The last decade has witnessed dramatic growth in the number of reactions catalyzed by electrophilic gold complexes. While proposed mechanisms often invoke the intermediacy of gold-stabilized cationic species, the nature of bonding in these intermediates remains unclear. Herein, we propose that the carbon-gold bond in these intermediates is comprised of varying degrees of both σ and π-bonding; however, the overall bond order is generally less than or equal to unity. The bonding in a given gold-stabilized intermediate, and the position of this intermediate on a continuum ranging from gold-stabilized singlet carbene to gold-coordinated carbocation, is dictated by the carbene substituents and the ancillary ligand. Experiments show that the correlation between bonding and reactivity is reflected in the yield of gold-catalyzed cyclopropanation reactions.

This communication demonstrates the generation of over 300 phase-separated systems—ranging from two to six phases—from mixtures of aqueous solutions of polymers and surfactants. These aqueous multiphase systems (MuPSs) form self-assembling, thermodynamically stable, step-gradients in density using a common solvent (water). The steps in density between phases of a MuPS can be very small (Δρ ~ 0.001 g/cm3), do not change over time, and can be tuned by the addition of co-solutes. We use two sets of similar objects—glass beads and pellets of different formulations of Nylon—to demonstrate the ability of MuPSs to separate mixtures of objects by differences in density. The stable interfaces between phases facilitate the convenient collection of species after separation. These results suggest that the stable, sharp, step-gradients in density provided by MuPSs can enable new classes of fractionations and separations based on density.

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Abstract:The liquid-metal eutectic of gallium and indium (EGaIn) is a useful electrode for making soft electrical contacts to self-assembled monolayers (SAMs). This electrode has, however, one feature whose effect on charge transport has been incompletely understood: a thin (approximately 0.7 nm) film-consisting primarily of Ga 2 O 3 -that covers its surface when in contact with air. SAMs that rectify current have been measured using this electrode in Ag TS -SAM//Ga 2 O 3 /EGaIn junctions. This paper organizes evidence, both published and unpublished, showing that the molecular structure of the SAM (specifically, the presence of an accessible molecular orbital asymmetrically located within the SAM), not the difference between the electrodes or the characteristics of the Ga 2 O 3 film, causes the observed rectification. By examining and ruling out potential mechanisms of rectification that rely either on the Ga 2 O 3 film, or on the asymmetry of the electrodes, this paper demonstrates that the structure of the SAM dominates charge transport through Ag TS -SAM//Ga 2 O 3 /EGaIn junctions, and that the electrical characteristics of the Ga 2 O 3 film have a negligible effect on these measurements.3

Recently, gold-carbenoid species have been proposed as intermediates in gold-catalyzed enyne rearrangements. 1 Additionally, the reaction of gold complexes with propargyl esters has been developed as an alternative approach to metal carbenoids capable of effecting olefin cyclopropanation; 2,3 however, to date, the reaction of electrophilic metals with alkynes has not been amenable to the generation of R-carbonyl carbenoids analogous to those traditionally formed in situ from transition-metal-catalyzed decomposition of R-diazocarbonyl compounds. 4,5 We recently described a rearrangement of homopropargyl azides to pyrroles in which gold(I) promotes addition of a leaving-group-bearing nucleophile (Nu ) N) to an acetylene and subsequent loss of the leaving group (LG ) N 2 ) (eq 1). 6 On the basis of this reactivity principle, we envisioned that R-carbonyl metal carbenoids could be generated from alkynes via a gold(I)-catalyzed rearrangement in which sulfoxides serve the role of nucleophile (Nu ) O) and latent leaving group (LG ) R 2 S).In order to explore this hypothesis, homopropargyl sulfoxide 1 was treated with 5 mol % of Ph 3 PAuCl/AgSbF 6 in dichloromethane. Gratifyingly, this reaction produced 1-benzothiepin-4-one 3, presumably via the desired R-carbonyl carbenoid intermediate 2, albeit in only 34% yield (eq 2). While switching to an electron-deficient phosphine ligand resulted in a decreased yield, the use of an N-heterocyclic carbene (IMes) ligated gold(I) complex as catalyst dramatically improved the yield of ketone 3 to 94%. Under these conditions, a wide range of homopropargyl arylsulfoxides underwent the gold-catalyzed rearrangement to benzothiepinones (Table 1). The reaction proceeded smoothly when the aryl group of the sulfoxide was substituted with electron-withdrawing (entry 1) or electron-donating groups (entry 2), although the latter underwent the gold-catalyzed rearrangement with increased yield. Substitution at the homopropargyl (entry 3) and propargyl (entry 4) position of the sulfoxide is tolerated; however, the latter required slightly elevated temperatures to afford 11 in 76% yield. Notably, one diastereomer (illustrated) of phenyl-substituted propargyl sulfoxide (()-12 was significantly more reactive than the other in the gold-(I)-catalyzed rearrangement, affording 13 in 94% yield (entry 5). 7 The triphenylphosphinegold(I)-catalyzed reaction of a sulfoxide (14) containing an alkyne substituted with an electron-deficient aryl group 8 (entry 6) or an ester (entry 7) produced the anticipated benzothiepinones 15 and 17 in 63 and 91% yield, respectively. In sharp contrast, the gold(I)-catalyzed reaction of alkyl-substituted alkyne 18 afforded benzothiopine 19 in 64% yield (entry 8). 9 On the basis of this reaction, 1,4-diyne 20 was converted to furan 22 in 56% yield by a gold(I)-catalyzed sulfoxide rearrangement and subsequent cycloisomerization of propargyl ketone 21 (eq 3). 10 A proposed mechanism for the gold-catalyzed rearrangement of homopropargyl sulfoxides is detailed in Scheme 1. Coordin...

This paper characterizes the rates of charge transport by tunneling across a series of molecules—arrayed in self-assembled monolayers—containing a common head group and body (HS(CH2)4CONH(CH2)2-) and structurally varied tail groups (-R). These molecules are assembled in junctions of the structure AgTS/SAM//Ga2O3/EGaIn. Over a range of common aliphatic, aromatic, and heteroaromatic organic tail groups, changing the structure of R does not significantly influence the rate of tunneling.

The chemical community has recently witnessed a dramatic increase in the application of cationic gold(I)-phosphine complexes as homogeneous catalysts for organic synthesis. The majority of gold(I)-catalyzed reactions rely on nucleophilic additions to carbon-carbon multiple bonds, which have been activated by coordination to a cationic gold(I) catalyst. However, structural evidence for coordination of cationic gold(I) complexes to alkynes has been limited. Here, we report the crystal structure of a gold(I)-phosphine 2 -coordinated alkyne. Related Ag(I) and Cu(I) complexes have been synthesized for comparison. The crystallization of these complexes was enabled by tethering a labile alkyne ligand to a strongly coordinating triarylphosphine. This approach also proved applicable to crystallization of the first gold(I)-phosphine 2 -coordinated alkene.alkyne complexes ͉ DFT calculations ͉ gold catalysts ͉ homogenous catalysis ͉ x-ray structures

This paper describes the use of magnetic levitation (MagLev) to measure the association of proteins and ligands. The method starts with diamagnetic gel beads that are functionalized covalently with small molecules (putative ligands). Binding of protein to the ligands within the bead causes a change in the density of the bead. When these beads are suspended in a paramagnetic aqueous buffer and placed between the poles of two NbFeB magnets with like poles facing, the changes in the density of the bead on binding of protein result in changes in the levitation height of the bead that can be used to quantify the amount of protein bound. This paper uses a reaction-diffusion model to examine the physical principles that determine the values of rate and equilibrium constants measured by this system, using the well-defined model system of carbonic anhydrase and aryl sulfonamides. By tuning the experimental protocol, the method is capable of quantifying either the concentration of protein in a solution, or the binding affinities of a protein to several resin-bound small molecules simultaneously. Since this method requires no electricity and only a single piece of inexpensive equipment, it may find use in situations where portability and low cost are important, such as in bioanalysis in resource-limited settings, point-of-care diagnosis, veterinary medicine, and plant pathology. It still has several practical disadvantages. Most notably, the method requires relatively long assay times and cannot be applied to large proteins (> 70 kDa), including antibodies. The design and synthesis of beads with improved characteristics (e.g., larger pore size) has the potential to resolve these problems.