1,721,031 research outputs found
Mechanism of the Mukaiyama Aldol Reaction: The First Solid-State Characterization of a Trichlorotitanium Aldolate
No full textThe reaction of the silyl ketene acetal Me2C=C(OMe)OSiMes (1) with preformed TiCl4 carbonyl adducts (the monomeric [PhC(O)C(O)PhTiCl4] (2) and the dimeric {[MesCHOTi(Cl)3]2(mu-Cl)2} (Mes = 2,4,6-Me3C6H2; 3)) led to the isolation and structural characterization of the corresponding titanium aldolate complexes {[(PhCO)PhC(O)C(Me2)C(OMe)OTi(Cl)2]2(mu-Cl)2} (4) and {[MesCH(O)C(Me2)C(OMe)OTi(Cl)2]2(mu-Cl)2} (5), in which the aldolate fragment functions as a bidentate ligand in dimeric titanium complexes bridged by two chlorine atoms. Crystallographic details: 4 is monoclinic, space group P2(1)/n, with a = 9.029(1) angstrom, b = 17.468(2) angstrom, c - 13.165(2) angstrom, beta = 98.81(2)-degrees, Z = 2, and R = 0.53
Oxazoline Early Transition Metal Complexes: Functionalizable Achiral Titanium(IV), Titanium(III), Zirconium(IV), Vanadium(III), and Chiral Zirconium(IV) Bis(oxazoline) Complexes
No full textLarge-scale syntheses of the (hydroxyphenyl)oxazoline ligands L(1) = 6, L(2) = 7, L(3) = 8, L(4) = 9, and L(5) = 10 are reported. The conversion of 6-10 into the corresponding sodium salts has been employed for metal complexation. The reaction of 6 with TiCl4 . 2THF, TiCl3 . 3THF, ZrCl4 . 2THF, and VCl3 . 3THF in a 1:2 metal:ligand ratio afforded the corresponding hexacoordinate functionalizable metal complexes: cis-[(Cl)(2)Ti(L(1))(2)], 11; cis-[(THF)(Cl)Ti-(L(1))(2)], cis-[(Cl)(2)Zr(L(1))(2)], 13; trans-[(THF)(Cl)V(L(1))(2)], 14. The structures of 11, 12, and 14 have been clarified by X-ray analysis. The reaction of the 8 and 9 sodium salts with ZrCl4 . 2THF in a 1:2 metal:ligand ratio led to the corresponding chiral functionalizable metal complexes, whose structures were clarified by X-ray analysis: cis-[(Cl)(2)Zr(L(3))(2)], 15; cis-[(Cl)(2)Zr(L(4))(2)], 16. A common significant feature discovered from these studies is the existence of intramolecular hydrogen bonding, which is probably responsible for the high conformational rigidity of the oxazoline ligand. Crystallographic details are as follows: 11 is monoclinic, space group I2/a, with a = 16.443(2) Angstrom, b = 8.365(1) Angstrom, c = 17.945(2) Angstrom, beta = 106.93(1)degrees, Z = 4, and R = 0.034; 12 is orthorhombic, space group Pca2(1) with a = 15.669(2) Angstrom, b = 17.750(3) Angstrom, c = 18.971(3) Angstrom, Z = 8, and R = 0.050; 14 is monoclinic, space group P2(1)/n, with a = 9.391(4) Angstrom, b = 28.618(6) Angstrom, c = 12.077(4) Angstrom, beta = 105.79(2)degrees, Z = 4, and R = 0.052; 16 is monoclinic, space group P2(1), with a = 10.161(2) Angstrom, b = 14.151(3) Angstrom, c = 12.361(2) Angstrom, beta = 93.76(2)degrees, Z = 2, and R = 0.033
alpha-Sulfonyl Carbanion-Transition-Metal Bonds. Alkali-Metal alpha-Sulfonyl Carbanions and Their Reactivity with Metal Complexes
No full textDeprotonation of p-MeC(6)H(4)SO(2)Me (1) with KH in THF in the presence of 18-crown-6 led to the isolation of the ion contact pair [p-MeC(6)H(4)S(CH2)O-2- - -K(18-crown-6)] (2). The alpha-sulfonyl carbanion has been involved in the derivatization of both early- and late-transition-metal complexes. When the deprotonation of 1 in situ was followed by a reaction with Cp(2)TiCl(2) (Cp eta(5)-C5H5), we obtained a Ti-C-bonded alpha-sulfonyl complex, [Cp(2)Ti(Cl)(CH2-SO (2)C(6)H(4)Me-p)] (4), while the reaction of 2 with Cp(2)ZrCl(2) led to a 1:1 mixture of Zr-C- and Zr-O-bonded alpha-sulfonyl complexes, [Cp(2)Zr(Cl)(CH(2)SO(2)C(6)H(4)Me-p)] (5a) and [Cp(2)Zr(Cl)-(O2S(CH2)C(6)H(4)Me-p)] (5b). The reaction of the alpha-sulfonyl carbanion derived from 1 with [cis-Pd(PPh(3))(2)Cl-2] gave a Pd-C-bonded alpha-sulfonyl complex occurring with the isomerization of the metal center, [trans-(PPh(3))(2)(Cl)Pd-CH2SO2(p-MeC(6)H(4))] (7). Crystallographic details: 2 is monoclinic, space group P2(1)/c, with a = 8.635(2) Angstrom, b = 14.879(3) Angstrom, c = 19.589(3) Angstrom, alpha = gamma = 90 degrees, beta = 96.30(2)degrees, Z = 4, and R = 0.057; 4 is monoclinic, space group C2/c, with a = 22.318(3) Angstrom, b = 9.727(2) Angstrom, c = 16.267(3) Angstrom, alpha = gamma = 90 degrees, beta 100.08(2)degrees, Z = 8, and R = 0.047
A Novel Homogeneous Lewis Acid Catalyst: Bistriflatedibenzotetramethyltetraaza annulenezirconium(IV) in a Cationic Form
No full textThe catalytic properties of bistriflatedibenzotetramethyltetraazaannulenezirconium(IV), [Zr(tmtaa)(OTf) 2 ] a prototype of a novel class of organometallic Lewis acids capable of promoting aldol condensations and allylation reactions in fair to good yields have been disclosed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
beta-Keto Amino Enolates Binding to Transition Metals: Synthesis and Structure of the Ion-Pair Form and Its Mono- and Bidentate Coordination to Zirconium and Nickel
No full textThe 2-piperidinoacetophenone, 1, was deprotonated in THF by KH, and the resulting beta-keto amino enolate-potassium ion pair, 2, complexed with 18-crown-6 to give 3, in which the enolate group chelates the [K(18-crown-6)]+ moiety. The reaction of 2 with cp2-ZrCl2 [cp = eta5-C5H5] afforded complex 4 in which the beta-keto amino enolate exhibits an eta1-O bonding mode to zirconium(IV). The enolate 2 acts as a N-O bidentate ligand complexing Ni(II) in the form of a square planar, diamagnetic complex. Crystallographic details: 3 is monoclinic, space group P2(1)/n, with a = 11.836(2) angstrom, b = 16.757(3) angstrom, c = 13.899(2) angstrom, beta = 95-55(1)degrees, Z = 4, and R = 0.081; 5 is triclinic, space group P1BAR, with a = 9.415(1) angstrom, b = 11.533(1) angstrom, c = 17.892(3) angstrom, alpha = 102.81(2)degrees, beta = 92.00(2)degrees, gamma = 67.28(2)degrees, Z = 3, and R = 0.057
(eta6-Acetophenone)Cr(CO)3 Enolates Complexed to Bis(cyclopentadienyl) titanium(IV) and -zirconium(IV)
No full textThis is a synthetic and structural report on substituted acetophenone-Cr(CO)(3) enolate complexes of titanium(IV) and zirconium(IV). The deprotonation of[(eta(6)-2,4,6-R'R''R'''C6H2-COMe)Cr(CO)(3)] [R' = R'' = R''' = H, 1; R' = Me, R'' = R''' = H, 2; R' = R'' = Me, R''' = H, 3, R' = R'' = R''' = Me, 4] with KH in the presence of 18-crown-6 in THF gave the ion-pair enolates [{eta(6)-2,4,6-R'R''R'''C6H2-C(CH2)O-K(18-crown-6)} Cr(CO)(3)] [R' = R'' = Me, R''' = H, 5; R' = R'' = R''' = Me, 6]. When the deprotonation of 1-4, carried out using LiNPr2i in THF, was followed by reaction with (cp)(2)MCl(2) [M = Ti, Zr], dimetallic enolates were isolated and characterized [eta(6)-2,4,6-R'R''R'''C6H2-C(CH2)O-M(Cl)(cp)(2)][R' = R'' = R''' = H, M = Ti, 7; R' Me, R'' = R''' = H, M = Zr, 8; R' = R'' = Me, R''' = H, M = Zr, 9;R' = R'' = R''' Me, M = Ti, 10; R' = R'' = R''' = Me, M = Zr, 11]. In all of these complexes, the enolato functionality is O-bonded to the oxophilic [(cp)(2)M(Cl)] fragment. Complexes 10 and 11 have been isolated as an 85:15 diastereoisomeric mixture with a significant diastereoselection for the most hindered form. The two diastereoisomers cannot be interconverted thermally because of the very high barrier to free rotation around the phenyl-C(enolate) bond, as from our calculations. An X-ray crystallographic analysis has been carried out on the most hindered diastereoisomer of 11 and compared with that of the starting complex 4, where the oxygen is directed to the opposite face of the arene ring. In order to clarify the mechanism of the diastereoisomerization, the protolysis of the potassium and lithium enolates, including that of the pyridine adduct 12[{eta(6)-2,4,6-Me(3)C(6)H(2)-C(CH2)OLiPy(2)}Cr(CO)(3)], has been carried out. The protolysis, which produces exclusively the starting diastereoisomer of 4 having the oxygen syn to the [Cr(CO)(3)] group, supports strongly the fact that there is no change in the conformation at the level of the enolato formation. Then the comparison between results from the reaction of the lithium enolate of 4 and 12 with Me(3)SiCl, MeBu(2)(t)SiCl, Me(3)SiOSO(2)-CF3, and (cp)(2)ZrCl2 shows that the diastereoselection depends mostly on steric,rather than on electronic; factors. Further support for the steric argument comes when some steric hindrance has been introduced in the starting ketone, i.e. using [(eta(6)-2,4,6-R'R''R'''C6H2COCH2-CH3)Cr(CO)(3)]. The corresponding lithium enolate reacted with (cp)(2)ZrCl2 gave a diastereoselection with a 67:33 ratio. The only plausible hypothesis for explaining the diastereoisomerization occurring at the stage of the reaction of the enolate with the electrophile is an intermolecular migration of the [Cr(CO)(3)] fragment from the more to the less hindered face of the arene ring. Crystallographic details: 4 is monoclinic, space group P2(1)/c, a 6.858(1) Angstrom, b = 13.977(2) Angstrom, c 14.557(2) Angstrom, alpha = gamma = 90 degrees, beta = 94.46(1)degrees, Z = 4, and R = 0.034; 11 is monoclinic, space group P2(1)/c, a = 11.238(1) Angstrom, b = 14.144(1) Angstrom, c = 14.808(1) Angstrom, alpha = gamma = 90 degrees, beta = 95.12(1)degrees, Z = 4, and R = 0.033
- …
