Robert A. Moss

We describe the intermolecular reactivity toward alkenes of several extremely reactive carbenes as determined by experimental and computational methods. Among the examined species are the electrophiles trifluoromethylchlorocarbene and (N‐methyl‐3‐pyridinium)chlorocarbene and the nucleophiles adamantanylidene and dimethylcarbene. Phenomena discussed include negative activation energies for addition reactions and apparent compensation between the enthalpies and entropies of activation. Copyright © 2016 John Wiley & Sons, Ltd.

Equilibrium constants and the associated thermodynamic parameters are reported for the equilibria established between the cationic carbene (N-methyl-3-pyridinium)chlorocarbene tetrafluoroborate (MePyr⁺CCl BF₄^–, 3) and 1,3,5-trimethoxybenzene (TMB) to form a carbene–TMB complex, as well as between carbene 3 and chloride ion to form the zwitterion, N-methyl-3-pyridinium dichloromethide (10). These equilibrium constants and thermodynamic parameters are contrasted with analogous data for several related carbenes, and the influence of the pyridinium unit in carbene 3 is thereby highlighted. Computational studies augment and elucidate the experimental results.

(N-Methyl-3-pyridinium)chlorocarbene tetrafluoroborate (MePyr⁺CCl BF₄^–, 4) is generated by laser flash photolysis (LFP) of the corresponding diazirine (5) and reacted with tetramethylethylene, cyclohexene, 1-hexene, 2-ethyl-1-butene, methyl acrylate, and acrylonitrile. Absolute rate constants are measured for these carbene–alkene addition reactions, and activation parameters are obtained for additions of MePyr⁺CCl BF₄^– to tetramethylethylene, cyclohexene, and 1-hexene. MePyr⁺CCl BF₄^– is computed to be a highly reactive, electrophilic, singlet carbene, and experiments are in accord with expectations. Its activation parameters are compared with those of CF₃CCl, CCl₂, CClF, and CF₂. In all cases, enthalpy–entropy compensation is observed, with ΔH^‡ and ΔS^‡ decreasing in tandem as carbenic stability decreases. A qualitative explanation is offered for this phenomenon.

Reactions of p-nitrophenylchlorocarbene (PNPCC) with various dibenzo crown ethers produce O-ylides and π-complexes; the reactions can be followed via the spectral signatures of the carbene and the products. The O-ylides form most rapidly, but over time they decay in favor of the more stable π-complexes. Extensive computational studies support and refine appropriate structural and mechanistic conjectures. Reactions of PNPCC with monobenzo crown ethers afford only the spectral signatures of O-ylides; monobenzo π-complexes are either not formed in significant concentrations or are spectroscopically silent.

Computational and experimental results demonstrate that adamantanylidene (1) behaves as a highly reactive nucleophile toward common alkenes. It is the only known saturated nucleophilic carbene that lacks direct or vinylogous heteroatomic substitution. The activation energy and enthalpy for addition of 1 to methyl acrylate are the most negative values yet encountered in any carbene–alkene addition.

Decades of research on the structure of the 2‐norbornyl cation afford a new understanding of bonding in carbon compounds and of the electron‐donor capacity of carbon–carbon σ bonds. The bridged or “nonclassical” structure of the 2‐norbornyl cation, which features 3‐center, 2‐electron bonding, is supported by (inter alia) X‐ray crystallography, NMR spectroscopy, calculations, thermochemical studies, and copious mechanistic evidence. Copyright © 2014 John Wiley & Sons, Ltd.

Reviewed herein are equilibria between halocarbenes and halocarbanions, carbenes and carbene complexes and carbenes and O‐ylides. The transient species were visualized by laser flash photolysis coupled with UV–Visible spectroscopy. This methodology enabled the determination of equilibrium constants and the extraction of associated thermodynamic parameters. Parallel computational studies provided anticipated structures and energies for the transient species, as well as electronic absorption wavelengths and oscillator strengths.

p-Nitrophenylchlorocarbene (PNPCC) reacted reversibly with tetrahydrofuran (THF), tetrahydropyran (THP), 1,3-dioxane (1,3-D), and 1,4-dioxane (1,4-D) to form O-ylides 8, 9, 10, and 11, respectively. The O-ylides were visualized by their characteristic UV–vis spectroscopic signatures. Equilibrium constants (K) were determined spectroscopically, and studies of K as a function of pentane/1,2-dichloroethane (DCE) solvent blends illustrated the dependence of K on solvent polarity. Electronic structure calculations based on density functional theory provided carbene/ether O-ylide structures and energetics, as well as electronic spectroscopic parameters for use in the determination of K. Comparisons of the computed and experimental data were generally satisfactory.

p-Nitrophenylchlorocarbene reacted reversibly with diethyl ether, di-n-propyl ether, or tetrahydrofuran (THF) to form O-ylides, which were visualized by their UV–visible spectroscopic signatures. Equilibrium constants (K_eq) were determined spectroscopically and ranged from 0.10 M^–1 (di-n-propyl ether) to 7.5 M^–1 (THF) at 295 K. Studies of K_eq as a function of temperature afforded ΔH^o, ΔS^o, and ΔG^o for the di-n-propyl ether and THF/O-ylide equilibria. ΔH^o was favorable for ylide formation, but ΔS^o was quite negative, so that ΔG^os for the equilibria were small. Electronic structure calculations based on density functional theory provided structures, spectroscopic signatures, and energetics for the carbene/ether O-ylides.