Highly functionalized five-membered carbocycles from (3-dialkylamino-1-ethoxyalkenylidene)pentacarbonylchromium complexes and alkynes: The effects of substituents, solvents, ligand additives, and reagent concentrations on the product distribution

The cocyclization reaction of pentacarbonyl(β-amino-1-ethoxyalkenylidene)chromium complexes I with alkynes has been studied with respect to the effects of substituents, solvents, ligand additives, and reagent concentrations upon the product distribution. This reaction proceeds either as a formal [2 + 2 + 1] cycloaddition to give 5-(1′-dialkylaminoalkylidene)-4-ethoxycyclopent-2-enones 8 or a formal [3 + 2] cycloaddition to give 5-dialkylamino-3-ethoxy-1,3-cyclopentadienes 9. A working hypothesis for the mechanism of this reaction is proposed on the basis of that previously determined for the Dötz reaction. The effects of the aforementioned parameters upon the product distribution of this current reaction are explained in terms of this model. A pronounced ligand-induced allochemical effect has been observed. Conditions for the selective preparation of both classes of cycloadducts 8 and 9 have been determined.