Intermediacy of a Persistent Urazole Radical and an Electrophilic Diazenium Species in the Acid-Catalyzed Reaction of MeTAD with Anisole

The reaction of <i>N</i>-methyl-1,2,4-triazoline-3,5-dione (MeTAD) with anisole in the presence of trifluoroacetic acid affords unexpected disubstituted urazole products instead of the expected monosubstituted urazole as typically observed in the reactions of MeTAD with other substituted benzenes. Our investigation into the mechanism of formation of these disubstituted products suggests that MeTAD is capable of further reaction with the initially formed monosubstituted urazole to afford a persistent urazole radical. The identity of this radical has been established by UV–vis spectroscopy, the nature of its self-dimerization reaction, and via independent generation. Electrochemical oxidation of this radical was carried out, and the resulting diazenium ion was demonstrated to be reactive with added substituted benzenes, including anisole. When oxidation was carried out chemically using thianthrenium perchlorate in the presence of anisole it was shown to produce the same disubstituted products (and in the same ratio) as observed in the acid-catalyzed reaction. A common diazenium species is proposed to be active in both cases. We also report the synthesis and characterization of three interesting tetrazane dimers resulting from unstable urazole radicals

Substituted 2‑(Dimethylamino)biphenyl-2′-carboxaldehydes as Substrates for Studying n→π* Interactions and as a Promising Framework for Tracing the Bürgi–Dunitz Trajectory

The Bürgi–Dunitz trajectory traces points along the pathway of bond formation between a nucleophile and electrophile. Previous X-ray crystallographic studies of some molecules containing a nucleophilic nitrogen atom and electrophilic carbonyl group provided some initial evidence for various degrees of bond formation via initial n→π* interactions. Observation of a complete set of points along the trajectory, however, has not yet been attained. In this paper, we present a DFT computational study investigating substituted 2-(dimethylamino)­biphenyl-2′-carboxaldehydes as substrates for further examination of n→π* interactions and as a potential framework for more complete tracing of the Bürgi–Dunitz trajectory. These compounds are particulary suitable for study because of the rotational freedom granted by the C–C bond connecting the two aromatic rings allowing the molecule to choose the degree of interaction between the two complementary groups. The extent of interaction is measured by interatomic distance, NBO second-order perturbative analysis energies, volume of transferred electron density as provided by ETS-NOCV analysis, and differences in energies between models that allow for n→π* interactions and those that do not. A series of substituted biphenyls are ultimately identified as future synthetic targets that have maximum potential for providing improved tracing of the Bürgi–Dunitz trajectory

Application of Radical Cation Spin Density Maps toward the Prediction of Photochemical Reactivity between <i>N</i>‑Methyl-1,2,4-triazoline-3,5-dione and Substituted Benzenes

Visible light irradiation of <i>N</i>-methyl-1,2,4-triazoline-3,5-dione in the presence of substituted benzenes is capable of inducing substitution reactions where no reaction takes place thermally. In addition to the formation of 1-arylurazole products resulting from ring substitution, side-chain substitution occurs in some cases where benzylic hydrogens are accessible to form benzylic urazole products. Formation of both types of products is most consistent with the involvement of a common intermediate, a radical ion pair, generated from photoexcitation of an initially formed charge-transfer complex. The charge-transfer complexes have been observed spectroscopically. Additionally, application of a modified Rehm–Weller model suggests that the electron-transfer processes are feasible for all of the substrates examined. In most cases, the spin density maps of the aromatic radical cation intermediates calculated at the DFT UB3LYP/6-31G* level are excellent predictors of the observed product distributions

An <i>ab Initio</i> Study of the Effect of Substituents on the <i>n</i> → π* Interactions between 7‑Azaindole and 2,6-Difluorosubstituted Pyridines

The <i>n</i> → π* interaction is a weak but important noncovalent interaction present in biomolecules and other compounds. Complexes between 7-azaindole and 2,6-difluorinated pyridines were demonstrated earlier to interact not only via an expected strong hydrogen bond but also by a weaker and unexpected <i>n</i> → π* interaction between the nucleophilic nitrogen atom of the 7-azaindole and the electrophilic π-system of the pyridine ring. This system provides a unique and convenient framework upon which to investigate the effect that distal substitution on the 7-azaindole ring has on the strength of the <i>n</i> → π* interaction. Herein we describe our thorough analysis of these effects by applying a variety of diverse methods including NBO, ETS-NOCV, and AIM. Very good agreement in trends was observed among all these diverse methods of analysis. Substitution at the position para to the nucleophilic nitrogen atom of the 7-azaindole ring with electron-donating groups weakened the hydrogen bond interaction with the 2,6-difluoropyridine but enhanced the <i>n</i> → π* interaction. Substitution with electron-withdrawing groups had the opposite effect. In addition, good correlation of the results of the calculations with the substituents’ Hammett σ_p values was observed. Energy decomposition analysis (EDA) corroborated the conclusions derived by the other methods of analysis

Unexpected σ Bond Rupture during the Reaction of <i>N</i>‑Methyl-1,2,4-triazoline-3,5-dione with Acenaphthylene and Indene

The reaction of <i>N</i>-methyl-1,2,4-triazoline-3,5-dione (MeTAD) with acenaphthylene and indene leads not only to the formation of the expected [2 + 2] diazetidine cycloadducts but also to unexpected 2:1 adducts of MeTAD with substrate. The structures of the products derived from acenaphthylene were confirmed by X-ray crystallography. A similar distribution of products was afforded from indene. The 2:1 adducts appear to derive from a diradical intermediate, the radical centers of which are strongly stabilized by the bridging urazoyl ring and benzylic delocalization. The triplet states of these diradical intermediates may be trapped via exposure to molecular oxygen to afford oxygen-containing adducts. Computational studies at the (U)­B3LYP/6-31G* level provide additional support for the conclusions of our experimental work