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Published on: May 16, 2014
Alkynes and azides: beyond click chemistry
Christina N Wiswell1, Mitchell P Croatt1
1Chemistry and Biochemistry, The University of North Carolina at Greensboro, Greensboro, North Carolina, USA. mpcroatt@uncg.edu.
Abstract:
The relatively weak π-bonds in alkynes and azides present a unique opportunity to more easily access high energy reactive intermediates. This opportunity has been harnessed through copper-catalyzed azide-alkyne cycloadditions (CuAAC), one of the well-known reactions associated with "click chemistry." This review covers the additional chemistry of alkynes and azides that is beyond the CuAAC realm. This includes cascades and interconversions of reactive intermediates that can be accessed through mild means, due to the combination of weak bonds in both reactants. The products of these reactions, often heterocycles, are core structures of molecules of interest and the processes to obtain the products are of mechanistic value. Thus, the chemistry of alkynes and azides included herein is a valuable addition to the arsenal of synthetic chemists.
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01:22Preparation of 1° Amines: Azide Synthesis
Azide ions act as good nucleophiles and react with unhindered alkyl halides to form alkyl azides. Alkyl azides do not participate in further nucleophilic substitution reactions, thereby eliminating the chances of polyalkylated products. Alkyl azides are reduced by hydride-based reducing agents, like lithium aluminum...
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.
02:34Preparation 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.
02:38Electrophilic 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.
02:40Alkynes 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.