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Updated: Oct 21, 2025
Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
Published on: June 21, 2017
Selective difunctionalization of electron-deficient alkynes: access to (E)-2-iodo-3-(methylthio)acrylate
Ling-Hui Lu1, Guangchuan Ou1, Xiongjie Zhao1
1College of Chemistry and Bioengineering, Hunan University of Science and Engineering, Yongzhou 425100, China. chwu_chem@yeah.net.
Abstract:
A convenient and efficient approach to (E)-2-iodo-3-(methylthio)acrylate has been developed through direct iodothiomethylation of alkynes with aqueous HI and DMSO under mild conditions. This novel protocol has demonstrated a unique difunctionalization of electron-deficient alkynes with a broad substrate scope and excellent functional-group tolerance. Preliminary mechanistic studies indicated that prior diiodination of alkynes, followed by nucleophilic substitution with in situ generated DMS led to the formation of (E)-2-iodo-3-(methylthio)acrylate.
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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.
02:38Electrophilic 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.
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