Correlation between the structure and wettability of photoswitchable hydrophilic azobenzene monolayers on silicon

Photoresponsive monolayers of hydrophilically substituted azobenzenes have been prepared by reaction on aminosilane monolayers on silicon surfaces. Grafting densities in the 0.2-1.0 molecule/nm(2) range were determined by X-ray reflectometry. The monolayers exhibit reversible photoisomerization, switching from a more hydrophilic trans state to a less hydrophilic cis state upon UV irradiation, in contrast with the usual behavior of most azobenzene monolayers that switch from a less to a more hydrophilic state. This indicates that the wettability is not dominated by the change in the dipole moment of the azobenzene moiety but originates from variations in the composition of the outer surface of the monolayers resulting from the reorientation of the substituent groups. The light-driven change in the water contact angle correlates linearly with the grafting density but remains small. However, the wettability contrast can be increased by forcing the molecules to stand in an improved vertical orientation, either by densifying the underlying aminosilane monolayer or by filling the voids left at the bottom of the layer of grafted azobenzene molecules.     

Enhanced chemical reworkability of DGEBA thermosets cured with rare earth triflates using aromatic hyperbranched polyesters (HBP) and multiarm star HBP‐b‐poly(ε‐caprolactone) as modifiers

A hyperbranched aromatic polyester (HBPOH) has been synthesized, and poly(ε‐caprolactone) arms have been grown on some of its end hydroxyl groups (HBPCL). These modifiers have been used in cationic diglycidyl ether of bisphenol A formulations cured with ytterbium triflate as cationic initiator. The effect of HBPOH and HBPCL on the curing kinetics has been studied using differential scanning calorimetry (DSC). The obtained materials have been characterized by dynamomechanical analysis, DSC, thermogravimetric analysis and mechanical tests. The modifiers are incorporated into the thermosetting network because of the participation of the end hydroxyl groups in the cationic curing of epoxides by the activated monomer mechanism. Homogeneous thermosets have been obtained with a remarkable increase in impact strength without sacrificing elastic modulus or hardness. A compromise between the rigid structure of the aromatic hyperbranched core and the flexibilizing effect of the poly(ε‐caprolactone) arms is believed to be responsible for the overall thermal and mechanical properties of the materials. The use of these polymeric modifiers increases the thermal stability of the resulting materials because of the low degradability of the aromatic ester groups in the hyperbranched core and the incorporation of the modifier into the network structure. However, the presence of such ester groups makes them reworkable by hydrolysis or alcoholysis in an alkaline medium, thus opening a way for recovery of valuable substrates. Copyright © 2013 John Wiley & Sons, Ltd.     

Remarkably facile solvolyses of triflates via carbocationic processes in dimethyl sulfoxide

A number of triflates have been shown to undergo clean pseudo-first-order solvolysis reactions in DMSO-d(6) to give products derived from carbocationic intermediates. Thus, t-BuCH(OTf)CO-t-Bu (5) and t-BuCH(2)OTf (9) react readily in DMSO-d(6) at 25 degrees C to give a rearranged oxosulfonium salts, and subsequent alkene products where methyl migration to the incipient cationic center occurs. t-BuCH(OTf)CO(2)CH(3) (14) gives analogous rearranged products, and 1-methylcyclopropyl triflate (21) gives a ring-opened allylic oxosulfonium salt. These triflates react primarily via k(Delta) pathways. 6-Methylbicyclo[3.1.0]hex-6-yl triflate (23), bicyclo[2.2.1hept-1-yl triflate (24), 1,6-methano[10]annulen-11-yl triflate (25), (CH(3))(2)C(OTf)CO(2)CH(3) (26), and (CH(3))(2)CCN(OTf) (29) all react in DMSO-d(6) to give carbocation-derived products. PhCH(OTf)CF(3) (33) and substituted analogues also react readily in DMSO-d(6), and the Hammett rho(+) value is -3.7. This suggests a "borderline" mechanism where the transition state has substantial charge development. The primary feature of these solvolyses is the high reactivity of all of these triflates in DMSO-d(6). Thus, these triflates are all more reactive in DMSO-d(6) than in HOAc, and for most, rates are faster than in CF(3)CH(2)OH. Triflates 5, 21, 29, and 33 are 10(8)-10(9) times more reactive in DMSO-d(6) than the corresponding mesylates. It is suggested that the decreased need for electrophilic solvation of triflate anion, and the high cation solvating ability of DMSO, are the reasons for the high triflate reactivity in DMSO-d(6).     

The role of intermolecular polarization for the stability of lithium intercalation compounds of alpha- and beta-perylene

Lithium intercalation compounds of alpha- and beta-perylene are investigated by photoelectron spectroscopy. Spectroscopic data together with a Born-Haber cycle provide information on the formation enthalpy of those materials. This approach allows understanding the amount of charge transferred from the alkali metal atoms to the pi system, and illuminates the role of molecular versus solid-state properties in the formation of the intercalation compounds. In the bulk of alpha-perylene material, molecular dimerization survives upon intercalation which reduces the Madelung energy of the intercalation compound but increases the electron-accepting capability of the organic system and facilitates the ionization of lithium atoms in the molecular solid environment. The lower ionization potential results in a larger charge transfer (about two electrons per molecule) in alpha-perylene compared to the monomeric system, beta-perylene.     

Regioselective cyclomanganation of Schiff bases. An unexpected effect of chloro substituents

In this paper we describe the synthesis of new metallacycles by the cyclomanganation reaction of benzyl-benzylidene-amines by using [MnMe(CO)₅] as metallating agent. These ligands can undergo metallation on different aromatic carbon atoms but no important differences have been found in the regioselectivity of the process, that can be related to the electronic effect of the substituents, and in all the cases studied the endo-cyclomanganated complexes of para-substituted imines have been obtained. The corresponding exo-metallacycles were obtained by reaction of [MnMe(CO)₅] on the imine 2,6-Cl₂C₆H₃CHNCH₂Ph and 2,4,6-Me₃C₆H₂CHNCH₂Ph, derived from 2,6-dichlorobenzaldehyde and 2,4,6-trimethylbenzaldehyde, respectively.The results described suggest that the mechanism of the cyclomanganation is similar to that of cyclopalladation and it can be proposed that cyclomanganation takes place by the formation of a four-centered transition state, involving the C-H and Mn-C_acetyl bonds, in the acetyl coordination complex formed in the first step of the reaction.     

Design and characterization of libraries of molecular fragments for use in NMR screening against protein targets

We have designed four generations of a low molecular weight fragment library for use in NMR-based screening against protein targets. The library initially contained 723 fragments which were selected manually from the Available Chemicals Directory. A series of in silico filters and property calculations were developed to automate the selection process, allowing a larger database of 1.79 M available compounds to be searched for a further 357 compounds that were added to the library. A kinase binding pharmacophore was then derived to select 174 kinase-focused fragments. Finally, an additional 61 fragments were selected to increase the number of different pharmacophores represented within the library. All of the fragments added to the library passed quality checks to ensure they were suitable for the screening protocol, with appropriate solubility, purity, chemical stability, and unambiguous NMR spectrum. The successive generations of libraries have been characterized through analysis of structural properties (molecular weight, lipophilicity, polar surface area, number of rotatable bonds, and hydrogen-bonding potential) and by analyzing their pharmacophoric complexity. These calculations have been used to compare the fragment libraries with a drug-like reference set of compounds and a set of molecules that bind to protein active sites. In addition, an analysis of the overall results of screening the library against the ATP binding site of two protein targets (HSP90 and CDK2) reveals different patterns of fragment binding, demonstrating that the approach can find selective compounds that discriminate between related binding sites.     

Easy access to diastereomerically pure platinacycles

The synthesis of the first examples of diastereomerically pure platinacycles having simultaneously a chiral carbon and a sigma[Pt-C(sp(2), ferrocene)] bond is described.