Karsten Krogh-Jespersen

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.

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.

Laser flash photolysis of fluorophenyldiazirine incarcerated in hemicarcerand 2 affords incarcerated fluorophenylcarbene [2 circle dot 3], which forms a meta-stable, innermolecular pi-complex with aryl moieties of 2. This carbene complex can be observed spectroscopically. Extensive computational studies provide insights into the structure, spectroscopy, energetics, and kinetics of the 2 circle dot 3 carbene complex.

Carbon-fluorine bonds are the strongest known single bonds to carbon and as a consequence can prove very hard to cleave. Alhough vinyl and aryl C-F bonds can undergo oxidative addition to transition metal complexes, this reaction has appeared inoperable with aliphatic substrates. We report the addition of C(sp³)-F bonds (including alkyl-F) to an iridium center via the initial, reversible cleavage of a C-H bond. These results suggest a distinct strategy for the development of catalysts and promoters to make and break C-F bonds, which are of strong interest in the context of both pharmaceutical and environmental chemistry.

Pentafluorophenylchlorocarbene, generated by laser flash photolysis (LFP) of pentafluorophenylchlorodiazirine, formed π-type complexes with 1,3,5-trimethoxybenzene in pentane. The carbene and carbene complexes were in equilibrium with K = 3.21 × 10⁵ M^–1 at 294 K. From the temperature dependence of K, ΔH° = −10.2 kcal/mol, ΔS° = −9.5 eu, and ΔG° = −7.4 kcal/mol at 298 K. The carbene complexes were characterized by UV–vis spectroscopy and computational analysis.

Phenylchlorocarbene, generated by laser flash photolysis of phenylchlorodiazirine, formed highly stable π-type complexes with 1,3,5-trimethoxybenzene in pentane. The carbene and carbene complexes were in equilibrium. We measured the equilibrium constant (K = 1264 M⁻¹ at 294 K) and, from its temperature dependence, extracted the associated thermodynamic parameters: ΔH° = −7.1 kcal/mol, ΔS° = −10.2 eu, and ΔG° = −4.1 kcal/mol. The carbene complexes were characterized by UV−vis spectroscopy and computational analysis.