Here we report the use of 10-phenylphenothiazine (PTH) as an inexpensive, highly reducing metal-free photocatalyst for the reduction of carbon-halogen bonds via the trapping of carbon-centered radical intermediates with a mild hydrogen atom donor. Dehalogenations were carried out on various substrates with excellent yields at room temperature in the presence of air.In recent years, photoredox chemistry has enabled the development of a wide variety of synthetic transformations. 1 These methods are based on photocatalysts which, upon absorption of light, enter either a highly reducing or oxidizing excited state capable of facilitating redox-based transformations. In particular, the reduction of activated carbon-halide (C-X) bonds has generated wide interest, largely because of the broad synthetic utility of resulting carbon-centered radical intermediates. [1][2][3][4][5][6][7][8][9][10] One example includes subsequent trapping of these intermediates with a mild H-atom source to achieve radical dehalogenations. 3,5,6,9 In this case, the power of using a photoredox approach is that it offers a more efficient and safer alternative to traditional dehalogenation protocols involving metal-halogen exchange, 11,12 stoichiometric tin hydride, 13 and various other highly toxic reagents. [14][15][16] However, despite the notable advantages of photoredox catalysis, 1 a number of major challenges still exist. This includes the use of catalysts based on rare-earth transition metals such as Ru and Ir, which have inherent limitations due to the cost of the catalyst itself (B$1 mg À1 for Ir(ppy) 3 ), 17 as well as the expense associated with the removal of trace metals from the desired products -critical for applications from pharmaceuticals to micro-electronics. In addition, although an assortment of activated carbon-halogen bonds have been accessed using these catalysts, 1 higher energy unactivated halides are a significantly more challenging task, with only unactivated iodides being explored to date. 5,18 To this end, a more affordable gold-based photocatalyst has been developed, 10 and although offering broader substrate scope, the disadvantages of metal-based systems remain. In addressing this, the use of an organic perylene diimide (PDI)-based photocatalyst was recently reported, and while providing a metal-free alternative, it requires elevated temperatures and has a scope limited to activated aryl-halides. 8 In this context, we envisioned the development of a highly reducing, inexpensive, metal-free photocatalyst that could offer access to a wide range of carbon-halogen substrates under markedly mild conditions (Fig. 1).Our groups previously employed 10-phenylphenothiazine (PTH) as a metal-free catalyst for photomediated atom transfer radical polymerizations (ATRP). 19 In this system, PTH acts as a photoreductant in a similar manner to Ir(ppy) 3 with a reduction potential (E 1/2 * = À2.1 V vs. SCE) significantly higher than Ir(ppy) 3 (E 1/2 * = À1.7 V vs. SCE). Based on our interest in metalfree ATRP, we envisioned that th...

ABSTRACT:The development of an operationally simple, metal-free surface-initiated atom transfer radical polymerization (SI-ATRP) based on visible-light mediation is reported. The facile nature of this process enables the fabrication of well-defined polymer brushes from flat and curved surfaces using a "benchtop" setup that can be easily scaled to four-inch wafers. This circumvents the requirement of stringent air-free environments (i.e., glovebox), and mediation by visible light allows for spatial control on the micron scale, with complex three-dimensional patterns achieved in a single step. This robust approach leads to unprecedented access to brush architectures for nonexperts.

Thermoplastic elastomers (TPEs) are designed by embedding discrete glassy or semicrystalline domains in an elastomeric matrix. Typical styrenic-based amorphous TPEs are made of linear ABA-type triblock copolymers, where the volume fraction f of the glassy domains A is typically less than 0.3. This limitation ultimately restricts the range of mechanical strength attainable with these materials. We had previously predicted using self-consistent field theory (SCFT) that A(BA′) n miktoarm block copolymers with an approximately 8:1 ratio of the A to A′ block molecular weights and n ≥ 3 should exhibit discrete A domains at considerably larger f and offer potential for the combination of high modulus, high recoverable elasticity, and high strength and toughness. Using transmission electron microscopy and small-angle X-ray scattering on model polystyrene-bpolyisoprene (PS−PI) miktoarm copolymers, we show that such polymers indeed possess discrete PS domains for f values considerably higher than 0.3. The hexagonal morphology with PS cylinders was achieved for f = 0.5 and n = 3. Mechanical testing indicates that these miktoarm materials are strong, tough, and elastic and thus may be potential candidates for a new generation of thermoplastic elastomers.

We report herein the modular synthesis and nanolithographic potential of poly­(dimethylsiloxane-block-methyl methacrylate) (PDMS-b-PMMA) with self-assembled domains approaching sub-10 nm periods. A straightforward and modular coupling strategy, optimized for low molecular weight diblocks and using copper-catalyzed azide–alkyne “click” cycloaddition, was employed to obtain a library of PDMS-b-PMMA and poly­(dimethylsiloxane-block-styrene) (PDMS-b-PS) diblock copolymers. Flory–Huggins interaction parameters, determined from small-angle X-ray scattering experiments, were high for PDMS-b-PMMA (χ ∼ 0.2 at 150 °C), suggesting this diblock copolymer system has promise for sub-10 nm lithographic applications when compared to the corresponding PDMS-b-PS diblock copolymers (χ ∼ 0.1 at 150 °C). Performance evaluation in thin film self-assembly experiments allowed domain periods as small as 12.1 nm to be obtained, which is among the smallest highly ordered nanoscale patterns reported hitherto for thermally annealed materials.

2,4-Oxazole is an important structural motif in various natural products. An efficient modular synthesis of this structure is achieved via a [3+2] annulation between a terminal alkyne and a carboxamide by using a gold-catalyzed oxidation strategy. The postulated reactive intermediate, a terminal α-oxo gold carbene, previously known to be highly electrophilic and hence impropable to be trapped by stoichiometric external nucleophiles, is coerced to react smoothly with a carboxamide en route to the oxazole ring by a P,N- or P,S-bidentate ligand such as Mor-DalPhos; in stark contrast, often used ligands including monodentate phosphines and NHCs are totally ineffective. The role of these bidentate phosphines in this reaction is attributed to the formation of a tricoordinated gold carbene intermediate, which is less electrophilic and hence more chemoselective when reacting with nucleophiles. The success in using bidentate phosphine ligands to temper the reactivities of in-situ generated gold carbenes would likely open many new opportunities to apply the oxidative gold catalysis to the development of novel methods, and the implication of tricoordinated gold intermediates in homogeneous gold catalysis should stimulate further advance in gold catalysis.

The effect of dispersity on block polymer selfassembly was studied in the monodisperse limit using a combination of synthetic chemistry, matrix-assisted laser desorption ionization spectroscopy, and small-angle X-ray scattering. Oligo-(methyl methacrylate) (oligoMMA) and oligo(dimethylsiloxane) (oligoDMS) homopolymers were synthesized by conventional polymerization techniques and purified to generate an array of discrete, semidiscrete, and disperse building blocks. Coupling reactions afforded oligo(DMS−MMA) block polymers with precisely tailored molar mass distributions spanning single molecular systems (Đ = 1.0) to low-dispersity mixtures (Đ ≈ 1.05). Discrete materials exhibit a pronounced decrease in domain spacing and sharper scattering reflections relative to disperse analogues. The order−disorder transition temperature (T ODT ) also decreases with increasing dispersity, suggesting stabilization of the disordered phase, presumably due to the strengthening of composition fluctuations at the low molar masses investigated.

Despite the number of methods available for dehalogenation and carbon-carbon bond formation using aryl halides, strategies that provide chemoselectivity for systems bearing multiple carbon-halogen bonds are still needed. Herein, we report the ability to tune the reduction potential of metal-free phenothiazine-based photoredox catalysts and demonstrate the application of these catalysts for chemoselective carbon-halogen bond activation to achieve C-C cross-coupling reactions as well as reductive dehalogenations. This procedure works both for conjugated polyhalides as well as unconjugated substrates. We further illustrate the usefulness of this protocol by intramolecular cyclization of a pyrrole substrate, an advanced building block for a family of natural products known to exhibit biological activity.

A pi-acceptor phosphine-electron-deficient olefin ligand was found effective in promoting Pd-catalyzed C(sp)-C(sp) cross-coupling reactions. The new protocol realized the cross-coupling of a broad scope of terminal alkynes and haloalkynes in good to excellent yields with high selectivities. Electron-rich alkynes, which are normally difficult substrates in Glaser couplings, could be employed as either nucleophiles or electrophiles. Alkynes bearing similar substituents, such as n-C5H11CCBr and n-C4H9CCH, which usually suffer from homocoupling side reactions under Cadiot-Chodkiewicz conditions, were successfully cross-coupled in the system. Preliminary kinetic studies revealed that the reaction rate was zero-order in the concentrations of both haloalkynes and terminal alkynes and first order in the loading of Pd(dba)2 and exhibited no obvious dependence on the loading of the copper salt. Control experiments with other phosphines such as PPh3 and DPPF as the ligand were carried out. All the kinetic evidence indicated that the phosphine-olefin ligand facilitated the reductive elimination in the catalytic cycle.