Instantaneous Conversion of [11 C]CO2 to [11 C]CO via Fluoride-Activated Disilane Species

Unlocking full and fast conversion in photocatalytic carbon dioxide reduction for applications in radio-carbonylation

Harvesting sunlight to drive carbon dioxide (CO₂) valorisation represents an ideal concept to support a sustainable and carbon-neutral economy. While the photochemical reduction of CO₂ to carbon monoxide (CO) has emerged as a hot research topic, the full CO₂-to-CO conversion remains an often-overlooked criterion that prevents a productive and direct valorisation of CO into high-value-added chemicals. Herein, we report a photocatalytic process that unlocks full and fast CO₂-to-CO conversion (<10 min) and its straightforward valorisation into human health related field of radiochemistry with carbon isotopes. Guided by reaction-model-based kinetic simulations to rationalize reaction optimisations, this manifold opens new opportunities for the direct access to ¹¹C- and ¹⁴C-labeled pharmaceuticals from their primary isotopic sources [¹¹C]CO₂ and [¹⁴C]CO₂.

Computational predictions on Brønsted acidic ionic liquid-catalyzed carbon dioxide conversion to five-membered heterocyclic carbonyl derivatives

Experimentally conducted reactions between CO₂ and various substrates (i.e., ethylenediamine (EDA), ethanolamine (ETA), ethylene glycol (EG), mercaptoethanol (ME), and ethylene dithiol (EDT)) are considered in a computational study. The reactions were previously conducted under harsh conditions utilizing toxic metal catalysts. We computationally utilize Brønsted acidic ionic liquid (IL) [Et₂NH₂]HSO₄ as a catalyst aiming to investigate and propose 'greener' pathways for future experimental studies. Computations show that EDA is the best to fixate CO₂ among the tested substrates: the nucleophilic EDA attack on CO₂ is calculated to have a very small energy barrier to overcome (TS1EDA, ΔG^‡ = 1.4 kcal mol^-1) and form I1EDA (carbamic acid adduct). The formed intermediate is converted to cyclic urea (PEDA, imidazolidin-2-one) via ring closure and dehydration of the concerted transition state (TS2EDA, ΔG^‡ = 32.8 kcal mol^-1). Solvation model analysis demonstrates that nonpolar solvents (hexane, THF) are better for fixing CO₂ with EDA. Attaching electron-donating and -withdrawing groups to EDA does not reduce the energy barriers. Modifying the IL via changing the anion part (HSO₄^-) central S atom with 6 A and 5 A group elements (Se, P, and As) shows that a Se-based IL can be utilized for the same purpose. Molecular dynamics (MD) simulations reveal that the IL ion pairs can hold substrates and CO₂ molecules via noncovalent interactions to ease nucleophilic attack on CO₂.

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

Silicon compounds in carbon-11 radiochemistry: present use and future perspectives

Positron emission tomography (PET) is a powerful functional imaging technique that requires the use of positron emitting nuclides. Carbon-11 (¹¹C) radionuclide has several advantages related to the ubiquity of carbon atoms in biomolecules and the conservation of pharmacological properties of the molecule upon isotopic exchange of carbon-12 with carbon-11. However, due to the short half-life of ¹¹C (20.4 minutes) and the low scale with which it is produced by the cyclotron (sub-nanomolar concentrations), quick, robust and chemospecific radiolabelling strategies are required to minimise activity loss during incorporation of the ¹¹C nuclide into the final product. To address some of the constraints of working with ¹¹C, the use of silicon-based chemistry for ¹¹C-labelling was proposed as a rapid and effective route for radiopharmaceutical production due to the broad applicability and high efficiency showed in organic chemistry. In the past years several organic chemistry methodologies have been successfully applied to ¹¹C-chemistry. In this short review, we examine silicon-based ¹¹C-chemistry, with a particular emphasis on the radiotracers that have been successfully produced and potential improvements to further expand the applicability of silicon in radiochemistry.

The use of silicon-based reagents and precursors for carbon-11 labelling has shown wide applicability and robustness with short reaction times using mild conditions. In this review, recent advances and future perspectives are examined.

The development of 11C-carbonylation chemistry: A systematic view

The prospects for using carbon-11 labelled compounds in molecular imaging has improved with the development of diverse synthesis methods, including 11C-carbonylations and refined techniques to handle [11C]carbon monoxide at a nanomole scale. Facilitating biological research and molecular imaging was the driving force when [11C]carbon monoxide was used in the first in vivo application with carbon-11 in human (1945) and when [11C]carbon monoxide was used for the first time as a chemical reagent in the synthesis of [11C]phosgene (1978). This review examines a rich plethora of labelled compounds synthesized from [11C]carbon monoxide, their chemistry and use in molecular imaging. While the strong development of the 11C-carbonylation chemistry has expanded the carbon-11 domain considerably, it could be argued that the number of 11C-carbonyl compounds entering biological investigations should be higher. The reason for this may partly be the lack of commercially available synthesis instruments designed for 11C-carbonylations. But as this review shows, novel and greatly simplified methods to handle [11C]carbon monoxide have been developed. The next important challenge is to make full use of these technologies and synthesis methods in PET research. When there is a PET-tracer that meets a more general need, the incentive to implement 11C-carbonylation protocols will increase.

[11C]Carbon monoxide: advances in production and application to PET radiotracer development over the past 15 years

[¹¹C]Carbon monoxide is an appealing synthon for introducing carbon-11 at a carbonyl position (C=O) in a wide variety of chemotypes (e.g., amides, ketones, acids, esters, and ureas). The prevalence of the carbonyl group in drug molecules and the present-day broad versatility of carbonylation reactions have led to an upsurge in the production of this synthon and in its application to PET radiotracer development. This review focuses on the major advances of the past 15 years.

Recent progress in [11 C]carbon dioxide ([11 C]CO2 ) and [11 C]carbon monoxide ([11 C]CO) chemistry

[11 C]Carbon dioxide ([11 C]CO2 ) and [11 C]carbon monoxide ([11 C]CO) are 2 attractive precursors for labelling the carbonyl position (C═O) in a vast range of functionalised molecules (eg, ureas, amides, and carboxylic acids). The development of radiosynthetic methods to produce functionalised 11 C-labelled compounds is required to enhance the radiotracers available for positron emission tomography, molecular, and medical imaging applications. Following a brief summary of secondary 11 C-precursor production and uses, the review focuses on recent progress with direct 11 C-carboxylation routes with [11 C]CO2 and 11 C-carbonylation with [11 C]CO. Novel approaches to generate [11 C]CO using CO-releasing molecules (CO-RMs), such as silacarboxylic acids and disilanes, applied to radiochemistry are described and compared with standard [11 C]CO production methods. These innovative [11 C]CO synthesis strategies represent efficient and reliable [11 C]CO production processes, enabling the widespread use of [11 C]CO chemistry within the wider radiochemistry community.

Instantaneous Conversion of [11 C]CO2 to [11 C]CO via Fluoride-Activated Disilane Species

The development of a fast and novel methodology to generate carbon-11 carbon monoxide ([11 C]CO) from cyclotron-produced carbon-11 carbon dioxide ([11 C]CO2 ) mediated by a fluoride-activated disilane species is described. This methodology allows up to 74 % conversion of [11 C]CO2 to [11 C]CO using commercially available reagents, readily available laboratory equipment and mild reaction conditions (room temperature). As proof of utility, radiochemically pure [carbonyl-11 C]N-benzylbenzamide was successfully synthesized from produced [11 C]CO in up to 74 % radiochemical yield (RCY) and >99 % radiochemical purity (RCP) in ≤10 min from end of [11 C]CO2 delivery.