First photochemical envelope isomerization of late-transition-metal 1,3-butdiene complex: a triple stereochemical labeling experiment

Several reports have appeared concerning a fundamentally new thermal isomerization reaction of early-transition-metal eta4-butadiene complexes in which exo and endo substituents at C-1 and/or C-4 exchange positions. A common intermediate that has been proposed in these transformations is the metallacyclopent-3-ene 2 characteristic of a so-called envelope flip process. However, mechanistic evidence for this rare species has been scarce and stepwise pathways, such as those involving (perhaps concerted) reversible butadiene-cyclobutene closures and others, have not been ruled out. The authors report here the first photochemical isomerization of this type, the first to be observed for a late transition metal (e.g., cobalt), and a stereochemical experiment clearly linking simultaneous double exo-endo exchange with a switch of the metal from one π face of the ligand to the other (inversion at the metal). 16 references, 1 table