REACTION OF A CYCLIC RHODIUM CARBENOID WITH AROMATIC-COMPOUNDS AND VINYL ETHERS

Further investigation has been made of the reactions between the cyclic diazo compound 2-diazo-1,3-cyclohexanedione and aromatic heterocycles or vinyl ethers, catalyzed by rhodium carboxylates. The extraordinary reactivity of the carbenoid derived from this diazo compound is shown by its ready reaction with solvents such as dichloromethane, dichloroethane, and fluorobenzene. Detailed investigation of its reactions with furans have shown that steric interactions dominate, both in terms of regioselectivity with unsymmetrical substrates and yield. This reaction provides a useful entry to the furo[2,3-b]furan ring system found in a number of naturally-occurring compounds and is formally a 1,3-dipolar cycloaddition. Products of net C-H insertion and with reverse regiochemistry (furo[3,2-b]furan ring system) were also detected. With pyrroles and thiophenes, cycloadducts were seen in a few cases, but were generally the exception; C-H insertion products dominate these reactions. Vinyl ethers proved reliable reactants in providing dipolar cycloadducts. The results of this study have been interpreted in terms of four pathways: an initial cyclopropanation would produce a spirocyclic dicarbonyl system that on heterolytic cleavage of one of the two cyclopropane bonds would give a zwitterion. The partitioning of such a zwitterion between ring closure and proton transfer would define the ratio of C-H insertion and dipolar cycloaddition products. Both thermodynamic and stereoelectronic arguments have been advanced to explain the observations and were supported by calculations.