Showing papers by "Peter S. White published in 2001"
TL;DR: The one in the authors' garden continued its growth peacefully, as did thousands like it in neglected spots all over the world, and some little time later that the first one picked up its roots and walked.
Abstract: There must have been plenty of them about, growing up quietly and inoffensively, with nobody taking any particular notice of them…. And so the one in our garden continued its growth peacefully, as did thousands like it in neglected spots all over the world…. It was some little time later that the first one picked up its roots and walked. John Wyndham, The Day of the Triffids
745 citations
01 Jan 2001
TL;DR: In this paper, the significance of disturbance as an ecological factor in various habitats and communities has been discussed, and a few theories and synthetic concepts have been proposed, but we do not yet have an inclusive general paradigm for this important body of work.
Abstract: Studies of disturbance have a long tradition in vegetation ecology (Cooper 1926; Raup 1941; White 1979) and have increased lly during the last 30 years (Dayton 1971; Heinselman 1973; Levin and Paine 1974; Borman and Likens 1979; Sousa 1979a,b, 1984; Pickett 1980; Pickett and White 1985; Van der Maarel 1993; Bornette and Amoros 1996; Paine et al. 1998; Freiich and Reich 1999; White et al. 1999). We have learned a tremendous amount about the significance of disturbance as an ecological factor in various habitats and communities (Knapp 1974; Grubb 1977; Miles 1979; Oliver 1981; Pickett and White 1985; Goldberg 1988; Frelich and Lorimer 1991; Milton et al. 1997), about disturbance regimes (Romme 1982; Turner et al. 1993; White et al. 1999), about functional adaptations of plants (Garcia-Mora et al. 1999; Walker et al. 1999), about responses of ecosystems (Bornette and Amoros 1996; Johnson et al. 1998; Engelmark et al. 1999) and about restoring disturbance as an ecosystem process (White and Walker 1997; Covington et al. 1999). During this period, a few theories and synthetic concepts have been proposed, but we do not yet have an inclusive general paradigm for this important body of work.
535 citations
TL;DR: The pendant protonated amine, in conjunction with the carbonyl face of the Fe(III) coordination shell, is proposed to play an important role in the recognition and membrane transport processes.
Abstract: Ferrioxamine B was successfully co-crystallized with ethanolpentaaquomagnesium(II) and perchlorate ions as counter ions, C27H62Cl3FeMgN6O26, and the crystal structure has been determined by single-crystal X-ray diffraction. The crystals are monoclinic, space group P21/n, four molecules per unit cell with dimensions a=21.1945(7) A, b=10.0034(3) A, c=106.560(1) A, and β=106.560(1)°. The crystal structure contains a racemic mixture of Λ-N-cis,cis and Δ-N-cis,cis coordination isomers. The structural parameters and the conformational features of ferrioxamine B compare very well with those of ferrioxamines D1 and E, with an exception of the orientation of the pendant protonated amine, which is pointing away from the connecting amide chains and towards the carbonyl face of the inner coordination shell distorted octahedron. This pendant protonated amine, in conjunction with the carbonyl face of the Fe(III) coordination shell, is proposed to play an important role in the recognition and membrane transport processes.
102 citations
TL;DR: The synthetic methodology for generating an anionic methylidene complex by hydride addition to neutral Tp'(CO)(2)W triple bond C--H contrasts with routes that utilize alpha-hydrogen abstraction or hydrides removal from neutral methyl precursors to generate methylidenes complexes.
Abstract: Treatment of [Tp‘(CO)2W⋮C−PPh3][PF6] (Tp‘ = hydridotris(3,5-dimethylpyrazolylborate)) with Na[HBEt3] in THF forms the methylidyne complex Tp‘(CO)2W⋮C−H via formyl and carbene intermediates Tp‘(CO)(C(O)H)W⋮C−PPh3 and Tp‘(CO)2WC(PPh3)(H), respectively. Spectroscopic features reported for Tp‘(CO)2W⋮C−H include the W⋮C stretch (observed by both IR and Raman spectroscopy) and the 183W NMR signal (detected by a 1H, 183W 2D HMQC experiment). Protonation of the Tp‘(CO)2W⋮C−H methylidyne complex with HBF4·Et2O yields the cationic α-agostic methylidene complex [Tp‘(CO)2WCH2][BF4]. The methylidyne complex Tp‘(CO)2W⋮C−H can be deprotonated with alkyllithium reagents to provide the anionic terminal carbide Tp‘(CO)2W⋮C−Li; a downfield resonance at 556 ppm in the 13C NMR spectrum has been assigned to the carbide carbon. The terminal carbide Tp‘(CO)2W⋮C−Li adds electrophiles at the carbide carbon to generate Tp‘(CO)2W⋮C−R (R = CH3, SiMe3, I, C(OH)Ph2, CH(OH)Ph, and C(O)Ph) Fischer carbynes. A pKa of 28.7 was determined f...
91 citations
87 citations
TL;DR: The catalyst was developed through the combination of arrayed catalyst evaluation and informed ligand design and converted in an enantioselective aldol-Tishchenko reaction directly into the chiral adduct by using catalytic amounts of a chiral base prepared in situ.
Abstract: Unmodified carbonyl compounds are converted in an enantioselective aldol-Tishchenko reaction directly into the chiral adduct by using catalytic amounts of a chiral base prepared in situ [Eq. (1)]. The catalyst was developed through the combination of arrayed catalyst evaluation and informed ligand design. Ad=adamantyl.
85 citations
83 citations
TL;DR: Labile (P−P)Pd(CH3)(OEt2)+BAr‘4- complexes (2) have been prepared via protonation of PdMe2 as discussed by the authors.
78 citations
68 citations
55 citations
TL;DR: The remarkably stable cationic, three-coordinate, 14-electron 14electron Rhodium complex has been synthesized, isolated and used as a catalyst for cyclopropanation reactions as discussed by the authors.
TL;DR: Tp‘PtMeH2 reacts with substoichiometric amounts of B(C6F5)3 in aromatic solvents to yield platinum(IV) aryl dihydride complexes via borane-induced reductive elimination of methane from 1 and subsequent C−H activation of the aromatic solvent.
TL;DR: Oxo-like reactivity exists for a new series of osmium complexes such as [Os(IV) (bpy)(Cl)3 (NCN)](-) ( bpy=2,2'-bipyridine, see structure) containing the cyanoimido ligand.
Abstract: Oxo-like reactivity exists for a new series of osmium complexes such as [Os(IV) (bpy)(Cl)3 (NCN)](-) (bpy=2,2'-bipyridine, see structure) containing the cyanoimido ligand. This ligand is formed directly at the metal center by the reaction of Os(VI) -nitrido precursors with tetraethylammonium cyanide. In the cyanoimido complexes there is an extensive electron-transfer chemistry at the metal center and an extensive functional-group chemistry based on the ligand.
TL;DR: DFT calculations supplement intuitive expectations regarding 3-center-2-electron bridging orbital descriptions for the electronic structure of this complex and X-ray structure determinations for the monomeric acetonitrile adduct 4 and the dicationic dimer 6 are reported.
Abstract: Tp‘PtMe(H)2 (2) [Tp‘ = hydridotris(3,5-dimethylpyrazolyl)borate] has been prepared from Tp‘PtMe(CO) (1) via reaction with water in a basic acetone/water mixture. Protonation of 2 at one of the pyra...
TL;DR: Preliminary electrochemical results for the isomers of [Ru(tpy)(4-CH2PO3H2-4'-Mebpy)(H2O)]2+, 4, adsorbed on ITO (In2O3:Sn) surfaces add support to a recently proposed electron-transfer mechanism involving cross-surface proton-coupled electron transfer.
Abstract: A series of polypyridyl ruthenium complexes of the general formula [Ru(tpy)(bpy‘)Cl]+ where tpy is 2,2‘:6‘,2‘ ‘-terpyridine and bpy‘ is 4-carboxy-4‘-methyl-2,2‘-bipyridine (4-CO2H-4‘-Mebpy), a proline derivative (4-CO-Pra(Boc)(OMe)-4‘-Mebpy), or 4-((diethoxyphosphinyl)methyl)-4‘-methyl-2,2‘-bipyridine (4-CH2PO3Et2-4‘-Mebpy) are prepared. For each complex, two isomers exist, and these are separated chromatographically. The structure of the hexafluorophosphate salt of cis-[Ru(tpy)(4-CO2H-4‘-Mebpy)Cl]+, cis-1, is determined by X-ray crystallography. The salt crystallizes in the monoclinic space group Cc with a = 12.4778(6) A, b = 12.6086(6) A, c = 20.1215(9) A, β = 107.08200(1)°, Z = 4, R = 0.058, and Rw = 0.072. The structures of the remaining complexes are assigned by 1H NMR comparisons with cis-1. The complexes are potentially important precursors for the incorporation of RuIVO2+ oxidants into polymers or peptides or for their adsorption onto oxide surfaces. Preliminary electrochemical results for the iso...
TL;DR: The Os(III)-ammine complexes are reoxidized electrochemically to the nitrido complexes by stepwise mechanisms involving the loss of both electrons and protons and sequential Os( III-->IV) and Os(IV-->VI) oxidations.
Abstract: This paper describes the 4e-/3H+ interconversion between NH3 and N3-, which is reversible in the coordination spheres of Os complexes containing either tpm (tpm = tris(1-pyrazolyl)methane) or Tp (Tp = hydrotris(1-pyrazolyl)borate anion) ligands. Electrochemical or chemical reduction of the nitrido complexes [OsVI(tpm)(Cl)2(N)]+ (1) and OsVI(Tp)(Cl)2(N) (2) in acidic aqueous solution gives the corresponding OsII−ammine complexes, which, after air oxidation and workup, are isolated and structurally characterized as [OsIII(tpm)(Cl)2(NH3)](PF6) (3) and OsIII(Tp)(Cl)2(NH3) (4). The OsIII−ammine complexes are reoxidized electrochemically to the nitrido complexes by stepwise mechanisms involving the loss of both electrons and protons and sequential Os(III→IV) and Os(IV→VI) oxidations.
TL;DR: Conditions are reported that allow OsV ≡N species to be trapped either in acidic, aqueous solutions as a neutral osmium(V) imido complex (OsV =NH, see structure) or in non-aqueous solvents, with high concentrations of added reductant, by N- ion transfer before coupling can occur.
Abstract: Trapped by a proton: In the formation of OsII -N2 -OsII dimers, an OsV ≡N complex has been invoked as a transient intermediate but not isolated. Herein conditions are reported that allow OsV ≡N species to be trapped either in acidic, aqueous solutions as a neutral osmium(V) imido complex (OsV =NH, see structure) or in non-aqueous solvents, with high concentrations of added reductant, by N- ion transfer before coupling can occur.
Journal Article•
TL;DR: In this paper, the authors used remote sensing and a geographic informa- tion system to model the distribution of evergreen shrub communities in the Great Smoky Moun- tains, North Carolina and Tennessee, USA.
Abstract: We used remote sensing and a geographic informa- tion system to model the distribution of evergreen shrub communities, called 'heath balds', in the Great Smoky Moun- tains, North Carolina and Tennessee, USA. The 421 heath balds averaged 1.8 ha in size and covered 0.3% of the land- scape. They reached their greatest importance on upper slopes (92% had relative slope positions > 80), convex topography (82% occurred on sites with a curvature greater than 2.6), and elevations between 1100 and 1600 m (94% of the balds). Although heath balds were found in old-growth watersheds, the two watersheds with the greatest number of balds burned extensively after logging in the early 1900s. Bald occurrence was positively correlated with burned sites, old growth condi- tion, and a highly acidic rock type. Heath balds showed a striking geographic pattern, with 88.1% of the area of this community found in six watersheds comprising only 35.4% of the study area. Despite similar topography, geology, and his- tory, the eleven other watersheds had only 11.9% of the bald area while comprising 64.4% of the study area. Multivariate models showed that this community occurs on only 0.4 to 9.0% of the seemingly appropriate sites. Once established, this shrub community, with its dense evergreen canopy and thick leaf litter, is resistant to tree invasion. Both forest and shrub communities are stable on sites that are seemingly ideal for heath bald occurrence.
01 Jan 2001
TL;DR: The key features of the suggested mechanisms are the extraordinary electron withdrawing effect of Os(VI) on tpy and the ability of the metal to undergo intramolecular Os( VI) to Os(IV) electron transfer.
Abstract: Reaction between the Os(VI)−hydrazido complex, trans-[OsVI(tpy)(Cl)2(NN(CH2)4O)]2+ (tpy = 2,2‘:6‘,2”-terpyridine and O(CH2)4N- = morpholide), and a series of N- or O-bases gives as products the substituted Os(VI)−hydrazido complexes, trans-[OsVI(4‘-RNtpy)(Cl)2(NN(CH2)4O)]2+ or trans-[OsVI(4‘-ROtpy)(Cl)2(NN(CH2)4O)]2+ (RN- = anilide (PhNH-); S,S-diphenyl sulfilimide (Ph2SN-); benzophenone imide (Ph2CN-); piperidide ((CH2)5N-); morpholide (O(CH2)4N-); ethylamide (EtNH-); diethylamide (Et2N-); and tert-butylamide (t-BuNH-) and RO- = tert-butoxide (t-BuO-) and acetate (MeCO2-). The rate law for the formation of the morpholide-substituted complex is first order in trans-[OsVI(tpy)(Cl)2(NN(CH2)4O)]2+ and second order in morpholine with kmorp(25 °C, CH3CN) = (2.15 ± 0.04) × 106 M-2 s-1. Possible mechanisms are proposed for substitution at the 4‘-position of the tpy ligand by the added nucleophiles. The key features of the suggested mechanisms are the extraordinary electron withdrawing effect of Os(VI) on tpy and...
TL;DR: The oxidations of benzyl alcohol, PPh3, and the sulfides by the Os(VI)-hydrazido complex trans-[Os(VI)(tpy)(Cl)2(NN(CH2)4O)](2+) have been investigated in CH3CN solution by UV-visible monitoring and product analysis by gas chromatography-mass spectrometry.
Abstract: The oxidations of benzyl alcohol, PPh3, and the sulfides (SEt2 and SPh2) (Ph = phenyl and Et = ethyl) by the Os(VI)-hydrazido complex trans-[Os(VI)(tpy)(Cl)2(NN(CH2)4O)](2+) (tpy = 2,2':6',2' '-terpyridine and O(CH2)4N(-) = morpholide) have been investigated in CH3CN solution by UV-visible monitoring and product analysis by gas chromatography-mass spectrometry. For benzyl alcohol and the sulfides, the rate law for the formation of the Os(V)-hydrazido complex, trans-[Os(V)(tpy)(Cl)2(NN(CH2)4O)](+), is first order in both trans-[Os(VI)(tpy)(Cl)2(NN(CH2)4O)](2+) and reductant, with k(benzyl) (25.0 +/- 0.1 degrees C, CH3CN) = (1.80 +/- 0.07) x 10(-4) M(-1) s(-1), k(SEt2) = (1.33 +/- 0.02) x 10(-1) M(-1) s(-1), and k(SPh2) = (1.12 +/- 0.05) x 10(-1) M(-1) s(-1). Reduction of trans-[Os(VI)(tpy)(Cl)2(NN(CH2)4O)](2+) by PPh3 is rapid and accompanied by isomerization and solvolysis to give the Os(IV)-hydrazido product, cis-[Os(IV)(tpy)(NCCH3)2(NN(CH2)4O)](2+), and OPPh3. This reaction presumably occurs by net double Cl-atom transfer to PPh3 to give Cl2PPh3 that subsequently undergoes hydrolysis by trace H2O to give the final product, OPPh3. In the X-ray crystal structure of the Os(IV)-hydrazido complex, the Os-N-N angle of 130.9(5) degrees and the Os-N bond length of 1.971(7) A are consistent with an Os-N double bond.
TL;DR: In this paper, the structure of a tungsten(II) methylene complex was obtained by taking advantage of the stabilizing, weakly coordinating counterion, BAr′ 4 −, to form X-ray quality single crystals.
01 Jan 2001
TL;DR: In this paper, the authors focus on the short and long-term dynamics of the populations and ecosystems of the sites that are assessed, and the effects of dynamic patterns and processes can lead to erroneous conclusions concerning a given site's condition and functional nature.
Abstract: Ecological assessments often begin with the inventory of a site’s biological resources and culminate in the generation of a static, two-dimensional map or other fixed report of present site conditions. Using this approach, we may fail to appreciate the short- and long-term dynamics of the populations and ecosystems that we are assessing. Spatial and temporal dynamics of patterns and processes are important to consider for at least three reasons. First, the very resources we assess may be changing, with consequences for the conclusions of the assessment itself. Second, the dynamics have implications for whether and under what conditions the populations and ecosystems are sustainable on the site of interest. Finally, key processes may occur at broader spatial scales or on longer temporal scales than we would otherwise consider in the assessment; that is, the populations and communities at one site may be attributed in part to the spatial context surrounding that site or the occurrence of rare past events. In essence, the processes responsible for the biological resources on the site may themselves not be contained within the site being assessed, and the resources may not be in balance with current site processes. Overlooking the effects of dynamic patterns and processes can therefore lead to misinterpretations of observations and erroneous conclusions concerning a given site’s condition and functional nature.
TL;DR: In this paper, an enantioselective aldol-Tishchenko reaction was used to convert modified carbonyl compounds into chiral adducts using catalytic amounts of a chiral base.
Abstract: Unmodified carbonyl compounds are converted in an enantioselective aldol-Tishchenko reaction directly into the chiral adduct by using catalytic amounts of a chiral base prepared in situ [Eq. (1)]. The catalyst was developed through the combination of arrayed catalyst evaluation and informed ligand design. Ad=adamantyl.
TL;DR: The remarkably stable cationic, three-coordinate, 14-electron 14electron Rhodium complex has been synthesized, isolated and used as a catalyst for cyclopropanation reactions as mentioned in this paper.
Abstract: The remarkably stable cationic, three-coordinate, 14-electron
rhodium complex 1 has been synthesized, isolated and used as a catalyst for
hydrosilation, Mukaiyama aldol and cyclopropanation reactions.