Buchner ring expansion

The Buchner ring expansion is a two-step organic C-C bond forming reaction used to access 7-membered rings. The first step involves formation of a carbene from ethyl diazoacetate, which cyclopropanates an aromatic ring. The ring expansion occurs in the second step, with an electrocyclic reaction opening the cyclopropane ring to form the 7-membered ring. The Buchner ring expansion is a two-step organic C-C bond forming reaction used to access 7-membered rings. The first step involves formation of a carbene from ethyl diazoacetate, which cyclopropanates an aromatic ring. The ring expansion occurs in the second step, with an electrocyclic reaction opening the cyclopropane ring to form the 7-membered ring. The Buchner ring expansion reaction was first used in 1885 by E. Buchner and T. Curtius who prepared a carbene from ethyl diazoacetate for addition to benzene using both thermal and photochemical pathways in the synthesis of cycloheptatriene derivatives. The resulting product was a mixture of four isomeric carboxylic acids. Variations in the reaction arise from methods of carbene preparation. Advances in organometallic chemistry have resulted in increased selectivity of cycloheptatriene derivatives. In the 1980s it was found that dirhodium catalysts provide single cyclopropane isomers in high yields. Applications are found in medicine (drug syntheses) and material science (fullerene derivatives). Preparation of ethyl-diazoacetate: Buchner's first synthesis of cycloheptatriene derivatives in 1885 used photolysis and thermal conditions to generate the carbene. A procedure for preparation of the hazardous starting material needed for carbene generation in the Buchner reaction, ethyl-diazoacetate, is available in Organic Syntheses. In the procedure provided, Searle includes cautionary instructions due to the highly explosive nature of diazoacetic esters. Preparation of the metal carbenoid: Synthesis of the carbene in the 1960s was focused on using copper catalysts for stereoselective propanation. In the 1980s, dirhodium catalysts have been used to generate the carbenoid for cyclopropanation. The advent of metallochemistry has improved the selectivity of the product ratios of the cyclohexatriene derivatives through choice of ligand on the carbenoid catalyst. Step 1: The reaction mechanism of a Buchner ring expansion begins with carbene formation from ethyl-diazoacetate generated initially through photochemical or thermal reactions with extrusion of nitrogen.