Phenyloxenium Ions: More Like Phenylnitrenium Ions than Isoelectronic Phenylnitrenes?

Threshold Photoelectron Spectrum of m-Benzyne

Due to their unusual electronic structure, the biradical m-benzyne, C₆H₄, and its cation are of considerable interest in chemistry. Here, the photoion mass-selected threshold photoelectron spectrum of the m-benzyne biradical is presented. An adiabatic ionization energy of 8.65 ± 0.015 eV is derived, while a vibrational progression of 0.10 eV is assigned to the ν₉⁺ ring breathing mode, in excellent agreement with computations. The experimental spectrum was reproduced well by Franck-Condon spectral modeling of the ²A₁ ← X ¹A₁ transition, in which the cation retains a monocyclic C₆ framework. The energetically close-lying bicyclic ²A₂ cation state exhibits low Franck-Condon factors, due to the large change in geometry, and thus cannot be observed.

Electrochemical and Spectroscopic Characterization of Oxidized Intermediate Forms of Vitamin E

Vitamin E, a collection of lipophilic phenolic compounds based on chroman-6-ol, has a rich and fascinating oxidative chemistry involving a range of intermediate forms, some of which are proposed to be important in its biological functions. In this review, the available electrochemical and spectroscopic data on these oxidized intermediates are summarized, along with a discussion on how their lifetimes and chemical stability are either typical of similar phenolic and chroman-6-ol derived compounds, or atypical and unique to the specific oxidized isomeric form of vitamin E. The overall electrochemical oxidation mechanism for vitamin E can be summarized as involving the loss of two-electrons and one-proton, although the electron transfer and chemical steps can be controlled to progress along different pathways to prolong the lifetimes of discreet intermediates by modifying the experimental conditions (applied electrochemical potential, aqueous or non-aqueous solvent, and pH). Depending on the environment, the electrochemical reactions can involve single electron transfer (SET), proton-coupled electron transfer (PCET), as well as homogeneous disproportionation and comproportionation steps. The intermediate species produced via chemical or electrochemical oxidation include phenolates, phenol cation radicals, phenoxyl neutral radicals, dications, diamagnetic cations (phenoxeniums) and para-quinone methides. The cation radicals of all the tocopherols are atypically long-lived compared to the cation radicals of other phenols, due to their relatively weak acidity. The diamagnetic cation derived from α-tocopherol is exceptionally long-lived compared to the diamagnetic cations from the other β-, γ- and δ-isomers of vitamin E and compared with other phenoxenium cations derived from phenolic compounds. In contrast, the lifetime of the phenoxyl radical derived from α-tocopherol, which is considered to be critical in biological reactions, is typical for what is expected for a compound with its structural features. Over longer times via hydrolysis reactions, hydroxy para-quinone hemiketals and quinones can be formed from the oxidized intermediates, which can themselves undergo reduction processes to form intermediate anion radicals and dianions. Methods for generating the oxidized intermediates by chemical, photochemical and electrochemical methods are discussed, along with a summary of how the final products vary depending on the method used for oxidation. Since the intermediates mainly only survive in solution, they are most often monitored using UV-vis spectroscopy, FTIR or Raman spectroscopies, and EPR spectroscopy, with the spectroscopic techniques sometimes combined with fast photoinitiated excitation and time-resolved spectroscopy for detection of short-lived species.

Exploiting the p-Bromodienone Route for the Formation and Trapping of Calixarene Oxenium Cations with Enamine Nucleophiles

This study shows that calixarene p-bromodienone derivatives can act as precursors for the formation of oxenium cations, which can be trapped with enamine C-nucleophiles. When calixarene p-bromodienones were treated with enamines, in the presence of AgClO₄, the lower rim-substituted C-O-C products were obtained by an electrophilic attack of the intermediate calixarene-oxenium cation with a contemporary cone-to-partial-cone inversion of the involved aromatic ring.

Concerning the mechanism of iodine(iii)-mediated oxidative dearomatization of phenols

The iodine(iii)-mediated oxidative dearomatization of phenols has proven to be a general method for the preparation of cyclohexadienones. While this is a widely used reaction, there is still a great deal of uncertainty regarding the mechanistic pathway followed by these reactions. In part, this is due to the highly unstable nature of many of the key intermediates, which makes their direct detection extremely difficult. In order to gain some insight into these mechanistic questions, DFT calculations [M06-2X/6-31+G(d) for C, H, and O and LANL2DZdp for iodine] were used to evaluate the two most commonly proposed reaction mechanisms. These results show that unimolecular fragmentation of an oxygen-bound intermediate to give a phenoxenium ion (TS1) is preferred over direct addition of the nucleophile to the aromatic ring of the activated phenol (TS3). In addition, results are presented that suggest protonation and/or hydrogen bonding may play a key role in lowering the energy of the unimolecular fragmentation pathway.

Mechanistic insights into the formation of oxenium ions and radical intermediates through the photolysis of phenylhydroxylamine and its derivatives

The photolysis of photoprecursors to produce oxenium ions has been the subject of extensive experimental studies from femtosecond to microsecond time scales. However, mechanistic insights into the generation of activated intermediate species remain elusive. Herein, we present a theoretical investigation to comprehensively elucidate the possible reaction channels for the formation of oxenium ions and radical intermediates at the multi-configuration perturbation level of theory. Computational results show that photo-initiated electron donation from the phenyl moiety to the repulsive N-O σ* orbital leads to the formation of a diradical intermediate in ground state, and further triggers intramolecular electron transfer from the phenyl moiety to the ammonia radical cation (˙NH₃⁺). This affords closed-shell singlet oxenium ions and neutral :NH₃ as the major products. However, the generation of open-shell triplet outcomes is shown to rely on the energetically accessible single-triplet crossings and spin-orbital interaction among the involved electronic states. Taken together, these data can be used to determine the electronic structures and related properties, as well as reactivities, of oxenium ions and radicals generated by the photolysis of phenylhydroxylamine and its derivatives.

Aryl Nitrenium and Oxenium Ions with Unusual High-Spin π,π* Ground States: Exploiting (Anti)Aromaticity

Nitrenium and oxenium ions are important reactive intermediates in synthetic and biological processes, and their ground electronic configurations are of great interest due to having distinct reactivities and properties. In general, the closed-shell singlet state of these intermediates usually react as electrophiles, while reactions of the triplet states of these ions react like typical diradicals (e.g., H atom abstractions). Nonsubstituted phenyl nitrenium ions (Ph-NH⁺) and phenyl oxenium ions (Ph-O⁺) have closed-shell singlet ground states with large singlet-triplet gaps resulting from a strong break in the degeneracy of the p orbitals on the formal nitrenium/oxenium center. Remarkably, we find computationally (CBS-QB3 and G4MP2) that azulenyl nitrenium and oxenium ions can have triplet ground states depending upon the attachment position on the azulene core. For instance, CBS-QB3 predicts that 1-azulenyl nitrenium ion and 1-azulenyl oxenium ion are singlet ground-state species with considerable singlet-triplet gaps of -47 and -45 kcal/mol to the lowest-energy triplet state, respectively. In contrast, 6-azulenyl nitrenium ion and 6-azulenyl oxenium ion have triplet ground states with a singlet-triplet gap of +7 and +10 kcal/mol, respectively. Moreover, the triplet states are π,π* states, rather than the typical n,π* states seen for many aryl nitrenium or oxenium ions. This dramatic switch in favored electronic states can be ascribed to changes in ring aromaticity/antiaromaticity, with the switch from ground-state singlet ions to triplet-favored ions resulting from both a destabilized singlet state (Hückel antiaromatic) and a stabilized triplet (Baird aromatic) state. Density functional theory (UB3LYP/6-31+G(d,p)) was used to determine substituent effects on the singlet-triplet energy gap for azulenyl nitrenium and oxenium ions, and we find that the unusual ground triplet states can be further tuned by employing electron-donating or -withdrawing groups on the azulene ring. This work demonstrates that azulenyl nitrenium and oxenium ions can have triplet π,π* ground states and provides a simple recipe for making ionic intermediates with distinct electronic configurations and consequent prediction of unique reactivity and magnetic properties from these species.

Direct Detection of the Open-Shell Singlet Phenyloxenium Ion: An Atom-Centered Diradical Reacts as an Electrophile

A new photoprecursor to the phenyloxenium ion, 4-methoxyphenoxypyridinium tetrafluoroborate, was investigated using trapping studies, product analysis, computational investigations, and laser flash photolysis experiments ranging from the femtosecond to the millisecond time scale. These experiments allowed us to trace the complete arc of the photophysics and photochemistry of this photoprecursor beginning with the initially populated excited states to its sequential formation of transient intermediates and ultimate formation of stable photoproducts. We find that the excited state of the photoprecursor undergoes heterolysis to generate the phenyloxenium ion in ∼2 ps but surprisingly generates the ion in its open-shell singlet diradical configuration (¹A₂), permitting an unexpected look at the reactivity of an atom-centered open-shell singlet diradical. The open-shell phenyloxenium ion (¹A₂) has a much shorter lifetime (τ ∼ 0.2 ns) in acetonitrile than the previously observed closed-shell singlet (¹A₁) phenyloxenium ion (τ ∼ 5 ns). Remarkably, despite possessing no empty valence orbitals, this open-shell singlet oxenium ion behaves as an even more powerful electrophile than the closed-shell singlet oxenium ion, undergoing solvent trapping by weakly nucleophilic solvents such as water and acetonitrile or externally added nucleophiles (e.g., azide) rather than engaging in typical diradical chemistry, such as H atom abstraction, which we have previously observed for a triplet oxenium ion. In acetonitrile, the open-shell singlet oxenium ion is trapped to generate ortho and para Ritter intermediates, one of which (para) is directly observed as a longer-lived species (τ ∼ 0.1 ms) in time-resolved resonance Raman experiments. The Ritter intermediates are ultimately trapped by either the 4-methoxypyridine leaving group (in the case of para addition) or trapped internally via an essentially barrierless rearrangement (in the case of ortho addition) to generate a cyclized product. The expectation that singlet diradicals react similarly to triplet or uncoupled diradicals needs to be reconsidered, as a recent study by Perrin and Reyes-Rodríguez (J. Am. Chem. Soc. 2014, 136, 15263) suggested the unsettling possibility that singlet p-benzyne could suffer nucleophilic attack to generate a naked phenyl anion. Now, this study provides direct spectroscopic observation of this phenomenon, with an atom-centered open-shell singlet diradical reacting as a powerful electrophile. To the question of whether a nucleophile can attack a singly occupied molecular orbital, the answer is apparently yes, at least if another partially occupied orbital is available to avoid violation of the rules of valence.

Anomalous effect of non-alternant hydrocarbons on carbocation and carbanion electronic configurations

Carbocations are widely viewed to be closed-shell singlet electrophiles. Here, computations show that azulenyl-substituted carbocations have triplet ground states. This triplet ground state for azulenyl carbocations stands in striking contrast to the isomeric naphthenyl carbocation, which is a normal closed-shell singlet with a large singlet-triplet gap. Furthermore, substitution of the azulenyl carbocation can substantially alter the energy gap between the different electronic configurations and can manipulate the ground state towards either the singlet or the triplet state depending on the nature and location of the substituent. A detailed investigation into the origin of this spin state reversal, including NICS calculations, structural effects, substitution patterns, orbital analysis, and detailed linear free-energy relationships allowed us to distill a set of principles that caused these azulenyl carbocations to have such low-lying diradical states. The fundamental origin of this effect mostly centers on singlet state destabilization from increasing antiaromatic character, in combination with a smaller, but important, Baird triplet state aromatic stabilization. We find that azulene is not unique, as extension of these principles allowed us to generate simple rules to predict an entire class of analogous non-alternant carbocation and carbanion structures with low-energy or ground state diradical states, including a purely hydrocarbon triplet cation with a large singlet-triplet gap of 8 kcal mol-1. Although these ions have innocuous-looking Lewis structures, these triplet diradical ions are likely to have substantially different reactivity and properties than typical closed-shell singlet ions.

New photoheterolysis precursors to study oxenium ions: combining experiment and theory

The combination of theoretical calculations and laser flash photolysis experiments has aided in understanding the reactivity and properties of oxenium ions.

(18)F-Labeling of Arenes and Heteroarenes for Applications in Positron Emission Tomography

Diverse radiochemistry is an essential component of nuclear medicine; this includes imaging techniques such as positron emission tomography (PET). As such, PET can track diseases at an early stage of development, help patient care planning through personalized medicine and support drug discovery programs. Fluorine-18 is the most frequently used radioisotope in PET radiopharmaceuticals for both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.8 min half-life, 635 keV positron energy) and high specific activity make it an attractive nuclide for labeling and molecular imaging. Arenes and heteroarenes are privileged candidates for (18)F-incorporation as they are metabolically robust and therefore widely used by medicinal chemists and radiochemists alike. For many years, the range of (hetero)arenes amenable to (18)F-fluorination was limited by the lack of chemically diverse precursors, and of radiochemical methods allowing (18)F-incorporation in high selectivity and efficiency (radiochemical yield and purity, specific activity, and radio-scalability). The appearance of late-stage fluorination reactions catalyzed by transition metal or small organic molecules (organocatalysis) has encouraged much research on the use of these activation manifolds for (18)F-fluorination. In this piece, we review all of the reactions known to date to install the (18)F substituent and other key (18)F-motifs (e.g., CF3, CHF2, OCF3, SCF3, OCHF2) of medicinal relevance onto (hetero)arenes. The field has changed significantly in the past five years, and the current trend suggests that the radiochemical space available for PET applications will expand rapidly in the near future.

Optimizing the lifetimes of phenoxonium cations derived from vitamin E via structural modifications

Systematic synthesis of a number of new phenolic compounds with structures similar to vitamin E led to the identification of several sterically hindered compounds that when electrochemically oxidised in acetonitrile in a -2e(-)/-H(+) process formed phenoxonium diamagnetic cations that were resistant to hydrolysis reactions. The reactivity of the phenoxonium ions was ascertained by performing cyclic voltammetric scans during the addition of carefully controlled quantities of water into acetonitrile solutions, with the data modelled using digital simulation techniques.

Direct Spectroscopic Detection and EPR Investigation of a Ground State Triplet Phenyl Oxenium Ion

Oxenium ions are important reactive intermediates in synthetic chemistry and enzymology, but little is known of the reactivity, lifetimes, spectroscopic signatures, and electronic configurations of these unstable species. Recent advances have allowed these short-lived ions to be directly detected in solution from laser flash photolysis of suitable photochemical precursors, but all of the studies to date have focused on aryloxenium ions having closed-shell singlet ground state configurations. To study alternative spin configurations, we synthesized a photoprecursor to the m-dimethylamino phenyloxenium ion, which is predicted by both density functional theory and MRMP2 computations to have a triplet ground state electronic configuration. A combination of femtosecond and nanosecond transient absorption spectroscopy, nanosecond time-resolved Resonance Raman spectroscopy (ns-TR(3)), cryogenic matrix EPR spectroscopy, computational analysis, and photoproduct studies allowed us to trace essentially the complete arc of the photophysics and photochemistry of this photoprecursor and permitted a first look at a triplet oxenium ion. Ultraviolet photoexcitation of this precursor populates higher singlet excited states, which after internal conversion to S1 over 800 fs are followed by bond heterolysis in ∼1 ps, generating a hot closed-shell singlet oxenium ion that undergoes vibrational cooling in ∼50 ps followed by intersystem crossing in ∼300 ps to generate the triplet ground state oxenium ion. In contrast to the rapid trapping of singlet phenyloxenium ions by nucleophiles seen in prior studies, the triplet oxenium ion reacts via sequential H atom abstractions on the microsecond time domain to ultimately yield the reduced m-dimethylaminophenol as the only detectable stable photoproduct. Band assignments were made by comparisons to computed spectra of candidate intermediates and comparisons to related known species. The triplet oxenium ion was also detected in the ns-TR(3) experiments, permitting a more clear assignment and identifying the triplet state as the π,π* triplet configuration. The triplet ground state of this ion was further supported by photolysis of the photoprecursor in an ethanol glass at ∼4 K and observing a triplet species by cryogenic EPR spectroscopy.

Asymmetric oxidative dearomatizations promoted by hypervalent iodine(III) reagents: an opportunity for rational catalyst design?

The use of and λ³- and λ⁵-iodanes in the oxidative dearomatization of phenols is a well-established and general procedure for the construction of cyclohexadienone structures. However, their use in asymmetric dearomatization reactions is quite underdeveloped and, despite work by several research groups over the past several years, a general chiral aryl iodide catalyst has yet to emerge. This article will serve to highlight the significant progress that has been made in this area and will reveal some of deficiencies in the literature that the author believes may be hindering further progress.

Potential nonadiabatic reactions: ring-opening 4,6-dimethylidenebicyclo[3.1.0]hex-2-ene derivatives to aromatic reactive intermediates

Potential singlet-triplet surface crossings for the ring opening of 4,6-dimethylidenebicyclo[3.1.0]hex-2-ene derivatives were explored using density functional theory (DFT) and complete active space self-consistent field (CASSCF) methods. Since these ring openings involve relatively high energy species that lead to relatively stable aromatic species, a good scenario for potential nonadiabatic events, we posited that the reaction paths of these ring openings might come close to or cross excited state surfaces. At the DFT level of theory, all reaction paths exhibited characteristics suggestive of singlet-triplet intersections along their paths. 6-Methylidenebicyclo[3.1.0]hex-3-en-2-one and a closely related derivative (4-methylidenebicyclo[3.1.0]hex-2-en-6-one) were explored at the CASSCF level of theory; CASSCF results were qualitatively similar to DFT results and yielded spin-orbit couplings of 1.1-1.4 cm(-1) at the singlet-triplet crossing points.