Cyclopropanation of Styrene with Ethyl Diazoacetate Catalyzed by Chiral and Achiral Ruthenium 2,6-Bis(imino)pyridyl Complexes

Abstract

The optically pure 2,6-bis(imino)pyridyl ligands (R)-2,6-bis{1-[α-methylbenzylimine]ethyl}pyridine [(R)-2,6-py(NCHMePh)2] and (S)-2,6-bis{1-(1-naphthyl)ethylimine]ethyl}pyridine [(S)-2,6-py(NCHMeNaph)2] have been prepared by condensation of 2,6-diacetylpyridine with (R)-(+)-α-methylbenzylamine and (S)-(−)-1-(1-naphthyl)ethylamine, respectively. These two ligands and others bearing different substituents on the imine nitrogen atoms (i.e., c-C6H11, C6H5, 2,6-Me2Ph) form, in combination with ruthenium(II) fragments, efficient catalysts for the cyclopropanation of styrene with ethyl diazoacetate (EDA). The catalyst precursors have the general formula RuCl2(PPh3){2,6-py(NR)2} (R = Cy,1; Ph, 6; CHMeNaph, 7; CHMePh, 8). It is generally found that the catalytic activity increases with the size of the substituents on the imine nitrogen atoms. Consistently, the solvento complex RuCl2{2,6-py(N(2,6-Me2Ph2)2}·CH2Cl2 is the most efficient in the series. Best results in terms of activity, diastereoselectivity, and enantioselectivity have been obtained with the chiral precursor 7. In the presence of AgPF6 as cocatalyst, a remarkable improvement in both productivity and chemoselectivity of the cyclopropanation reactions was observed with the catalysts 1 and 6, whereas a substantial decrease in enantioselectivity occurred with the chiral precursors 7 and 8. The carbene complex trans-RuCl2(CHCO2Et){2,6-py(NCy)2} (trans-3) has been isolated upon reaction of 1 with EDA. A kinetic product, cis-RuCl2(CHCO2Et){2,6-py(NCy)2}, was intercepted at low temperature. Compound trans-3 was also obtained by reaction of the ethylene complex RuCl2(η2-H2CCH2)[2,6-py(NCy)2] with EDA. Treatment of trans-3 with AgPF6 led to the formation of the unsaturated complex [RuCl(CHCO2Et){2,6-py(NCy)2}]PF6. The overall reactivity of the Ru(II) 2,6-bis(imino)pyridyl six-coordinate complexes toward either EDA alone or EDA/styrene mixtures suggests that the carbene transfer from EDA to the olefin is apparently mediated by Ru(II) carbene species in an intermolecular fashion. This mechanistic view may not be true for the reactions performed in the presence of a halide scavenger.