Crosslinking Vinyl-Addition Polynorbornenes via Difunctional Diazirines to Generate Low Dielectric-Constant and Low Dielectric-Loss Thermosets.

Thermosets having low dielectric constant (Dk < 3) and low dielectric dissipation factor (Df < 0.003), high glass transition temperature (Tg > 150 °C), and good adhesion to copper are desirable for the low loss layers of the copper clad laminates (CCL) in next generation printed circuit boards (PCB). Three different difunctional diazirines were evaluated for both thermal and photochemical crosslinking of a high Tg vinyl-addition polynorbornene resin: poly(5-hexyl-1-norbornene) (poly(HNB)). The substrate polymer, crosslinked by the carbenes generated from the activated diazirines, formed thermosets with Dk < 2.3 and Df < 0.001 at 10 GHz depending on the identity of the diazirine and the loading. The Dk and Df values for one composition were stable for 1600 h at 125 °C in air and for 1400 h at 85 °C and 85% relative humidity, suggesting good long-term reliability of this thermoset. Adhesion of poly(HNB) to copper can be enhanced by priming the copper surface with a diazirine prior to high temperature lamination; peel strength values of greater than 7.5 N/cm were achieved. Negative-tone photopatterning of poly(HNB) with diazirines upon exposure to 365 nm light was demonstrated. This article is protected by copyright. All rights reserved.

Functionalization of Polydimethylsiloxane with Diazirine-Based Linkers for Covalent Protein Immobilization.

Biomolecule attachment to solid supports is critical for biomedical devices, such as biosensors and implants. Polydimethylsiloxane (PDMS) is commonly used for these applications due to its advantageous properties. To enhance the biomolecule immobilization on PDMS, a novel technique is demonstrated using newly synthesized diazirine molecules for the surface modification of PDMS. This nondestructive process involves a reaction between diazirine molecules and PDMS through C-H insertion with thermal or ultraviolet activation. The success of the PDMS modification is confirmed by various surface characterization techniques. Bovine serum albumin (BSA) and immunoglobulin G (IgG) are strongly attached to the modified PDMS surfaces, and the amount of protein is quantified using iodine-125 radiolabeling. The results demonstrate that PDMS is rapidly functionalized, and the stability of the immobilized proteins is significantly improved with multiple types of diazirine molecules and activation methods. Confocal microscopy provides three-dimensional images of the distribution of immobilized IgG on the surfaces and the penetration of diazirine-based linkers through the PDMS substrate during the coating process. Overall, this study presents a promising new approach for functionalizing PDMS surfaces to enhance biomolecule immobilization, and its potential applications can extend to multimaterial modifications for various diagnostic and medical applications such as microfluidic devices and immunoassays with relevant bioactive proteins.

Arylboronic Acid-Catalyzed Racemization of Secondary and Tertiary Alcohols.

The use of 2-carboxyphenylboronic acid (5 mol %) and oxalic acid (10 mol %) with 2-butanone as a solvent for the racemization of a range of enantiomerically pure secondary and tertiary alcohols is demonstrated. The process is postulated to proceed via reversible Brønsted acid-catalyzed C-O bond cleavage through an achiral carbocation intermediate.

Light-Induced Anti-Bacterial Effect Against Staphylococcus aureus of Porphyrin Covalently Bonded to a Polyethylene Terephthalate Surface.

Antimicrobial photodynamic inactivation represents a promising and potentially greener alternative to conventional antimicrobials, and a solution for multidrug-resistant strains. The current study reports the development and characterization of tetra-substituted diazirine porphyrin covalently bonded to polyethylene terephthalate (PET) and its use as an antimicrobial surface. The diazirine moiety on the porphyrin was activated using a temperature of 120 °C, which initiated a C-H insertion mechanism that irreversibly functionalized the PET surface. Activation of the surface with white LED light in phosphate-buffered saline (PBS) led to singlet oxygen generation, which was detected via the degradation of 9,10-anthracenediylbis(methylene)dimalonic acid (ADMA) over time. The bactericidal effect of the 1O2-producing surface against Staphylococcus aureus was determined qualitatively and quantitatively. The growth of the pathogen beneath porphyrin-functionalized PET coupons was reduced; moreover, the PET coupons resulted in a 1.76-log reduction in cell counts after exposure to white LED light for 6 h. This is a promising material and platform for the development of safer antimicrobial surfaces, with applications in healthcare, food packaging, marine surfaces, and other surfaces in the environment.

Structure-function relationships in aryl diazirines reveal optimal design features to maximize C-H insertion.

Diazirine reagents allow for the ready generation of carbenes upon photochemical, thermal, or electrical stimulation. Because carbenes formed in this way can undergo rapid insertion into any nearby C-H, O-H or N-H bond, molecules that encode diazirine functions have emerged as privileged tools in applications ranging from biological target identification and proteomics through to polymer crosslinking and adhesion. Here we use a combination of experimental and computational methods to complete the first comprehensive survey of diazirine structure-function relationships, with a particular focus on thermal activation methods. We reveal a striking ability to vary the activation energy and activation temperature of aryl diazirines through the rational manipulation of electronic properties. Significantly, we show that electron-rich diazirines have greatly enhanced efficacy toward C-H insertion, under both thermal and photochemical activation conditions. We expect these results to lead to significant improvements in diazirine-based chemical probes and polymer crosslinkers.

Flexible polyfluorinated bis-diazirines as molecular adhesives.

Motivated by a desire to develop flexible covalent adhesives that afford some of the same malleability in the adhesive layer as traditional polymer-based adhesives, we designed and synthesized two flexible, highly fluorinated bis-diazirines. Both molecules are shown to function as effective crosslinkers for polymer materials, and to act as strong adhesives when painted between two polymer objects of low surface energy, prior to thermal activation. Data obtained from lap-shear experiments suggests that greater molecular flexibility is correlated with improved mechanical compliance in the adhesive layer.

Synthesis of the natural product descurainolide and cyclic peptides from lignin-derived aromatics.

Alternative sources of potential feedstock chemicals are of increasing importance as the availability of oil decreases. The biopolymer lignin is viewed as a source of useful mono-aromatic compounds as exemplified by the industrial scale production of vanillin from this biomass. Alternative lignin-derived aromatics are available in pure form but to date examples of the use of these types of compounds are rare. Here we address this issue by reporting the conversion of an aromatic keto-alcohol to the anti- and syn-isomers of Descurainolide A. The key step involves a rhodium-catalyzed allylic substitution reaction. Enantio-enriched allylic alcohols were generated via an isothiourea-catalyzed kinetic resolution enabling access to both the (2R,3R) and (2S,3S) enantiomers of anti-Descurainolide A. In addition we show that the lignin-derived keto-alcohols can be converted into unnatural amino acid derivatives of tyrosine. Finally, these amino acids were incorporated into cyclic peptide scaffolds through the use of both chemical and an enzyme-mediated macrocylisation.

Isothiourea-Catalysed Acylative Kinetic Resolution of Aryl-Alkenyl (sp2 vs. sp2 ) Substituted Secondary Alcohols.

The non-enzymatic acylative kinetic resolution of challenging aryl-alkenyl (sp2 vs. sp2 ) substituted secondary alcohols is described, with effective enantiodiscrimination achieved using the isothiourea organocatalyst HyperBTM (1 mol %) and isobutyric anhydride. The kinetic resolution of a wide range of aryl-alkenyl substituted alcohols has been evaluated, with either electron-rich or naphthyl aryl substituents in combination with an unsubstituted vinyl substituent providing the highest selectivity (S=2-1980). The use of this protocol for the gram-scale (2.5 g) kinetic resolution of a model aryl-vinyl (sp2 vs. sp2 ) substituted secondary alcohol is demonstrated, giving access to >1 g of each of the product enantiomers both in 99:1 e.r.

Isothiourea-catalysed enantioselective pyrrolizine synthesis: synthetic and computational studies.

The catalytic enantioselective synthesis of a range of cis-pyrrolizine carboxylate derivatives with outstanding stereocontrol (14 examples, >95 : 5 dr, >98 : 2 er) through an isothiourea-catalyzed intramolecular Michael addition-lactonisation and ring-opening approach from the corresponding enone acid is reported. An optimised and straightforward three-step synthetic route to the enone acid starting materials from readily available pyrrole-2-carboxaldehydes is delineated, with benzotetramisole (5 mol%) proving the optimal catalyst for the enantioselective process. Ring-opening of the pyrrolizine dihydropyranone products with either MeOH or a range of amines leads to the desired products in excellent yield and enantioselectivity. Computation has been used to probe the factors leading to high stereocontrol, with the formation of the observed cis-steroisomer predicted to be kinetically and thermodynamically favoured.