes 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.
Electrophilic Addition to Alkynes: Hydrohalogenation

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