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

Further investigation has been made of the reactions between the cycli c diazo compound 2-diazo-1,3-cyclohexanedione and aromatic heterocycle s or vinyl ethers, catalyzed by rhodium carboxylates. The extraordinar y reactivity of the carbenoid derived from this diazo compound is show n by its ready reaction with solvents such as dichloromethane, dichlor oethane, and fluorobenzene. Detailed investigation of its reactions wi th furans have shown that steric interactions dominate, both in terms of regioselectivity with unsymmetrical substrates and yield. This reac tion 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-di polar cycloaddition. Products of net C-H insertion and with reverse re giochemistry (furo[3,2-b]furan ring system) were also detected. With p yrroles and thiophenes, cycloadducts were seen in a few cases, but wer e generally the exception; C-H insertion products dominate these react ions. Vinyl ethers proved reliable reactants in providing dipolar cycl oadducts. 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 cycl opropane bonds would give a zwitterion. The partitioning of such a zwi tterion between ring closure and proton transfer would define the rati o of C-H insertion and dipolar cycloaddition products. Both thermodyna mic and stereoelectronic arguments have been advanced to explain the o bservations and were supported by calculations.