Metal−Olefin Interactions in M(CO)5(cycloolefin) (M = Cr, Mo, W; Cycloolefin = Cyclopropene to Cyclooctene): Strain Relief and Metal−Olefin Bond Strength

Trans-cyclooctene-a Swiss army knife for bioorthogonal chemistry: exploring the synthesis, reactivity, and applications in biomedical breakthroughs

BACKGROUND

Trans-cyclooctenes (TCOs) are highly strained alkenes with remarkable reactivity towards tetrazines (Tzs) in inverse electron-demand Diels-Alder reactions. Since their discovery as bioorthogonal reaction partners, novel TCO derivatives have been developed to improve their reactivity, stability, and hydrophilicity, thus expanding their utility in diverse applications.

MAIN BODY

TCOs have garnered significant interest for their applications in biomedical settings. In chemical biology, TCOs serve as tools for bioconjugation, enabling the precise labeling and manipulation of biomolecules. Moreover, their role in nuclear medicine is substantial, with TCOs employed in the radiolabeling of peptides and other biomolecules. This has led to their utilization in pretargeted nuclear imaging and therapy, where they function as both bioorthogonal tags and radiotracers, facilitating targeted disease diagnosis and treatment. Beyond these applications, TCOs have been used in targeted cancer therapy through a "click-to-release" approach, in which they act as key components to selectively deliver therapeutic agents to cancer cells, thereby enhancing treatment efficacy while minimizing off-target effects. However, the search for a suitable TCO scaffold with an appropriate balance between stability and reactivity remains a challenge.

CONCLUSIONS

This review paper provides a comprehensive overview of the current state of knowledge regarding the synthesis of TCOs, and its challenges, and their development throughout the years. We describe their wide ranging applications as radiolabeled prosthetic groups for radiolabeling, as bioorthogonal tags for pretargeted imaging and therapy, and targeted drug delivery, with the aim of showcasing the versatility and potential of TCOs as valuable tools in advancing biomedical research and applications.

Advances in the Synthesis of Bioorthogonal Reagents: s-Tetrazines, 1,2,4-Triazines, Cyclooctynes, Heterocycloheptynes, and trans-Cyclooctenes

Aligned with the increasing importance of bioorthogonal chemistry has been an increasing demand for more potent, affordable, multifunctional, and programmable bioorthogonal reagents. More advanced synthetic chemistry techniques, including transition-metal-catalyzed cross-coupling reactions, C-H activation, photoinduced chemistry, and continuous flow chemistry, have been employed in synthesizing novel bioorthogonal reagents for universal purposes. We discuss herein recent developments regarding the synthesis of popular bioorthogonal reagents, with a focus on s-tetrazines, 1,2,4-triazines, trans-cyclooctenes, cyclooctynes, hetero-cycloheptynes, and -trans-cycloheptenes. This review aims to summarize and discuss the most representative synthetic approaches of these reagents and their derivatives that are useful in bioorthogonal chemistry. The preparation of these molecules and their derivatives utilizes both classical approaches as well as the latest organic chemistry methodologies.

Halogen bonding in crystals of free 1,2-diiodo-ethene (C2H2I2) and its π-complex [CpMn(CO)2](π-C2H2I2)

1,2-trans-diiodo-ethene (C2H2I2) – is an overlooked halogen bond donor, which demonstrate the distinct similarity of the geometry and directionality of I···I halogen bonds around the iodine atoms in its native and CpMn(CO)2(C2H2I2) π-complex crystals. Distortion of the planar geometry of C2H2I2 upon the π-coordination result the distortion of the native planar layered geometry of C2H2I2, so that [CpMn(CO)2](π-C2H2I2) features more complex I···I XB assisted 3D network. Unusual structural parallels between the native C2H2I2 crystals and solid iodine are discussed.

Flow Photochemical Syntheses of trans-Cyclooctenes and trans-Cycloheptenes Driven by Metal Complexation

trans-Cyclooctenes and trans-cycloheptenes have long been the subject of physical organic study, but the broader application had been limited by synthetic accessibility. This account describes the development of a general, flow photochemical method for the preparative synthesis of trans-cycloalkene derivatives. Here, photoisom erization takes place in a closed-loop flow reactor where the reaction mixture is continuously cycled through Ag(I) on silica gel. Selective complexation of the trans-isomer by Ag(I) during flow drives an otherwise unfavorable isomeric ratio toward the trans-isomer. Analogous photoreactions under batch-conditions are low yielding, and flow chemistry is necessary in order to obtain trans-cycloalkenes in preparatively useful yields. The applications of the method to bioorthogonal chemistry and stereospecific transannulation chemistry are described.

Transient anions of cis- and trans-cyclooctene studied by electron-impact spectroscopy

Electron attachment to the π* orbitals of trans- and cis-cyclooctene was measured in the quest to understand the frontier orbitals and their role in reactivity.

A DFT study of the interaction between olefins and Cu2+ on silica and MCM-41 model surfaces

The interaction between ethylene and Cu(2+) on a silica model surface was studied by density functional theory (DFT) with nine popular functionals. It is found that B3LYP with BSSE correction is the best method by comparing the calculated results with reported experimental data. This method was also used to study the interactions of Cu(2+) with β-carotene, 1,3,5,7,9,11,13-tetradecaheptaene and ethylene on a MCM-41 model surface. The relationship between the reorganization energy of an olefin and its conjugation length was studied, and the roles of the electrostatic interaction between the olefin and the Cu(2+) were investigated. It is also found that the different environments of Cu(2+) affect the Cu(2+)-olefin interaction significantly.

BLOCK-LOCALIZED WAVEFUNCTION ENERGY DECOMPOSITION (BLW-ED) ANALYSIS OF σ/π INTERACTIONS IN METAL-CARBONYL BONDING

The bonding features in metal-carbonyls including neutral M CO (M = Ni , Pd , Pt ) and M CO + (M+ = Cu +, Ag +, Au +) complexes have been elucidated at the DFT level with relativistic compact effective potentials for transition metals and 6-311+G(d) basis sets for C and O by the block-localized wavefunction (BLW) method. The BLW method can decompose the intermolecular interactions in terms of Heitler–London, polarization, and charge transfer energy contributions. Since the metal– CO bonding involves two synergic interactions, namely the σ-dative bond from the carbon lone electron pair to an empty d σ orbital on the metal, and the π back-donation from filled d π orbitals to the empty 2π* orbital on CO , the present BLW-ED analyses quantitatively demonstrated that in neutral M CO complexes the π-bonding dominates over the σ-bonding, whereas in cationic M CO + complexes, the σ-bonding plays a major role. But in both neutral and cationic species, the CO polarization induced by the metals enhances the C–O bond and increases the C–O vibrational frequencies, while the π back-donation tends to weaken the C–O bond and decrease the C–O vibrational frequencies. For neutral complexes, the latter is more prominent than the former, and consequently, there is a red-shifting of the C–O vibrational frequencies. In contrast, the π back-donation is insignificant in M CO + cations, and the C–O eventually vibrates at higher frequencies than the free CO frequency.

Synthesis and structural characterisation of stable cationic gold(I) alkene complexes

Treatment of [AuCl(PBu(t)(3))] with AgSbF(6) in CH(2)Cl(2) at room temperature in the presence of the alkenes norbornene, norbornadiene, trans-cyclooctene, and also interestingly, isobutylene, leads to the formation of stable crystalline complexes [Au(PBu(t)(3))(alkene)][SbF(6)], characterised by NMR spectroscopy and X-ray crystallography.

Ethylene receptor antagonists: strained alkenes are necessary but not sufficient

Plants use ethylene as a hormone to control many physiological processes. Ethylene perception involves its binding to an unusual copper-containing, membrane-bound receptor. Inhibitors of ethylene action are valuable to study signaling and may have practical use in horticulture. Past investigation of alkene ligands for this receptor has identified strain as the key factor in antagonism of ethylene binding and action, consistent with known trends in metal-alkene complex stability. However, in this work, this principle could not be extended to other alkenes, prompting development of the proposal that a ring-opening reaction accounts for the unusual potency of cyclopropene ethylene antagonists. Another factor augmenting the affinity of alkenes for the copper binding site is pyramidalization, as in trans-cycloalkenes. The enantiomeric selectivity in the binding of one such alkene to the ethylene receptor demonstrates its protein-composed asymmetric environment.