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=Noyori asymmetric hydrogenation=

Introduction
Noyori Asymmetric Hydrogenation is a chemical reaction used for the catalytic reduction of olefins, functionalized ketones and simple ketones. This reaction is catalyzed using chiral ruthenium complex catalysts introduced by Ryoji Noyori since 1987 . He won the Nobel Prize in Chemistry in 2001 for his study of the asymmetric hydrogenation. Noyori Assymetric hydrogenation has an excellent stereoselectivity and very wide scope of reactions depends on the catalyst. BINAP-Ru catalyst is used for the asymmetric hydrogenation of olefins and functionalized ketones. BINAP/diamine-Ru catalyst is used for simple ketones.



Due to its high stereoselectivity, the BINAP-Ru catalytic system is widely used in the pharmaceutical industry. The anti-inflammatory drug, naproxen is synthesized from α-aryl-acrylic acid. An insect repellant, citronellol is synthesized from geraniol or nerol. The painkiller, morphine is also obtained using BINAP-Ru asymmetric hydrogenation.

History
Hydrogenation has a long history in organic synthesis. At the end of the 19th century, P. Sabitier initiated the hydrogenation using fine metal particles in the heterogeneous catalytic system . In 1961, the hydrogenation of olefins were also achieved in the homogenous catalytic system using RuCl3 by B.R. James and co-workers .However, any of reactions could achieve a significant stereoselectivity until 1971 when H. B. Kagan introduced DIOP, a chiral diphosphane ligand. The DIOP-Rh complex was used for the asymmetric hydrogenation of dehydro amino acid and could achieve 72% ee.



In 1980, Noyori introduced a cationic BINAP-Rh catalyst for the asymmetric hydrogenation of olefin. The Rh catalyst could achieve high ee(>90%), but the reaction was relatively slow and only worked under specific conditions. The scope of the reaction was also limited to the stereoselective hydrogenation of α-(acylamino)-acrylic acid or esters to make amino acid derivatives. In 1986, Noyori and coworkers discovered the BINAP-Ru dicarboxylate catalyst. The scope of the asymmetric hydrogenation was not limited to the synthesis of amino acid derivatives anymore, but also the hydrogenation of α,β- and β,γ- unsaturated carboxylic acids and allyric/homoallyric alcohols with high ee around 90%.



In 1991, Noyori and coworkers also found that the BINAP-Ru dihalide can catalyze the asymmetric hydrogenation of various functionalized ketones. . While the BINAP-Ru dicarboxylate could only efficiently catalyze the hydrogenation of olefins, the BINAP-Ru dihalide could effectively catalyze both the hydrogenation of olefins and the hydrogenation of functionalized C=O bond.



Even though the BINAP-Ru dihalide catalyst could reduce functionalized ketones, the hydrogenation of simple ketones has remaind as problem to solve. Since 1953, H. C Brown and coworkers have succesfully achieved and developed the chemo-, and stereo-selective reduction of C=O bond. The chemoselectivity over C=C bond was achieved using NaBH4 through the coordination of boron to oxygen atom, the diastereoselectivity was achieved using Selectrides , and enantioselectivity was achieved using chiral stoichiometric reagents. . However, all three selectivity could not be achieved at the same time. In 1995, Noyori discovered that the RuCl2(phosphane)2(diamine)2 complex can catalyze the hydrogenation of unfunctionalized ketones. This system also had chemoselectivity on C=O bond over the more reactive C=C bond. . The diastereoselectivity and the enantioselectivity could be achieved as well.



BINAP-Ru
In 1986, Noyori and coworkers discovered the BINAP-Ru dicarboxylate catalyst. This complex catalyzed the asymmetric hydrogenation of some olefins. The catalytic reaction proceeded via the coordination of the functional group to the ruthenium center. This could catalyze the asymmetric hydrogenation of amino acids, α,β- and β,γ- unsaturated carboxylic acids, and allyric/homoallyric alcohols. Eventhough the BINAP-Ru dicarboxylate catalyst showed high stereoselectivity in mild conditions, the scope of reaction was still limited to functionalized olefins. The progress was made when Noyori discovered the BINAP-Ru dihalide catalyst in 1991. Noyori found that the BINAP-Ru dihalide can catalyze the asymmetric hydrogenation of various functionalized ketones. The stereoselectivity was achieved by the steric hindrance of substituents and the coordination of the Ru center to the N, O, or X atoms near the C=O bond.



BINAP/diamine-Ru
In 1995, Noyori has discovered that the RuCl2(phosphane)2(diamine)2 complex can catalyze the hydrogenation of unfunctionalized ketones . This system had the chemoselectivity on C=O bond over the more reactive C=C bond through a favored pericyclic transition state(See Mechanism and Selectivity section). The diastereoselectivity was acheived for the asymmetric hydrogenation of simple cyclic ketones. By coordinating the chiral BINAP ligand, the enantioselectivity was also acheived for the hydrogenation of simple ketones.



BINAP-Ru
The BINAP-Ru dihalide precatalyst facilitate hydride through Ru-monohydride intermediate and gives off HCl. The ruthenium center of the catalyst coordinates to the oxygen atoms in the ester compound. Because of the excellent chirality of the BINAP ligand, one of the two possible transition states is favored. The transition state on the left is favored over the other because of the large R1/ Ph steric hindrance. The the (R)-BINAP-Ru catalyze the synthesis the (S)-Product, and the (S)-BINAP Ru catalyze the synthesis the (R)-product with high ee. Again, the dehydrated BINAP-Ru catalyst is utilized by the addition of another hydrogen atom from H2. The newly activated Ru-monohydride participates in the catalytic cycle again.



BINAP/diamine-Ru
In BINAP-Ru catalytic system, the hydrogenation of functionalized ketone is catalyzed by [2+2] transition state(left transition state model) which forms a metal alkoxide intermediate. Unlike the BINAP-Ru, the BINAP/diamine-Ru catalytic system forms the six membered pericyclic transition state(Right Transition state model) which directly leads to the product. This non-classical metal-ligand bifunctional transition state causes the hydrogenation of C=O bond with higher rate and the higher chemoselectivity.



The Ru diphosphane diamine catalyst, including the BINAP/diamine-Ru catalyst, can hydrogenate simple cyclic ketones diastereoselectively. In the transition state, the ruthenium monohydride moiety acts as a bulky group (marked red on the scheme below). The product is predictable in the way that the catalyst approaches from the less hindered side.



The excellent chirality of the BINAP ligand made it possible for the BINAP/diamine-Ru complex possible to reduce simple ketones enantioselectively. Due to the steric hindrance between the BINAP ligand and the large substituent group on ketone, the less steric hindered transition state is favored as expected. The simple ketones include aromatic, heteroaromatic, and olefinic ketones.



The ruthenium diphosphane diamine complex is activated by the addition of the hydrogen gas. The activated catalyst transfers hydrogen chemoselectively through the pericyclic transition state. The dehydrated ruthenium complex is supplied hydrogen and re-participate in the catalytic cycle with the assistance of base. The diastereoselectivity of cyclic ketones is predicted by the steric hindrance between substituents and ruthenium monohydride. The stereoselectivity of simple ketone is attained by using the chiral ligand, such as BINAP.



Application
One of the most important applications of Noyori asymmetric hydrogenation is the synthesis of naproxen, which is a widely used anti-inflammatory drug. (S)-naproxen is synthesized from the asymmetric hydrogenation of 2-(6-methoxynaphthalen-2-yl)acrylic acid. BINAP-Ru dicarboxylate catalyst is used in this reaction. The ruthenium center of the catalyst coordinates to the carboxylic functional group in the reactant and selectively reduce C=C bond. (S)-naproxen is collected with 97% ee and 92% yield.



Morphine is a naturally occurring painkiller. Benzomorphane, an non-addictive artificial morphine, can be synthesized using the Noyori asymmetric hydrogenation as an key step. In the synthesis, ruthenium center on the catalyst coordinates to the O atom in the aldehyde group and reduce the nearby C=C bond. Benzomorphane is produced after several steps.



An antipsychotic agent BMS 181000 can be synthesized using BINAP/diamine-Ru catalyst. Enantioselectivity is achieved by steric hindrance between the chiral BINAP ligand and the aromatic group on the reactant.