First Stepwise Dötz Reaction: Isolation and Characterization of a Chelated Metallatriene Intermediate. Influence of Its Pattern of Substitution in Product Partition

A series of chelated tetracarbonyl aminovinyl carbene complexes 5 are prepared by high-yielding decarbonylation reaction. γ-Alkyl-substituted complexes 5a − d are stable in solid form while γ-aryl-substituted analogues 5e − g are stable only in solution. NMR studies determined that complexes 5 undergo a dynamic process in solution, in which the unsaturated η 1 species are present in low concentration. Reaction of complexes 5 with electron-deficient alkynes under controlled conditions afforded the 1,4,5-η 3 -dienylcarbene complexes 9 arising from the insertion of the alkyne in the metal−carbon bond. The stability and behavior of these complexes were studied and found to be related to their electronic structure and pattern of substitution. Thermal decomposition of carbenes 9 produced either cyclopentadienes 13 or phenol derivatives 15, the major products in the Dötz reaction, thus demonstrating that they can be the actual intermediates in this reaction. The overall process can be considered a Dötz reaction carried out in three separate steps at low temperature.