Cross-cyclotrimerization with two nitriles as a synthetic pathway to unsymmetrically 3,3'-disubstituted bis(tetrahydroisoquinolines)

Fine-tuning the reactivity and stability by systematic ligand variations in CpCo(I) complexes as catalysts for [2+2+2] cycloaddition reactions

CpCo(I)-olefin-phosphite and CpCo(I)-bisphosphite complexes were systematically prepared and their reactivity in [2+2+2] cycloaddition reactions compared with highly active [CpCo(H(2)C=CHSiMe(3))(2)] (1). Whereas 1 is an excellent precursor for the synthesis of [CpCo(olefin)(phosphite)] complexes (2 a-f), [CpCo(phosphite)(2)] complexes (3 a-e) 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 °C were sufficient to perform the cycloaddition reaction. A direct comparison showed that the order of reactivity for the complex ligands was olefin(2)>olefin/phosphite>phosphites(2). 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(H(2)C=CHSiMe(3)){P(OPh)(3)}] (2 a) was investigated for the co-cyclization of diynes and nitriles and found to be an efficient catalyst at rather mild temperatures.