Die präparative Chemie der Cyclobutendione; I. Synthese von Cyclobutendion und dessen Alkyl-, Alkenyl-und Aryl-Derivaten

The particular chemical properties of cyclobutenedione and its derivatives make the cyclobutenedione ring system worthy of special attention. In spite of the ring tension characteristic of four-membered carbocyclic systems, the cyclobutenediones (which may formally be regarded as quinones of the extremely unstable cyclobutadiene) exhibit high thermal stability. This fact has favored the broad investigation of the cyclobutenediones. The preparative results of these investigations are summarized in a tripartite review article. I. Synthesis of Cyclobutenedione and its Alkyl, Alkenyl, and Aryl Derivatives II. Reactions of Alkyl-, Alkenyl-, and Arylcyclobutenediones III. Synthesis of Squaric Acid, Benzocyclobutenedione, and Derivatives thereof In the following Part I of the review, the syntheses of cyclobutenediones from open-chain precursors are described. 1. Cyclobutenediones by Photochemical [2+2] Cycloaddition 1.1. Addition of Alkynes to Alkenes 1.2. Cyclodimerization of Olefinic Compounds (Photodimerization) 2. Cyclobutenediones by Thermal [2+2] Cycloaddition 2.1. Addition of Alkynes to Tetrahaloethylenes 2.2. Addition of Alkynes to Ketenes 2.3. Dimerization of Tetrahaloethylenes 2.4. Thermal [2+2] Cycloaddition of Electron-rich to Electron-poor Ethylene Derivatives 2.5. Thermal Intramolecular Cycloaddition of 1,2,4,5-Hexatetraenes 3. Cyclobutenediones by Ring-Enlargement of Cyclopropene Derivatives 3.1. Ring-Enlargement of 3-Alkoxycarbonyl-3-hydroxycyclopropenes 3.2. Ring-Enlargement by Thermal Cycloaddition of Dichlorocarbene to Cyclopropenes 3.3. Thermal Addition of Isocyanides to Cyclopropenones 4. Diphenylcyclobutenedione from Diphenylacetylene and Tetrakis[arylisocyanide]-nickel 5. Cyclobutenediones from Other Cyclobutenediones 5.1. Halo-arylcyclobutenediones 5.2. 2-Substituted 1-Arylcyclobutenediones from Arylhalocyclobutenediones 5.3. Alkyl- and Arylcyclobutenediones from Squaric Acid Derivatives