TANDEM CYCLIZATION CYCLOADDITION REACTION OF RHODIUM CARBENOIDS - STUDIES DEALING WITH INTRAMOLECULAR CYCLOADDITIONS

A series of 5-alkenyl-1-diazo-2,5-pentanediones, when treated with a catalytic quantity of rhodium(II) acetate, were found to give cycloadducts derived from the intramolecular trapping of a carbonyl ylide intermediate. Tethers of three or four methylenes readily enter into intramolecular cycloaddition, while longer and shorter tethers were reluctant to do so. Alkenes attached to the formally cationic terminus of the carbonyl ylide readily undergo internal cycloaddition if the tether allows for a relatively strain-free transition state. The internal cycloaddition reaction does not occur when the olefinic side chain is attached by means of an ester functionality. Bimolecular trapping experiments established that carbonyl ylide formation occurred, but the dipole does not undergo intramolecular cycloaddition. The inability of these alpha-diazo keto esters to undergo internal cycloaddition is related to conformational factors. The equilibrium between the two possible conformations of the dipole lies predominantly on the side of the Z-isomer. In this orientation, intramolecular dipolar cycloaddition cannot occur, and instead the dipole collapses by means of a proton transfer to give an enol ether.