Cyclopropanation of styrene with ethyl diazoacetate catalyzed by chiral and achiral ruthenium 2,6-bis(imino)pyridyl complexes

The optically pure 2,6-bis(imino)pyridyl ligands (R)-2,6-bis{1-[alpha-methy lbenzylimine]ethyl}pyridine [(R)-2,6-py(NCHMePh)(2)] and (S)-2,6-bis{1-(1-n aphthyl)ethylimine]ethyl}pyridine [(S)-2,6-py(NCHMeNaph)(2)] have been prep ared by condensation of 2,6-diacetylpyridine with (R)-(+)-alpha-methylbenzy lamine and (S)-(-)-1-(1-naphthyl)ethylamine, respectively. These two ligand s and others bearing different substituents on the imine nitrogen atoms (i. e., C-C6H11, C6H5, 2,6-Me2Ph) form, in combination with ruthenium(II) fragm ents, efficient catalysts for the cyclopropanation of styrene with ethyl di azoacetate (EDA). The catalyst precursors have the general formula RuCl2(PP h3){2,6-py(NR)(2)} (R = Cy, 1; Ph, 6; CHMeNaph, 7; CHMePh, 8). It is genera lly found that the catalytic activity increases with the size of the substi tuents on the imine nitrogen atoms. Consistently, the solvento complex RuCl 2{2,6-py(N(2,6-Me2Ph2)(2)}. CH2Cl2 is the most efficient in the series. Bes t results in terms of activity, diastereoselectivity, and enantioselectivit y have been obtained with the chiral precursor 7. In the presence of AgPF6 as cocatalyst, a remarkable improvement in both productivity and chemoselec tivity of the cyclopropanation reactions was observed with the catalysts 1 and 6, whereas a substantial decrease in enantioselectivity occurred with t he 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 kin etic product, cis-RuCl2(=CHCO2Et){2,6-py(NCy)(2)}, was intercepted at low t emperature. Compound trans-3 was also obtained by reaction of the ethylene complex RuCl2(eta(2)-H2C=CH2)[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)p yridyl six-coordinate complexes toward either EDA alone or EDA/styrene mixt ures suggests that the carbene transfer from EDA to the olefin is apparentl y mediated by Ru(II) carbene species in an intermolecular fashion. This mec hanistic view may not be true for the reactions performed in the presence o f a halide scavenger.