Enantioselective Intramolecular Cyclopropanation of cis-Alkenes by Chiral Ruthenium(II) Schiff Base Catalysts and Crystal Structures of (Schiff base)ruthenium Complexes Containing Carbene, PPh3, and CO Ligands

Abstract

The enantioselective intramolecular cyclopropanation of cis-substituted allylic diazoacetates catalyzed by the chiral ruthenium Schiff base complexes [RU(Schiff base)(PPh(3)h] (1) is described. Among this class of complexes examined, [Ru(2-Br-salen)(PPh3)(2)] (1a) is the most effective, catalyzing intramolecular cyclopropanation of cis-allylic diazoacetates cis-(CRH=CH)CH2OC(O)CHN2, (R = alkyl, aryl) in CHCl3 solution to give [3.1.0.]-bicyclic lactones with yields and ee values up to 71 and 90%, respectively. The analogous reactions of cis-alkenyl diazoacetates using [Ru(Schiff base)(CO)] (2) as catalyst gave comparable enantioselectivities (up to 91% ee) but lower product yields of 20-38%. Treatment of [Ru(2,4-X-salen)(PPh3)(2)] (1d, X = Br; 1e, X = Cl; 1f, X = 1) with N2C(p-YC6H4)(2) (Y = H, MeO) and N-methylimidazole (MeIm) or pyridine (py) gave the monocarbene complexes [Ru(2,4-X-salen)(C(pYC(6)H(4))(2))(MeIm)] (3a, X = Br, Y = H; 3b, X = Cl, Y = H; 3c, X = I, Y = H; 3d, X = Br, Y = OMe) and [Ru(2,4-Br-salen)(CPh2)(py)] (4, H-2(2,4-Br-salen) = bis(3,5-dibromosalicylidene)-(1R,2R)-cyclo-hexanediamine), respectively. X-ray crystal structure determinations revealed Ru=C(carbene) distances of 1.921(12) angstrom for 3a, 1.913(5) angstrom for 3b, 1.919(14) angstrom for 3c, 1.910(2) angstrom for 3d, and 1.917(4) angstrom for 4. A comparison of the structures and electrochemistry of 1, [Ru(Schiff base) (CO) (MeIm)], 3, and 4 with those of the porphyrin analogues is presented.