Chiral pyridylamino-ruthenium(II) complexes: synthesis, structure and catalytic properties in Diels-Alder reactions

Half-sandwich complexes [(eta(6)-arene) RuCl( pyam)][SbF6] (pyam= L-n = N-( 2- pyridylmethyl)-( R)- 1- phenylethylamine (L-1), N-( 2- pyridylmethyl)-( R)- 1- naphthylethylamine ( L-2), N-( 2- quinolylmethyl)-( R)- 1- naphthylethylamine ( L-3), N-( 2- pyridylmethyl)-( R)- 1- cyclohexylethylamine ( L-4), N-( 2- pyridylmethyl)-( 1R, 2S, 4R)- 1- bornylamine ( L-5)) have been synthetised and characterised. Treatment of these compounds with AgSbF6 generates dicationic complexes [(eta(6)- arene) Ru( pyam)(H2O)](2+) which act as enantioselective catalysts for the Diels- Alder reactions of methacrolein and cyclopentadiene. The catalytic reactions occur quickly at room temperature with good exo : endo selectivity ( from 84 : 16 to 98 : 2) and moderate enantioselectivity ( up to 74% ee). The molecular structures of the chloride complexes (R-Ru, S-N, R-C)-[(eta(6)-p-MeC(6)H(4)iPr) RuClL1][SbF6], (R-Ru, S-N, S-C2)-[(eta(6)- p- MeC(6)H(4)iPr)RuClL5][SbF6], and that of the aqua complex (R-Ru, S-N, S-C2)-[(eta(6)- p- MeC(6)H(4)iPr)RuL5(H2O)][SbF6](2), were determined by X- ray diffractometric methods. The distinctive variations observed in the molecular structures of these complexes only concern the puckering parameters of the metallacycle and the relative disposition of substituents within this ring. A clear trend to localise the most steric demanding substituents at equatorial positions is evident from the structural study.