Fine-Tuning the Reactivity and Stability by Systematic Ligand Variations in CpCoI Complexes as Catalysts for [2+2+2] Cycloaddition Reactions

CpCoI-olefin-phosphite and CpCoI-bisphosphite complexes were systematically prepared and their reactivity in [2+2+2] cycloaddition reactions compared with highly active [CpCo(H2CCHSiMe3)2] (1). Whereas 1 is an excellent precursor for the synthesis of [CpCo(olefin)(phosphite)] complexes (2af), [CpCo(phosphite)2] complexes (3ae) were prepared photochemically from [CpCo(cod)]. The complexes were evaluated in the cyclotrimerization reaction of diynes with nitriles yielding pyridines. For [CpCo(olefin)(phosphite)], as well as some of the [CpCo(phosphite)2] complexes, reaction temperatures as low as 50 degrees C were sufficient to perform the cycloaddition reaction. A direct comparison showed that the order of reactivity for the complex ligands was olefin2>olefin/phosphite>phosphites2. The complexes with mixed ligands favorably combine reactivity and stability. Calculations on the ligand dissociation from [CpCo(olefin)(phosphite)] proved that the phosphite is dissociating before the olefin. [CpCo(H2CCHSiMe3){P(OPh)3}] (2a) was investigated for the co-cyclization of diynes and nitriles and found to be an efficient catalyst at rather mild temperatures.