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

The enantioselective intramolecular cyclopropanation of cis-substituted allylic diazoacetates catalyzed by the chiral ruthenium Schiff base complexes [Ru(Schiff base)(PPh3)2] (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 = I) 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(p-YC6H4)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, H2(2,4-Br-salen) = bis(3,5-dibromosalicylidene)-(1R,2R)-cyclohexanediamine), respectively. X-ray crystal structure determinations revealed RuC(carbene) distances of 1.921(12) Å for 3a, 1.913(5) Å for 3b, 1.919(14) Å for 3c, 1.910(2) Å for 3d, and 1.917(4) Å 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.