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Updated: Jun 12, 2025
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Intermolecular 1,2-Aminoboration of Alkynes and the Critical Role of Electron-Rich Alkynes
Simon Dotzauer1,2, Arumugam Jayaraman1,2,3, David Reinhart1,2
1Institute for Inorganic Chemistry, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074, Würzburg, Germany.
Abstract:
The intramolecular 1,2-aminoboration of alkynes by aminoboranes is rare and invariably requires a catalyst to proceed, while the intermolecular aminoboration of alkynes is yet entirely unknown. Through an exploration of the significance of electronics in alkynes for activating the B-N σ-bond of aminoboranes, we demonstrate in this work the first intermolecular 1,2-aminoboration of alkynes. These reactions employ a series of (amino)dihaloboranes and aminoboronic esters, mild reaction conditions, and no catalysts, yielding syn-addition alkene products with the incorporation of two crucial functionalities: amino and boryl. While highly electron-rich examples can afford the aminoborated products (Z)-2-borylethenamines, other alkynes, including unactivated and less electron-rich examples, do not lead to the corresponding aminoborated products due to the fundamental impediment that the reactions are significantly endergonic.
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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.
02:47Alkynes 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.
02:27Preparation 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.
02:42Acidity of 1-Alkynes
The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.
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.
02:40Alkynes 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.