Alkoxyamines are heat-labile molecules, widely used as in-situ source of nitroxides in polymer and materials sciences. Here we show that the one-electron oxidation of an alkoxyamine leads to a cation radical intermediate that even at room temperature rapidly fragments releasing a nitroxide and carbocation. Digital simulations of experimental voltammetry and current-time transients suggest the unimolecular decomposition which yields the "unmasked" nitroxide (TEMPO) is exceedingly rapid and irreversible. High-level quantum computations indicate the collapse of the alkoxyamine cation radical is likely to yield a neutral nitroxide radical and a secondary phenylethyl cation. However, this fragmentation is predicted to be slow and energetically very unfavorable. To attain qualitative agreement between the experimental kinetics and computational modelling for this fragmentation step the explicit electrostatic environment within the double layer must be accounted for. Single-molecule break-junction experiments in a scanning tunneling microscope using solvent of low dielectric (STM-BJ technique) corroborate the role played by electrostatics forces on the lysis of the alkoxyamine CON bond. This work highlights the electrostatic aspects played by charged species in a chemical step that follows an electrochemical reaction, defines the magnitude of this catalytic effect by looking at an independent electrical technique in non-electrolyte systems (STM-BJ), and suggests a redox on/off switch to guide the cleavage of alkoxyamines at an electrified interface.

Natural biopolymers, such as DNA and proteins, have uniform microstructures with defined molecular weight, precise monomer sequence, and stereoregularity along the polymer main chain that affords them unique biological functions. To reproduce such structurally perfect polymers and understand the mechanism of specific functions through chemical approaches, researchers have proposed using synthetic polymers as an alternative due to their broad chemical diversity and relatively simple manipulation. Herein, we report a new methodology to prepare sequence-controlled and stereospecific oligomers using alternating radical chain growth and sequential photoinduced RAFT single unit monomer insertion (photo-RAFT SUMI). Two families of cyclic monomers, the indenes and the N-substituted maleimides, can be alternatively inserted into RAFT agents, one unit at a time, allowing the monomer sequence to be controlled through sequential and alternating monomer addition. Importantly, the stereochemistry of cyclic monomer insertion into the RAFT agents is found to be trans-selective along the main chains due to steric hindrance from the repeating monomer units. All investigated cyclic monomers provide such trans-selectivity, but analogous acyclic monomers give a mixed cis- and trans-insertion.

The wavelength-dependent conversion of two rapid photoinduced ligation reactions, i.e., the light activation of o-methylbenzaldehydes, leading to the formation of reactive o-quinodimethanes (photoenols), and the photolysis of 2,5-diphenyltetrazoles, affording highly reactive nitrile imines, is probed via a monochromatic wavelength scan at constant photon count. The transient species are trapped by cycloaddition with N-ethylmaleimide, and the reactions are traced by high resolution mass spectrometry and nuclear magnetic resonance spectroscopy. The resulting action plots are assessed in the context of Beer-Lambert's law and provide combined with time-dependent density functional theory and multireference calculations an in-depth understanding of the underpinning mechanistic processes, including conical intersections. The π → π* transition of the carbonyl group of the o-methylbenzaldehyde correlates with a highly efficient conversion to the cycloadduct, showing no significant wavelength dependence, while conversion following the n → π* transition proceeds markedly less efficient at longer wavelengths. The influence of absorbance and reactivity has critical consequences for an effective reaction design: At high concentrations of o-methylbenzaldehydes (c = 8 mmol L), photoligations with N-ethylmaleimide (possible for λ ≤ 390 nm) are ideally performed at 330 nm, whereas at high light penetration regimes at lower concentrations (c = 0.3 mmol L), 315 nm irradiation leads to the highest conversion. Activation and trapping of 2,5-diphenyltetrazoles (possible for λ ≤ 322 nm) proceeds best at a wavelength shorter than 295 nm, irrespective of concentration.

Oriented electrostatic fields can exert catalytic effects upon both the kinetics and thermodynamics of chemical reactions; however, the vast majority of studies thus far have focused upon ground state chemistry and rarely consider any more than a single class of reaction. In the present study, we first use density functional theory (DFT) calculations to clarify the mechanism of CO2 storage via photochemical carboxylation of o-alkylphenyl ketones, originally proposed by Murakami et al. (J. Am. Chem. Soc. 2015, 137, 14063); we then demonstrate that oriented internal electrostatic fields arising from remote charged functional groups (CFGs) can selectively and cooperatively promote both ground-and excited-state chemical reactivity at all points along the revised mechanism, in a manner otherwise difficult to access via classical substituent effects. What is particularly striking is that electrostatic field effects upon key photochemical transitions are predictably enhanced in increasingly polar solvent, thus overcoming a central limitation of the electrostatic catalysis paradigm. We explain these observations, which should be readily extendable to the ground state.

This work demonstrates the effect of electrostatic interactions on the electroactivity of a persistent organic free radical. This was achieved by chemisorption of molecules of 4-azido-2,2,6,6-tetramethyl-1-piperdinyloxy (4-azido-TEMPO) onto monolayer-modified Si(100) electrodes using a two-step chemical procedure to preserve the open-shell state and hence the electroactivity of the nitroxide radical. Kinetic and thermodynamic parameters for the surface electrochemical reaction are investigated experimentally and analyzed with the aid of electrochemical digital simulations and quantum-chemical calculations of a theoretical model of the tethered TEMPO system. Interactions between the electrolyte anions and the TEMPO grafted on highly doped, i.e., metallic, electrodes can be tuned to predictably manipulate the oxidizing power of surface nitroxide/oxoammonium redox couple, hence showing the practical importance of the electrostatics on the electrolyte side of the radical monolayer. Conversely, for monolayers prepared on the poorly doped electrodes, the electrostatic interactions between the tethered TEMPO units and the semiconductor-side, i.e., space-charge, become dominant and result in drastic kinetic changes to the electroactivity of the radical monolayer as well as electrochemical nonidealities that can be explained as an increase in the self-interaction "a" parameter that leads to the Frumkin isotherm.

The fundamental influence of the structure and substitution of radical photoinitiators was investigated via a trifold combination of pulsed-laser polymerization with subsequent electrospray-ionization mass spectrometry (PLP-ESI-MS), femtosecond transient absorption (fs-TA) spectroscopy, and quantum chemistry. For the first time, a library of benzoin-derived photoinitiators with varied substitution patterns was synthesized. In the PLP-ESI-MS study, different photoinitiators were compared pairwise in so-called cocktail experiments, enabling the direct comparison of their initiation efficiency. In the fs-TA experiments, the transient response was observed after UV excitation in the visible spectral region, allowing for a description of excited state dynamics, which was analyzed with the aid of TD-DFT calculations. Ab initio calculations were undertaken to determine the reactivity of the radical fragments generated from these photoinitiators and to quantify the influence of various substituents on the rate of addition to monomer. In summary, the influence of the substituent on the initiation efficiency, intersystem crossing (ISC) behavior, excited state dynamics, and the extinction coefficients were analyzed. Hence, relaxation pathways and reaction mechanisms were optimized to explain disparate initiation efficiencies of a wide range of newly designed photoinitiators with varying substitution patterns. The strongly divergent absorptivities of the different photoinitiators and their corresponding initiation efficiencies underline that the absorptivity of a molecule is by no means an unequivocal measure for its reactivity when excited at a specific wavelength. In fact, the most efficient initiators are governed by one nπ* singlet state with a very low extinction coefficient at the excitation wavelength and one or two triplet states with nπ* character.

A test set of 14 TEMPO-based alkoxyamines was studied via a combination of cyclic voltammetry (CV) and accurate quantum chemistry to assess the effect of substituents on electrochemical cleavage. The experimental oxidation potentials of the alkoxyamines fell into the range of 1.1-1.6 V versus Ag/AgCl in acetonitrile, were well reproduced by theory (MAD 0.04V), with values showing good correlation with the sR Hammett parameters of both the R group and the OR group in TEMPO-R. Importantly, most of the studied alkoxyamines underwent oxidative cleavage to form either TEMPO • and R + or TEMPO + and R • , with the former favored by electron donating substituents on R (e.g., 2-oxolane, Ac, CH(CH3)Ph, i-Pr, t-Bu) and the latter by electron withdrawing substituents (Bn, allyl, CH(CH3)C(O)OCH3, C(CH3)2C(O)OCH3, CH(CH3)CN). Where R is not stabilized (e.g. R = CH2C(O)OCH3, Me, Et), fully or almost fully reversible oxidationwithout cleavagewas observed, making these species promising candidates for battery applications. Finally, in the case of R = Ph where NO cleavage occurred, a phenoxy cation and an aminyl radical were generated. Based on these results, TEMPO-based alkoxyamines can provide a variety of electrochemically generated carbon-centered radicals and carbocations for use in synthesis, polymerization and surface modification.

Bench-and air-stable 1-methoxy-2,2,6,6-tetramethylpiperidine (TEMPO-Me) is relatively unreactive at ambient temperature in the absence of an electrochemical stimulus. In this report, we demonstrate that the one-electron electrochemical oxidation of TEMPO-Me produces a powerful electrophilic methylating agent in situ. Our computational and experimental studies are consistent with methylation proceeding via a SN2 mechanism, with a strength comparable to the trimethyloxonium cation. A protocol is developed for the electrochemical methylation of aromatic acids using TEMPO-Me.