The Cope Rearrangement of 1,5‐Dimethylsemibullvalene‐2(4)‐d1: Experimental Evidence for Heavy‐Atom Tunneling

As an experimental test of the theoretical prediction that heavy‐atom tunneling is involved in the degenerate Cope rearrangement of semibullvalenes at cryogenic temperatures, monodeuterated 1,5‐dimethylsemibullvalene isotopomers were prepared and investigated by IR spectroscopy using the matrix isolation technique. As predicted, the less thermodynamically stable isotopomer rearranges at cryogenic temperatures in the dark to the more stable one, while broadband IR irradiation above 2000 cm⁻¹ results in an equilibration of the isotopomeric ratio. Since this reaction proceeds with a rate constant in the order of 10⁻⁴ s⁻¹ despite an experimental barrier of E_a=4.8 kcal mol⁻¹ and with only a shallow temperature dependence, the results are interpreted in terms of heavy‐atom tunneling.     

Persistent Organic High‐Spin Trinitrenes

The septet ground state trinitrenes 1,3,5‐trichloro‐2,4,6‐trinitrenobenzene and 1,3,5‐tribromo‐2,4,6‐trinitrenobenzene were isolated in inert (Ar, Ne, and Xe) as well as reactive matrices (H₂, O₂, and H₂O) at cryogenic temperatures. These trinitrenes were obtained in high yields by UV photolysis of the corresponding triazides and characterized by IR and UV/Vis spectroscopy. The trinitrenes, despite bearing six unpaired electrons, are remarkably unreactive towards molecular oxygen and hydrogen and are persistent in water ice up to 160 K where the water matrix starts to sublime off.     

Heavy‐Atom Tunneling Calculations in Thirteen Organic Reactions: Tunneling Contributions are Substantial,and Bell's Formula Closely Approximates Multidimensional Tunneling at ≥250 K

Multidimensional tunneling calculations are carried out for 13 reactions, to test the scope of heavy‐atom tunneling in organic chemistry, and to check the accuracy of one‐dimensional tunneling models. The reactions include pericyclic, cycloaromatization, radical cyclization and ring opening, and S_N2. When compared at the temperatures that give the same effective rate constant of 3×10⁻⁵ s⁻¹, tunneling accounts for 25–95 % of the rate in 8 of the 13 reactions. Values of transmission coefficients predicted by Bell's formula, κ_Bell , agree well with multidimensional tunneling (canonical variational transition state theory with small curvature tunneling), κ_SCT. Mean unsigned deviations of κ_Bell vs. κ_SCT are 0.08, 0.04, 0.02 at 250, 300 and 400 K. This suggests that κ_Bell is a useful first choice for predicting transmission coefficients in heavy‐atom tunnelling.     

Heavy‐Atom Tunneling in Organic Reactions

In the past few years, numerous investigations have been reported on the role of heavy‐atom tunneling in the area of pericyclic reactions, π‐bond‐shifting, and other processes. These studies illustrate unique strategies for the experimental detection of heavy‐atom tunneling and the increased use of calculations to predict it. This Minireview focuses primarily on carbon tunneling in ground‐state processes but also highlights nitrogen tunneling and the first example of excited‐state heavy‐atom tunneling. Salient features of these reactions along with potential limitations are discussed, as well as challenges and directions for future investigation.     

Activation of the B−F Bond by Diphenylcarbene: A Reversible 1,2‐Fluorine Migration between Boron and Carbon

Experiments in low‐temperature matrices reveal that triplet diphenylcarbene inserts into the very strong B−F bond of BF₃ in a two‐step reaction. The first step is the formation of a strongly bound Lewis acid–base complex between the singlet state of diphenylcarbene and BF₃. This step involves an inversion of the spin state of the carbene from triplet to singlet. The second step requires visible‐light photochemical activation to induce a 1,2‐F migration from boron to the adjacent carbon atom under formation of the formal insertion product of the carbene center into BF₃. The 1,2‐F migration is reversible under short‐wavelength UV irradiation, thus leading back to the Lewis acid–base adduct.     

Heavy‐Atom Tunneling in the Ring Opening of a Strained Cyclopropene at Very Low Temperatures

The highly strained 1H‐bicyclo[3.1.0]‐hexa‐3,5‐dien‐2‐one 1 is metastable, and rearranges to 4‐oxacyclohexa‐2,5‐dienylidene 2 in inert gas matrices (neon, argon, krypton, xenon, and nitrogen) at temperatures as low as 3 K. The kinetics for this rearrangement show pronounced matrix effects, but in a given matrix, the reaction rate is independent of temperature between 3 and 20 K. This temperature independence means that the activation energy is zero in this temperature range, indicating that the reaction proceeds through quantum mechanical tunneling from the lowest vibrational level of the reactant. At temperatures above 20 K, the rate increases, resulting in curved Arrhenius plots that are also indicative of thermally activated tunneling. These experimental findings are supported by calculations performed at the CASSCF and CASPT2 levels by using the small‐curvature tunneling (SCT) approximation.     

Redox Self‐Adaptation of a Nitrene Transfer Catalyst to the Substrate Needs

The development of iron catalysts for carbon–heteroatom bond formation, which has attracted strong interest in the context of green chemistry and nitrene transfer, has emerged as the most promising way to versatile amine synthetic processes. A diiron system was previously developed that proved efficient in catalytic sulfimidations and aziridinations thanks to an Fe^IIIFe^IV active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a Fe^IIIFe^IVL^. active species able to catalyze aliphatic amination. Extensive DFT calculations show that this activation event drastically enhances the electron affinity of the active species to match the substrates requirements. Overall this process consists in a redox self‐adaptation of the catalyst to the substrate needs.     

Formation of a Tunneling Product in the Photorearrangement of o‐Nitrobenzaldehyde

The photochemical rearrangement of o ‐nitrobenzaldehyde to o ‐nitrosobenzoic acid, first reported in 1901, has been shown to proceed via a distinct ketene intermediate. In the course of matrix isolation experiments in various host materials at temperatures as low as 3 K, the ketene was re‐investigated in its electronic and vibrational ground states. It was shown that hitherto unreported H‐tunneling dominates its reactivity, with half‐lives of a few minutes. Unexpectedly, the tunneling product is different from o ‐nitrosobenzoic acid formed in the photoprocess: Once prepared by irradiation, the ketene spontaneously rearranges to an isoxazolone via an intriguing mechanism initiated by H‐tunneling. CCSD(T)/cc‐pVTZ computations reveal that this isoxazolone is neither thermodynamically nor kinetically favored under the experimental conditions, and that formation of this unique tunneling product constitutes a remarkable and new example of tunneling control.     

Heavy‐Atom Tunneling Calculations in Thirteen Organic Reactions: Tunneling Contributions are Substantial,and Bell's Formula Closely Approximates Multidimensional Tunneling at ≥250 K

Multidimensional tunneling calculations are carried out for 13 reactions, to test the scope of heavy‐atom tunneling in organic chemistry, and to check the accuracy of one‐dimensional tunneling models. The reactions include pericyclic, cycloaromatization, radical cyclization and ring opening, and S_N2. When compared at the temperatures that give the same effective rate constant of 3×10⁻⁵ s⁻¹, tunneling accounts for 25–95 % of the rate in 8 of the 13 reactions. Values of transmission coefficients predicted by Bell's formula, κ_Bell , agree well with multidimensional tunneling (canonical variational transition state theory with small curvature tunneling), κ_SCT. Mean unsigned deviations of κ_Bell vs. κ_SCT are 0.08, 0.04, 0.02 at 250, 300 and 400 K. This suggests that κ_Bell is a useful first choice for predicting transmission coefficients in heavy‐atom tunnelling.     

Intermolecular C−H Amidation of (Hetero)arenes to Produce Amides through Rhodium‐Catalyzed Carbonylation of Nitrene Intermediates

Amide bond formation is one of the most important reactions in organic chemistry because of the widespread presence of amides in pharmaceuticals and biologically active compounds. Existing methods for amides synthesis are reaching their inherent limits. Described herein is a novel rhodium‐catalyzed three‐component reaction to synthesize amides from organic azides, carbon monoxide, and (hetero)arenes via nitrene‐intermediates and direct C?H functionalization. Notably, the reaction proceeds in an intermolecular fashion with N₂ as the only by‐product, and either directing groups nor additives are required. The computational and mechanistic studies show that the amides are formed via a key Rh‐nitrene intermediate.     

Heavy‐Atom Tunneling Through Crossing Potential Energy Surfaces: Cyclization of a Triplet 2‐Formylarylnitrene to a Singlet 2,1‐Benzisoxazole

Not long ago, the occurrence of quantum mechanical tunneling (QMT) chemistry involving atoms heavier than hydrogen was considered unreasonable. Contributing to the shift of this paradigm, we present here the discovery of a new and distinct heavy‐atom QMT reaction. Triplet syn‐2‐formyl‐3‐fluorophenylnitrene, generated in argon matrices by UV‐irradiation of an azide precursor, was found to spontaneously cyclize to singlet 4‐fluoro‐2,1‐benzisoxazole. Monitoring the transformation by IR spectroscopy, temperature‐independent rate constants (k≈1.4×10^?3 s^?1; half‐life of ≈8 min) were measured from 10 to 20 K. Computational estimated rate constants are in fair agreement with experimental values, providing evidence for a mechanism involving heavy‐atom QMT through crossing triplet to singlet potential energy surfaces. Moreover, the heavy‐atom QMT takes place with considerable displacement of the oxygen atom, which establishes a new limit for the heavier atom involved in a QMT reaction in cryogenic matrices.     

Locking the Conformation of a Paddlewheel Rhodium Complex: Design,Synthesis, and Applications in Catalytic Nitrene Transfers

The exponential proliferation of conformers makes it impossible to examine the entire population in most systems. Controlling conformational ensembles is thus pivotal in many areas of chemistry. Rh₂(esp)₂, a dicarboxylate-derived paddlewheel rhodium complex, is one of the most effective catalysts for nitrene chemistry. Its enormous success has led to preparing many analogous complexes. However, there has been little consideration for the conformational dynamics of the parent catalyst. Herein, we report a new ligand modification principle that prevents conformer interconversion. The resulting complex comprises two isolable conformers, whose structures have been determined by X-ray diffraction. Combined experimental and computational data has revealed similarities and dissimilarities between the conformationally confined and parent complexes. Three model cases have demonstrated the utility of conformational fixation in the development of stereoselective catalysts for nitrene transfer reactions. The design principle described in this study can be combined with other established modification strategies, serving as a springboard for further advancement of the chemistry of paddlewheel metal complexes.     

Atom Tunneling in Chemistry

Quantum mechanical tunneling of atoms is increasingly found to play an important role in many chemical transformations. Experimentally, atom tunneling can be indirectly detected by temperature‐independent rate constants at low temperature or by enhanced kinetic isotope effects. In contrast, the influence of tunneling on the reaction rates can be monitored directly through computational investigations. The tunnel effect, for example, changes reaction paths and branching ratios, enables chemical reactions in an astrochemical environment that would be impossible by thermal transition, and influences biochemical processes.