The foam analogy: from phases to elasticity

By mapping the interactions of colloidal particles onto the problem of minimizing areas, the physics of foams can be used to understand the phase diagrams of both charged and fuzzy colloids. We extend this analogy to study the elastic properties of such colloidal crystals and consider the face-centered cubic, body-centered cubic and A15 lattices. We discuss two types of soft interparticle potentials corresponding to charged and fuzzy colloids, respectively, and we analyze the dependence of the elastic constants on density as well as on the parameters of the potential. We show that the bulk moduli of the three lattices are generally quite similar, and that the shear moduli of the two non-close-packed lattices are considerably smaller than in the face-centered cubic lattice. We find that in charged colloids, the elastic constants are the largest at a finite screening length, and we discuss a shear instability of the A15 lattice.     

From crown ethers to zeolites: Reaction of EtAlCl2 with crown ethers

In contrast to aluminum alkyls, alkyl aluminum halides such as EtAlCl₂ react with crown ethers to form cation-anion pairs which exhibit the liquid clathrate effect. Specifically, [12-C-4·AlCl₂][AlCl₃Et] and [18-C-6·AlCl₂][AlCl₃Et] have been isolated and characterized by X-ray diffraction techniques. The cations show aluminum in an octahedral environment made up of four of the oxygen atoms from the crown and two chlorine atoms. The 12-C-4 derivative crystallizes in the monoclinic space group P2₁/c with cell constants of a=7.497(4), b=22.121(8), c=12.339(5) Å, =94.99(3)^o, and Z=4 for =1.43 g cm^–3. Least-squares refinement based on 1413 observed reflections led to a final conventional R value of 0.093. The 18-C-6 complex belongs to the triclinic space group P1 with a=8.414(4), b=12.193(6), c=12.394(6) Å, =73.14(4), =86.07(4), =81.52(4)^o, and Z=2 for =1.45 g cm^–3. Refinement based on 2605 observed reflections led to R=0.063. The complex aluminum-containing species are related to a class of compounds called aluminoxanes.     

Spectral Properties of Fluorescein in Solvent-Water Mixtures: Applications as a Probe of Hydrogen Bonding Environments in Biological Systems

Although fluorescein is a widely used fluorescent probe in the biosciences, the effect of solvent environment on its spectral properties is poorly understood. In this paper we explore the use of fluorescein as a probe of the state of hydrogen bonding in its local environment. This application is based on the observation, originally made by Martin ( Chem. Phys. Lett . 35, 105–111, 1975), that the absorption maximum of fluorescein undergoes substantial shifts in organic solvents related to the hydrogen bonding power of the solvents. We have extended this work by studying the spectral properties of the dianion form of the probe in solvent–water mixtures. We show that the magnitude of the shift correlates with the α and β parameters of Kamlet and Taft ( J. Am. Chem. Soc . 98, 377–383; 2886–2894, 1976), which provide a scale of the hydrogen bond donor acidities and acceptor basicities, respectively, of the solvents. In solvent–water mixtures, these shifts reflect general effects of the solvents on the hydrogen bonding environment of the fluorescein through water–solvent hydrogen bonding and specific effects due to fluorescein–solvent hydrogen bonding. Indeed, both the absorption and fluorescence properties appear to be dominated by these effects indicating that the spectral shifts of the dianion can be used as an indicator of its hydrogen bonding environment. We discuss the application of fluorescein as a probe of hydrogen bonding in the microenvironment immediately surrounding the fluorophore, and we illustrate the effect with reference to the fluorescein–antifluorescein antibody complex where it appears that antibodies selected during the immune response possess binding sites that are increasingly dehydrated and hydrophobic.     

Deazapurine solid-phase synthesis: combinatorial synthesis of a library of N3,N5,C6-trisubstituted pyrrolo[3,2-d]pyrimidine derivatives on cross-linked polystyrene bearing a cysteamine linker

Solid-phase methodology for the preparation of pyrrolo[3,2-d]pyrimidine-6-carboxylates with diversity at the N3 pyrmidine nitrogen has now been elaborated to allow for the generation of pyrrolopyrimidine libraries with members possessing diversity at the N3, N5, and C6 positions. The diversification of the N5 position was achieved by treating the parent resin-bound pyrrolo[3,2-d]pyrimidines 3 with an alkyl halide in the presence of Cs2CO3 in DMF. Modification of the C6 carboxylate of resin-bound pyrrolopyrimidines 3-5 was first achieved by hydrolysis of the benzyl ester using LiOH in a mixture of THF/H2O/MeOH. Further alteration of the C6 position of resin-bound pyrrolo[3,2-d]pyrimidine-6-carboxylic acids 6-8 was then performed by activation with triphosgene and treatment with an amine to furnish resin-bound pyrrolo[3,2-d]pyrimidine-6-amides. Twenty-two pyrrolo[3,2-d]pyrimidines 1a-v with different substituents at the N3, N5, and C6 positions were obtained in yields of 21-83% and purities of 61-98% after cleavage from the solid support.     

Electric-field-induced patterns in thin polymer films: weakly nonlinear and fully nonlinear evolution

A thin polymer melt on a substrate can be unstable to an electric field normal to the interface, a phenomenon that can be harnessed as a patterning technique with a range of potential applications. Motivated by the variety of patterns observed in experiments for polymers under both unpatterned and patterned masks, we describe here, from theoretical and numerical analyses, how nonlinear effects govern the growth of the instability and determine the final patterns. In particular, we discuss the nonlinear growth in terms of interactions among different Fourier modes and show that the second- and third-order nonlinearities favor the growth of hexagonal patterns under a featureless mask, in agreement with experimental observations. Also, numerical simulations based on the fully nonlinear model validate the prediction of the weakly nonlinear analysis: hexagonal patterns do emerge under an unpatterned mask. Furthermore, in one-dimensional simulations, we demonstrate the energetic evolution of this patterning process and reveal several "kinetically stable structures" along the path to the thermodynamically stable state. Two-dimensional simulations allow us to study the effects of both mask patterns and the initial film thickness. Generally, patterns on the mask guide the growth such that the pattern conforms to the geometric shapes. Interestingly, a small cylindrical protrusion at the center of the mask can produce exactly the same pattern as a large, flat, circular protrusion. The initial film thickness or the thickness ratio of the polymer layer to the air gap plays an important role in determining the final pattern formed. Finally, we demonstrate, by two simple examples, that the simulations can provide insights on "smart" mask designs for producing large areas of well-ordered patterns.     

Reactivity of (bicyclo[5.1.0]octadienyl)iron(1+) cations: application to the synthesis of cis-2-(2'-carboxycyclopropyl)glycines

The addition of carbon and heteroatom nucleophiles to (bicyclo[5.1.0]octadienyl)Fe(CO)(2)L(+) cations 5 or 8 (L = CO, PPh(3)) generally proceeds via attack at the dienyl terminus on the face of the ligand opposite to iron to generate 6-substituted (bicyclo[5.1.0]octa-2,4-diene)iron complexes (11 or 13). In certain cases, these products are unstable with respect to elimination of a proton and the nucleophilic substituent to afford (cyclooctatetraene)Fe(CO)(2)L (4 or 7). Decomplexation of 13f, arising from addition of phthalimide to 8, gave N-(bicyclo[5.1.0]octa-3,5-dien-2-yl)phthalimide (19). Oxidative cleavage of 19 (RuCl(3)/NaIO(4)) followed by esterification gave the cyclopropane diester 22, which upon hydrolysis gave cis-2-(2'-carboxycyclopropyl)glycine (CCG-III, 18) (eight steps from 4, 43% overall yield). This methodology was also utilized for preparation of stereospecifically deuterated CCG-III (d-18) and optically enriched (-)-18. Deprotonation of 22 resulted in cyclopropane ring opening to afford the benzoindolizidine (23).     

Iminophosphines: synthesis, formation of 2,3-dihydro-1H-benzo[1,3]azaphosphol-3-ium salts and N-(pyridin-2-yl)-2-diphenylphosphinoylaniline, coordination chemistry and applications in platinum group catalyzed Suzuki coupling reactions and hydrosilylations

The aprotic and protic bi- and multidentate iminophosphines 2-Ph₂PC₆H₄N=CR¹R² (R¹=H, R²=Ph=2a; R¹=Me R²=Ph=2b; R¹=H, R²=2-thienyl=2c; R¹=H, R²=C₆H₄-2-PPh₂=2d; R¹=H, R²=C₆H₄-2-OH=2e, R¹=H, R²=C₆H₄-2-OH-3-Bu^t=2f; R¹=H, R²=CH₂C(O)Me=2g) have been prepared by the acid catalyzed condensation of 2-(diphenylphosphino)aniline with the corresponding aldehyde–ketone. Iminophosphine 2d can be reduced with sodium cyanoborohydride to give the corresponding amino-diphosphine 2-Ph₂PC₆H₄N(H)CH₂C₆H₄-2-PPh₂ (2h). In the presence of a stoichiometric quantity of acid, 2-(diphenylphosphino)aniline reacts in an unexpected manner with benzaldehyde, salicylaldehyde, or acetophenone to give the corresponding 2,3-dihydro-1H-benzo[1,3]azaphosphol-3-ium salts and with pyridine-2-carboxaldehyde to give N-(pyridin-2-ylmethyl)-2-diphenylphosphinoylaniline, the latter of which has been characterized by single-crystal X-ray crystallography, as its palladium dichloride derivative. The attempted condensation of 2-(diphenylphosphino)aniline with pyridine-2-carboxaldehyde to give the corresponding pyridine-functionalized iminophosphine resulted in an unusual transformation involving the diastereoselective addition of two equivalents of aldehyde to give 1,2-dipyridin-2-yl-2-(o-diphenylphosphinoyl)phenylamino-ethanol, which has been characterized by a single-crystal X-ray structure determination. The bidentate iminophosphine 2-Ph₂PC₆H₄N=C(H)Ph reacts with [(cycloocta-1,5-diene)PdClX] X=Cl, Me) to give [Pd{2-Ph₂PC₆H₄N=C(H)Ph}ClX] and the imino-diphosphine 2-Ph₂PC₆H₄N=C(H)C₆H₄-PPh₂ reacts with [(cycloocta-1,5-diene)PdClMe] to give [Pd{2-Ph₂PC₆H₄N=C(H)C₆H₄---PPh₂}ClMe] and each has been characterized by single-crystal X-ray crystallography. The monobasic iminophosphine 2-Ph₂PC₆H₄N=C(Me)CH₂C(O)Me reacts with [Ni(PPh₃)₂Cl₂] in the presence of NaH to give the phosphino–ketoiminate complex [Ni{2-Ph₂PC₆H₄N=C(Me)CHC(O)Me}Cl], which has been structurally characterized. Mixtures of iminophosphines 2a–h and a palladium source catalyze the Suzuki cross coupling of 4-bromoacetophenone with phenyl boronic acid. The efficiency of these catalysts show a marked dependence on the palladium source, catalysts formed from [Pd₂(OAc)₆] giving consistently higher conversions than those formed from [Pd₂(dba)₃] and [PdCl₂(MeCN)₂]. Catalysts formed from neutral bi- and terdentate iminophosphines 2a–d gave significantly higher conversions than those formed from their monobasic counterparts 2e–f. Notably, under our conditions the conversions obtained with 2a–c compare favorably with those of the standards; catalysts formed from tris(2-tolyl)phosphine and tris(2,4-di-tert-butylphenyl)phosphite and a source of palladium. In addition, mixtures of [Ir(COD)Cl]₂ and 2a–h are active for the hydrosilylation of acetophenone; in this case catalysts formed from monobasic iminophosphines 2e–f giving the highest conversions.     

Generation of 3-pyridyl biaryl systems via palladium-catalyzed Suzuki cross-couplings of aryl halides with 3-pyridylboroxin

The synthesis of 3-pyridyl biaryl systems can be readily achieved by means of palladium-catalyzed Suzuki cross-coupling reactions between aryl halides and 3-pyridylboroxin. A series of cross-couplings were conducted in order to investigate the scope and limitations of this protocol.     

Co2(CO)8 Catalyzed reaction of oxetanes with trialkylsilanes

Dicobalt octacarbonyl catalyzed silyl-hydroformylation of oxetanes can be controlled by the choice of trialkylsilane. Triethylsilane gives 1,4-$\text{bis}$-silyl ethers while $\text{t}$-butyl-dimethylsilane yields silyl enol ethers.     

Forward and backward pericyclic photochemical reactions have intermediates in common, yet cyclobutenes break the rules.

Photochemical pericyclic reactions are believed to proceed via a so-called pericyclic minimum on the lowest excited potential surface (S(1)), which is common to both the forward and backward reactions. Such a common intermediate has never been directly detected. The photointerconversion of 1,3-butadiene and cyclobutene is the prevailing prototype for such reactions, yet only diene ring closure proceeds with the stereospecificity that the Woodward-Hoffmann rules predict. This contrast seems to exclude a common intermediate. Using ultrafast spectroscopy, we show that the excited states of two cyclobutene/diene isomeric pairs are linked by not one, but by two common minima, p* and ct*. Starting from the diene side (cyclohepta-1,3-diene and cycloocta-1,3-diene), electrocyclic ring closure passes via the pericyclic minimum p*, whereas ct* is mainly responsible for cis-trans isomerization. Starting from the corresponding cyclobutenes (bicyclo[3.2.0]heptene-6 and bicyclo[4.2.0]octene-7), the forbidden isomer is formed from ct*. The path branches at the first (S(2)/S(1)) conical intersection towards p* and ct*. The fact that the energetically unfavorable ct* path can compete is ascribed to a dynamic effect: the momentum in C=C twist direction, acquired--such as in other olefins--in the Franck-Condon region of the cyclobutenes.