MEPY ligand

The precursor to the Rh-carbene complex is a dirhodium system that typically has four bidentate ligands attached. One example of the dirhodium complex is shown as structure 63, which was discovered by Doyle and co-workers and is abbreviated Rh2(5.S -MEPY)4,76 Here the 5.S-MEPY ligand is chiral carboxami-date, which is derived from the amino acid L-proline. The use of 63 and similar complexes makes it possible to induce chirality into the cyclopropane product. Equation 10.51 is an example of an intramolecular chiral cyclopropanation... 

In the catalysis shown in Equation (a) a double stereoselection is involved. The formation of the new asymmetric centers in the cyclopropane ring is influenced by the men-thyl group contained in the substrate (15,35,4/ )-menthyl diazoacetate and by the mepy ligand contained in the catalyst. The two influences are referred to as substrate and catalyst control, respectively. With regard to the efficiency it has to be noted that whe-... 

Figure 15.2 Rh complexes inspired by the MEPY ligand scaffold [1g],...

Ligand abbreviations (6-Mepy)(py)2tren and (6-Mepy)2(py)tren = tris[4-[(6-/ )-2-pyridyl]-3-aza-6-butenyl]amme, R = H or CH3 HB(pz)3 = hydro-tris(pyrazolyl)borate paptH = 2-(2-pyridylamino)-4-(2-pyridyl)thiazole phenmethoxa = 3-[2-(l,10-phenanthrolyl)]-5-methyl-l,2,4-oxadiazole pyimH = 2-(2 -pyridyl)itnidazole pybimH = 2-(2 -pyridyl)benzimidazole ppa = JV -(2-pyridylmethyl)picolineamidine tpmbn = tetrakis(2-pyridylmcthyl)-meso-2,3-butane-diamine tppn = tetrakis-(2-pyridylmethyl)-l-methyl-l,2-propanediainine tpchxn = tetrakis(2-pyridylmethyl)-tra s-l,2-cyclohexanediamine biz = 2,2 -bi-1,4,5,6-tetrahydropyrimidine. 

Fe(6-Mepy)3tren](PF6)2- Within the series of iron(II) complexes of the type [Fe(6-Mepy)3 (py) tren](PFg)2 containing the hexadentate ligand tris[4-(6-R)-2-pyridyl]-3-aza-3-butenyl]amine, R = H or CH3, the compounds with X = 1, 2 show a similar HS <- LS transition in solution as well as in the solid state [170]. In solution, spin-state conversion rates have been obtained as shown in Table 3. The complex with x = 0 is pure HS in solution, whereas in the solid state a gradual transition is observed extending over the range... 

Diketonate cobalt(III) complexes with alkyl peroxo adducts have been prepared recently and characterized structurally, and their value in hydrocarbon oxidation and olefin epoxidation examined.980 Compounds Co(acac) 2(L) (O O / - B u) with L = py, 4-Mepy and 1-Meim, as well as the analog of the first with dibenzoylmethane as the diketone, were prepared. A distorted octahedral geometry with the monodentates cis is consistently observed, and the Co—O bond distance for the peroxo ligand lies between 1.860(3) A and 1.879(2) A. 

Kinetic studies on complex formation reactions of the tripodal tetra-mine complex [Co(Me6tren)(H20)]2+ with pyridine, 4-methylpyridine, and imidazole yielded activation parameters AH1, and AS. Activation parameters and dependences of rate constants on incoming ligand concentration indicated that the formation mechanism ranged from dissociative for the weaker and bulkier incoming ligands (py, 4-Mepy) to associative for the more basic and less bulky imidazole 2-methylimida-zole occupies an intermediate position (280). 

Electron-impact ionization potentials have been determined for [W(CO)sL] (L = 2,6-Mepy, 4-Mepy, or 2-CNpy) and for the free ligands. A good linear correlation was obtained between the two values in this and other series of compounds and the results were rationalized in terms of a synergic bonding mechanism. ... 

Scheme 11. Each ligand is monodentate and the methyl groups of the 2,6-Mepy molecules above and below the Cr02N2 plane block further coordination. ...

Reactions of the triamine complexes such as [cis-Pt(NHa)2(4-mepy)Cl]Cl with d(GpG) proved that there is no release of amine ligands and therefore the formation of bifunctional adducts, comparable to those induced by cis-Pt, is highly unlikely (71, 72). Only two mononu-... 

Acetone complexes are also prepared by the trans activation of the ammine ligands in [Osn(NH3)5(C-Mepy)]2 + to form (r[Pg.285]

As mentioned above, the hydrogen atoms at the 2- and 6-positions of the pyridine hamper octahedral coordination of pyridine to most metal ions. Only when a large counterion is present, such as [Fe4(C013)]2 or perhaps PF or AsF, do octahedral [M(py)6]2+ species have some stability.15,16 With a small cation such as Ni2+ even a weak ligand such as 4-methylpyridine (4-Mepy) may induce a low-spin square-planar geometry as found in [Ni(4-Mepy)4](PF6)2.54... 

Chiral Rh(II) oxazolidinones Rh2(BNOX)4 and Rh2(IPOX)4 (25a,b) were not as effective as Rh2(MEPY)4 for enantioselective intramolecular cyclopropanation, even though the steric bulk of their chiral ligand attachments (COOMe versus i-Pr or CH2Ph) are similar. Significantly lower yields and lower enantioselectivides resulted from dinitrogen extrusion from prenyl diazoacetate catalyzed by either Rh2(4.S -lPOX)4 or Rh2(4S-BNOX)4. This difference, and those associated with butenolide formation [91], can be attributed to the ability of the carboxylate substituents to stabilize the carbocation form of the intermediate metal carbene (3b), thus limiting the Rh2(MEPY)4-catalyzed reaction to concerted carbene addition onto both carbon atoms of the C-C double bond. 

Fish and co-workers have studied N-to-77 rearrangements of a number of cyclopentadienyl rhodium and ruthenium complexes of pyridines and quinolines. This behavior was first observed in the complex [Ru(C5H5)(CH3CN)2(2-Mepy)]+ where the cr-bound N-isomer (15) gave an equilibrium mixture of the N- and w-bound (16) complexes on heating in 1,2-dichloroethane (ratio 15 16 = 6 1). No free ligand was detected in... 

---