The chemistry of lithium enolates is used to demonstrate that complex structures held together by noncovalent bonds (“supramolecules”) may dramatically influence the result of seemingly simple standard reactions of organic synthesis. Detailed structural data have been obtained by crystallographic investigations of numerous Li enolates and analogous derivatives. The most remarkable features of these structures are aggregation to give dimers, tetramers, and higher oligomers, complexation of the metal centers by solvent molecules and chelating ligands, and hydrogen-bond formation of weak acids such as secondary amines with the anionoid part of the enolates. The presence in nonpolar solvents of the same supramolecules has been established by NMR-spectroscopic, by osmometric, and by calorimetric measurements. The structures and the order of magnitude of the interactions have also been reproduced by ab-initio calculations. Most importantly, supramolecules may be product-forming species in synthetic reactions of Li enolates. A knowledge of the complex structures of Li enolates also improves our understanding of their reactivity. Thus, simple procedures have been developed to avoid complications caused by secondary amines, formed concomitantly with Li enolates by the common methods. Mixtures of achiral Li enolates and chiral Li amides can give rise to enantioselective reactions. Solubilization by LiX is observed, especially of multiply lithiated compounds. This effect is exploited for alkylations of N-methylglycine (sarcosine) CH2 groups in open-chain oligopeptides. Thus, the cyclic undecapeptide cyclosporine, a potent immunosuppressant, is converted into a THF-soluble hexalithio derivative (without epimerization of stereogenic centers) and alkylated by a variety of electrophiles in the presence of either excess lithiumdiisopropyl amide or of up to 30 equivalents of lithium chloride. Depending on the nature of the LiX additive, a new stereogenic center of (R) or (S) configuration is created in the peptide chain by this process. A structure-activity correlation in the series of cyclosporine derivatives thus available is discussed.

Structure and Reactivity of Lithium Enolates. From Pinacolone to Selective C‐Alkylations of Peptides. Difficulties and Opportunities Afforded by Complex Structures

The effect of structural parameters on the exchange coupling constant of the title complexes of Cu(II), Ni(II), and Mn(II) is studied by means of density functional calculations on model compounds....

Magnetic Coupling in End-On Azido-Bridged Transition Metal Complexes: A Density Functional Study

Copper Binding to the Amyloid-β (Aβ) Peptide Associated with Alzheimer's Disease FOLDING, COORDINATION GEOMETRY, pH DEPENDENCE, STOICHIOMETRY, AND AFFINITY OF Aβ-(1–28): INSIGHTS FROM A RANGE OF COMPLEMENTARY SPECTROSCOPIC TECHNIQUES

Reactions of transition metal-coordinated nitriles

The secular determinants obtained in the previous paper are solved for the energy levels which are important in the absorption spectra of the normal complex ions, leaving the crystalline field strength as a parameter. The values of B and C (Racah's parameters) there needed are determined from the observed spectra of free ions or in some cases by extrapolation. The f -values of the transitions which connect the energy levels calculated are estimated and compared with the observed intensities. The difference of the spectral width among absorption bands and lines is also considered using the energy diagram obtained. Following the assignments determined by the above considerations, the calculated positions of lines and bands are rather in good agreement with the experimental data in divalent ions [MX 6 ] 2+ (M=Cr, Mn, Fe, Co, Ni), when we adjust the crystalline field parameter D q suitably. In trivalent ions [MX 6 ] 3+ (M=Ti, V, Cr, Mn, Fe), it is necessary besides the adjustment of D q to use smaller values ...

On the absorption spectra of complex ions. II

A number of palladium(II) and platinum(II) bis-chelates have been prepared which contain one kind of symmetric or unsymmetric β-diketonate anions, or involve two different β-diketonate anions. Existence of the cis and trans isomers in solution of the binary chelates of unsymmetric β-diketones was confirmed by 1H and 13C NMR spectroscopy. Geometrical isomerization of the palladium(II) complexes to attain equilibrium in solution is fast, while the platinum(II) complexes are quite inert. The mixed-ligand chelates are stable in solution, showing no sign of disproportionation. The isomeric pairs of bis(1-phenyl-1,3-butanedionato)palladium(II), bis(2,4-hexanedionato)palladium(II), and bis(1,1,1-trifluoro-2,4-pentanedionato)platinum(II) were isolated, and molecular structure of the cis isomer of the first complex was determined by X-ray analysis.

Bis(β-diketonato)-palladram(II) and -platinum(II) Complexes

The oxygenation of 2,6-di-t-butylphenols with five-co-ordinate cobalt(II) Schiff-base complexes in aprotic solvents, such as CH2Cl2, thf, and dmf (thf = tetrahydrofuran, dmf = dimethylformamide), has been found to result in regioselective formation of peroxyquinolatocobalt(III) complexes. The regioselectivity depends on the nature of the substituent at the 4-position of the phenol used: 4-alkyl-2,6-di-t-butylphenols (1) afford peroxy-p-quinolatocobalt(III) complexes, whereas peroxy-o-quinolato-complexes are formed from 4-aryl-2,6-di-t-butyl phenols (4). The initiation of the oxygenation is hydrogen abstraction by superoxocobalt(III) species from the phenols to give the corresponding phenoxy-radicals (10). Rapid reduction of (10) with cobalt(II) species follows giving rise to a phenolatocobalt(III) complex intermediate, within which dioxygen is incorporated. The regioselectivity of the oxygenation is attributable to the formation of the phenolatocobalt(III) complex intermediate. Crystals of the peroxy-p-quinolatocobalt(III) complex (2a) are orthorhombic, space group P212121, with a= 33.749(11), b= 11.844(5), c= 9.329(4)A, and Z= 4. The crystal structure has been refined from 3 018 diffractometer data to R= 0.067.

Regioselective formation of peroxyquinolatocobalt(III) complexes in the oxygenation of 2,6-di-t-butylphenols with cobalt(II) Schiff-base complexes

Synthesis and solution behavior of mononuclear palladium(II) and platinum(II) complexes containing pyridine-2-thiolate as a ligand. Crystal structure of chloro(pyridine-2-thiolato)- (triphenylphosphine)palladium(II)

A deep blue mixed valence complex [VO(nta)–O–VO(nta)]3–(nta = nitrilotriacetate) was found to be in equilibrium with [VIVO(nta)(H2O)]– and [VVO2(nta)]2– in aqueous solution; the crystal structure of its ammonium salt trihydrate has been determined.

A mixed valence binuclear complex of vanadium(IV) and vanadium(V). X-Ray crystal structure of (NH4)3[V2O3(nitrotriacetate)2]·3H2O

Mixed-valence complexes of the type [CoII(µ-SO3)2(µ-L)2CoIII2(H2O)n(ROH)2–n](n= 0, 1, or 2; R = Me or Prn) have been prepared and characterized, where the deprotonated Schiff-base ligands L2– are either symmetrical, such as N,N′-ethylenebis(salicylideneiminate)(salen), N,N′-ethylenebis(α-methylsalicylideneiminate)(α,α′-Me2-salen), propane-1,3-diylbis(salicylideneiminate)(salpd), and propane-1,3-diylbis(α-methylsalicylideneiminate)(α,α′-Me2-salpd), or unsymmetrical such as propane-1,3-diyl(α-methylsalicylideneiminate)(salicylideneiminate)(α-Me-salpd). An X-ray structure determination of [COII(µ-SO3)2(µ-α,α′Me2-salpd)2CoIII2(PrnOH)2]·2PrnOH showed the presence of bridging phenolic oxygens and sulphite ions. The mixed-valence state CoIII(S= 0)–CoII(S=)–CoIII(S= 0) in these trinuclear complexes has been demonstrated by cryomagnetic measurements because the effective magnetic moments (4.40–5.00) at room temperature correspond to the monomeric complex of a cobalt(II) ion in a high-spin state. Down to 77 K, no spin-exchange interaction was observed between the cobalt(III) and cobalt(II) ions. I.r. spectra and thermal gravimetric-differential thermal analysis data have been obtained and discussed with regard to the co-ordination modes of the solvent molecules water, methanol, ethanol, and propan-1-ol.

Mixed oxidation state trinuclear cobalt complexes with bridging sulphito and Schiff-base ligands. Part 1. Preparation of the complexes [CoII(µ-SO3)2(µ-L)2COIII2(ROH)2](L = Schiff base anion, R = alkyl) and structure determination of [CoII(µ-SO3)2(µ-α,α′-Me2-salpd)2CoIII2(PrnOH)2]·2PrnOH

Shun'ichiro Ooi

Papers

Bis(β-diketonato)-palladram(II) and -platinum(II) Complexes

Regioselective formation of peroxyquinolatocobalt(III) complexes in the oxygenation of 2,6-di-t-butylphenols with cobalt(II) Schiff-base complexes

Synthesis and solution behavior of mononuclear palladium(II) and platinum(II) complexes containing pyridine-2-thiolate as a ligand. Crystal structure of chloro(pyridine-2-thiolato)- (triphenylphosphine)palladium(II)

A mixed valence binuclear complex of vanadium(IV) and vanadium(V). X-Ray crystal structure of (NH4)3[V2O3(nitrotriacetate)2]·3H2O

Mixed oxidation state trinuclear cobalt complexes with bridging sulphito and Schiff-base ligands. Part 1. Preparation of the complexes [CoII(µ-SO3)2(µ-L)2COIII2(ROH)2](L = Schiff base anion, R = alkyl) and structure determination of [CoII(µ-SO3)2(µ-α,α′-Me2-salpd)2CoIII2(PrnOH)2]·2PrnOH