Chiral Ruthenium(II)–Bis(2-oxazolin-2-yl)pyridine Complexes. Asymmetric Catalytic Cyclopropanation of Olefins and Diazoacetates

Abstract

A chiral ruthenium(II)–bis(2-oxazolin-2-yl)pyridine catalyst prepared in situ from optically active bis(2-oxazolin-2-yl)pyridine (Pybox-ip) (2) and [RuCl2(p-cymene)]2 (1) exhibited efficient activity for the asymmetric cyclopropanation (ACP) of styrene and several diazoacetates to give the corresponding trans- and cis-2-phenylcyclopropane-1-carboxylates (3 and 4) in good yields (66—87%). A mixture of 1 and 2 in an atmosphere of ethylene produced the trans-RuCl2(Pybox-ip)(ethylene) complex (5), which also proved to be a powerful catalyst for ACP. The stereoselectivity of the trans- and cis-cyclopropanes were from 90 : 10 up to 98 : 2, and their enantioselectivities reached 97%. A catalytic system with 5 could be used for several olefins and internal olefins. A concerted mechanism of ACP with the Ru–Pybox catalyst was postulated on the basis of the stereospecificity with deuterated styrene. Other substituents, such as ethyl, s-butyl, benzyl, and phenyl on the oxazoline rings of Pybox in place of the isopropyl group were also examined, and were found to exhibit a similar high trans-stereoselection and a high enantioselectivity of the cyclopropane products; ex. with Ru–Pybox-ethyl 91 : 9 of 3 : 4 and 82% ee for 3. The non-chiral Ru–Pybox-dihydro catalyst exhibited asymmetric induction with 39% ees of 3 with d- and l-menthyl diazoacetates, and kept a high trans- and cis-stereoselection of 97 : 3.

Asymmetric Cyclopropanation of Styrene Catalyzed by Cu–(Chiral Schiff-Base) Complexes

“…These results are comparable to the results with the most efficient catalysts reported to date, including Cu-semicorrin, Cu-bisoxazoline, Cu-bipyridine, etc. [10][11][12][13][14][15][16][17][18][19] Furthermore, when one or two chlorine atoms or nitro groups are introduced, the ligands formed are easy to purify due to their high melting points. A recrystallization is sufficient to purify them.…”

Chiral Bis(dihydrooxazolyl)pyridineruthenium Complexes oftrans-Cyclooctene andtrans-Cycloheptene

“…A mixture of the complexes of (À)-(R)-and ()-(S)-cyclooctene (1 R and 1 S) was obtained under the same reaction conditions as those for pybox-ip: with pybox-me, 6 R 6 S (85 % after column chromatography, 76:24), 1 S (97 %, 25 % ee); pybox-bz, 7 R 7 S (87 % after column chromatography, 78:22), 1 S (68 %, 31 % ee); the olefinic protons: Alternatively, we employed [RuCl 2 (pybox-ip)(C 2 H 4 )] (8) [10] (0.2 mmol) as the starting ruthenium complex. The olefinexchange reaction between trans-cyclooctene (1) (0.42 mmol) and complex 8 occurred smoothly to give the desired complex 4 R in high yield (95 %) (Scheme 3).…”

“…The pybox and [RuCl 2 (pybox)(C 2 H 4 )] reagents were prepared by our method. [10] trans-Cyclooctene was synthesized by photoisomerization of cis-cyclooctene with a photosensitizer. [5] The photoreaction was carried out under argon in a quartz-glass vessel with a low-pressure mercury lamp (32 W, Riko ± Kagaku Sangyo Co. Ltd., UVL-32LP).…”

Enantioselective CH Amination Using Cationic Ruthenium(II)–pybox Catalysts

“…[15] Subsequent ethylene dissociation and bromide isomerization provides a second vacant coordination site cis to the pyridine ring, and a second oxidation provides the active Ru VI species. In the transition state, the sulfamate ester wraps around into the vacant quadrant created by the pybox ligand and the bulky aromatic group points away from the ligand, thus resulting in one of the two enantiotopic hydrogen atoms (H a ) pointing directly at the reactive metallonitrene species.…”