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

The cocyclization reaction of pentacarbonyl(beta -amino-1-ethoxyalkenyliden e)chromium complexes 1 with alkynes has been studied with respect to the ef fects of substituents, solvents, ligand additives, and reagent concentratio ns upon the product distribution. This reaction proceeds either as a formal [2 + 2 + 1] cycloaddition to give 5-(1'-dialkylaminoalkylidene)-4-ethoxycy cloopent-2-enous 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 t he Dotz reaction. The effects of the aforementioned parameters upon the pro duct distribution of this current reaction are explained in terms of this m odel. A pronounced ligand-induced allochemical effect has been observed. Co nditions for the selective preparation of both classes of cycloadducts 8 an d 9 have been determined.