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The periselectivity of a particular reaction depends on the structure of both the ketene and the substrate. Although the reaction is predominantly used to form four-membered rings, a limited number of substrates undergo [3+2] or [4+2] reactions with ketenes. Examples of all three modes of cycloaddition are discussed in this section.
An alkyne trimerisation is a [2+2+2] cycloaddition reaction in which three alkyne units (C≡C) react to form a benzene ring. The reaction requires a metal catalyst. The process is of historic interest as well as being applicable to organic synthesis. [1] Being a cycloaddition reaction, it has high atom economy.
In organic chemistry, enone–alkene cycloadditions are a version of the [2+2] cycloaddition. This reaction involves an enone and alkene as substrates. Although the concerted photochemical [2+2] cycloaddition is allowed, the reaction between enones and alkenes is stepwise and involves discrete diradical intermediates. [1]
For example, an analog for the Diels-Alder reaction is the quadricyclane-DMAD reaction: In the (i+j+...) cycloaddition notation i and j refer to the number of atoms involved in the cycloaddition. In this notation, a Diels-Alder reaction is a (4+2)cycloaddition and a 1,3-dipolar addition such as the first step in ozonolysis is a
The Staudinger synthesis, also called the Staudinger ketene-imine cycloaddition, is a chemical synthesis in which an imine 1 reacts with a ketene 2 through a non-photochemical 2+2 cycloaddition to produce a β-lactam 3. [1] The reaction carries particular importance in the synthesis of β-lactam antibiotics. [2]
While not a classic Diels-Alder reaction in the typical sense of a conjugated diene and a separate dienophile, Staudinger's observation of this [4+2] process, forming a six-membered ring, foreshadowed the later work of Diels and Alder. However, his focus remained primarily on the more common [2+2] ketene cycloaddition.
The Paternò–Büchi reaction, named after Emanuele Paternò and George Büchi, who established its basic utility and form, [1] [2] is a photochemical reaction, specifically a 2+2 photocycloaddition, which forms four-membered oxetane rings from an excited carbonyl and reacting with an alkene. [3]
Scheme of the Strain-promoted Azide-Alkyne Cycloaddition. This reaction proceeds as a concerted [3+2] cycloaddition to the triple bond in a cyclooctyne in the same mechanism as the Huisgen 1,3-dipolar cycloaddition. Substituents other than fluorines, such as benzene rings, are also allowed on the cyclooctyne.