Phenyloxenium Ions: More Like Phenylnitrenium Ions than Isoelectronic Phenylnitrenes?

Abstract

The geometries and energies of the electronic states of phenyloxenium ion 1 (Ph−O + ) were computed at the multireference CASPT2/pVTZ level of theory. Despite being isoelectronic to phenylnitrene 4, the phenyloxenium ion 1 has remarkably different energetic orderings of its electronic states. The closed-shell singlet configuration ( 1 A 1 ) is the ground state of the phenyloxenium ion 1, with a computed adiabatic energy gap of 22.1 kcal/mol to the lowest-energy triplet state ( 3 A 2 ). Open-shell singlet configurations ( 1 A 2 , 1 B 1 , 1 B 2 , 2 1 A 1 ) are significantly higher in energy (>30 kcal/mol) than the closed-shell singlet configuration. These values suggest a revision to the current assignments of the ultraviolet photoelectron spectroscopy bands for the phenoxy radical to generate the phenyloxenium ion 1. For para-substituted phenyloxenium ions, the adiabatic singlet−triplet energy gap (ΔE ST ) is found to have a positive linear free energy relationship with the Hammett-like σ + R /σ + substituent parameters; for meta substituents, the relationship is nonlinear and negatively correlated. CASPT2 analyses of the excited states of p-aminophenyloxenium ion 5 and pcyanophenyloxenium ion 10 indicate that the relative orderings of the electronic states remain largely unperturbed for these para substitutions. In contrast, meta-donor-substituted phenyloxenium ions have lowenergy open-shell states (open-shell singlet, triplet) due to stabilization of a π,π* diradical state by the donor substituent. However, all of the other phenyloxenium ions and larger aryloxenium ions (naphthyl, anthryl) included in this study have closed-shell singlet ground states. Consequently, ground-state reactions of phenyloxenium ions are anticipated to be more closely related to closed-shell singlet arylnitrenium ions (Ar−NH + ) than their isoelectronic arylnitrene (Ar−N) counterparts. Disciplines Materials Chemistry | Other Chemistry | Physical Chemistry CommentsReprinted (adapted) with permission from J. Am. Chem. Soc., 2011, 133 (13) Oxenium ions are hypovalent species of formula R-O þ . These reactive intermediates are isoelectronic with nitrenes, with a formally monovalent oxygen containing two nonbonding electron pairs. Like nitrenes, the electron-deficient nature of these intermediates makes them powerful electrophiles, further amplified by the formal positive charge on the electronegative oxygen. These species are increasingly being proposed as intermediates in useful umpolung synthetic transformations, such as the oxidative Hosomi-Sakurai reaction, 1,2 the oxidative Wagner-Meerwin transposition, 3 electrochemical oxidations of phenols and phenolates, 4,5 alkane oxidations, 6 and a wide variety of other phenolic oxidations and tautomerization reactions. 7-11 These reactions convert phenols and alkanes, many of which are derived from petrochemical feedstocks, into value-added products such as cyclohexadienones, substituted phenols, and oxidized alkanes, as well as petrochemicals like poly(1,4-phenylene ether), a high-va...

Direct Detection and Reactivity of the Short-Lived Phenyloxenium Ion

“…The computed absorption band for 1 4 gives a single band in the visible region of the UV−vis spectrum centered at 524 nm, in good agreement with the experimental band (see Figure 1).…”

Computational organic chemistry

“…Winter has reported a computational study of the singlet and triplet phenyloxenium cation and finds some very striking differences between phenyloxenium cation and phenylnitrene. 63 Phenylnitrene has a triplet ground state, with the 1 A 1 state about 18 kcal mol À1 higher in energy, and the 1 A 2 state higher still. CASPT2/pVTZ//CASSCF(8,8)/pVTZ computations of 40 find the singlet 1 A 1 to be the ground state.…”

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

“…17 Nucleophilic attack as shown results in oxidation of the phenoxyl group, reduction of the iodine(III) center, and formation of dienone 2 . 18 Alternatively, the direct fragmentation of λ 3 -iodane 3 (Scheme 1, path B) through a unimolecular redox reaction would form phenoxenium ion 4 , 19 the subsequent trapping of which gives rise to dienone 2 . Despite little experimental evidence for either of these two mechanisms, 15 practitioners have adopted these pathways to rationalize the observed reactivity.…”