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Alkyne difunctionalization via metal-nitrenoid and metal-carbenoid
Duy-Viet Vo1, Siyuan Su1, Daesung Lee1
1University of Illinois Chicago, 845 W. Taylor Street, Chicago, IL 60607, USA. dsunglee@uic.edu.
None:
The alkyne 1,2-difunctionalization represents a versatile transformation whereby one or both of the π-bonds of the alkyne are engaged in bond-forming processes to generate products that contain newly formed C-C or C-heteroatom bonds at both carbons. Depending on the characteristics of the employed reagents and the bond-forming processes, the C-C bond of the alkyne can serve as a saturated or unsaturated functionality within a cyclic framework or remain acyclic. The nature of the alkyne as a 1,2-dicarbene reveals its inherent propensity to participate in nitrene- and carbene-mediated transformations, especially in metal-associated forms, thereby improving reactivity and selectivity. In this review, representative 1,2-difunctionalization reactions involving metal-nitrenoids and metal-carbenoids (examples of 1,1-difunctionalization with copper-carbenoid) are described. The contents are organized into two main sub-parts, metal-nitrenoids and metal-carbenoids, and each part follows the order of the atomic number. This review is not intended to be comprehensive, but rather to provide an overview of the reactivity profiles of alkynes as a source or counterpart in nitrene- and carbene-mediated reactions.
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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.
02:34Preparation 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.
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.