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

Gong Yong Li, Jing Zhang, Philip Wai Hong Chan, Zhen Jiang Xu, Nianyong Zhu, Chi Ming Che

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Abstract

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] (la) is the most effective, catalyzing intramolecular cyclopropanation of c/s-allylic diazoacetates cis-(CRH=CH)CH2OC(O)CHN2 (R - alkyl, aryl) in CHCI3 solution to give [3.1.0]-bicyclic lactones with yields and ee values up to 71 and 90%, respectively. The analogous reactions of c/s-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)-cyclo-hexanediamine), 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.

Original languageEnglish
Pages (from-to)1676-1688
Number of pages13
JournalOrganometallics
Volume25
Issue number7
DOIs
Publication statusPublished - 27 Mar 2006
Externally publishedYes

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