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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. and material science (fullerene derivatives).
The Buchner ring expansion reaction was first used in 1885 by Eduard Buchner and Theodor 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.
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.
Synthesis of the carbene in the 1960s was focused on using copper catalysts for stereoselective cyclopropanation. 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.
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.
The generated carbene adds to one of the double bonds of benzene to form the cyclopropane ring.
The advent of transition metal catalyzed reagents provides alternative stereospecific methods for cyclopropanation. The choices for metals include Cu, Rh and Ru with a variety of ligands. The use of rhodium catalysts in the Buchner reaction for carbene generation reduces the number of products by producing predominantly the kinetic cycloheptatrienyl esters. Product mixtures of Buchner reactions resulting from thermal Rhodium II-catalysts are less complicated. Wyatt et al. have studied the regioselectivity of the thermal Buchner reaction using Rh2(O2CCF3)4 and demonstrated that the electrophilic character of the rhodium carbene prefers reaction at the more nucleophilic π-bonds of the aromatic ring.
The accepted carbene catalytic cycle was proposed by Yates in 1952. Initially the diazo compound oxidatively adds to the metal ligand complex. Following the extrusion of nitrogen the metal carbene is generated and reacts with an electron rich aromatic substance to reductively regenerate the metal catalyst completing the catalytic cycle.
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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. and material science (fullerene derivatives).
The Buchner ring expansion reaction was first used in 1885 by Eduard Buchner and Theodor 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.
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.
Synthesis of the carbene in the 1960s was focused on using copper catalysts for stereoselective cyclopropanation. 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.
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.
The generated carbene adds to one of the double bonds of benzene to form the cyclopropane ring.
The advent of transition metal catalyzed reagents provides alternative stereospecific methods for cyclopropanation. The choices for metals include Cu, Rh and Ru with a variety of ligands. The use of rhodium catalysts in the Buchner reaction for carbene generation reduces the number of products by producing predominantly the kinetic cycloheptatrienyl esters. Product mixtures of Buchner reactions resulting from thermal Rhodium II-catalysts are less complicated. Wyatt et al. have studied the regioselectivity of the thermal Buchner reaction using Rh2(O2CCF3)4 and demonstrated that the electrophilic character of the rhodium carbene prefers reaction at the more nucleophilic π-bonds of the aromatic ring.
The accepted carbene catalytic cycle was proposed by Yates in 1952. Initially the diazo compound oxidatively adds to the metal ligand complex. Following the extrusion of nitrogen the metal carbene is generated and reacts with an electron rich aromatic substance to reductively regenerate the metal catalyst completing the catalytic cycle.