Showing papers on "Steric effects published in 2004"

Iron catalyzed polyethylene chain growth on zinc: a study of the factors delineating chain transfer versus catalyzed chain growth in zinc and related metal alkyl systems.

Abstract: The bis(imino)pyridine iron complex, [[2,6-(MeC=N-2,6-iPr2C6H3)2C5H)N]FeCl2] (1), in combination with MAO and ZnEt2 (> 500 equiv.), is shown to catalyze polyethylene chain growth on zinc. The catalyzed chain growth process is characterized by an exceptionally fast and reversible exchange of the growing polymer chains between the iron and zinc centers. Upon hydrolysis of the resultant ZnR2 product, a Poisson distribution of linear alkanes is obtained; linear alpha-olefins with a Poisson distribution can be generated via a nickel-catalyzed displacement reaction. Other dialkylzinc reagents such as ZnMe2 and ZniPr2 also show catalyzed chain growth; in the case of ZnMe2 a slight broadening of the product distribution is observed. The products obtained from Zn(CH2Ph)2 show evidence for chain transfer but not catalyzed chain growth, whereas ZnPh2 shows no evidence for chain transfer. The Group 13 metal alkyl reagents AlR3 (R = Me, Et, octyl, IBu) and GaR3 (R = Et, nBu) act as highly efficient chain transfer agents, whereas GaMe3 exhibits behavior close to catalyzed chain growth. LinBu, MgnBu2 and BEt3 result in very low activity catalyst systems. SnMe4 and PbEt4 give active catalysts, but with very little chain transfer to Sn or Pb. The remarkably efficient iron catalyzed chain growth reaction for ZnEt2 compared to other metal alkyls can be rationalized on the basis of: (1) relatively low steric hindrance around the zinc center, (2) their monomeric nature in solution, (3) the relatively weak Zn-C bond, and (4) a reasonably close match in Zn-C and Fe-C bond strengths.

Nucleophilic catalysis by 4-(dialkylamino)pyridines revisited--the search for optimal reactivity and selectivity.

Abstract: 4-(Dimethylamino)pyridine (4-DMAP, 1) is well known as a catalyst for the esterification of alcohols by acid anhydrides and for various other synthetically useful transformations involving acyl transfer. 2] Its catalytic potential was first discovered by the groups of Litvinenko and Steglich in the late 1960s and its synthetic utility and that of its congeners, including polymeric variants, have been reviewed. Recently, attention has been focused on the development of enantiomerically pure chiral 4-(dialkylamino)pyridines for the kinetic resolution of alcohols and related enantioselective transformations. As a result of this interest, the detailed mechanism of catalysis by 4-(dialkylamino)pyridines and the factors that influence their reactivity have come under renewed scrutiny. In particular, Steglich and co-workers reported pyridonaphthyridine 3 as being the most catalytically active 4-DMAP analogue yet prepared for the acetylation of tertiary alcohols, and work by Kattnig and Albert has illustrated the key role of the anion and general base catalysis in regulating the rate and regioselectivity of polyol acetylation by 1 (Scheme 1). Herein, the detailed mechanism of catalysis of esterification by 4-(dialkylamino)pyridines is reexamined in light of these findings, and the complexity associated with this apparently straightforward process is highlighted. That pyridine and 4-substituted derivatives act primarily as nucleophilic rather than general base catalysts for alcohol esterification follows from the dramatic loss of activity that accompanies 2-alkyl substitution despite the relatively marginal effect that this substitution has on the pKa value of these derivatives. Such steric inhibition of catalysis was first shown to be characteristic of nucleophilic catalysis in work by Gold and Jefferson in the early 1950s on the hydrolysis of Ac2O with a series of methyl-substituted pyridines. The effect was quantified by Litvinenko and co-workers in 1981 for the catalysis of benzoylation of benzyl alcohol with BzCl. In addition to confirming the nucleophilic nature of the catalysis, this work also highlighted the particularly high catalytic activity of 1, which exhibits a rate of 3.4 ; 10 relative to the uncatalyzed reaction (Scheme 2). The high catalytic reactivity of 1 had previously been noted by Litvinenko and co-workers in the benzoylation of 3chloroaniline and subsequently, but independently, 1 was shown by Steglich and co-workers to enable esterification of even hindered tertiary alcohols with Ac2O. [4] Esterification reactions of tertiary alcohols are relatively slow and particularly susceptible to steric factors and therefore proved to be useful for exploring structure–activity relations for catalysis by 4-DMAP analogues. Accordingly, Hassner et al. found that 4pyrrolidinopyridine (4-PPY, 2) was the most efficient of a series of 4-aminopyridine derivatives, including 1, for the acetylation of 1-methylcyclohexanol with Ac2O (Scheme 3). [17] Hassner et al. noted the lack of correlation between the pKa value and the catalytic activity; they suggested that the relative efficiencies of the various catalysts reflected the stabilities of the respective derived acyl pyridinium intermediates in a mechanistic scenario involving equilibrium formation of these salts followed by rate-determining reaction with the alcohol (Scheme 4). Additionally, Hassner et al. noted that the order of catalytic activity of 4aminopyridine derivatives [4-pyrrolidino (2)> 4-dimethylamino (1)> 4-piperidino (5)> 4-morpholino (7)] mirrored the order of reactivity of cyclohexanone-derived enamines towards electrophiles. This order has been rationalized as a balance of stereoelectronic (nN!p*C=C) and steric effects (A strain) which dictates the efficiency with which the lone pair of electrons on the enamine nitrogen atom interacts with the C C double bond. By analogy with the enamine series, Hassner et al. noted that there was a qualitative correlation between the degree of shielding of the pyridyl b-hydrogen atoms in the H NMR spectra of the catalytically active 4-aminopyridine derivatives and their catalytic efficiency (see Scheme 3). They inferred that the extent of electronic communication between the lone pair of electrons of the exocyclic nitrogen atom and the carbonyl function through the pyridyl ring was a key factor in stabilizing the acyl pyridinium intermediate. The design of pyridonaphthyridine 3 (Scheme 1), recently disclosed by Steg[*] Dr. A. C. Spivey, S. Arseniyadis Department of Chemistry South Kensington Campus Imperial College London SW7 2AZ (UK) Fax: (+44)20-7594-5841 E-mail: a.c.spivey@imperial.ac.uk Highlights

Structure, Reactivity, and Density Functional Theory Analysis of the Six-Electron Reductant, [(C5Me5)2U]2(μ-η6:η6-C6H6), Synthesized via a New Mode of (C5Me5)3M Reactivity

Abstract: The sterically crowded (C5Me5)3U complex reacts with KC8 or K/(18-crown-6) in benzene to form [(C5Me5)2U]2(-6:6-C6H6), 1, and KC5Me5. These reactions suggested that (C5Me5)3U could be susceptible to (C5Me5)1- substitution by benzene anions via ionic salt metathesis. To test this idea in the synthesis of a more conventional product, (C5Me5)3U was treated with KN(SiMe3)2 to form (C5Me5)2U[N(SiMe3)2] and KC5Me5. 1 has long U-C(C5Me5) bond distances comparable to (C5Me5)3U, and it too is susceptible to (C5Me5)1- substitution via ionic metathesis: 1 reacts with KN(SiMe3)2 to make its amide-substituted analogue {[(Me3Si)2N](C5Me5)U}2(-6:6-C6H6), 2. Complexes 1 and 2 have nonplanar C6H6-derived ligands sandwiched between the two uranium ions. 1 and 2 were examined by reactivity studies, electronic absorption spectroscopy, and density functional theory calculations. [(C5Me5)2U]2(-6:6-C6H6) functions as a six-electron reductant in its reaction with 3 equiv of cyclooctatetraene to form [(C5Me5)(C8H8)U]2(-3:3-C8H8), (C5Me5)2, and benzene. This multielectron transformation can be formally attributed to three different sources: two electrons from two U(III) centers, two electrons from sterically induced reduction by two (C5Me5)1- ligands, and two electrons from a bridging (C6H6)2- moiety.

Mechanism of homogeneous Ir(III) catalyzed regioselective arylation of olefins.

Abstract: The mechanism of hydroarylation of olefins by a homogeneous Ph-Ir(acac)_2(L) catalyst is elucidated by first principles quantum mechanical methods (DFT), with particular emphasis on activation of the catalyst, catalytic cycle, and interpretation of experimental observations. On the basis of this mechanism, we suggest new catalysts expected to have improved activity. Initiation of the catalyst from the inert trans-form into the active cis-form occurs through a dissociative pathway with a calculated ΔH(0 K)^⧧ = 35.1 kcal/mol and ΔG(298 K)^⧧ = 26.1 kcal/mol. The catalytic cycle features two key steps, 1,2-olefin insertion and C−H activation via a novel mechanism, oxidative hydrogen migration. The olefin insertion is found to be rate determining, with a calculated ΔH(0 K)^⧧ = 27.0 kcal/mol and ΔG(298 K)^⧧ = 29.3 kcal/mol. The activation energy increases with increased electron density on the coordinating olefin, as well as increased electron-donating character in the ligand system. The regioselectivity is shown to depend on the electronic and steric characteristics of the olefin, with steric bulk and electron withdrawing character favoring linear product formation. Activation of the C−H bond occurs in a concerted fashion through a novel transition structure best described as an oxidative hydrogen migration. The character of the transition structure is seven coordinate Ir^V, with a full bond formed between the migrating hydrogen and iridium. Several experimental observations are investigated and explained: (a) The nature of L influences the rate of the reaction through a ground-state effect. (b) The lack of β-hydride products is due to kinetic factors, although β-hydride elimination is calculated to be facile, all further reactions are kinetically inaccessible. (c) Inhibition by excess olefin is caused by competitive binding of olefin and aryl starting materials during the catalytic cycle in a statistical fashion. On the basis of this insertion-oxidative hydrogen transfer mechanism we suggest that electron-withdrawing substituents on the acac ligands, such as trifluoromethyl groups, are good modifications for catalysts with higher activity.

Steric retardation of SN2 reactions in the gas phase and solution.

Abstract: The gas-phase SN2 reactions of chloride with ethyl and neopentyl chlorides and their α-cyano derivatives have been explored with B3LYP, CBS-QB3, and PDDG/PM3 calculations. Calculations predict that the steric effect of the tert-butyl group raises the activation energy by about 6 kcal/mol relative to methyl in both cases. Solvent effects have been computed with QM/MM Monte Carlo simulations for DMSO, methanol, and water, as well as with a polarizable continuum model, CPCM. Solvents cause a large increase in the activation energies of these reactions but have a very small differential effect on the ethyl and neopentyl substrates and their cyano derivatives. The theoretical results contrast with previous conclusions that were based upon gas-phase rate measurements.

New progress in the chemistry of stable metallaaromatic compounds of heavier group 14 elements.

Abstract: By taking advantage of kinetic stabilization afforded by introduction of an efficient steric protection group, several kinds of silaaromatic compounds, such as silabenzene, 1- and 2-silanaphthalenes, and 9-silaanthracene, were successfully synthesized and isolated as stable compounds. The first stable germaaromatics, such as germabenzene and 2-germanaphthalene, were also synthesized by the use of a similar synthetic approach. Most of their molecular structures were definitively determined by X-ray crystallographic analysis. The novel molecular structures and unique reactivities of these metallaaromatic species are discussed on the basis of the experimental results together with those obtained by theoretical calculations, focusing on their aromaticity.

Controlled Self-Assembly of Hexa-peri-hexabenzocoronenes in Solution

Abstract: Disc-shaped hexa-peri-hexabenzocoronenes (HBCs) peripherally substituted by flexible dodecyl chains (molecule 1) or rigid polyphenylene dendrons (molecules 2a,b and 3) were efficiently synthesized. Steric hindrance arising from the substituents, from less hindered dodecyl to bulky dendrons, was utilized to program the self-assembly of the HBC cores in solution. The high tendency of the hexadodecyl-substituted HBC 1 to aggregate was determined by concentration and temperature-dependent 1H NMR spectroscopic measurements and nonlinear least-squares analysis of the experimental data. The rigid dendrons in molecule 2a suppress the π−π interactions of the HBC cores to a certain extent, and a slow (with respect to the NMR time scale) monomer−dimer equilibrium is observed. This unique equilibrium was further controlled by temperature, concentration, and solvent to afford discrete monomeric or dimeric species. Further structural modifications such as the replacement of dodecyl groups in 2a with hydrogen atoms resu...

Density functional theory investigation of benzenethiol adsorption on Au(111)

Abstract: We have studied the adsorption of benzenethiol molecules on the Au(111) surface by using first principles total energy calculations. A single thiolate molecule is adsorbed at the bridge site slightly shifted toward the fcc-hollow site, and is tilted by 61° from the surface normal. As for the self-assembled monolayer (SAM) structures, the (2∛×∛)R30° herringbone structure is stabilized against the (∛×∛)R30° structure by large steric relaxation. In the most stable (2∛×∛)R30° SAM structure, the molecule is adsorbed at the bridge site with the tilting angle of 21°, which is much smaller compared with the single molecule adsorption. The van der Waals interaction plays an important role in forming the SAM structure. The adsorption of benzenethiolates induces the repulsive interaction between surface Au atoms, which facilitates the formation of surface Au vacancy.

Self-Assembly of Benzene-Dicarboxylic Acid Isomers at the Liquid Solid Interface: Steric Aspects of Hydrogen Bonding

Abstract: We compare the self-assembly of the various isomers of benzene−dicarboxylic acids at the interface between solution and graphite substrate. In the case of planar benzene−dicarboxylic acids it was possible to observe long-range ordered monolayers by STM. However, no ordered adsorption was observed for the nonplanar 1,2-benzene−dicarboxylic acid. By means of a control experiment with 1,2,4,5-benzene-tetracarboxylic acid, it was possible to demonstrate that the nonplanar structure is not the decisive reason for the absence of self-assembly. In fact, the direct neighborhood of the two carboxylic groups in 1,2-benzene−dicarboxylic acid does not allow for extended hydrogen-bound aggregations. Thus, the stabilization due to intermolecular hydrogen bonding is too weak for STM investigations, at least at room temperature. It has been shown that a periodic, infinitely extendable hydrogen-bonding scheme is a requirement.

Origin of enantioselectivity in the Ru(arene)(amino alcohol)-catalyzed transfer hydrogenation of ketones.

Abstract: The origin of the enantioselectivity in the ruthenium-catalyzed transfer hydrogenation has been studied by means of experiment and density functional theory calculations. The results clearly show that electrostatic effects are of importance, not only in the T-shaped arene-aryl interaction in the favored transition state but also between the aryl of the substrate and the amine ligand in the disfavored TS. In addition, the electrostatic interaction between the alkyl substituent of the substrate and the catalyst is of importance to the enantioselectivity. The major cause of enantioselection is found to be of nonelectrostatic origin. This inherent property of the catalytic system is discussed in terms of dispersion forces and solvent effects. Finally, a minor but well-characterized steric effect was identified. The success of this class of catalysts in the reduction of alkyl aryl ketones is based on the fact that all factors work in the same direction.

Solvation effects on alanine dipeptide: A MP2/cc‐pVTZ//MP2/6‐31G** study of (Φ, Ψ) energy maps and conformers in the gas phase, ether, and water

Abstract: The effects of solvation on the conformations and energies of alanine dipeptide (AD) have been studied by ab initio calculations up to MP2/cc-pVTZ//MP2/6-31G**, utilizing the polarizable continuum model (PCM) to mimic solvation effects. The energy surfaces in the gas phase, ether, and water bear similar topological features carved by the steric hindrance, but the details differ significantly due to the solvent effects. The gas-phase energy map is qualitatively consistent with the Ramachandran plot showing seven energy minima. With respect to the gas-phase map, the significant changes of the aqueous map include (1) the expanded low-energy regions, (2) the emergence of an energy barrier between C5-β and αR-β2 regions, (3) a clearly pronounced αR minimum, a new β-conformer, and the disappearance of the gas-phase global minimum, and (4) the shift of the dominant region in LEII from the gas-phase C7ax region to the αL region. These changes bring the map in water to be much closer to the Ramachandran plot than the gas-phase map. The solvent effects on the geometries include the elongation of the exposed NH and CO bonds, the shortening of the buried HNCO peptide bonds, and the enhanced planarity of the peptide bonds. The energy surface in ether has features similar to those both in the gas phase and in water. The free energy order computed in the gas phase and in ether is in good agreement with experimental studies that concluded that C5 and C7eq are the dominant species in both the gas phase and nonpolar solvents. The free energy order in water is consistent with the experimental observation that the dominant C7eq in the nonpolar solvent was largely replaced by PII-like (i.e., β) and αR in the strong polar solvents. Based on calculations on AD + 4H2O and other AD–water clusters, we suggest that explicit water–AD interactions may distort C5 and β (or αR and β) to an intermediate conformation. Our analysis also shows that the PCM calculations at the MP2/cc-pVTZ//MP2/6-31G** level give good descriptions to the bulk solvent polarization effect. The results presented in this article should be of sufficient quality to characterize the peptide bonds in the gas phase and solvents. The energy surfaces may serve as the basis for developing of strategies enabling the inclusion of solvent polarization in the force field. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1699–1716, 2004

Low-Coordinate Iron(II) Amido Complexes of β-Diketiminates: Synthesis, Structure, and Reactivity

Abstract: The synthesis, structure, and reactivity of a series of low-coordinate Fe(II) diketiminate amido complexes are presented. Complexes LRFeNHAr (R = methyl, tert-butyl; Ar = para-tolyl, 2,6-xylyl, and 2,6-diisopropylphenyl) bind Lewis bases to give trigonal pyramidal and trigonal bipyramidal adducts. In the adducts, crystallographic and 1H NMR evidence supports the existence of agostic interactions in solid and solution states. Complexes LRFeNHAr may be oxidized using AgOTf, and the products LRFe(NHAr)(OTf) are characterized with 19F NMR spectroscopy, UV/vis spectrophotometry, solution magnetic measurements, elemental analysis, and, in one case, X-ray crystallography. In the structures of the iron(III) complexes LRFe(NHAr)(OTf) and LRFe(OtBu)(OTf), the angles at nitrogen and oxygen result from steric effects and not π-bonding. The reactions of the amido group of LRFeNHAr with weak acids (HCCPh and HOtBu) are consistent with a basic nitrogen atom, because the amido group is protonated by terminal alkynes and ...

Catalytic Hydrohydrazination of a Wide Range of Alkenes with a Simple Mn Complex

Abstract: Enhanced activity, lower catalyst loading, shorter reaction time, and expanded substrate scope are the advantages of [Mn(dpm)3] over Co catalysts in the hydrohydrazination reaction of alkenes. Thus, sterically hindered alkenes, including tetrasubstituted alkenes, can now also readily undergo this reaction. [on SciFinder (R)]

The molecular structure of tetra-tert-butyldiphosphine: an extremely distorted, sterically crowded molecule

Abstract: The molecular structure of tetra-tert-butyldiphosphine has been determined in the gas phase by electron diffraction using the new DYNAMITE method and in the crystalline phase by X-ray diffraction. Ab initio methods were employed to gain a greater understanding of the structural preferences of this molecule in the gas phase, and to determine the intrinsic P–P bond energy, using recently described methods. Although the P–P bond is relatively long [GED 226.4(8) pm; X-ray 223.4(1) pm] and the dissociation energy is computed to be correspondingly small (150.6 kJ mol−1), the intrinsic energy of this bond (258.2 kJ mol−1) is normal for a diphosphine. The gaseous data were refined using the new Edinburgh structure refinement program ed@ed, which is described in detail. The molecular structure of gaseous P2But4 is compared to that of the isoelectronic 1,1,2,2-tetra-tert-butyldisilane. The molecules adopt a conformation with C2 symmetry. The P–P–C angles returned from the gas electron diffraction refinement are 118.8(6) and 98.9(6)°, a difference of 20°, whilst the C–P–C angle is 110.3(8)°. The corresponding parameters in the crystal are 120.9(1), 99.5(1) and 109.5(1)°. There are also large deformations within the tert-butyl groups, making the DYNAMITE analysis for this molecule extremely important.

HETEROLYTIC SPLITTING OF HH, SiH, AND OTHER σ BONDS ON ELECTROPHILIC METAL CENTERS

Abstract: Publisher Summary This chapter discusses heterolytic splitting of H-H, Si-H, and other σ bonds on electrophilic metal centers. Extensive computational analyses of the structure, bonding, and reactions of coordinated dihydrogen are carried out because of the innate ‘‘simplicity’’ of the H2 ligand. However, this is quite deceptive because M-H2 systems have proven to exhibit astonishingly complex structure, bonding, and dynamics, including quantum mechanical behavior. In principle, any X-Y σ bond can coordinate to a metal center, providing that steric and electronic factors are favorable—for example, substituents at X and Y do not block the metal's access. Heterolytic splitting of X-H bonds via proton transfer to a basic site on a cis ligand or to an external base is a crucial step in both industrial and biological processes. Calculations show that for highly electrophilic M the reduction in back donation is almost completely offset by increased electron donation from H2 to the electron-poor M. One of the old, most significant, and widespread reactions of H2 on metal centers is heterolytic cleavage, which involves essentially breaking the H-H bond into H+ and H– fragments.

Synthesis and Reactivity of New Chelate-N-Heterocyclic Biscarbene Complexes of Ruthenium

Abstract: The carbene-ligand precursors methylenebis(N-alkylimidazolium) iodide (alkyl = methyl, neo-pentyl) and ethylenebis(N-methylimidazolium) chloride have been used in the preparation of several new Ru(II)-p-cymene complexes where the ligand behaves as mono- and bidentate. The molecular structures of the two biscarbene-complexes are reported. From the data reported, we can conclude that steric reasons (mainly the bisimidazolium linkers, methylene/ethylene) are the main factors determining both reactivity and synthetic difficulties of the products reported.

Insights into CO/Styrene Copolymerization by Using PdII Catalysts Containing Modular Pyridine–Imidazoline Ligands

Abstract: Continuing our studies into the effect that N-N' ligands have on CO/styrene copolymerization, we prepared new C(1)-symmetrical pyridine-imidazoline ligands with 4',5'-cis stereochemistry in the imidazoline ring (5) and 4',5'-trans stereochemistry (6-10) and compared them with our previously reported ligands (1-4). Their coordination to neutral methylpalladium(II) (5 a-10 a) and cationic complexes (5 b-10 b), investigated in solution by NMR spectroscopy, indicates that both the electronic and steric properties of the imidazolines determine the stereochemistry of the palladium complexes. The crystal structures of two neutral palladium precursors [Pd(Me)(2-n)Cl(n)(N-N')] (n=1 for 8 a; n=2 for 9 a') show that the Pd-N coordination distances and the geometrical distortions in the imidazoline ring depend on the electronic nature of the substituents in the imidazoline fragment. Density functional calculations performed on selected neutral and cationic palladium complexes compare well with NMR and X-ray data. The calculations also account for the formation of only one or two stereoisomers of the cationic complexes. The performance of the cationic complexes as catalyst precursors in CO/4-tert-butylstyrene copolymerization under mild pressures and temperatures was analyzed in terms of the productivity and degree of stereoregularity of the polyketones obtained. Insertion of CO into the Pd-Me bond, which was monitored by multinuclear NMR spectroscopy, shows that the N ligand influences the stereochemistry of the acyl species formed.

Effects of methyl substitution on radiative and solvent quenching rate constants of [Ru(phen)2dppz]2+ in polyol solvents and bound to DNA.

Abstract: Methyl substituents on the distant benzene ring of the dppz ligand in the "light switch" complex [Ru(phen)(2)dppz](2+) have profound effects on the photophysics of the complexes in water as well as in the polyol solvents ethylene glycol, glycerol, and 1,2- and 1,3-propanediol. Whereas 11,12-dimethyl substitution decreases the rate of quenching by diminishing hydrogen bonding by solvent, the 10-methyl substituent in addition also decreases both the radiative and the nonradiative rate constant for decay to the ground state of the non-hydrogen-bonded excited state species. For both the 10-methyl and the 11,12-dimethyl derivatives, the effect of methyl substitution on the equilibrium of solvent hydrogen bonding to the excited state is due to changes in the entropy terms, rather than in the enthalpy; indicating that the effect is a steric perturbation of the solvent cage around the molecule. When intercalated into DNA, the effects of methyl substitution is smaller than those in polyol solvent or water, suggesting that the water molecules that quench the excited state by hydrogen bonding to the phenazine aza nitrogens mainly access them from the same groove as in which the Ru(II) ion resides. Since the Delta-enantiomer of [Ru(phen)(2)10-methyl-dppz](2+) has an absolute quantum yield of up to 0.23 when bound to DNA, a value 7000 times higher than in pure water solution, it is promising as a new luminescent DNA probe.

Molecular Crystals with Moving Parts: Synthesis, Characterization, and Crystal Packing of Molecular Gyroscopes with Methyl-Substituted Triptycyl Frames

Abstract: We report a highly convergent synthesis for the preparation of molecular gyroscopes consisting of para-phenylene rotors linked by triple bonds to methyl-substituted triptycenes acting as pivots and encapsulating frames. The desired 1,4-bis[2-(2,3,6,7,12,13-hexamethyl-10-alkyl-9-triptycyl)ethynyl]benzenes were prepared from 2,3-dimethyl-1,3-butadiene using Diels−Alder cycloadditions and Pd(0)-catalyzed coupling as the key reactions. The main challenge in the synthesis came about in the preparation of 9-alkynyl-triptycenes by Diels−Alder reaction of benzynes and 9-alkynyl-2,3,6,7-tetramethylanthracenes. These reactions occurred with chemical yields and regioselectivities that were strongly influenced by steric and electronic effects of substituents at C10 of the anthracene core. Anthracenes with methyl, propyl, and phenyl substituents were utilized to complete the synthesis of their corresponding molecular gyroscopes, and their solid-state structures were determined by single-crystal X-ray diffraction analy...

Adsorption of Organic Matter at Mineral/Water Interfaces. 2. Outer-Sphere Adsorption of Maleate and Implications for Dissolution Processes

Abstract: The effects of the adsorption of a simple dicarboxylate low molecular weight organic anion, maleate, on the dissolution of a model aluminum oxide, corundum (α-Al2O3), have been examined over a range of different maleate concentrations (0.125−5.0 mM) and pH conditions (2−10). In situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic measurements indicate that maleate binds predominantly as an outer-sphere, fully deprotonated complex (⋮AlOH2+---Mal2-) at the corundum surface over the entire range of maleate concentrations and pH conditions investigated. In accordance with the ATR-FTIR findings, macroscopic adsorption data can be modeled as a function of maleate concentration and pH using an extended constant capacitance approach and a single ⋮AlOH2+---Mal2- species. Outer-sphere adsorption of maleate is found to significantly reduce the protolytic dissolution rate of corundum under acidic conditions (pH < 5). A likely mechanism involves steric protection of dissolution-active ...