Reaction Optimization and Mechanism in Maleimide [5 + 2] Photocycloaddition: A Dual Approach Using Tunable UV Lasers and Time-Dependent DFT

Mechanisms of photoisomerization of the prenylated flavin mononucleotide cofactor: a theoretical study

The enzymatic decarboxylation of α,β-unsaturated acids using the ferulic acid decarboxylase (Fdc1) enzyme and prenylated flavin mononucleotide (prFMN) cofactor is a potential, environmentally friendly reaction for the biosynthesis of styrene and its derivatives. However, experiments showed that the enzyme activity of Fdc1 depends on the ring structure of prFMN, namely, the iminium and ketimine forms, and the loss of enzyme activity results from prFMNim → prFMNket photoisomerization. To obtain insight into this photochemical process and to improve the enzyme efficiency of Fdc1, two proposed photoisomerization mechanisms with different proton sources for the acid-base reaction were studied herein using theoretical methods. The potential energy surfaces calculated using the density functional theory method with the Becke, 3-parameter, and Lee-Yang-Parr hybrid functionals and DZP basis set (DFT/B3LYP/DZP) and TD-DFT/B3LYP/DZP methods confirmed that the light-dependent reaction occurs in the rate-determining proton transfer process and that the mechanism involving intermolecular proton transfer between prFMNim and Glu282 (external base) is energetically more favorable than that involving intramolecular proton transfer in prFMNim (internal base). The thermodynamic results obtained from the transition state theory method suggested that the exothermic relaxation energy in the photo-to-thermal process can promote the spontaneous formation of a high-energy-barrier transition state, and an effective enzymatic decarboxylation could be achieved by slowing down the formation of the undesirable thermodynamically favorable product (prFMNket). Because the rate constant for formation of the high-energy-barrier transition state varies exponentially over the temperature range of 273-298 K, and experimental results have shown that incubating Fdc1 on ice results in a complete loss of enzyme activity, it is recommended to perform the decarboxylation reaction at 285 K to strike a balance between minimizing enzyme stability loss at 273 K and mitigating the effects of UV irradiation. The computational strategy and fundamental insights obtained in this study could serve as guidelines for future theoretical and experimental investigations on the same and similar photochemical systems.

Mind the GAP: quantifying the breakdown of the linear vibronic coupling Hamiltonian

Excited state dynamics play a critical role across a broad range of scientific fields. Importantly, the highly non-equilibrium nature of the states generated by photoexcitation means that excited state simulations should usually include an accurate description of the coupled electronic-nuclear motion, which often requires solving the time-dependent Schrödinger equation (TDSE). One of the biggest challenges for these simulations is the requirement to calculate the PES over which the nuclei evolve. An effective approach for addressing this challenge is to use the approximate linear vibronic coupling (LVC) Hamiltonian, which enables a model potential to be parameterised using relatively few quantum chemistry calculations. However, this approach is only valid provided there are no large amplitude motions in the excited state dynamics. In this paper we introduce and deploy a metric, the global anharmonicity parameter (GAP), which can be used to assess the accuracy of an LVC potential. Following its derivation, we illustrate its utility by applying it to three molecules exhibiting different rigidity in their excited states.

Intramolecular thiomaleimide [2 + 2] photocycloadditions: stereoselective control for disulfide stapling and observation of excited state intermediates by transient absorption spectroscopy

Thiomaleimides undergo efficient intermolecular [2 + 2] photocycloaddition reactions and offer applications from photochemical peptide stapling to polymer crosslinking; however, the reactions are limited to the formation of the exo head-to-head isomers. Herein, we present an intramolecular variation which completely reverses the stereochemical outcome of this photoreaction, quantitatively generating endo adducts which minimise the structural disturbance of the disulfide staple and afford a 10-fold increase in quantum yield. We demonstrate the application of this reaction on a protein scaffold, using light to confer thiol stability to an antibody fragment conjugate. To understand more about this intriguing class of [2 + 2] photocycloadditions, we have used transient absorption spectroscopy (electronic and vibrational) to study the excited states involved. The initially formed S₂ (π₁π*) excited state is observed to decay to the S₁ (n₁π*) state before intersystem crossing to a triplet state. An accelerated intramolecular C–C bond formation provides evidence to explain the increased efficiency of the reaction, and the impact of the various excited states on the carbonyl vibrational modes is discussed.

Tethered bromomaleimides are described which undergo thiol conjugation and rapid regio- and stereoselective photocycloadditions to serve as disulfide staples; with the excited state intermediates observed by transient absorption spectroscopies.

The role of vibronic coupling in the electronic spectroscopy of maleimide: a multi-mode and multi-state quantum dynamics study

The first two excitation bands below 7 eV in the electronic absorption spectrum of maleimide are investigated using a model Hamiltonian including four low-lying singlet excited states within the manifold of 24 vibrational modes. The role of non-adiabatic effects is studied and shines light on both the broad, inter-state coupling-dominated spectral band as well as the fine-structured, not-so-strong coupled band. Calculations have been performed using the Multiconfigurational Time-Dependent Hartree (MCTDH) wavepacket propagation method as well as its multilayer version (ML-MCTDH) using a quadratic vibronic coupling (QVC) Hamiltonian model where parameters are obtained from fitting adiabatic potential energy surfaces computed by ab initio methods. The quantum dynamics calculations provide information on the relaxation dynamics and the vibrational modes involved. Already with a low-order vibronic coupling model and only a few modes being considered, a quantitative agreement with the experimental spectrum is obtained. However, it is found that all modes need to be considered to get a full picture of the photo-excited relaxation dynamics of this molecule.

Tale of Twisted Molecules. Atropselective Photoreactions: Taming Light Induced Asymmetric Transformations through Non-biaryl Atropisomers

Photochemical transformations are a powerful tool in organic synthesis to access structurally complex and diverse synthetic building blocks. However, this great potential remains untapped in the mainstream synthetic community due to the challenges associated with stereocontrol originating from excited state(s). The finite lifetime of an excited state and nearly barrierless subsequent processes present significant challenges in manipulating the stereochemical outcome of a photochemical reaction. Several methodologies were developed to address this bottleneck including photoreactions in confined media and preorganization through noncovalent interactions resulting in stereoenhancement. Yet, stereocontrol in photochemical reactions that happen in solution in the absence of organized assemblies remained largely unaddressed. In an effort to develop a general and reliable methodology, our lab has been exploring non-biaryl atropisomers as an avenue to perform asymmetric phototransformations. Atropisomers are chiral molecules that arise due to the restricted rotation around a single bond (chiral axis) whose energy barrier to rotation is determined by nonbonding interactions (most often by steric hindrance) with appropriate substituents. Thus, atropisomeric substrates are chirally preorganized during the photochemical transformation and translate their chiral information to the expected photoproducts. This strategy, where "axial to point chirality transfer" occurs during the photochemical reaction, is a hybrid of the successful Curran's prochiral auxiliary approach involving atropisomers in thermal reactions and the Havinga's NEER principle (nonequilibrating excited-state rotamers) for photochemical transformations. We have investigated this strategy in order to probe various aspects such as regio-, enantio-, diastereo-, and chemoselectivity in several synthetically useful phototransformations including 6π-photocyclization, 4π-ring closure, Norrish-Yang photoreactions, Paternò-Büchi reaction, and [2 + 2]- and [5 + 2]-photocycloaddition. The investigations detailed in this Account clearly signify the scope of our strategy in accessing chirally enriched products during phototransformations. Simple design modifications such as tailoring the steric handle in atropisomers to hold reactive units resulted in permanently locked/traceless axial chirality in addition to incorporating multiple stereocenters in already complex scaffolds obtained from phototransformation. Further improvements allowed us to employ low energy visible light rather than high energy UV light without compromising the stereoenrichment in the photoproducts. Continued investigations on atropisomeric scaffolds have unraveled new design features, with outcomes that are unique and unprecedented for excited state reactivity. For example, we have established that reactive spin states (singlet or triplet excited state) profoundly influence the stereochemical outcome of an atropselective phototransformation. In general, the photochemistry and photophysics of atropisomeric substrates differ significantly from their achiral counterparts irrespective of having the same chromophore initiating the excited state reactivity. The ability of axially chiral chromophores to impart stereoenrichment in the intramolecular photoreactions appears to be promising. A challenging endeavor for the "axial to point chirality transfer" strategy is to enhance stereoenrichment or alter chemical reactivity in intermolecular photoreactions. Insights gained from our investigations will serve as a platform to venture into more complicated yet fruitful research in terms of broad synthetic utility.

Engaging electronic effects for atropselective [5+2]-photocycloaddition of maleimides

Atropisomeric maleimides were synthesized and subjected to atropselective [5+2]-photocycloaddition under direct irradiation to yield azepinone products with high enantio- (ee >98%) and diastereoselectivity (dr >98%). While the ee was dictated by the axial chirality, the dr was influenced by the substituent on the maleimide ring. Interestingly, by tuning the electronics of the substituent, the dr of the product can be reversed.

Photochemistry with laser radiation in condensed phase using miniaturized photoreactors

Miniaturized microreactors enable photochemistry with laser irradiation in flow mode to convert azidobiphenyl into carbazole with high efficiency.

A protecting group free synthesis of (+/-)-neostenine via the [5 + 2] photocycloaddition of maleimides

A concise, linear synthesis of the Stemona alkaloid (+/-)-neostenine is reported. Key features include an organocopper-mediated bislactone C2-desymmetrization for the stereoselective construction of the cyclohexane-lactone C,D-rings. The assembly of the fused pyrrolo[1,2-a]azepine core was achieved by application of a [5 + 2] maleimide photocycloaddition. A custom FEP flow reactor was used to successfully overcome the scale limitations imposed by a classical immersion well batch reactor. The synthesis was completed in 14 steps from furan, in 9.5% overall yield, without the use of any protecting groups.

Photochemical transformations of pyridinium salts: mechanistic studies and applications in synthesis

The discovery, understanding and synthetic exploitation of the photochemical transformation of pyridinium salts are described. The investigations surrounding the remarkable transformation of pyridinium salts into a host of structurally complex motifs have helped extend the comprehension of aromatic and heteroaromatic photochemistry. The synthetic community has, in recent years, recognised the potential inherent in these compounds and has since exploited the irradiation of variously substituted pyridinium salts as key steps in the preparation of advanced intermediates in numerous synthetic programmes.