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Updated: May 17, 2025
Metal-free Synthesis of Ynones from Acyl Chlorides and Potassium Alkynyltrifluoroborate Salts
Published on: February 24, 2015
Silver- and Base-Enabled SOMOphilic Alkynylation of Alkylboronic Acids with Bromoalkynes
Wenqian Du1, Rongjie Yang1, Jinhong Bai2
1School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China.
Abstract:
Alkynyl-containing molecules hold significant importance in organic synthesis, drug discovery, and materials science. Herein, we report an efficient protocol for the C(sp)-C(sp3) cross-coupling of bromoalkynes with primary, secondary, and tertiary alkylboronic acids. Mechanistic experiments suggest that silver and base enable the generation of an alkyl radical intermediate from the alkylboronic acid and then undergo a cascade of α-addition and β-elimination with alkynyl bromides, leading to the construction of C(sp)-C(sp3) bonds. This strategy features readily available starting materials, diverse substrates, and mild and easily handled reaction conditions.
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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.
02:42Acidity 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.
01:27Radical Substitution: Allylic Bromination
00:51Base-Promoted α-Halogenation of Aldehydes and Ketones
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.
02:46Halogenation of Alkenes
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.