Highly enantioselective zirconium-catalyzed cyclization of aminoalkenes

Aminoalkenes are catalytically cyclized in the presence of cyclopentadienylbis(oxazolinyl)borato group 4 complexes {PhB(C5H4)(Ox(R))2}M(NMe2)2 (M = Ti, Zr, Hf; Ox(R) = 4,4-dimethyl-2-oxazoline, 4S-isopropyl-5,5-dimethyl-2-oxazoline, 4S-tert-butyl-2-oxazoline) at room temperature and below, affording five-, six-, and seven-membered N-heterocyclic amines with enantiomeric excesses of >90% in many cases and up to 99%. Mechanistic investigations of this highly selective system employed synthetic tests, kinetics, and stereochemistry. Secondary aminopentene cyclizations require a primary amine (1-2 equiv vs catalyst). Aminoalkenes are unchanged in the presence of a zirconium monoamido complex {PhB(C5H4)(Ox(4S-iPr,Me2))2}Zr(NMe2)Cl or a cyclopentadienylmono(oxazolinyl)borato zirconium diamide {Ph2B(C5H4)(Ox(4S-iPr,Me2))}Zr(NMe2)2. Plots of initial rate versus [substrate] show a rate dependence that evolves from first-order at low concentration to zero-order at high concentration, and this is consistent with a reversible substrate-catalyst interaction preceding an irreversible step. Primary kinetic isotope effects from substrate conversion measurements (k'obs((H))/k'obs((D)) = 3.3 ± 0.3) and from initial rate analysis (k2((H))/k2((D)) = 2.3 ± 0.4) indicate that a N-H bond is broken in the turnover-limiting and irreversible step of the catalytic cycle. Asymmetric hydroamination/cyclization of N-deutero-aminoalkenes provides products with higher optical purities than obtained with N-proteo-aminoalkenes. Transition state theory, applied to the rate constant k2 that characterizes the irreversible step, provides activation parameters consistent with a highly organized transition state (ΔS(++) = -43(7) cal·mol(-1) K(-1)) and a remarkably low enthalpic barrier (ΔH(++) = 6.7(2) kcal·mol(-1)). A six-centered, concerted transition state for C-N and C-H bond formation and N-H bond cleavage involving two amidoalkene ligands is proposed as most consistent with the current data.

Four Polymorphs of Methyl Paraben: Structural Relationships and Relative Energy Differences

Four polymorphic forms of methyl paraben (methyl 4-hydroxybenzoate, 1), denoted 1-I (melting point 126 °C), 1-III (109 °C), 1-107 (107 °C), and 1-112 (112 °C), have been investigated by thermomicroscopy, infrared spectroscopy, and X-ray crystallography. The crystal structures of the metastable forms 1-III, 1-107, and 1-112 have been determined. All polymorphs contain the same O-H···O=C connected catemer motif, but the geometry of the resulting H-bonded chain is different in each form. The Z' = 3 structure of 1-I (stable form; space group Cc) contains local symmetry elements. The crystal packing of each of the four known crystal structures of 1 was compared with the crystal structures of 12 chemical analogues. Close two-dimensional relationships exist between 1-112 and a form of methyl 4-aminobenzoate and between 1-107 and dimethyl terephthalate. The lattice energies of the four methyl paraben structures have been calculated with a range of methods based on ab initio electronic calculations on either the crystal or single molecule. This shows that the differences in the induction energy of the different hydrogen-bonded chain geometries have a significant effect on relative lattice energies, but that conformational energy, repulsion, dispersion, and electrostatic also contribute.

Unusual structural motif in a zwitterionic Fe(II) complex of a tetradentate phosphine

The reaction of meso-DPPEPM (DPPEPM = bis(diphenylphosphino-ethylphenylphosphino)methane) with one equivalent of FeBr(2) in tetrahydrofuran generates a zwitterionic compound [FeBr(κ(2)-DPPEPM)(κ(3)-DPPEPM-FeBr(3))] (1). Compound 1 exhibits an unusual bonding arrangement with two meso-DPPEPM ligands and one bromide coordinated to a single Fe(II) center. One of the DPPEPM ligands binds to iron in a κ(2) mode via two central phosphorus atoms, leaving the terminal phosphines dangling. The second DPPEPM binds through three phosphines, whereas the fourth one coordinates to the iron center of an external FeBr(3)(-) unit. A 1 : 2 reaction of FeBr(2) and meso-DPPEPM in tetrahydrofuran generates [FeBr(κ(2)-DPPEPM)(κ(3)-DPPEPM)]Br ([2]Br) in which the positive charge on the pseudo-octahedral unit is balanced by free Br(-) as opposed to the phosphine-bound FeBr(3)(-) in 1. The compound [2]PF(6) was obtained from [2]Br and TlPF(6). Solution structures for 1, [2]Br and [2]PF(6) were assigned on the basis of (31)P NMR. For all three compounds the data are consistent with five phosphorus atoms bound to the metal.

Mapping the formation areas of giant molybdenum blue clusters: a spectroscopic study

The self-assembly of soluble molybdenum blue species from simple molybdate solutions has primarily been associated with giant mixed-valent wheel-shaped cluster anions, derived from the {Mo(V/VI)(154/176)} archetypes, and a {Mo(V/VI)(368)} lemon-shaped cluster. The combined use of Raman spectroscopy and kinetic precipitation as self-assembly monitoring techniques and single-crystal X-ray diffraction is key to mapping the realm of molybdenum blue species by establishing spherical {Mo(V/VI)(102)}-type Keplerates as an important giant molybdenum blue-type species. We additionally rationalize the empirical effect of reducing agent concentration on the formation of all three relevant skeletal types: wheel, lemon and spheres. Whereas both wheels and the lemon-shaped {Mo(V/VI)(368)} cluster are obtained from weakly reduced molybdenum blue solutions, considerably higher reduced solutions lead to {Mo(V/VI)(102)}-type Keplerates.

[Co(II)4Mo(V)12O28(OH)12(H2O)12]·12H2O: facilitating single-crystal growth by deuteration

The structure of the neutral heterometal oxide cluster dodecaaqua-di-μ(3)-hydroxido-deca-μ(2)-hydroxido-octacosaoxidotetracobalt(II)dodecamolybdenum(V) dodecahydrate, [Mo(12)O(28)(μ(2)-OH)(10)(μ(3)-OH)(2){Co(H(2)O)(3)}(4)], is virtually identical to the previously reported Ni(II) analogue [Mo(12)O(28)(μ(2)-OH)(10)(μ(3)-OH)(2){Ni(II)(H(2)O)(3)}(4)] [Müller, Beugholt, Kögerler, Bögge, Budko & Luban (2000). Inorg. Chem. 39, 5176-5177], the first molecular magnet to exhibit signs of magnetostriction. The formation kinetics of the neutral cluster species, which is insoluble in water, can be significantly slowed by the use of deuterated reactants in order to grow single crystals of sufficient size for single-crystal X-ray diffraction studies using standard diffractometers. One half of the main cluster and six solvent water molecules constitute the asymmetric unit. The main cluster is located on a mirror plane.

An octanuclear iron(III) isobutyrate wheel

The reaction of the μ(3)-oxido-centred trinuclear isobutyrate cluster [Fe(3)O(O(2)CCHMe(2))(6)(H(2)O)(3)](+) with an excess of phenol (PhOH) in chloroform produces a novel octanuclear Fe(III) cluster, cyclo-tetra-μ(2)-hydroxido-dodeca-μ(2)-isobutyrato-κ(24)O:O'-octa-μ(2)-phenolato-κ(16)O:O'-octairon(III) phenol hexasolvate monohydrate, [Fe(8)(C(4)H(7)O(2))(12)(C(6)H(5)O)(8)(OH)(4)]·6C(6)H(5)OH·H(2)O. The neutral cluster is located about a centre of inversion and consists of a planar ring of eight Fe(III) centres with two types of bridges between adjacent Fe atoms: each Fe atom is bridged to one of its neighbours by a μ-hydroxide and two 1,3-bridging carboxylates, or by two phenolate and one 1,3-bridging isobutyrate ligand. The cavity within the {Fe(8)} wheel is occupied by a disordered water molecule. Intermolecular O-H···O hydrogen bonds and C-H···π interactions connect the clusters and the phenol solvent molecules to form a three-dimensional network.

Rationalizing the effect of halogenation on the molecular structure of simple cyclobutene derivatives by topological real-space analysis of their electron density

The accurate gas-phase equilibrium structures on the ground-state potential energy surface of the complete series of fluorinated and chlorinated cyclobutene derivatives with C(2v) symmetry have been evaluated at DFT PBE0/6-311++G(d,p) theory level. The optimized geometries have been compared with all the available experimental data reported in the literature, as obtained by microwave spectroscopy (MW) and gas-phase electron diffraction (GED) techniques. For hexafluorocyclobutene and 1,2-dichloro-3,3',4,4'-tetrafluorocyclobut-1-ene, the results of accurate low-temperature single-crystal X-ray diffraction experiments have also been considered. Structural changes within the cyclobutene ring, as induced by fluorination and chlorination at allylic and vinylic positions, have been correlated with changes in the corresponding theoretical charge densities. To this aim, several local and nonlocal topological descriptors provided by the quantum theory of atoms in molecules, QTAIM, have been employed, with particular emphasis on the delocalization indices and integrated source function decomposition schemes. Key factors for the resulting molecular structures are the chemical nature and the steric hindrance of the substituents, as well as quantum-mechanical effects, such as delocalization and partial conjugation. When fluorine atoms replace hydrogens at allylic or vinylic positions, the corresponding Csp(3)-Csp(3) or Csp(2)═Csp(2) bonds between the substituted carbons undergo a significant strengthening, while chlorination has just the opposite effect. In the latter case the steric hindrance between bulky chlorine atoms occupying vicinal positions is crucial in determining the single Csp(3)-Csp(3) bond length. These findings are discussed in the context of the reactivity of chemically related chlorofluorocarbon compounds.

A spin-frustrated cobalt(II) carbonate pyrochlore network

The crystal structure of the cobalt(II) carbonate-based compound cobalt(II) dicarbonate trisodium chloride, Co(CO(3))(2)Na(3)Cl, grown from a water-ethanol mixture, exhibits a three-dimensional network of corner-sharing {Co(4)(μ(3)-CO(3))(4)} tetrahedral building blocks, in which the Co(II) centres define a pyrochlore lattice and reside in a slightly distorted octahedral Co(O-CO(2))(6) environment. The space outside the hexagonal framework defined by these interlinked groups is occupied by Na(+) and Cl(-) ions. Antiferromagnetic coupling between adjacent Co(II) centres, mediated by carbonate bridges, results in geometric spin frustration which is typical for pyrochlore networks. The Co and Cl atoms reside on the special position 3, one Na atom on position 2 and a carbonate C atom on position 3.

Synthesis of monomeric Fe(II) and Ru(II) complexes of tetradentate phosphines

rac-Bis[{(diphenylphosphino)ethyl}-phenylphosphino]methane (DPPEPM) reacts with iron(II) and ruthenium(II) halides to generate complexes with folded DPPEPM coordination. The paramagnetic, five-coordinate Fe(DPPEPM)Cl(2) (1) in CD(2)Cl(2) features a tridentate binding mode as established by (31)P{(1)H} NMR spectroscopy. Crystal structure analysis of the analogous bromo complex, Fe(DPPEPM)Br(2) (2) revealed a pseudo-octahedral, cis-α geometry at iron with DPPEPM coordinated in a tetradentate fashion. However, in CD(2)Cl(2) solution, the coordination of DPPEPM in 2 is similar to that of 1 in that one of the external phosphorus atoms is dissociated resulting in a mixture of three tridentate complexes. The chloro ruthenium complex cis-Ru(κ(4)-DPPEPM)Cl(2) (3) is obtained from rac-DPPEPM and either [RuCl(2)(COD)](2) [COD = 1,5-cyclooctadiene] or RuCl(2)(PPh(3))(4). The structure of 3 in both the solid state and in CD(2)Cl(2) solution features a folded κ(4)-DPPEPM. This binding mode was also observed in cis-[Fe(κ(4)-DPPEPM)(CH(3)CN)(2)](CF(3)SO(3))(2) (4). Addition of an excess of CO to a methanolic solution of 1 results in the replacement of one of the chloride ions by CO to yield cis-[Fe(κ(4)-DPPEPM)Cl(CO)](Cl) (5). The same reaction in CH(2)Cl(2) produces a mixture of 5 and [Fe(κ(3)-DPPEPM)Cl(2)(CO)] (6) in which one of the internal phosphines has been substituted by CO. Complexes 2, 3, 4, and 5 appear to be the first structurally characterized monometallic complexes of κ(4)-DPPEPM.

P[N(i-Bu)CH(2)CH(2)](3)N: nonionic Lewis base for promoting the room-temperature synthesis of α,β-unsaturated esters, fluorides, ketones, and nitriles using Wadsworth-Emmons phosphonates

The bicyclic triaminophosphine P(RNCH(2)CH(2))(3)N (R = i-Bu, 1c) serves as an effective promoter for the room-temperature stereoselective synthesis of α,β-unsaturated esters, fluorides, and nitriles from a wide array of aromatic, aliphatic, heterocyclic, and cyclic aldehydes and ketones, using a range of Wadsworth-Emmons (WE) phosphonates. Among the analogues of 1c [R = Me (1a), i-Pr (1b), Bn (1d)], 1a and 1b performed well, although longer reaction times were involved, and 1d led to poorer yields than 1c. Functionalities such as cyano, chloro, bromo, methoxy, amino, ester, and nitro were well tolerated. We were able to isolate and characterize (by X-ray means; see above) the reactive WE intermediate species formed from 2b and 1c.