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 ( 11 C) 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 11 C (20.4 minutes) and the low scale with which it is produced by the cyclotron (sub-nanomolar concentrations), quick, robust and chemospeci ﬁ c radiolabelling strategies are required to minimise activity loss during incorporation of the 11 C nuclide into the ﬁ nal product. To address some of the constraints of working with 11 C, the use of silicon-based chemistry for 11 C-labelling was proposed as a rapid and e ﬀ ective route for radiophar-maceutical production due to the broad applicability and high e ﬃ ciency showed in organic chemistry. In the past years several organic chemistry methodologies have been successfully applied to 11 C-chemistry. In this short review, we examine silicon-based 11 C-chemistry, with a particular emphasis on the radiotra-cers that have been successfully produced and potential improvements to further expand the applicability of silicon in radiochemistry.


Introduction
Positron emission tomography (PET) is a powerful functional imaging technique that allows the in vivo detection of normoand patho-physiological changes in humans by using mole-cules radiolabelled with positron (β + ) emitting nuclides (radiotracers). 1 To achieve radiopharmaceutical targeting, radiotracers are often derived from biologically-active compounds with a known pharmacological profile, possessing high selectivity for a molecular target or physiological process. 2 The inclusion of a positron-emitting nuclide in the molecule of interest enables the in vivo visualization of the molecules biodistribution and kinetics metabolism. 2,3Of all the available PET nuclides, carbon-11 ( 11 C) is of particular interest due to the ubiquity of carbon atoms in biomolecules and because isotopic substitution of carbon-12 for carbon-11 preserves the biological properties of the non-radioactive isotope. 2,4,5However, due to the rapid radioactive decay of carbon-11 (radioactive half-life t 1/2 = 20.4minutes), the radiosynthesis, purification, formulation and quality control of carbon-11 radiopharmaceuticals must be accomplished in short times (the whole process should not exceed 60 minutes), hence quick and robust chemistry are needed to avoid substantial activity loss. 2,6The sub-nanomolar scale with which the radioisotope is produced from the cyclotron also represents a burden when performing 11 C-labelling, with the non-radioactive reactants being in large stoichiometric excess.Each minor impurity in the solvents and the reagents may generate side-products or degradation of reagents resulting in unwanted intermediates, so a high degree of chemospecificity is required, as well. 2 To meet criteria suitable for 11 C-labelling, silicon-containing compounds have received increased interest in the field.][9][10][11][12] Moreover, silyl compounds act as effective protecting groups due to the large number of functional groups that can be protected (e.g.alcohols, alkynes, amines, carboxylic acids…) and the ease of the protecting/deprotecting steps. 13,146][17][18] In the hydrosilylation reaction, CO 2 is used as a building block for the synthesis of a variety of functional groups such as formamides, 15 methylamines, 15,16 aldehydes 17 and aminals. 18Moreover, organosilicates have shown to be optimal substrates for the electrophilic fluorination of aryl and alkenyl substrates under mild conditions (e.g.room temperature, 18 hours) whilst having regio-and enantio-selectivity on the final product. 19Besides the large number of reactions available, silicon chemistry is costeffective (the silylated reagents are easily synthesized or commercially available) 20 and eco-friendly (organosilicon compounds are ultimately catabolised into silica gel in the environment). 7,8iven the high versatility and ease of handling of organosilane compounds, several methodologies have been successfully translated into carbon-11 and fluorine-18 chemistry in the past years.In particular, silicon-based compounds were applied in the production of a variety of fluorine-18 labelled small molecules and peptides as prosthetic groups, where the radionuclide was attached on via isotopic exchange (obtaining silicon-fluoride-acceptors -SiFAs), 21 or as substrates for electrophilic fluorination. 19The application of silicon in fluorine-18 chemistry was recently reviewed by Bernard-Gauthier et al. and Tredwell et al. 19,21 Herein we report the latest advances in the 11 C-chemistry field based on silicon starting materials or reagents (Scheme 1).This review will initially disclose the atomic properties of silicon to then discuss the main uses of organosilane compounds in carbon-11 chemistry: (i) conversion from [ 11 C]CO 2 to [ 11 C]CO; (ii) trapping and activating agents for [ 11 C]CO 2 and (iii) precursors of the target radiopharmaceutical.This review will also report the biologically-active molecules that were successfully radiolabelled with the discussed reactions.
Scheme 1 Schematic representation of the 11 C functional groups and synthons obtainable via silicon chemistry.

Atomic properties of silicon
The versatility of silicon in organic chemistry is explainable by examining the strength of the bonds that this atom creates with other elements.3][24] The higher electropositivity of silicon also favours the formation of hydride ions upon Si-H bond cleavage, making organosilicon compounds functional reducing agents. 253][24] The involvement of d-orbitals further expands the applicability of silicon in radiochemistry, allowing the formation of penta-and hexa-coordinate compounds (e.g. the formation transition metal silylene complexes) 26 which opens to more reaction pathways. 26,27

Use of silyl compounds as [ 11 C]CO 2 -to-[ 11 C]CO converting agents
9][30] Within these compounds, silacarboxylic acids can produce CO upon exposure to high temperature or reaction with nucleophiles (e.g.fluoride anions, bases, water) following the 1,2-Brook rearrangement [28][29][30][31][32][33] and can be readily synthesized by the reaction of the relative silyl lithium derivative with CO 2 . 29,30The produced CO was then employed in carbonylative coupling of aryl iodides and amines to yield amides, esters and α,β-unsaturated ketones. 29,30onsidering these advantageous characteristics, silacarboxylic acids were examined as potential [ 11 C]CO 2 -to-[ 11 C]CO converting agents as an alternative strategy to gas-phase reduction (reduction over thin molybdenum wires at 850 °C), 34 photo-chemical (a ruthenium/cobalt solution irradiated by visible light) 35 and electro-chemical methods (using transitionmetal triflates as cathodes). 36nitial efforts focused on identifying the organosilyl chloride with the highest reactivity towards [ 11 C]CO 2 .8][39] Follow-up studies on the 11 Csilacarboxylate production of [ 11 C]CO and subsequent 11 Ccarbonylation showed that the whole process can be fully automated using a commercially available radiochemistry synthesis module. 38With an initial delivery of 10 GBq of [ 11 C]CO 2 , the radiotracer [ 11 C]FPS-ZM1 (Scheme 2) was achieved with a RCY of 34% (isolated, decay-corrected to end of [ 11 C]CO 2 delivery (EOD)) and molar activity (A m ) 28 GBq per μmol within 25 minutes from end of bombardment (EOB). 38nother approach to produce [ 11 C]CO was reported by reacting disilanes with [ 11 C]CO 2 .Four different disilane species were tested for their ability of trapping [ 11 C]CO 2 and converting it into [ 11 C]CO, with 1,2-diphenyl-1,1,2,2-tetramethyldisilane ((Me 2 PhSi) 2 , 6, Scheme 3) being the most effective. 40ith the aid of catalytic amounts of TBAF (0.1 equiv.)as a fluoride source, the production of [ 11 C]CO reached 74% using mild reaction conditions and within 3 minutes from EOB. 40 The reaction mechanism is believed to proceed by the initial formation of a pentavalent fluorosilyl anion which rapidly interacts with [ 11 C]CO 2 forming an unstable intermediate that spontaneously rearranges into a silylfluoride derivative (7), a silyloxide derivative (8) and [ 11 C]CO (Scheme 3).As a proof-ofconcept, the applicability of the produced [ 11 C]CO in 11 Ccarbonylation reactions was tested via the radiosynthesis of [ 11 C]benzylbenzamide which was obtained with a good RCY (74%, estimated by radioHPLC). 40

Reductive functionalisation of [ 11 C]CO 2 using organosilanes
The high reactivity that organosilanes have towards CO 2 enabled other interesting applications, such as the conversion of the cyclotron-produced [ 11 C]CO 2 to a 11 C-N-methyl group without the need of using [ 11 C]CH 3 I.This approach was vastly exploited in synthetic chemistry to use CO 2 as a C 1 building block in hydrosilylation reactions of CO 2 . 16,41,42The gas is initially trapped by the organosilicon compound in the form of silyl formate (Scheme 4) 41 with the aid of transition-metal catalysis (e.g.Cu, Zn, Ni) and bulky ligands (e.g.σ-donor N-heterocyclic carbenes, NHCs). 16,41,42Then, the carbonyl group reacts with the target amino compound to form formamides (Scheme 4). 42The presence of an excess of silane provokes the reduction of the formamide to N-methylamine (Scheme 4). 16he hydrosilylation of CO 2 was later translated into carbon-11 chemistry by Liger et al. 43 Similarly to the aforementioned non-radioactive reactions, 16 this methodology exploited a NHC (1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene, iPr, Scheme 5) as ligand, zinc catalysis and high temperature (150 °C) to yield 11 C-N-methylamines within 20 minutes from EOD (Scheme 5).The radiolabelling of the amyloid-β plaque imaging agent [ 11 C]PiB was achieved, as well, with an overall time of 50 minutes from EOD and in good Scheme 4 Hydrosilylation of CO 2 forming a silyl formate.The reaction of an amine with a silyl formate produces a formamide which can be reduced by an excess of hydrosilane to yield an N-methylamine. 16heme 5 Hydrosilylation of [ 11 C]CO 2 and application on the 11 C-N-methylation. 43heme 3 Proposed mechanism for the fluoride-promoted [ 11 C]CO production from disilane species. 40CY (38%, isolated, decay-corrected) but low A m (15 GBq per µmol). 43 simpler set up that would not require the use of unstable NHCs was lately proposed for the synthesis of 11 C-N-methylamines. 44Inspired by non-radioactive experiments, 45 the radiolabelling step required the use of only hydrosilane and TBAF as fluoride source. 44The hydrosilane and the fluoride source were initially mixed in order to form an activated pentavalent fluorosilyl anion (11, Scheme 6) which instantly reduced the delivered [ 11 44 This fluoride-activated radiosynthesis of 11 C-N-methylamines was also fully automated and exploited for the synthesis of [ 11 C]PiB with RCY of 15% (isolated, decay-corrected) and A m 61 GBq per µmol within 32 min from EOB. 44

Organosilicon as precursors for carbon-11 radiolabelling
Recent findings revealed that organosilicon compounds are also viable radiolabelling precursors, especially in the form of trialkylsilyl and trialkoxysilyl arenes.The synthesis of these versatile organosilicon precursors is readily achieved via transition metal catalysis, such as Ni, Rh, Cu, allowing the silylation of a large number of aromatic compounds (e.g.][48][49] Trialkylsilyl and trialkoxysilyl arenes showed high reactivity towards copper-catalysed desilylative carboxylation reactions. 50ith the aid of a fluoride source, the precursor was initially converted into a pentavalent silyl fluoride anion which then undergoes oxidative addition onto the Cu catalyst and react with the cyclotron-produced [ 11 C]CO 2 .The highest reactivity was achieved when using DMF as solvent, KF/Kryptofix® 222 (crypt-222, 0.25 equiv.)as fluoride source, 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine (BEMP, 0.6 equiv.)as CO 2 -trapping agent and a temperature of 140 °C for 5 minutes (Scheme 7).This method resulted equally efficient in the radiolabelling of alkynyl, aryl and heteroaryl precursors in short times (12 min) and with RCYs ranging between 19% and 93% (estimated by radioHPLC). 50 similar methodology that did not require copper catalysis was developed, as well.When CsF (1 equiv.) was employed as fluoride source and the reaction proceeded for 2-8 minutes at temperatures between r.t. and 200 °C, a large number of alkynyl, aryl and heteroaryl precursors (26 examples, Scheme 7) were successfully 11 C-carboxylated with RCYs from 11% to 99%. 51A variety of solvents (DMF, DMA, DMSO/THF) were also tested returning similar trapping efficiency and reactivity.Using this strategy, eleven alkyl silyl precursors were successfully 11 C-carboxylated (RCYs = 19%-97%), as well, including the mitochondrial kinase inhibitor dichloroacetic acid

Review
Organic & Biomolecular Chemistry ([ 11 C]dichloroacetic acid) and [ 11 C]succinic acid (isolated decay-corrected RCYs = 32% and 50%, respectively, Scheme 7). 51esides the production of 11 C-carboxylic acids, the interaction between alkylsilyl precursors and [ 11 C]CO 2 showed to effectively yield other moieties, such as 11 C-N-methylamines.Following a protocol that was initially developed with nonradioactive CO 2 , 52 Ram et al. established a reliable method for 11 C-N-methylation of secondary amines to methyl-11 C-tertiary amines via direct use of [ 11 C]CO 2 and silyl amine precursors.The hydrochloride salt of the amine precursor (13, Scheme 8) was initially treated with hexamethyldisilazane (HMDS, Scheme 8) in the presence of either n-butyl lithium or ammonium sulphate to yield the respective silyl amine (14, Scheme 8).Upon delivery, [ 11 C]CO 2 would then be incorporated in the precursor as 11 C-silyl carbamate (15, Scheme 8) when reacted for 8-10 minutes at 60-65 °C.3][54][55][56] This method was applied in the radiolabelling of a variety of tertiary alkylic amines, including the biologically-active molecules [ 11 C]tamoxifen, 53 56 The interaction of alkylsilyl precursors with [ 11 C]CO 2 showed to be effective in the radiosynthesis of 11 C-ureas, as well. 57The reaction uses lithium bis(trimethylsilyl)amide (LBTMSA, Scheme 9) as a precursor which reacts with  57 The radiolabelling of 11 C-ureas could also be achieved by coupling organosilicon compounds with [ 11 C]CO.This reaction requires the presence of a silylazide (18) and a (silyl) hydroxylamine (19, Scheme 10) and proceeds via transition metal catalysis in short times. 58In particular, the reaction was initially developed using trimethylsilyl azide and O-(trimethylsilyl)hydroxylamine in THF and in the presence of chloro(1,5cyclooctadiene)rhodium(I) dimer ([RhCl(cod)] 2 ) and 1,2-bis (diphenylphosphino)ethane (dppe) as catalyst and ligand, respectively (Scheme 10).The reaction proceeded at a temperature of 120 °C for 5 minutes and the resulting [ 11 C]hydroxyurea was obtained with a RCY of 38% (isolated, decay-corrected, based on delivered [ 11 C]CO). 58The suggested reaction mechanism proceeds with the formation of a Rh(I)-bound nitrene intermediate ([ 11 C]20) which then reacts with the delivered [ 11 C]CO to yield a Rh(III)-coordinated 11 C-isocyanate ([ 11 C]21, Scheme 10).Then, the hydroxylamine acting as nucleophile reacts with the 11 C-isocyanate forming a silyl-protected 11 Curea (Scheme 10).The cleavage of the silyl groups was then easily achieved with a mixture of water and ethanol. 58he synthesised [ 11 C]hydroxyurea (Scheme 10), which in the body acts as a ribonucleoside reductase inhibitor, was then uti-lised to study its pharmacokinetics across the blood brain barrier in vivo and explore the interaction with multidrug resistance transporters. 59This was achieved by measuring the brain activity in rats with and without the simultaneous administration of multidrug resistance protein inhibitors (such as cyclosporin A and probenecid).The influx of [ 11 C] hydroxyurea in the rat brain, however, was not significantly modified by the used intervention drugs, suggesting that hydroxyurea is not a substrate for active efflux transporters at the blood brain barrier. 59he coupling of organosilicon compounds with [ 11 C]CO was also employed in the synthesis of 1-hydroxy-3-phenyl[ 11 C] urea by using phenylazide (19, Scheme 11) and O-(trimethylsilyl)hydroxylamine as reagents (22, Scheme 11) whilst keeping the same conditions. 581-Hydroxy-3-phenyl[ 11 C]urea was successfully radiolabelled with RCY of 35% (isolated, decay-corrected, based on delivered [ 11 C]CO) and A m of 686 GBq per µmol at 21 minutes from EOB. 58 The use of trialkylsilyl and trialkoxysilyl precursors was then tested for 11

Conclusion
In the past years, the interesting chemistry of silicon-containing compounds has been embraced for radiolabelling purposes, meeting the need for simpler, cost-effective and more efficient radiolabelling methodologies.The versatility of the silicon atom unlocked a plethora of different applications, from [ 11 C]CO 2 converting agents to radiolabelling precursors, and enabled the production of a large number of biologicallyactive radiopharmaceuticals. Nonetheless, the applicability of compounds bearing silicon is yet to be fully explored.Taking inspiration from conventional organic chemistry, where silicon-based chemistry is already widely utilised, many more applications could be developed for 11 C-labelling.For example, the hydrosilylation of CO 2 have been used for the synthesis of a larger number of chemical structures (formamides, aldehydes, aminals) [10][11][12][13] whereas in 11 C-chemistry it's been used only for the production of 11 C-N-methylamines. 43,44nother potential route for widening the use of 11 C-Si chemistry would be taking inspiration from the more established 11 C-boron chemistry. 60Boron and silicon share many similarities with respect to bond energy and general chemical properties, 61,62 suggesting that they may also have similar applicability in radiochemistry.Whilst some of these methodologies showed to be suitable for both boron-and silicon-containing molecules (e.g.  60 Regarding the potential coupling with [ 11 C]CO, evidence from non-radioactive studies already show its feasibility: the synthesis of unsymmetrical diaryl ketones was achieved by combining a silyl arene, a iodoarene and CO in the presence of KF as activator. 63Likewise, non-radioactive experiments showed that the synthesis of thioesters is achievable by coupling CO with organosilicon precursors. 64he development of novel organosilicon radiolabelling strategies could also take inspiration from the well-established radiochemistry of organostannic compounds. 65Exploiting the Stille coupling reaction, the use of organostannanes as radiolabelling precursors found several applications and was coupled with a variety of carbon-11 synthons, such as [ 11 C] CH 3 I, [ 11 C]CO and [ 11 C]acyl chloride, for the production of a variety of functional groups (e.g.aryl 11 C-methylated compounds and 11 C-ketones). 65The translation of these methodologies into silicon-based 11 C-chemistry would represent a big step forward.
Besides the development of novel methodologies, organosilicon radiochemistry may also benefit from broadening the pool of radiolabel-able substrates.The previously discussed techniques are indeed still limited to certain classes of precursors, like the reductive functionalisation of [ 11 C]CO 2 (Scheme 6) which was poorly tested on alkyl precursors.The development of a "universal" radiolabelling tool, applicable to a larger number of radiotracer regardless of their chemical nature, should then also be taken into consideration.
The use of the radiochemistry of organosilicon compounds should also be enhanced in regular radiopharmaceutical production.The aforementioned methods are still restricted to the research world and not have impacted the broader clinical production of radiotracers yet.Although some clinically-used radiotracers were successfully produced with the discussed methods (e.g.CO 2 delivery to end of synthesis (EOS)), 44,68 a critical improvement considering the short half-life of 11 C.The use of organosilicate methodologies would also lower the costs as not requiring the purchase and maintenance of costly infrastructure such as a standalone methyl iodide production unitpotentially increasing the availability of carbon-11 labelled compounds in laboratories.The RCY of the final radiopharmaceutical would also benefit from the use of the aforementioned methods (12% with [ 11 C]CH 3 I versus 26% with silicon chemistry). 44,68Hence, the translation of silicon radiochemistry into PET laboratory routine production is another important step that is required.
Dr. Bongarzone attained his PhD at the International School for Advanced Studies (SISSA, Trieste, Italy, 2007).Subsequent postdoctoral positions were held at the Institute for Research in Biomedicine, Barcelona (IRB) and the School of Biomedical Engineering and Imaging Sciences at King's College London (KCL).At KCL, Dr. Bongarzone conceived novel radiochemical reactions for developing PET imaging probes ([ 11 C]Niacin, [ 11 C]Biotin, [ 18 F]FAMTO, and [ 11 C]FPSZM1) and their preclinical characterization.
Scheme 12 The use of trialkylsilyl and trialkoxysilyl as precursors in the 11 C-methylation with [ 11 C]CH 3 I.