Showing papers in "Journal of the American Chemical Society in 1986"

The use of global and local molecular parameters for the analysis of the gas-phase basicity of amines.

Abstract: It is demonstrated that the variation of the gas-phase basicities of amines can be analyzed by using two parameters: one global and one local (that is, site-dependent). Two global quantities (the average ”effective” electronegativity and the geometric average of the isolated-atom electronegativities) and two local quantities (the fukui function and the residual charges) are tested. A two-parameter linear model containing one global and one local quantity produces satisfactory correlations with the experimental gas-phase basicities. It is shown how to express the fukui function, which reflects the site reactivity in density functional theory f l i t ) = [ap(F)/aNl,, ,) , in terms of the variation of the Mulliken gross charges (4,) of an atom in a molecule, which is accomDanied with a change in the total number of electrons ( N ) in this molecule: f,’ = q, (N + 1) q,(N); f,= q,(N) qi(N .l) andf,’ = 1/2[qiTN + 1 ) qi (N I ) ] . The electron density distribution is fundamental for understanding chemical reactivity and naturally leads to explaining nucleophylic and electrophylic attack on the basis of electrostatic interactions. However, the electron density by itself does not explain everything, and it was recognized that the change in electron density under the influence of an approaching reagent is also of importance. The frontier-electron theory of chemical reactivity by Fukui’*2 recognizes the key role of the valence electrons in forming molecules and considers therefore the distribution of the highest energy orbital electron density as being most important for electrophilic attack and the lowest energy vacant orbitals in nucleophilic substitution reactions. In reactions with radicals both orbitals become important. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are in this way considered as the principal factors governing the easiness of chemical reactions and the stereoselective path. It was recently demonstrated by Parr et aL3q4 that most of the frontier-electron theory of chemical reactivity can be rationalized from the density functional theory of the electronic structure of molecules.5%6 For a system of N electrons with ground-state energy E[n,v] , where G‘ is the potential energy acting on an electron due to the presence of all nuclei, several quantities of fundamental importance can be defined. The chemical potential p of the electrons, Le., the negative of the electronegativity x, is given by p = (aE/am,( , and has the same value everywhere. The change of p with the number of electrons was defined by Parr and Pearson’ as a measure for the “absolute hardness” as 7 = (&.L/c~N),(, = (C3*E/aM),(,. The frontier function or fukui function for a University of North Carolina. *University of North Carolina and K. U. Leuven. molecule, reflecting the reactivity of a site, was defined by Parr and Yang3 as Le., the functional derivative of the chemical potential with respect to a change in the external potential or, because of the Maxwell relations, identical with the change in electron density accompanied with a change in the number of electrons. The hardness and the fukui function are local quantities and reflect the properties of the different sites within the molecule. Probing the reactivity of the different sites within a molecule, being of importance for understanding reaction mechanisms, only partially meets the chemists aspirations. A comparison of different molecules is also of importance. The object of the present study is to explore different ways to make this comparison by using easily obtainable parameters. Substituent group properties have been used extensively in this context. However, as the atomic properties change from molecule to molecule, so will substituent groups be subject to variations. Alkyl groups, e.g., may act either as electron donors or as electron acceptors, depending on the substrate. One of the properties of interest of substituent groups is their tendency to attract electrons, i.e., their electronegativity. Depending on the actual molecular configuration, this may qualify a group as electron donor or electron acceptor. In a study of the gas-phase (!) Fukui, K. Theory of Orientation and Stereoselection; Springer-Verlag: (2) Fukui, K. Science (Washington, DC) 1982, 218, 747-754. (3) Parr, R. G.; Yang, W. J. Am. Chem. SOC. 1984, 106, 4049-4050. (4) Yang, W.; Parr, R. G.; Pucci, R. J . Chem. Phys. 1984,81,2862-2863. ( 5 ) Hohenberg, P.; Kohn, W. Phys. Reu. 1964, 136, B864-8871. (6) Parr, R. G. Annu. Reu. Phys. Chem. 1983, 34, 631-656. (7) Parr, R. G.; Pearson, R. G. J . Am. Chem. So?. 1983,105, 7512-7516. Berlin, 1973; p 134. 0002-7863/86/1508-5708$01.50/0

Epoxidation of olefins with cationic (salen)manganese(III) complexes. The modulation of catalytic activity by substituents.

Abstract: Augmentation d'activite avec des substituants attracteurs d'electrons (dichloro ou dinitro)

High-resolution aluminum-27 NMR of aluminosilicates

Abstract: Recent advances in instrumentation and techniques have made a precise determination of isotropic chemical shifts of quadrupolar nuclei in solids practicable. Aluminum-27 chemical shifts in aluminosilicates closely parallel those of silicon-29 and display a similar dependence upon changes in both the first and second coordination spheres and the Al-O-Si bond angles.

Binding modes and base specificity of tris(phenanthroline)ruthenium(II) enantiomers with nucleic acids: tuning the stereoselectivity

Abstract: Binding of tris(phenanthroline)ruthenium(II), R~(phen),~+, enantiomers to nucleic acids of different base compositions and structure was examined by equilibrium dialysis and photophysical methods. Measurement of enantioselectivity combined with photophysical experiments permits the structural characterization of two noncovalent binding modes of the ruthenium(I1) complexes to the DNA helix, one intercalatively bound mode showing a strong chiral preference for A-R~(phen)~'+ and the other, a surface-bound mode along the DNA major groove, showing a weak preference for A-R~(phen),~+. Luminescence decay of R~(phen),~+ isomers in the presence of DNA shows components of two different lifetimes. Quenching of the emission with ferrocyanide results in nonlinear Stern-Volmer plots. Finite polarization in the emission of both A- and A-R~(phen)~~+ in the presence of DNA is indicative of intercalation; greater polarization is found consistently for A-R~(phen)~~+ with DNA. The total binding affinity of R~(phen),~+ to DNA is ionic strength dependent in a manner consistent with the release of 2.2 counterions per bund ruthenium. Although binding to DNA of Ru(phen)p shows no clear dependence on the guanine-cytosine (GC) content of the DNA, variations in enantiomeric preferences both as a function of GC content and as a function of ionic strength are observed. Chiral discrimination for A-Ru(phen),2+ increases both with the percent GC and with increasing Na+ concentration. Based upon the stereoselectivities found by steady-state emission polarization, the variations are attributed to changes in chiral preferences for intercalation. This variation may indicate local changes in DNA groove size, e.g. a compression along the helix axis direction with increasing ionic strength or increasing percent GC. Weak surface binding, having a preference for A-R~(phen),~+, is observed with double-stranded RNA. For both R~(phen)~~+ and RU(DIP),~+ (DIP = 4,7-diphenyl- phenanthroline), binding to T4 DNA glycosylated in the major groove is markedly diminished compared to binding to calf thymus DNA. The chiral ruthenium complexes, with luminescence characteristics indicative of binding modes, and stereo- selectivities that may be tuned to the helix topology, may be useful molecular probes in solution for nucleic acid secondary structure

Electrocatalytic reduction of carbon dioxide by nickel cyclam2+ in water: study of the factors affecting the efficiency and the selectivity of the process.

Abstract: The electrocatalytic properties of Ni cyclam/sup 2 +/ in CO/sub 2/ reduction have been studies. The influence of various experimental factors on the course of the reaction has been investigated. Under given conditions, Ni cyclam/sup 2 +/ is remarkably efficient and selective for electroreduction of CO/sub 2/ into CO, even in water. The stability of the system has been tested in long range electrolysis, showing that even after thousands of catalytic cycles no deactivation occurs. The mechanistic investigation presently described points to the importance of molecular species adsorbed on the cathode surface. The contribution of reduced species in the bulk is marginal. In addition, during the electrocatalytic process, a nickel(I) carbonyl complex is present: (Ni(cyclam)(CO))/sup +/. The latter complex may be involved in the catalytic reaction leading to CO evolution. Surprisingly, the behavior of Ni cyclam/sup 2 +/ as electrocatalyst is unique. Among the numerous complexes investigated, only Ni cyclam/sup 2 +/ shows such a large selectivity for the electroreduction of CO/sub 2/ over the electroreduction of water. The size of the cyclam ligand and the presence of secondary amine groups (NH) might account for the very special properties of the electrocatalyst.

Palladium-catalyzed coupling of vinyl triflates with organostannanes. Synthetic and mechanistic studies

Abstract: The palladium-catalyzed coupling reaction of vinyl triflates with acetylenic, vinyl, allyl, and alkyl tin reagents in the presence of lithium chloride or another suitable salt takes place in high yields under mild reaction conditions; however, benzyl and phenyl tin reagents give poor yield of coupled product. The utilization of a tin or silicon hydride reagent in place of the organotin partner yields the alkene by reductive cleavage of the triflate group. The palladium-catalyzed reaction of vinyl triflates with hexamethylditin gives vinyl stannanes in high yields. Regioselectively formed vinyl triflates can be used to produce 1,3-dienes as the regioisomeric coupled products.

Hexasulfonated calix[6]arene derivatives: a new class of catalysts, surfactants, and host molecules.

Abstract: Water-soluble hexasulfonated calix[6]arenes with various substituents (I-R) have been synthesized for the first time and applied as host molecules in an aqueous system. Dynamic ‘H NMR studies established that calb[6]arene-p-hexasulfonate (1-H) adopts a “winged” or “hinged” conformation in D20-Me2SO-d6 (2:l v/v) owing to hydrogen bonding among the OH groups, while 5,11,17,23,29,35-hexasulfonato-37,38,39,40,41,42-hexakis(hexyloxy)calix[6]arene (l-C6) adopts a similar conformation in D20 owing to hydrophobic bonding among the hexyl groups. The aggregation behavior in water was examined about I-C6 and I-C12. Physical (light-scattering, surface tension, and conductance) and spectral (fluorescence and absorption spectroscopies) studies established that 1-C, has a cmc at ca. 6 X IO4 M, as does sodium dodecyl sulfate (SDS), while I C l 2 has no detectable cmc and rather acts as a “unimolecular” micelle. In fact, l-C6 associated with small molecules (pyrene, 2-anilinonaphthalene, and Orange OT) according to the micelle-like biphasic concentration dependence, while 1-C12 formed host-guest-type 1: 1 complexes with these molecules. It was found that these calix[6]arene derivatives efficiently accelerate acid-catalyzed hydration of I-benzyl1,4-dihydronicotinamide and the reaction proceeds according to the Michaelis-Menten kinetics. In particular, the rate constants for 1-H and 1-CH2COOH, which both have acidic protons to catalyze the reaction and anionic sulfonates to stabilize the cationic intermediate at the two edges of the cavity, were greater by 426-1220-fold than those for noncyclic analogues. These findings indicate that hexasulfonated calix[6]arenes serve as a new class of catalysts, surfactants, and host molecules. This is the first example for the host-guest-type behavior of calixarenes observed in an aqueous system. The chemistry of cyclodextrins and cyclophanes has occupied a central interest in host-guest chemistry for the last two decades, and many functionalized host molecules which can mimic the in vivo action of enzymes by means of simple in vitro chemical systems have been exploited.2d Recently, Gutsche and coworker~’,~ have reported on a series of new cyclic molecules called “calixarenes” which are cyclic oligomers made up of benzene units as cyclodextrins are made up of glucose units. Since calixarenes possess a cylindrical architecture similar to cyclodextrins, they are expected to be useful to design enzyme mimics in totally synthetic systems.’.* Several groups have reported on the ionophoric properties of calixarenes which were obtained by introducing ether and/or ester groups into the edge of the cylindrical a rch i te~ture .~’~ In contrast, almost nothing is known with certainty as to the inclusion properties of calixarenes in solution, which should be more important in the design of the enzyme mimics. The data reported so far have been limited to only the solid and in fact, Gutsche stated in his recent review (1) Preliminary communications: (a) Shinkai, S.; Mori, S.; Tsubaki, T.; Sone, T.; Manabe, 0. Tetrahedron L e f f . 1984, 25, 5315. (b) Shinkai, S.; Koreishi, H.; Mori, S.; Sone, T.; Manabe, 0. Chem. Letf. 1985, 1033. (2) Breslow, R. Acc. Chem. Res. 1980, 13, 170. (3) Tabushi, I. Acc. Chem. Res. 1982, 15, 66. (4) Komiyama, M.; Hirai, H. J . Am. Chem. SOC. 1983, 105, 2018. (5) Bender, M. L.; Komiyama, M. In ‘Cyclodextrin Chemistry”; Spring(6) Murakami, Y. ‘Cyclophanes 11”; Springer-Verlag: Berlin, 1983; p 107. (7) Gutsche, C. D. Arc. Chem. Res. 1983, 16, 161. (8) Gutsche, C. D. In “Host Guest Complex Chemistry/Macrocycles”; (9) Chang, S.-K.; Cho, I. Chem. Lett. 1984, 477. (10) Ungaro, R.; Pochini, A.; Andreetti, G. D. J. Inclusion Phenom. 1984, er-Verlag: New York, 1977. Springer-Verlag: Berlin, 1985, p 375. 2. 199. -, . (1 1) Bocchi, V.; Foina, D.; Pochini, A,; Ungaro, R.; Andreetti, G. D. (12) McKervey, M. A.; Seward, E. M.; Ferguson, G.; Ruhl, B.; Harris, S. Tetrahedron 1982, 38, 373. J. J . Chem. SOC., Chem. Commun. 1985, 388. (1 3) Calixarenes can extract certain metal cations into the organic phase as their counterions: (a) Izatt, R. M.; Lamb, J. D.; Hawkins, R. T.; Brown, P. R.; Izatt, S. R.; Christensen, J. J. J. Am. Chem. Soc. 1983, 105, 1782. (b) Izatt, S. R.; Hawkins, R. T.; Christensen, J. J.; Zatt, R. M. Ibid. 1985, 107, 63. (14) Andreetti, G. D.; Ungaro, R.; Pochini, A. J. Chem. Soc., Chem. Commun. 1979, 1005. (15) (a) Coruzzi, M.; Andreetti, G. D.; Bocchi, V.; Pochini, A,; Ungaro, R. J . Chem. Soc., Perkin Trans. 2 1982, 1133. (b) Ungaro, R.; Pochini, A,; Andreetti, G. D.; Domiano, P. Ibid. 1985, 197. 0002-7863/86/ 1508-2409$01.50/0 article that there are no published data in support of solution complexes of calixarenes.8*16 This is in sharp contrast to cyclodextrins, which can form a variety of host-guest-type solution complexes. We noticed that the difference would stem mainly from the poor solubility of calixarenes; they are sparingly soluble in several organic solvents but insoluble in aqueous solutions. Therefore, the experimental efforts should be directed toward solubilization of calixarenes, which would eventually lead to tHe exploitation of calixarene-based host molecules and enzyme mimics. Here, we wish to report the synthesis and the solution properties of new water-soluble hexasulfonated calix[6]arenes (1-R). We have found that they serve not only as host molecules in an aqueous system but also as a new class of surfactants and acid catalysts.

A new group of ruthenium complexes: structure and catalysis

Abstract: Structure ORTEP et activite pour l'hydrogenation de la cyclohexanone. Cycle d'hydrogenation-deshydrogenation de butylcyclohexanol

Surface-enhanced Raman spectroscopy of amino acids and nucleotide bases adsorbed on silver

Abstract: Interpretation des spectres SERS de la glycine, des α- et β-alanines, de l'acide amino-6 caproique et des acides o- et m-aminobenzoiques adsorbes sur des particules d'argent colloidal. Donnees, aussi, sur les spectres SERS de l'adenine, de la cytosine et de l'uracile

Ionization potentials and electron affinities in aqueous solution

Abstract: By combining gas-phase proton affinities and aqueous pK/sub a/ values, it is possible to obtain free energies of hydration for a large number of cations and anions. With some corrections these numbers can be used to convert gas-phase ionization potentials, I, and electron affinities, A, into the corresponding aqueous solution values, I' and A'. An assumption must be made about the free energy of hydration of H/sup +/, taken as -259.5 kcal/mol. The absolute potential of the hydrogen electrode is then 4.50 V, and values of I' and A' can be converted to standard oxidation or reduction potentials. The absolute electronegativity, (I' + A')/2, in solution is estimated and found to be little different from the gas-phase value, (I + A)/2. The absolute hardness, (I'-A')/2, is much smaller than the gas-phase value, but it is shown that the solution value has little utility. The gas-phase value should be used even in solution.

Nonlinear effects in asymmetric synthesis. Examples in asymmetric oxidations and aldolization reactions.

Abstract: L'oxydation asymetrique du methyl p-tolyl sulfure ou l'epoxydation asymetrique du geraniol en presence de divers complexes chiraux du titane, ainsi que la reaction Hajos-Parrish catalysee par la proline, sont etudiees en detail

Reactions of alkenylchromium reagents prepared from alkenyl trifluoromethanesulfonates (triflates) with chromium(II) chloride under nickel catalysis.

Abstract: philicities in these complexes, with vanadium in complexes 4 and 5 exhibiting greater electrophilicity. Given the inclination of other 17-electron metal carbonyl complexes to undergo associative ligand displacement reactions, it seems ironic that the incorporation of a pentadienyl ligand, which would be expected to promote associative attack via $ 73 coordination changes, should lead instead to a series of compounds for which much lower rates of substitution are observed and for which the substitutions also take place via dissociative means. Additional kinetic, EPR, and structural studies are under way in attempts to better understand the unexpected behavior of these 17-electron complexes.

Symmetric vs. asymmetric linear M-X-M linkages in molecules, polymers, and extended networks.

Abstract: Linear M-X-M linkages in which X is a nitride, oxide, or halide commonly occur in dimers, square tetramers, one-dimensional polymers, and extended three-dimensional solids. For low d electron counts a second-order Jahn- Teller mixing of metal d, and X p, orbitals favors asymmetric M-X-M bridges. M-X u bonding works against the distortion. Going to higher d electron counts also favors the symmetrical bridge by filling M-X x* levels. For the cyclic (MI,,X), tetramer and ;(ML.,X) chain, d electron counts greater than two favor a symmetric bridge; for perovskites, d" metals with n 1 1 are calculated to be symmetric. The extent of M-X bond length alternation can also be decreased by increasing the electronegativity difference between M and X to widen the HOMO-LUMO gap. Vertex-sharing polyhedra abound in solid-state and molecular transition-metal chemistry. Ever since Taube's classic work with Cr(OH2)62+, studies of inner-sphere electron-transfer reactions have focused on an intermediate with two octahedrally coordinated metal atoms sharing a common bridging ligand.' Similarly, some of the oldest, most extensively studied metallic compounds are the tungsten bronzes2 The cubic modification of the tungsten bronzes adopts the simple cubic array of corner linked WO, octahedra shown in 1. The octahedra are arranged to form linear W-0-W bridges and cube centers are randomly occupied by metal 2 3

Chiral synthesis via organoboranes. 7. Diastereoselective and enantioselective synthesis of erythro- and threo-.beta.-methylhomoallyl alcohols via enantiomeric (Z)- and (E)-crotylboranes

Abstract: Synthese par reaction de crotylboranes avec les acetaldehyde, acroleine, benzaldehyde, propanal. Mecanismes

Asymmetric glycine enolate aldol reactions: synthesis of cyclosporin's unusual amino acid, MeBmt

Abstract: Synthese de l'acide (hydroxy-3 methyl-4 methylamino-2) octene-6oique apr aldolisation entre l'enolate de la (benzyl-4 isothiocyanatoacetyl-3) oxazolidone-2 et des alcenals

Titanacyclobutanes derived from strained, cyclic olefins: the living polymerization of norbornene

Abstract: Mecanisme de la polymerisation par ouverture de cycle, par metathese en presence de ces derives