Photogeneration of carbocations: applications in physical organic chemistry and the design of suitable precursors

Carbocationoids, a concept for controlling highly reactive cationic species

Carbocations, which are positively charged highly electrophilic intermediates, are efficacious for the direct alkylation of low-reactive nucleophiles. The utilization of carbocations in SN1 reactions relies on the activation of their precursors in the presence of a nucleophile. However, undesirable interactions between the nucleophile and the leaving group activator limit the scope of acceptable nucleophiles. Here we report a strategy to conduct SN1 reactions involving unstable carbocations in an alternative stepwise procedure, which was demonstrated by the benzylation of various neutral nucleophiles. In the first step, carbocations were accumulated in a nucleophile-free solution in the form of carbocationoids utilizing the coordinative stabilization of triazinediones. Subsequently, the addition of these solutions in the second step enabled room-temperature alkylation without the need for acidic additives. This methodology overcomes the inherent challenges of carbocations in SN1 reactions.

Stabilization of Carbocation Intermediate by Cucurbit[7]uril Enables High Photolysis Efficiency

A cucurbit[7]uril-based host-guest strategy is employed to enhance the efficiency of photolysis reactions that release caged molecules from photoremovable protecting groups. The photolysis of benzyl acetate follows a heterolytic bond cleavage mechanism, thereby leading to the formation of a contact ion pair as the key reactive intermediate. The Gibbs free energy of the contact ion pair is lowered by 3.06 kcal/mol through the stabilization of cucurbit[7]uril, as revealed by DFT calculations, which results in a 40-fold increase in the quantum yield of the photolysis reaction. This methodology is also applicable to the chloride leaving group and the diphenyl photoremovable protecting group. We anticipate that this research presents a novel strategy to improve reactions involving active cationics, thereby enriching the field of supramolecular catalysis.

Advances in mass spectrometry-based footprinting of membrane proteins

Structural biology is entering an exciting time where many new high-resolution structures of large complexes and membrane proteins (MPs) are determined regularly. These advances have been driven by over 15 years of technological improvements, first in macromolecular crystallography, and recently in cryo-electron microscopy. Obtaining information about MP higher order structure and interactions is also a frontier, important but challenging owing to their unique properties and the need to choose suitable detergents/lipids for their study. The development of mass spectrometry (MS), both instruments and methodology in the past 10 years, has also advanced it as a complementary method to study MP structure and interactions. In this review, we discuss advances in MS-based footprinting for MPs and highlight recent methodologies that offer new promise for MP study by chemical footprinting and mass spectrometry.

Carbocation Footprinting of Soluble and Transmembrane Proteins

Here, we introduce carbocations (R₃C⁺) as laser-initiated footprinting reagents for proteins. We screened seven candidates and selected trifluomethoxy benzyl bromide (TFBB) as an effective precursor for the electrophilic trifluomethoxy benzyl carbocation (TFB⁺) under laser (248 nm) irradiation on the fast photochemical oxidation of proteins (FPOP) platform. Initial results demonstrate that this electrophilic cation reagent affords residue coverage of nucleophilic amino acids including H, W, M, and S. Further, the addition of TFB⁺ increases the hydrophobicity of the peptides so that separation of isomeric peptide products by reversed-phase LC is improved, suggesting opportunities for subresidue footprinting. Comparison of apo- and holo-myoglobin footprints shows that the TFB⁺ footprinting is sensitive to protein conformational change and solvent accessibility. Interestingly, because the TFB⁺ is amphiphilic, the reagent can potentially footprint membrane proteins as demonstrated for vitamin K epoxide reductase (VKOR) stabilized in a micelle. Not only does footprinting of the extra-membrane domain occur, but also some footprinting of the hydrophobic transmembrane domain is achieved owing to the interaction of TFB⁺ with the micelle. Carbocation precursors are stable and amenable for tailoring their properties and those of the incipient carbocation, enabling targeting their soluble or membrane-associated or embedded regions and distinguishing between the extra- and trans-membrane domains of membrane proteins.

Nucleophilicities and Nucleofugalities of Thio- and Selenoethers

Rate constants for the reactions of dialkyl chalcogenides with laser flash photolytically generated benzhydrylium ions have been measured photometrically to integrate them into the comprehensive benzhydrylium‐based nucleophilicity scale. Combining these rate constants with the previously reported equilibrium constants for the same reactions provided the corresponding Marcus intrinsic barriers and made it possible to quantify the leaving group abilities (nucleofugalities) of dialkyl sulfides and dimethyl selenide. Due to the low intrinsic barriers, dialkyl chalcogenides are fairly strong nucleophiles (comparable to pyridine and N‐methylimidazole) as well as good nucleofuges; this makes them useful group‐transfer reagents.

Ultrafast non-adiabatic dynamics of excited diphenylmethyl bromide elucidated by quantum dynamics and semi-classical on-the-fly dynamics

Carbocations and carboradicals are key intermediates in organic chemistry. Typically UV laser excitation is used to induce homolytical or heterolytical bond cleavage in suitable precursor molecules. Of special interest hereby are diphenylmethyl compounds (Ph2CH-X) with X = Cl, Br as a leaving group as they form diphenylmethyl radicals (Ph2CH˙) and cations (Ph2CH+) within a femtosecond time scale in polar solvents. In this work, we build on our methodology developed for the chlorine case and investigate the photodissociation reaction of Ph2CH-Br by state-of-the-art theoretical methods. On the one hand, we employ specially adapted reactive coordinates for a grid-based wave packet dynamics in reduced dimensionality using the Wilson G-matrix ansatz for the kinetic part of the Hamiltonian. On the other hand, we use full-dimensional semiclassical on-the-fly dynamics with Tully's fewest switches surface hopping routine for comparison. We apply both methods to explain remarkable differences in experimental transient absorption measurements for Cl or Br as the leaving group. The wave packet motion, visible only for the bromine leaving group, can be related to the crucial role of the central carbon atom, which undergoes rehybridization from sp3 to sp2 during the photoinduced bond cleavage. Comparable features are the two consecutive conical intersections near the Franck-Condon region controlling the product splitting to Ph2CH˙/Br˙ and Ph2CH+/Br- as well as the difference in delay time for the respective product formation.

Concerning the Role of Supercritical Carbon Dioxide in SN 1 Reactions

A series of S_N 1-type reactions has been studied under various conditions to clarify the role of supercritical carbon dioxide (scCO₂ ) as reaction medium for this kind of transformations. The application of scCO₂ did not result in higher yields in any of the experiments in comparison to those under neat conditions or in the presence of other inert compressed gases. High-pressure UV/Vis spectroscopic measurements were carried out to quantify the degree of carbocation formation of a highly S_N 1-active alkyl halide as a function of the applied solvent. No measureable concentration of carbocations could be detected in scCO₂ , just like in other low polarity solvents. Taken together, these results do not support the previously claimed activating effect via enhanced S_N 1 ionization due to the quadrupolar moment of the supercritical fluid.

Molecular features in complex environment: Cooperative team players during excited state bond cleavage

Photoinduced bond cleavage is often employed for the generation of highly reactive carbocations in solution and to study their reactivity. Diphenylmethyl derivatives are prominent precursors in polar and moderately polar solvents like acetonitrile or dichloromethane. Depending on the leaving group, the photoinduced bond cleavage occurs on a femtosecond to picosecond time scale and typically leads to two distinguishable products, the desired diphenylmethyl cations (Ph2CH(+)) and as competing by-product the diphenylmethyl radicals ([Formula: see text]). Conical intersections are the chief suspects for such ultrafast branching processes. We show for two typical examples, the neutral diphenylmethylchloride (Ph2CH-Cl) and the charged diphenylmethyltriphenylphosphonium ions ([Formula: see text]) that the role of the conical intersections depends not only on the molecular features but also on the interplay with the environment. It turns out to differ significantly for both precursors. Our analysis is based on quantum chemical and quantum dynamical calculations. For comparison, we use ultrafast transient absorption measurements. In case of Ph2CH-Cl, we can directly connect the observed signals to two early three-state and two-state conical intersections, both close to the Franck-Condon region. In case of the [Formula: see text], dynamic solvent effects are needed to activate a two-state conical intersection at larger distances along the reaction coordinate.

Scales of Lewis basicities toward C-centered Lewis acids (carbocations)

Equilibria for the reactions of benzhydryl cations (Ar2CH(+)) with phosphines, tert-amines, pyridines, and related Lewis bases were determined photometrically in CH2Cl2 and CH3CN solution at 20 °C. The measured equilibrium constants can be expressed by the sum of two parameters, defined as the Lewis Acidity (LA) of the benzhydrylium ions and the Lewis basicity (LB) of the phosphines, pyridines, etc. Least-squares minimization of log K = LA + LB with the definition LA = 0 for (4-MeOC6H4)2CH(+) gave a Lewis acidity scale for 18 benzhydrylium ions covering 18 orders of magnitude in CH2Cl2 as well as Lewis basicities (with respect to C-centered Lewis acids) for 56 bases. The Lewis acidities correlated linearly with the quantum chemically calculated (B3LYP/6-311++G(3df,2pd)//B3LYP/6-31G(d,p) level) methyl anion affinities of the corresponding benzhydrylium ions, which can be used as reference compounds for characterizing a wide variety of Lewis bases. The equilibrium measurements were complemented by isothermal titration calorimetry studies. Rates of SN1 solvolyses of benzhydryl chlorides, bromides, and tosylates derived from E(13-33)(+), i.e., from highly reactive carbocations, correlate excellently with the corresponding Lewis acidities and the quantum chemically calculated methyl anion affinities. This correlation does not hold for solvolyses of derivatives of the better stabilized amino-substituted benzhydrylium ions E(1-12)(+). In contrast, the correlation between electrophilic reactivities and Lewis acidities (or methyl anion affinities) is linear for all donor-substituted benzhydrylium ions E(1-21)(+), while the acceptor-substituted benzhydrylium ions E(26-33)(+) react more slowly than expected from their thermodynamic stabilities. The boundaries of linear rate-equilibrium relationships were thus defined.

Quantum Dynamics of a Photochemical Bond Cleavage Influenced by the Solvent Environment: A Dynamic Continuum Approach

In every day chemistry, solvents are used to influence the outcome of chemical synthesis. Electrostatic effects stabilize polar configurations during the reaction and in addition dynamic solvent effects can emerge. How the dynamic effects intervene on the ultrafast time scale is in the focus of this theoretical study. We selected the photoinduced bond cleavage of Ph2CH-PPh3(+) for which the electrostatic interactions are negligible. Elaborate ultrafast pump-probe studies already exist and serve as a reference. We compared quantum dynamical simulations with and without environment and noticed the necessity to model the influence of the solvent cage on the reactive motions of the solute. The frictional force induced by the dynamic viscosity of the solvent is implemented in the quantum mechanical formalism with a newly developed approach called the dynamic continuum ansatz. Only when the environment is included are the experimentally observed products reproduced on the subpicosecond time scale.