Reaction of Benzopinacol with Non-ionic Bases: Reversing the Pinacol Coupling

The reaction of benzopinacol with the non-ionic bases butyllithium and phosphazene P₄ leads to the formation of the corresponding ketyl radical anions, which have been characterized by EPR/ENDOR spectroscopy. This conversion has a high efficiency. Such a reversed pinacol reaction can be used for a controlled release of ketyl radicals. Moreover, the nature of the base has a marked effect on the association of the ketyl radical anion and the counterions. This illustrates the importance of ion pairing for reductive coupling

Time-Resolved EPR as a Tool to Investigate Oxygen Quenching in Photoinitiated Radical Polymerizations

It is challenging to obtain absolute rate constants for the quenching of organic radicals by molecular oxygen because they often do not present absorbance in the UV–vis range. Here, it is shown that time-resolved EPR (chemically induced dynamic electron polarization, or CIDEP) spectroscopy is useful in establishing rate constants for the addition of benzoyl radicals to molecular oxygen. It was found that benzoyl radicals are particularly reactive toward O₂ and can, therefore, act as oxygen scavengers in the initiating phase of radical polymerizations. Kinetic simulations underpin this reactivity

UV-Triggered End Group Conversion of Photo-Initiated Poly(methyl methacrylate)

The analysis of photo-initiated poly­(methyl methacrylate) via electrospray ionization-mass spectrometry (ESI–MS) (synthesized by pulsed laser polymerization (PLP, at λ = 351 nm) of methyl methacrylate (MMA) and benzoin as photoinitiator at 6 mJ/pulse laser energy) evidences the presence of unidentified species. The determination of the origin of these species requires a detailed investigation via size exclusion chromatography-electrospray ionization-mass spectrometry (SEC/ESI–MS) and chemically induced dynamic nuclear polarization-nuclear magnetic resonance spectroscopy (CIDNP–NMR). It was found that post-irradiation of benzoin-initiated poly­(methyl methacrylate) leads to α-cleavage of the benzoyl fragment leading to a sequence of cascade reactions, including the formation of an additional double bond within the polymer chain as evidenced via ESI–MS. Furthermore, the reaction products of the benzoyl radical post α-cleavage (e.g., benzaldehyde, phenyl methyl ketone, methyl formate, or methane) as well as the formed macroradical can be followed by CIDNP–NMR, which allows establishing a reaction mechanism for the UV-induced cleavage process. The study thus evidence thatif the integrity of UV initiated polymers is to be kept intact during their synthesisvery low irradiation energies need to be employed

β‑Allyl Sulfones as Addition–Fragmentation Chain Transfer Reagents: A Tool for Adjusting Thermal and Mechanical Properties of Dimethacrylate Networks

Dimethacrylates are known to have good photoreactivity, but their radical polymerization usually leads to irregular, highly cross-linked, and brittle polymer networks with broad thermal polymer phase transitions. Here, the synthesis of mono- and difunctional β-allyl sulfones is described, and those substances are introduced as potent addition–fragmentation chain transfer (AFCT) reagents leading to dimethacrylate networks with tunable properties. By controlling the content and functionality of said AFCT reagents, it is possible to achieve more homogeneous networks with a narrow glass transition and an adjustable glass transition temperature (<i>T</i>_g), rubber modulus of elasticity (<i>E</i>_r), and network density. In contrast to dimethacrylate networks containing monomethacrylates as reactive diluents, the network architecture of the β-allyl sulfone-based dimethacrylate networks is more homogeneous and the tunability of thermal and mechanical properties is much more enhanced. The reactivity and polymerization were investigated using laser flash photolysis, photo-DSC, and NMR, while DMTA and swellability tests were performed to characterize the polymer

Wavelength-Dependent Photochemical Stability of Photoinitiator-Derived Macromolecular Chain Termini

Herein, we report the uniqueand first timewavelength-dependent investigation with strictly monochromatic light of 305–405 nm wavelength into the stability of photoinitiator-derived chain termini of poly­(methyl methacrylate) using a tunable laser system fused with pulsed-laser irradiation and size exclusion chromatography hyphenated to high-resolution electrospray mass spectrometry (PLI-SEC-ESI-MS). We assess several substitution patterns of methyl groups on the common benzoyl-type radical fragment. Critically, methyl substitution in the 2- and 6-positions of the benzoyl moiety, i.e., in both <i>ortho</i>-positions, resulted in stable chain ends up to approximately 350 nm. The stability can be attributed to a blue-shift of the n−π* transitions (relevant for the end group reactivity) as predicted by earlier density functional theory (DFT) calculations on model species. In sharp contrast, our experiments show a far reduced stability of the end groups commencing from 400 nm onwards, when the dual <i>ortho</i>-methyl substitution in the benzoyl fragment is missing. Thus, we demonstrate that the substitution pattern on the phenyl ring of the benzoyl group dictates the chain end stability as a function of wavelength in excellent agreement with the quantum chemical predictions. Our study thus provides critical insights into selecting suitable photoinitiation systems for specific wavelength regimes

Monotrimethylene-Bridged Bis‑<i>p</i>‑phenylenediamine Radical Cations and Dications: Spin States, Conformations, and Dynamics

The properties of <i>p</i>-phenylenediamine- (PD-) based systems substantially depend on the molecular topology. The singly bridged PD analogues HMPD and OMPD in which the PD rings are connected by a flexible linker reveal particular electronic properties in their radical cations and dications. The EPR and UV–vis spectra of HMPD^2+•• were found to be exceptionally temperature-sensitive, following a change from the extended conformation (doublet–doublet state) predominant at room temperature to the π-stacked conformation (singlet state) prevailing at dry-ice temperature. Changing the single bridge from (CH₂)₃ to dimethylated CH₂CMe₂CH₂ in OMPD^2+•• causes considerably less of the π-stacked conformation to be present at low temperature as a result of the steric interactions with the methyl groups of the bridge. In contrast to HMPD^2+•• and OMPD^2+••, in which the positive charges are localized separately in each PD^+• ring, in the extended conformation, exchange of the electron (“hole hopping”) between the two PD units (fast at the time scale of EPR experiments) was observed for HMPD^+• and OMPD^+•. This process slows in a reversible manner with decreasing temperature, thus forming the radical cation with the unpaired electron spin density predominantly on one PD core, at low temperatures. Accordingly, a subtle balance between conformational changes, electron delocalization, and spin states could be established

Initiators Based on Benzaldoximes: Bimolecular and Covalently Bound Systems

Typical bimolecular photoinitiators (PIs) for radical polymerization of acrylates show only poor photoreactivity because of the undesired effect of back electron transfer. To overcome this limitation, PIs consisting of a benzaldoxime ester and various sensitizers based on aromatic ketones were introduced. The core of the photoinduced reactivity was established by laser flash photolysis, photo-CIDNP, and EPR experiments at short time scales. According to these results, the primarily formed iminyl radicals are not particularly active. The polymerization is predominantly initiated by C-centered radicals. Photo-DSC experiments show reactivities comparable to that of classical monomolecular type I PIs like Darocur 1173

Photoinitiators with β-Phenylogous Cleavage: An Evaluation of Reaction Mechanisms and Performance

Bimolecular photoinitiators based on benzophenone and <i>N</i>-phenylglycine ideally overcome limitations of classical two-component systems, such as the possibility of deactivation by a back electron transfer or the solvent cage effect. Furthermore, if they are covalently linked, loss of reactivity by diffusion limitation could be reduced. Here we show that such an initiator displays unusually high photoreactivity. This is established by photo-DSC experiments and mechanistic investigations based on laser flash photolysis, TR-EPR, and photo-CIDNP. The β-phenylogous scission of the C–N bond is highly efficient and leads to the production of reactive initiating radicals at a short time scale

Helicene Quinones: Redox-Triggered Chiroptical Switching and Chiral Recognition of the Semiquinone Radical Anion Lithium Salt by Electron Nuclear Double Resonance Spectroscopy

We present the synthesis and characterization of enantiomerically pure [6]­helicene <i>o</i>-quinones (<i>P</i>)-(+)-<b>1</b> and (<i>M</i>)-(−)-<b>1</b> and their application to chiroptical switching and chiral recognition. (<i>P</i>)-(+)-<b>1</b> and (<i>M</i>)-(−)-<b>1</b> each show a reversible one-electron reduction process in their cyclic voltammogram, which leads to the formation of the semiquinone radical anions (<i>P</i>)-(+)-<b>1</b>^•– and (<i>M</i>)-(−)-<b>1</b>^•–, respectively. Spectroelectrochemical ECD measurements give evidence of the reversible switching between the two redox states, which is associated with large differences of the Cotton effects [Δ­(Δε)] in the UV and visible regions. The reduction of (±)-<b>1</b> by lithium metal provides [Li⁺{(±)-<b>1</b>^•–}], which was studied by EPR and ENDOR spectroscopy to reveal substantial delocalization of the spin density over the helicene backbone. DFT calculations demonstrate that the lithium hyperfine coupling <i>A</i>(⁷Li) in [Li⁺{(±)-<b>1</b>^•–}] is very sensitive to the position of the lithium cation. On the basis of this observation, chiral recognition by ENDOR spectroscopy was achieved by complexation of [Li⁺{(<i>P</i>)-(+)-<b>1</b>^•–}] and [Li⁺{(<i>M</i>)-(−)-<b>1</b>^•–}] with an enantiomerically pure phosphine oxide ligand

Oxorhenium(V) Complexes with Phenolate–Oxazoline Ligands: Influence of the Isomeric Form on the O‑Atom-Transfer Reactivity

The bidentate phenolate–oxazoline ligands 2-(2′-hydroxyphenyl)-2-oxazoline (<b>1a</b>, Hoz) and 2-(4′,4′-dimethyl-3′,4′-dihydrooxazol-2′-yl)­phenol (<b>1b</b>, Hdmoz) were used to synthesize two sets of oxorhenium­(V) complexes, namely, [ReOCl₂(L)­(PPh₃)] [L = oz (<b>2a</b>) and dmoz (<b>2b</b>)] and [ReOX­(L)₂] [X = Cl, L = oz (<b>3a</b> or <b>3a′</b>); X = Cl, L = dmoz (<b>3b</b>); X = OMe, L = dmoz (<b>4</b>)]. Complex <b>3a′</b> is a coordination isomer (<i>N</i>,<i>N</i>-cis isomer) with respect to the orientation of the phenolate–oxazoline ligands of the previously published complex <b>3a</b> (<i>N</i>,<i>N</i>-trans isomer). The reaction of <b>3a′</b> with silver triflate in acetonitrile led to the cationic compound [ReO­(oz)₂(NCCH₃)]­(OTf) ([<b>3a′</b>]­(OTf)). Compound <b>4</b> is a rarely observed isomer with a <i>trans</i>-ORe–OMe unit. Complexes <b>3a</b>, <b>3a′</b>, [<b>3a′</b>]­(OTf), and <b>4</b> were tested as catalysts in the reduction of a perchlorate salt with an organic sulfide as the O acceptor and found to be active, in contrast to <b>2a</b> and <b>2b</b>. A comparison of the two isomeric complexes <b>3a</b> and <b>3a′</b> showed significant differences in activity: 87% <b>3a</b> vs 16% <b>3a′</b> sulfoxide yield. When complex [<b>3a</b>′]­(OTf) was used, the yield was 57%. Density functional theory calculations circumstantiate all of the proposed intermediates with <i>N</i>,<i>N</i>-trans configurations to be lower in energy compared to the respective compounds with <i>N</i>,<i>N</i>-cis configurations. Also, no interconversions between <i>N</i>,<i>N</i>-trans and <i>N</i>,<i>N</i>-cis configurations are predicted, which is in accordance with experimental data. This is interesting because it contradicts previous mechanistic views. Kinetic analyses determined by UV–vis spectroscopy on the rate-determining oxidation steps of <b>3a</b>, <b>3a′</b>, and [<b>3a′</b>]­(OTf) proved the <i>N</i>,<i>N</i>-cis complexes <b>3a′</b> and [<b>3a′</b>]­(OTf) to be slower by a factor of ∼4