||Combined Experimental and Computational Study on Ruthenium(II)-Catalyzed Reactions of Diynes with Aldehydes and N,N‑Dimethylformamide
Yoshihiko, Yamamoto ,
Yuta, Okude ,
Shota, MoriMasatoshi, Shibuya
Journal of Organic Chemistry
7973 , 2017-08-04 , ACS Publications
Cycloaddition reactions of 1,6-diynes bearingmethyl terminal groups with p-anisaldehyde were conductedusing a cationic ruthenium catalyst with a η^5-pentamethylcyclopentadienylligand in THF at room temperature to afforddienyl ketones via ring opening of the initially formed fusedpyrans. (Z)-Stereoisomers of dienyl ketones were selectivelyobtained using the ruthenium catalyst, whereas previously reported rhodium catalysts produced (E)-isomers. These (E)- and(Z)-selectivities are kinetically controlled as the control experiments showed that the E/Z-isomerization of (E)-dienylketoneoccurs at 70 °C for 10 h to afford an E/Z-ratio of almost 1:1. The origin of this characteristic stereoselectivity for the rutheniumcatalyst was attributed to the direct ring opening of the CpRu^+-coordinated pyran complex intermediates on the basis oftheoretical calculations [PCM (THF) M06L/SDD-6-311++G(d,p)//B3LYP/LanL2DZ-6-31G(d)] and control experiments.The (Z)-selectivity increased when the bulkiness of the diyne terminal substituents increased. Notably, the reaction of 1,6-diynesbearing tert-butyl terminal groups with various α,β-unsaturated aldehydes exclusively afforded (Z)-dienyl ketones even at 70 °Cwhen a cationic ruthenium complex with a smaller η^5-cyclopentadienyl (Cp) ligand was used as the catalyst. The same Cpcomplex was found to be also efficient for the hydrocarbamoylative cyclization of sterically demanding 1,6-diynes bearing tertiaryor quaternary carbon tethers with N,N-dimethylformamide.