Highly chemoselective nickel-catalyzed three-component cross-trimerization between two distinct terminal alkynes and
Kenichi Ogata1, Yuka Atsuumi, Shin-Ichi Fukuzawa
1Department of Applied Chemistry, Institute of Science and Enginnering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan. kogata@kc.chuo-u.ac.jp
Abstract:
The highly chemo-, regio-, and stereoselective three-component cross-trimerization reaction between triisopropylsilylacetylene, diarylacetylene, and a terminal alkyne was achieved by Ni(cod)(2)/P(p-CF(3)C(6)H(4))(3) catalyst at room temperature via selective C-H oxidative addition of the terminal silylacetylene. The reaction is applicable for various diarylacetylenes and terminal alkynes to yield the corresponding 1,3-diene-5-yne compounds.
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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.
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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: 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.
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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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