240803-86-5Relevant articles and documents
Catalytic nitrile hydration with [Ru(η6- p -cymene)Cl 2(PR2R′)] complexes: Secondary coordination sphere effects with phosphine oxide and phosphinite ligands
Knapp, Spring Melody M.,Sherbow, Tobias J.,Yelle, Robert B.,Juliette, J. Jerrick,Tyler, David R.
, p. 3744 - 3752 (2013/07/26)
The rates of nitrile hydration reactions were investigated using [Ru(η6-p-cymene)Cl2(PR2R′)] complexes as homogeneous catalysts, where PR2R′ = PMe 2(CH2P(O)Me2), PMe2(CH 2CH2P(O)Me2), PPh2(CH 2P(O)Ph2), PPh2(CH2CH 2P(O)Ph2), PMe2OH, P(OEt)2OH. These catalysts were studied because the rate of the nitrile-to-amide hydration reaction was hypothesized to be affected by the position of the hydrogen bond accepting group in the secondary coordination sphere of the catalyst. Experiments showed that the rate of nitrile hydration was fastest when using [Ru(η6-p-cymene)Cl2PMe2OH]: i.e., the catalyst with the hydrogen bond accepting group capable of forming the most stable ring in the transition state of the rate-limiting step. This catalyst is also active at pH 3.5 and at low temperatures - conditions where α-hydroxynitriles (cyanohydrins) produce less cyanide, a known poison for organometallic nitrile hydration catalysts. The [Ru(η6-p-cymene) Cl2PMe2OH] catalyst completely converts the cyanohydrins glycolonitrile and lactonitrile to their corresponding α-hydroxyamides faster than previously investigated catalysts. [Ru(η6-p-cymene) Cl2PMe2OH] is not, however, a good catalyst for acetone cyanohydrin hydration, because it is susceptible to cyanide poisoning. Protecting the -OH group of acetone cyanohydrin was shown to be an effective way to prevent cyanide poisoning, resulting in quantitative hydration of acetone cyanohydrin acetate.
Diastereoselectivity in chiral ruthenium complexes of bidentate bisphosphine monoxide ligands: Controlling epimerization in aldehyde complexes and 16-electron intermediates
Faller,Patel, Ben P.,Albrizzio, Mauricio A.,Curtis, Michael
, p. 3096 - 3104 (2008/10/08)
Heterobidentate and hemilabile ligands involving P,O-donor chelates produce chiral metal centers when hound to arene-ruthenium complexes. This chirality in cymene complexes produces diastereotopic methyl groups in the isopropyl ligand which serve as a detector of the chirality at the metal. [CyRu(η2-chelate-P,O)Cl]+ cations are precursors to strong 16-electron dicationic Lewis acids which have potential use in asymmetric catalysis. Sixteen-electron complexes of this type, however, provide a pathway with a low barrier to racemization or epimerization of the metal center in intermediates, such as [CyRu(η2-chelate-P,O)(PhCHO)]2+. Substitution of the central carbon in diphenylphosphinomethane monoxide (dppmO) forces the ligand to adopt a configuration with the substituent in an endo position, thus forcing the 16-electron intermediate to return diastereoselectively to its original configuration and prevents epimerization. Thus, an X-ray structure shows that (R*RuR*C)-[CyRu(η2- Ph2PCHR)Ph2PO-P,O)Cl]+ is the preferred diastereomeric pair. In the parent, [CyRu(η2-dppmO-P,O)(ligand)]2+, racemization occurs at the metal center, since there is nothing driving the preferential formation of either the R or S ruthenium center. When the ligands are chiral, however, the metal center epimerizes to minimize steric interactions in the two diastereomers. The equilibrium between [(RRu)-CyRu(η2-dppmO-P,O)(RC-ligand)] 2+ and [(SRu)-CyRu(η2-dppmO-P,O)(RC-ligand)] 2+ reflects a 37% de for (1R)-(-)-myrtenal. Since a substituent on the central carbon prevents epimerization at the metal center, this diastereoselectivity is reflected in a preference for binding of (RC)-ligand by either [(RRuRC)-CyRu(η2-Ph 2PCHPr)Ph2PO-P,O)]2+ or [(SRuSC)-CyRu(η2-Ph 2PCHPr)Ph2PO-P,O)]2+. An X-ray structure of rac-[(RRu*,RC*)-CyRu(η2-Ph 2PCHPr)Ph2PO-P,O)(PhCHO]2+ shows that the aldehyde assumes an orientation that would suggest one stereoface of the aldehyde may be more susceptible to attack.
