[11C]Carbon Monoxide in Palladium- / Selenium-Promoted Carbonylation Reactions

Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 769

Synthesis of 11C-Imides, Hydrazides, Amides, Carboxylic Acids, Carboxylic Esters, Carbothioates, Ketones and Carbamoyl Compounds

BY FARHAD KARIMI

ACTA UNIVERSITATIS UPSALIENSIS UPPSALA 2002 Dissertation for the Degree of Doctor of Philosophy in Organic Chemistry at Uppsala University in 2002.

ABSTRACT

Karimi, F., 2002. [11C]Carbon Monoxide in Palladium- / Selenium-Promoted Carbonylation Reactions. Synthesis of 11C-imides, hydrazides, amides, carboxylic acids, carboxylic esters, carbothioates, ketones and carbamoyl compounds. Acta Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 769. 43 pp. Uppsala. ISBN 91-554-5452-6.

[11C]Carbon monoxide in low concentrations has been used in palladium- or selenium- mediated carbonylation reactions such as the synthesis of 11C-imides, hydrazides, amides, carboxylic acids, esters, carbothioates, ketones and carbamoyl compounds.

In these reactions aryl iodides have been used in most cases. However, less reactive aryl triflate, chloride and bromides were activated using tetrabutylammonium iodide.

The reactivities of nucleophiles may have influence on the radiochemical yield of the 11C- labelled compounds. Carboxyamination of aryl halides using aniline derivatives yielded 10% of the corresponding 11C-amide. However, the radiochemical yields increased significantly when the aniline derivatives were treated with lithium bis(trimethylsilyl)amide. In contrast, this reagent did not improve the radiochemical yields when primary amines such as methylamine and benzylamine were used. In these cases the radiochemical yields were improved by using pempidine.

11C-Esterification usually gave low yields. However, the radiochemical yields of 11C-esters could be improved by using magnesium bromide and pempidine.

An excess of ligand may have a significant impact on palladium-promoted carbonylation reaction. The radiochemical yields of 11C-ketones were improved when using excess amounts of tri-o-tolylphosphine.

(13C)Carbon monoxide may be utilized for the synthesis of 13C-substituated compounds in order to confirm the position of 11C-labelling.

Farhad Karimi, Department of Organic Chemistry, Institute of Chemistry, Uppsala University, P.O. Box 531, SE-725 21 Uppsala, Sweden.

ISSN 1104-232X ISBN 91-554-5452-6

Printed in Sweden by Kopieringshuset, Uppsala 2002.

To my parents, my sister, my wife & my daughter

Papers included in the thesis

This thesis is based on the following papers, referred to by their Roman numerals I-X. I. [11C] / (13C)Carbon monoxide in palladium-mediated synthesis of imides. Karimi F., Kihlberg T., Långström B., J. Chem. Soc. Perkin Trans. 1, 2001, 1528-1531. II. Palladium-mediated synthesis of [carbonyl-11C]amides and hydrazides using [11C]carbon monoxide. Karimi F., Långström B., J. Chem. Soc. Perkin Trans. 1, 2002, 2111-2116. III. Synthesis of 11C-amides using [11C]carbon monoxide and in situ activated amines by palladium-mediated carboxaminations. Karimi F., Långström B., J. Chem. Soc. Perkin Trans. 1, 2002, in press. IV. Synthesis of 11 C-amides using [11C]carbon monoxide and in situ activated amines with 1,2,2,6,6-pentamethylpiperidine in palladium-mediated carboxamination. Karimi F., Långström B., submitted. V. Palladium-mediated carboxylation of aryl halides (triflates) or benzyl halides using (13C) / [11C]carbon monoxide with tetrabutylammonium hydroxide or trimethylphenylammonium hydroxide. Karimi F., Långström B., J. Chem. Soc. Perkin Trans. 1, 2002, 2256-2259. VI. The synthesis of [carbonyl-11C]esters using [11C]carbon monoxide in palladium- mediated reactions. Karimi F., Långström B., manuscript. VII. Appendix: Supplementary material for the synthesis of 11C-carbothioates. VIII. Palladium-mediated 11C-carbonylative cross coupling of aryl halides with organostannanes. An efficient synthesis of alkyl / aryl [carbonyl-11C]ketones. Karimi F., Långström B., manuscript. IX. Synthesis of 3-[(2S)-azetidin-2-ylmethoxy]-5-[11C]-methylpyridine, an analogue of A- 85380, via a Stille coupling. Karimi F., Långström B., J. Label. Compd. Radiopharm. 2002, 45, 1-12. X. [11C]Carbon monoxide in selenium-mediated synthesis of 11C-carbamoyl compounds. Kihlberg T., Karimi F., Långström B., J. Org. Chem., 2002, 67, 3687-3692.

Reprints were made with permission from the publishers.

Contribution report

Paper I: The experimental work and writing was shared with T. Kihlberg. Paper X: The author performed the syntheses of precursors and reference compounds, the labelling of compounds 5, 6, 7, 9 and 14, NMR and MS analyses, the determination of specific radioactivity and wrote part of the experimental section.

Contents

ABSTRACT PAPERS INCLUDED ABBREVIATIONS 1. Introduction ...... 7 1.1. Radionuclides...... 7 1.2. Specific radioactivity ...... 8 1.3. Purification, analysis and identification...... 9 2. Labelled precursors...... 11 2.1. Production of [11C]carbon monoxide...... 11 3. Substrates used in carbonylation reactions...... 12 3.1. Carbon monoxide...... 12 3.2. Palladium ...... 13 3.2.1. β-Hydride elimination...... 15 3.3. Selenium ...... 16 3.4. Nucleophiles ...... 17 4. [11C]Carbon monoxide in palladium-mediated carbonylation reactions...... 18 4.1. 11C-Imides (Paper I)...... 18 4.2. 11C-Primary amides and 11C-hydrazides (Paper II)...... 19 4.3. 11C-Amides(Paper III, IV) ...... 21 4.4. 11C-Carboxylic acids (Paper V)...... 30 4.5. 11C-Carboxylic esters (Paper VI) ...... 33 4.6. 11C-Carbothioate (Paper VII) ...... 34 4.7. 11C-Ketones (Paper VIII) ...... 35 5. [11C]Carbon monoxide in selenium-mediated carbonylation reactions ...... 37 5.1. 11C-Carbamoyl compounds (Paper X) ...... 37 6. The impact of tetrabutylammonium iodide on 11C-carbonylation reactions ...... 38 7. Conclusion...... 40 8. Acknowledgments ...... 41 9. References ...... 42

The synthesis of [carbonyl-11C]esters using [11C]carbon monoxide in palladium-mediated reactions………………………………………………………………….………………….....A Appendix: Supplementary material for the synthesis of [carbonyl-11C]carbothioates…….….F Palladium-mediated 11C-carbonylative cross coupling of aryl halides with organostannanes. An efficient synthesis of alkyl / aryl [carbonyl-11C]ketones……………………………….….G

ABBREVIATIONS

# 13C * 11C AsPh3 Triphenylarsine Bq Becquerel (decay per second) BuLi Butyl lithium CO Carbon monoxide DBU 2,3,4,6,7,8,9,10-octahydropyrimido[1,2-a]azepine DMAP N,N-Dimethylpyridine-4-amine DMSO Dimethylsulfoxide GBq Giga Bequerel HPLC High Performance Liquid Chromatography L Ligand LC-MS Liquid Chromatography - Mass Spectroscopy LiM Lithium bis(trimethylsilyl)amide NMR Nuclear Magnetic Resonance OTf Trifluoromethanesulfonate P(o-tol)3 Tri-o-tolylphosphine Pd(PPh3)4 Tetrakis(triphenylphosphine)palladium(0) Pd2(dba)3 Tris(dibenzylideneacetone)palladium(0) PET Positron Emission Tomography PMP 1,2,2,6,6-Pentamethylpiperidine, Pempidine PQOH Trimethylphenylammonium hydroxide QI Tetrabutylammonium iodide QOH Tetramethylammonium hydroxide RCY Decay-corrected isolated radiochemical yield t½ Physical half-life of radionuclide TE Trapping efficiency. The value indicates the amount of incorporated [11C]carbon in the crude product. (Decay-corrected, the fraction of radioactivity left in the crude product after purging with nitrogen). THF Tetrahydrofuran UV Ultraviolet µAh Micro-ampere-hour

1. Introduction Positron Emission Tomography (PET) is a powerful and well-established medical imaging technique, which may be used to follow the in vivo fate of a labelled compound, allowing visualisation of various aspects of small molecules interaction with biochemical and physiological processes.1 Important aspects to study are receptor binding, enzyme inhibition and drug distribution. In order to meet the increasing demand for PET-tracers in drug development and the screening of new potential radiotracers, advances in labelling chemistry are required. Many biologically active compounds contain a carbonyl group or functionalities that can be derived from a carbonyl group.

This thesis deals with the scope and limitations of various types of carbonylation reactions, using short-lived radionuclide [11C]carbon. The synthesis of 11C-imides, hydrazides, amides, carboxylic acids, esters, carbothioates, ketones and carbamoyl compounds using palladium(0) or selenium and [11C]carbon monoxide are presented. In some cases the labelling chemistry has been performed by activation of nucleophiles or palladium-acyl-complexes. In few examples, the same approach has been used for labelling with (13C)carbon monoxide to verify the position of the label.

1.1. Radionuclides The most commonly used positron-emitting radionuclides for PET-studies are 15O, 13N, 11C and 18F, with half-lives of 2.07, 9.97, 20.3 and 110 min, respectively. Positron-emitters decay by conversion of a proton into a neutron and an emitted positron (β+). The positrons reach a few millimetres in tissue and annihilate by collision with an electron to produce two high- energy photons (Figure 1). These photons travel in the opposite direction, penetrating the body and can be detected externally by PET-camera. If two opposite detectors are hit simultaneously (coincidence detection) it is assumed that the photons come from the same decay event.

Figure 1. The β + travels a few millimetres in the tissue before it interacts with an electron. Their masses will be converted into two annihilation photons, i.e. 2 x 511 keV γ-photons that move in an antiparallel direction.

7 1.2. Specific radioactivity Specific radioactivity expresses the amount of radioactivity per unit mass. Positron-emitting- radionuclides are produced through nuclear reactions using charged particles, which result in neutron deficient radionuclides with high specific radioactivity.1b The effective value for [11C]carbon is far below the theoretical figure (3.4 x 105 GBq / µmol)1c because of the dilution by stable isotopes originating from the target, delivery lines, chemical reagents, etc. Despite this isotopic dilution, the specific radioactivity is generally sufficient to allow study of biological systems without perturbation.1, 2

Using [11C]carbon monoxide, 11C-labelled compounds with high specific radioactivity may be obtained. The specific radioactivity of a 11C- labelled compound, labelled with [11C]carbon monoxide, is in the region of 1700 GBq / µmol which is equal to a 200 fold isotopic dilution.3a Generally the amount of labelled compound is in the range of 10-100 nmol.

The relations between the magnitude of the bombardment and the specific radioactivities of five compounds were investigated. The target molecules and the results are shown in Figure 2 and Table 1, respectively.

O O O NH R NH2 2 * N S * N * NH2 H H Z N (4) (5) (1) Z = C, R = H

(2) Z = C, R = OCH3 (3) Z = N, R = H

Figure 2. 11C- labelled compounds used for the study of specific radioactivity

14 11 Table 1. Effect of particle bombardment ( N (p, α) C) on specific radioactivity.

Compound Bombardment (µAh)b Specific radioactivitya(GBq/µmol) 1 3.0 (4)c 125±4 1 10(1) 389 2 1.2(3) 50±5 2 10(1) 412 3 1.0(3) 42±3 3 37.4(1) 1609 4 2.0(3) 387±13 4 3.0(3) 532±9 4 10(1) 1760 5 1.0(2) 63±3 5 7.0(2) 426±8 b a Based on concentration measurements determined by LC-MS-analysis. Bombardment expressed as beam current (µAh) c Number of runs.

8 The data show a linear relation between bombardment (µAh) and specific radioactivity as illustrated for compound 4 shown below (Figure 3). However, the linearity will reach a steady state with increasing bombardment. At that point the production of [11C]carbon and decay are equal.

Figure 3. Plot of specific radioactivity against bombardment (µAh) using the same beam current (45 µA)

1.3. Purification, analysis and identification In labelling synthesis, the necessity of fast and efficient purification in work with low concentrations of short-lived-labelled compounds is obvious. The most commonly used purification method is HPLC equipped with a radioactivity- and UV absorption- detector. Purification can be achieved within a short period of time (usually less than 10 minutes). Chromatographic methods such as analytical HPLC and LC-MS are suitable tools for the analysis of labelled compounds. Identification of the product can be confirmed by LC-MS, GC-MS and co-elution of the sample with the corresponding well- characterised stable reference material. Another commonly used technique is 13C NMR. This analysis method is used for verification of labelling positions using 13C-substituted compounds.

In order to demonstrate the labelling position in a molecule, compounds 1 - 5 were synthesized (Figure 4). 3a-d, Paper VIII In these cases the 13C and 11C-syntheses were performed simultaneously. The reaction mixture and (13C)carbon monoxide were transferred to the micro-autoclave pre-charged with [11C] carbon monoxide. Subsequently the micro-autoclave was heated at the desired temperature for about 20 minutes. The collected purified fraction was further analysed by LC-MS in order to confirm that the carrier addition had not changed the reaction itself. In these reactions the concentration of the (13C)carbon monoxide was much higher than in the 11C-labelling.

9 O COOH O O O O # NH2 O N # N # OH # N # COOH H O N O (1) (2) (3) (4) (5) Figure 4. 13C- labelled compounds used for 13C NMR analysis (# = 13C)

This is exemplified in Figure 5, showing the 13C-NMR spectra of N-(13C)phthaloyl-L- glutamic acid (1) and the authentic N-phthaloyl-L-glutamic acid.

Figure 5. 13C-NMR spectra of N-(13C)phthaloyl-L-glutamic acid and the authentic N-phthaloyl-L- glutamic acid.

10 2. Labelled precursors [11C]Carbon dioxide is produced by bombardment of nitrogen with protons, 14N(p, α) 11C, in the presence of trace amounts of oxygen (Scheme 1). Starting with [11C]carbon dioxide, a variety of labelled precursors such as [11C]methyl iodide, [11C]methyl lithium, [11C]methane, hydrogen [11C]cyanide and [11C]carbon monoxide have been prepared (Scheme 1).4

11 11 11 CO 11CCl 11 CH3OTf H2 CO 4 COCl2

11 11 11 11 11 11 11 CH3NO2 CH I H CN CNBr 3 CH3OH CO2 CH4 11 11 Scheme 1. Some examples of conversion of [ C]carbon dioxide into other C-precursors

2.1. Production of [11C]carbon monoxide [11C]Carbon monoxide can be easily prepared by online reduction of [11C]carbon dioxide. However, the major problem is the low trapping efficiency. If the [11C]carbon monoxide in low concentrations is transferred in a steam of nitrogen gas into the reaction mixture, usually less than 10% is trapped.5a

The target produced [11C]carbon dioxide was transferred to the chemistry lab in a stream of nitrogen gas. In order to concentrate the radioactivity, the [11C]carbon dioxide was trapped on a Porapak Q column (1) (Figure 6) and the carrier gas was changed to helium. The concentrated radioactivity was released by heating and the gas flow was passed through a quartz tube filled with zinc (2),6 in which the [11C]carbon dioxide was reduced to [11C]carbon monoxide. The [11C]carbon monoxide was then collected in a small silica trap (3) 5b-d and later released by warming. The released [11C]carbon monoxide was finally used in the 11C-labelling synthesis by performing reactions in an autoclave (4).5b-d

Figure 6. Schematic presentation of the carbon monoxide system. (1) = Porapak Q (-196 ºC), (2) = Zinc oven (400 ºC), (3) = Silica trap (-196 ºC), (4) = Micro-autoclave

11 3. Substrates used in carbonylation reactions

3.1. Carbon monoxide Carbon monoxide is an odourless, colourless, tasteless and highly poisonous gas (it reacts with iron in haemoglobin of the blood to form carboxyhaemoglobin). Carbon monoxide has low solubility in most common solvents. Its boiling point and melting point are -191.5ºC and -205ºC, respectively. The atmospheric concentration of carbon monoxide is around 0.1 ppm.7

The potential application of [11C]carbon monoxide is illustrated in Figure 7. Using [11C]carbon monoxide in palladium- / selenium-mediated carbonylation reactions could have great impact on the development of PET-tracers owing to the following criteria: - The labelling synthesis is usually performed in one step. - The labelling method tolerates most functional groups. - The radiotracers can be obtained with high specific radioactivity. - The large numbers of biological active compounds have a carbonyl group or other functional groups that can be derived from a carbonyl group.

O RCNR'R'' O Amides & imides O RR'NCNR''R''' RCOH Ureas Carboxylic acids O O ROCOR' RCOR Carbonates Carboxylic esters 11CO O O RR'NCSR'' RCSR Carbothioates Thiocarbamates

O O RR'NCOR'' O RCR' Carbamates Ketones RCH Aldehydes

11 Figure 7. Application of [ C]carbon monoxide in carbonylation reactions

According to an earlier report,8 palladium(0) can facilitate carbonylation reactions where the 11C-carbonyl group is bound to carbon, hydrogen, oxygen, sulphur or nitrogen. Compounds where the carbonyl group is bound to two heteroatoms (such as carbamates) can be synthesized using selenium8b (Scheme 2).

12 O 11CO R© RX + Y R * Z Pd(0)

R = aryl, vinyl, methyl Y = amine, thiol, Z = O, N, S, CH2, ..... X = I, Br, Cl, OTf alcohol, organotin,...

O 11CO R R© RXH + R© XH X * X Se X = O, N, S R = R© = H, organic moiety

Scheme 2. Palladium- / selenium-mediated carbonylation reactions

However, substituted hydantoins9a and azathiolation9b have been synthesized using palladium.

3.2. Palladium Carbonylation of organic halides with carbon monoxide, mediated with transition metals such as palladium (0), constitute a valuable approach for the direct incorporation of a carbonyl group into organic compounds (Section 4). According to the general mechanism for carbonylation reactions using palladium(0) and carbon monoxide in high concentrations,10a-c the following steps might be involved in labelling with [11C]carbon monoxide in low concentrations (Scheme 3).10d-e

NuH O 0 - Pd Ln R Nu Path a - HX 3

Path b Nu© O NuH II R© X© Pd R 0 RX II CO II Pd Ln Pd X Pd R 0 - HX Ln - Pd Ln R Nu Ln Ln 4 O 5 O 1 2

R©© O R©Sn II 4 Pd R - R©SnX 0 3 Ln - Pd Ln R R© O 5 6

Scheme 3. Mechanistic scheme for palladium-promoted carbonylation reactions. L = ligand, Nu = nucleophile, X = I, Br

1- Oxidative addition; Due to the property of a transition metal of having low activation energies between the different oxidation states, it can easily acquire and supply electrons to its environment. In order to generate an organo-palladium-complex, the electron density from a metal orbital with matching symmetry is transferred to a σ-antibonding orbital of an incoming molecule (R-X), such as aryl or alkyl halides. Thus, the nonbonding metal electrons are used to break a σ-bond in the approaching molecule followed by formation of two new σ-bonds. Consequently, Pd(0) is oxidized to Pd(II). However, oxidative addition does not occur for saturated complexes, 0 11 unless they undergo reversible dissociation in solution to form Pd L2. 2- Insertion; Insertion of carbon monoxide into the organo-palladium-complex will generate the corresponding σ-acyl-palladium-complex, RCOPdLnX. The insertion reaction takes place by migration of the organic moiety from the palladium to the coordinated carbon monoxide as shown in Scheme 3. An alternative route to the migratory insertion of a coordinated carbon monoxide, is displacement of the halide ligand by carbon monoxide to give a CO-coordinated cationic organo-palladium complex that can undergo nucleophilic attack to give the organo- acyl-complex. Reductive elimination from this species liberates RCONu as shown in Scheme 4.12a

+ R' R' R' O II CO II NuH Pd X Pd CO X - PdII Nu 0 Ln - HX R Nu Ln Ln - Pd Ln O Scheme 4. Insertion of carbon monoxide into the palladium-ligand bond.

3- Cleavage (path a); The σ-acyl-palladium-complex undergoes a nucleophilic attack by nucleophiles such as alcohols, amines, etc, forming the corresponding RCONu (Scheme 3). 4- Nucleophilic attack (path b); This palladium-promoted step is thought to proceed through nucleophilic substitution of the halide of the σ-acyl-palladium-complex to form RCOPdLnNu (Scheme 3). 5- Transmetallation; Aryl or alkyl groups from an organometallic reagent such as organotin compounds (R'4Sn) can be transferred to a σ-acyl-palladium-complex, forming RCOPdLnR' (Scheme 3) . 6- Reductive elimination;

In this step the alkyl, aryl group (R'1) or nucleophile (path b) migrates from the σ-palladium- complex to the carbonyl group (Scheme 3). However, it has been suggested that rearrangement of the trans-σ-palladium-complex, formed by oxidative addition, to cis- complex takes place prior to the reductive elimination. 12b The mechanism of reductive elimination has also been suggested to proceed from a “T “or “Y” shaped unsaturated palladium complex. 12c-d

14 3.2.1. β-Hydride elimination Insertion of carbon monoxide into the σ-organo-palladium-complex using palladium(0) is very facile. However, palladium-catalysed reactions are limited to substances such as organic halides or triflates that lack β-hydrogens on saturated sp3 carbons such as aryl compounds. This is so since the intermediate σ-organo-palladium species generated by oxidative addition of these substrates to active metal centres do not undergo β-hydride elimination. On the other hand, in the case of organohalides or triflates having β-hydrogens on saturated sp3 carbons, palladium catalysts do not produce the carbonylated products efficiently due to the β-hydride elimination of the σ-alkyl-palladium species as shown in Scheme 5. 10b

H H 0 - HI Pd II R X Pd R R X

Scheme 5. β-Hydride elimination of the σ-alkyl-palladium complex

11C-Labelling of the two following compounds having β-hydrogen has been studied: • 2-Phenyl[carbonyl-11C]propionic acid.3c (paper V) The synthesis of 2-phenyl[carbonyl-11C]propionic acid was performed using the corresponding bromo and iodo compounds. When (1-bromoethyl)benzene was used, the analytical yield was 14% and the trapping efficiency was 19% (Scheme 6). By contrast, when the iodide was applied the trapping efficiency increased to 35% but no product was obtained. This result was unexpected and may be due to the more reactive iodo-palladium-complex favouring side reaction rather than carbonylation. This hypothesis should be further investigated.

11 CO, Pd(PPh3)4, Reag., O Br * THF, 180 ° C, 5 min OH Reag. = PQOH, RCY = 14%, TE = 19%

Scheme 6.

The low trapping efficiency may be due to a competing reaction, which results in formation of the palladium hydride compound in the presence of water (Scheme 7).13

- CO , - HX 2 PdL (H)X PdL2X2 + CO + H2O n 0 PdLn(H)X L4Pd + HX

Scheme 7. Formation of palladium hydride species in the presence of CO and water

15 • 1-(2-Phenyl-[carbonyl-11C]propanoyl)pyrrolidine. 3e (paper III) The labelling of 1-(2-phenyl-[carbonyl-11C]propanoyl)pyrrolidine was also investigated (Scheme 8). As expected when (1-iodoethyl)benzene was used, the radiochemical yield was 1% at 150°C. In contrast, when (1-bromoethyl)-benzene was used as the starting material, the analytical radiochemical yield increased to 28% at 150°C. However, decreasing the reaction temperature to 100°C resulted in a higher yield (LC-purified radiochemical yield = 58%) as shown in Scheme 8.

H N 11 N CO, Pd(PPh3)4, * Br + THF, 100 ° C, 5 min O RCY = 58%, TE = 95%

Scheme 8.

One possible explanation for these observations could be that organoiodides, having β - hydrogens on saturated sp3 carbons, do not produce carbonylated products efficiently due to the β-hydride elimination of σ-alkyl-palladium species. The competitive reaction (β-hydride elimination) may be suppressed in some conditions i.e. using organobromides at lower temperatures.

3.3. Selenium Selenium-mediated carbonylation reactions (see Section 5) using carbon monoxide in high concentrations have been postulated to proceed through formation of the carbonyl selenide (1) shown in Scheme 9, followed by reaction with a nucleophile (such as alcohol and amine) to form a carbamoselenoate (2).14a The carbonylation of primary amines may proceed via nucleophilic attack on isocyanates (3) formed by the elimination of hydrogen selenide from 2.14b With secondary amines, it has been suggested that the nucleophilic attack may take place on bis(carbamoyl) diselenides (4) generated by oxidation of 2.14c It has been postulated that carbonates are formed by nucleophilic attack of alkoxides on 2.14d This is presented in Scheme 9. In the case of 11C- labelling using [11C]carbon monoxide in low concentrations, the mechanism is assumed to proceed in the same way (Scheme 9). O X = NH R'XH RNCO RHN NHR' 3

O O O X = R'N R''XH CO RXH Se SeCO RX Se RR'N Se RR'N NR'R'' 2 1 4 2 O X = O

X = O, NH, R'N R'O RX OR' Scheme 9. Selenium-mediated carbonylation reaction.

16 3.4. Nucleophiles Nucleophiles such as amines, thiols, alcohols and hydoxyide have been used in palladium- or selenium-mediated carbonylation reactions. • Amines: Amines like pyrrolidine were used in the synthesis of 11C-amides. However, less reactivity amines such as aniline, benzylamine and indole were activated using lithium bis(trimethylsilyl)amide, pempidine or the in situ generated corresponding organo-stannyl compound, respectively.(papers III & IV) • Alcohols: Alcohols usually gave low yields of 11C-esters. In order to increase the radiochemical yield, the appropriate alcohol has been activated using magnesium bromide and pempidine. The formed alkoxide acts as a nucleophile in the synthesis of 11C-esters. (paper VI) • Thiols: Thiols were used in the synthesis of are 11C-carbothioates. However, the radiochemical yields were slightly increased by using pempidine. (paper VII)

• Hydroxide: Hydroxide has lower solubility in organic solvents compared with the organic nucleophiles. Since the hydroxide source is an aqueous solution, the competitive side reaction may consume [11C]carbon monoxide and decrease the radiochemical yield of 11C- carboxylic acid. (paper V)

17 4. [11C]Carbon monoxide in palladium-mediated carbonylation reactions The carbonylation reaction using [11C]carbon monoxide in low concentrations was started with the production of [11C]carbon dioxide, followed by reduction to [11C]carbon monoxide as described in section 2.1. The reaction mixture, containing palladium(0), an aryl halide or triflate and a nucleophile was transferred using an anhydrous solvent (such as THF, dioxane or DMSO) to the micro-autoclave, pre-charged with [11C]carbon monoxide. The micro- autoclave was then heated at the desired temperature for 5 min. The crude reaction mixture was released into a flask at reduced pressure. After measuring the radioactivity before and after purging with nitrogen gas (for calculation of TE and RCY), the diluted crude product was purified on a semi-preparative LC. Using this approach, 11C-imides, hydrazides, amides, carboxylic acid, esters, carbothioates and ketones were synthesized. In the analysis of the 11C-compounds, reference substances were used for comparison in the LC runs. The identity of all labelled compounds was further investigated by LC-MS. In some cases the position of labelling was determined using 13C NMR and LC-MS. In all cases the syntheses were repeated between 2 - 10 times unless stated otherwise. In general, the mean values (ca ± 5) for isolated radiochemical yields (RCY) and trapping efficiencies (TE) have been presented.

4.1. 11C-Imides (Paper I) The palladium-mediated carbonylation reaction has not been previously used in the synthesis of imides. Imides with thalidomide structure possess pharmacophores with a wide range of biological activities such as anti-tumor-promoting activity and anti-cell invasion.15a In order to investigate the labelling of 11C-imides, four compounds were selected as shown in Figure 8.

O O O O COOH O NH * ( )5 * N * N O * N N COOH O O O O O (1) (2) (3) (4)

Figure 8. Target compounds (* = 11C)

Three of these compounds, i.e. thalidomide15b, (2-(2,6-dioxo-3-piperidyl)isoindoline-1,3- dione) (1), N-phtaloyl-L-glutamic acid16 (2) and aniracetam17 (1-(4-methoxybenzoyl)-2- pyrrolidinone) (4), have been reported to be biologically active, whilst compound 3 was a model molecule.

Using 2-bromobenzamide derivatives in a palladium-mediated synthesis with [11C]carbon monoxide, three cyclic imides were labelled. The results are presented in Table 2.

18 Table 2. Radiochemical yields and trapping efficiencies for the 11C-labelled imides shown in Figure 8.

Compound RCY TE 1 69% 88% 2 73% 85% 3 45% 80%

Labelling of compounds 1, 2 and 3 was performed at 140ºC. Compound (4) was labelled at 200ºC using 4-iodoanisole and 2-pyrrolidinone as shown in Scheme 10. The lower temperature in the former cases (i.e. compounds 1 - 3) was due to the favourable ring closure reaction.

O I H 11CO, Pd(PPh ) , + N 3 4 * N O ° O THF, 200 C, 5 min O O

(4) RCY = 77%, TE = 96%

Scheme 10.

4.2. 11C-Primary amides and 11C-hydrazides (Paper II) In a previously reported synthetic approach, 11C-benzamide (1) and 11C-nicotinamide (5) were 11 18 synthesized via [ C]cyanation followed by treatment with H2O2.

One of the potential problems using this approach is its unsuitability in cases where sensitive functional groups are present. Therefore, five benzamides (1 - 5) and seven hydrazides (6 - 12) were selected for investigation of the option to use carbonylation labelling (Figure 9).

O O O NH2 NH2 S N * NH2 * N * H H R Z R Z (11) (1) Z = C, R = H (6) Z = C, R = OCH3 O (2) Z = C, R = OCH3 (7) Z = C, R = NO2 NH2 Z = C, R = NO O * N (3) 2 (8) Z = C, R = Cl O2N H (4) Z = C, R = Cl (9) Z = C, R = H (5) Z = N, R = H (10) Z = N, R = H (12)

Figure 9. Target compounds (* = 11C)

The labelling of various 11C-benzamide using tetrakis(triphenylphosphine)-palladium(0), an aryl halide and ammonia is exemplified in Scheme 11.

O 11 I CO, Pd(PPh3)4, NH3 in * NH2 dioxane, 180 ° C, 5 min (1) RCY = 53%, TE = 97%

Scheme 11.

In order to optimise the reaction temperature, the 11C-labelling of compounds 1 and 2 at 180°C vs. 150°C was studied. Higher temperature resulted in a two fold increased yield, which reflects the relatively poor nucleophilic property of ammonia, compared to e.g. pyrrolidine and piperidine. Compounds 2 – 4 were synthesized using the corresponding organoiodide. The results are shown in Table 3. Further investigations were made using 3-bromopyridine in similar conditions, as presented in Scheme 12. The radiochemical yield of [carbonyl-11C]nicotinamide (5) was considerably higher using 3-iodopyridine. The trapping efficiency was around 95% in both cases, which may be due to the formation of a more stable and less reactive palladium-complex after insertion of [11C]carbon monoxide when 3-bromopyridine was used.

O X 11 CO, Pd(PPh3)4, NH3 in * NH2 ° N dioxane, 180 C, 5 min N X = Br (5) RCY = 11%, TE = 93% X = I (5) RCY = 54%, TE = 98%

Scheme 12.

The synthesis of 11C-hydrazides has not been reported before. Therefore, seven hydrazides were selected in order to investigate the scope and limitations of this 11C-labelling method as exemplified in Scheme 13. Increasing the reaction temperature from 150°C to 180°C resulted in a decreased radiochemical yield, most probably due to decomposition of either hydrazine or 11C-hydrazides.

O O I 11 O NH CO, Pd(PPh3)4, NH2-NH2, * N 2 THF, 150 ° C, 5 min H (6) RCY = 81%, TE = 98%

Scheme 13.

The corresponding organoiodide was used in the labelling synthesis of compounds 7 – 9 as presented in Table 3.

Table 3. Radiochemical yields and trapping efficiencies for the 11C-labelled primary amides and hydrazides shown in Figure 9.

Compound RCY TE 2 47% 98% 3 45% 97% 4 41% 95% 7 63% 98% 8 75% 98% 9 72% 99%

In the case of compounds 10 and 11, the relative reactivities of bromo- and iodo-heteroaryls were investigated in the synthesis of 11C-hydrazides as illustrated in Scheme 14.

O X 11 CO, Pd(PPh3)4, NH2-NH2, NH * N 2 THF, 150 ° C, 5 min H N N X = Br (10) RCY (Anal.) = 20%, TE = 89% X = I ( 5) RCY = 78%, TE = 98% O NH S X 11 S 2 CO, Pd(PPh3)4, NH2-NH2, * N H THF, 150 ° C, 5 min X = Br (11) RCY (Anal.) = 20%, TE = 90% X = I ( 5) RCY = 78%, TE = 98%

Scheme 14.

The synthesis of 5-nitro-2-[carbonyl-11C]furohydrazide (12) will be discussed in section 6.

4.3. 11C-Amides(Paper III, IV) 11C-Amides have been synthesized by the reaction of the corresponding 11C-acyl chloride with an amine. 19a Until recently, this synthetic approach was the preferred method for synthesis of 11C-amides. 11C-Acyl halides (such as 11C-acetyl chloride and 11C-benzoyl chloride) were synthesized using phenylmagnesium chloride or methylmagnesium bromide and [11C]carbon dioxide. The resulting magnesium salt was treated with phthaloyl dichloride.19a The acid chlorides were then reacted with the appropriate amines.

21 11C-Amides have also been synthesized by carbonylation of lithium dialkylamides followed by quenching with alkyliodide.19b However, with this approach the yields were only in the range of 10% and the synthesis is limited to dialkylamides in which sensitive functional groups are absent. A general method5c for the synthesis of 11C-amides has recently been reported using [11C]carbon monoxide in palladium-promoted carbonylative coupling of phenyl / benzyl halides with amines. However, this method is somewhat limited to relatively reactive amines such as pyrrolidine. Therefore, the versatility of this method was further exemplified by the 11C-labelling of amides using activated amines and palladium-mediated carbonylation with [11C]carbon monoxide. The above limitation may be overcome by performing the reaction in two steps. In the first, 11 the corresponding organo-acyl-complex (R COPd(PPh3)2)X) is generated, which in a second step is reacted with the appropriate anion of the amine.19c

Owing to the importance of reaction time due to the short half-life of [11C]carbon, 20 a one-pot carbonylation reaction is preferred. Therefore, it was assumed that in situ activation of the appropriate amines might be away to overcome this problem. A series of amides (Figure 10a & 10b) were selected to investigate the 11C-labelling synthesis of amides. (Paper III, IV) The target molecules were selected based on the following criteria: a) To use relatively less reactive amines where the previously published synthetic procedure5C gave low radiochemical yields or failed completely. b) To activate the relatively unreactive amines such as aniline derivatives and indole using lithium bis(trimethylsilyl)amide or the appropriate organostannanes. c) To activate the relatively less reactive amines e.g. methylamine with pempidine. d) To show that the organohalide having β-protons bound to sp3 carbons may be used for synthesis of the corresponding 11C-amide. e) To synthesize 11C-amides where the use of Grignard reagents would be obstructed. f) To show that the organo-acyl-complexes generated from organochlorides and triflates can be activated with tetrabutylammonium iodide. g) To label amides that could be of interest as radiotracers in PET-studies.

The 11C-amides selected for this study cover various types of amines in the carbonylation reactions: • Cyclic amines such as pyrrolidine, piperidine and piperazine derivatives. • Secondary amines e.g. N,N-dimethylamine and N,N-dipropylamine. • Primary amines such as glycine, 2-pyridin-2-ylethanamine and methanamine. • Aniline derivatives e.g. 2-chloro-4-nitroaniline, aniline, 2,6-dimethylaniline and pyridin-2-amine.

22 O ( )n* N * N O * N n( ) O O X Z R O (2) n = 1 (1a) n = 0, Z = C, R = COOEt, X = H (3a) n = 2 (1b) n = 1, Z = C, R = H, X = F (3b) (1c) n = 0, Z = N, R = X = H (1d) n = 0, Z = C, R = X = H (1e) n = 0, Z = C, R = H, X = Cl OH O n = 0, Z = C, R = H, X = OCH O (1f) 3 * (1g) n = 0, Z = C, R = H, X = NO2 N O (4) F O O S O N O N * * N N * N N O (5) (6) (7)

O O R© * N N COOH R© * X R H H2N (9) (8a) R = H, R© = CH3, X = COCH3 R = H, R© = CH , X = COOH (8b) 3 O (8c) R = H, R© = CH , X = H Cl 3 * (8d) R = H, R© = CH , X = NO N 3 2 H N O (8e) R = COOEt, R© = C H , X = H 3 7 (10) (8f) R = H, R© = CH3, X = COPh O O H O S N * N N * * N H O HO H N I (11) (12) H (13)

O O O * N * N N H N H (14) (15)

Figure 10a. Target compounds (* = 11C)

23 Cl H O O H O N N * N * Cl * N H O O R (16) (17a) R = CH3 (17b) R = Cl (18) O O H N * * N H N H N * O (19) H O (20) (21) H R1 O O Cl N * * N * O N H N NO2 HO R H H 2 (24) (22) (23a) R1 = Cl, R2 = OH

(23b) R1 = R2 = H

H R1 OR N * 2 * N O Z N N H H (25) R3 (26a) Z = C, R1 = R2 = R3 = H

(26b) Z = C, R1 = OCH3, R2 = R3 = CH3

(26c) Z = N, R1 = R2 = R3 = H

O O * N N * N N N * H O (27) (28) (29)

N * N * N H O N H (30) (31)

Figure 10b. Target compounds (* = 11C)

24 1) Cyclic amines such as pyrrolidine, piperidine and piperazine derivatives: The 11C-labelling synthesis is exemplified in Scheme 15.

EtOOC COOEt 11CO, Pd(PPh ) , * N I NH 3 4 + THF, 150 ° C, 5 min O (1a) RCY = 72%, TE = 97%

Scheme 15.

The lower yield of compound 1a (72%) in comparison to 1b – 1g, could be due to steric hindrance. 1-(Bromomethyl)-4-fluorobenzene was used in the synthesis of 1b. Compounds 1c – 1g were synthesized using the corresponding organoiodide (Table 4). Compound 2 has been discussed in Section 6. Piperidine, 5-bromo-1,3-benzodioxole, 6-bromo-2,3-dihydro-1,4-benzodioxine and 1-fluoro- 4-(bromomethyl)benzene were used in the labelling synthesis of compounds 3a, 3b and 4 respectively. The results are presented in Table 4. The cyclic amine (1-(2-methoxyphenyl)piperazine) seemed to be less reactive. The synthesis of 5 was performed using this amine and 1-iodo-3-methoxybenzene. When the synthesis was performed as above the radiochemical yield was less than 4%. Therefore, the use of PMP for activation of the amine was investigated. The approach was indeed successful and the radiochemical yield increased considerably (73%, Table 4).

Table 4. Radiochemical yields and trapping efficiencies for the 11C-labelled amides shown in Figure 10a.

Compound RCY TE 1b 91% 99% 1c 85% 99% 1d 89% 99% 1e 81% 99% 1f 88% 99% 1g 83% 99% 3a 59% 97% 3b 70% 94% 4 90% 99% 5 73% 99%

2) Secondary amines e.g. N,N-dimethylamine and N,N-dipropylamine. The synthesis of N,N-dimethylpyrazine-2-[carbonyl-11C]carboxamide is illustrated in Scheme 16.

25 O N I 11 CO, Pd(PPh3)4, N + N H * N ° N THF, 150 C, 5 min N (6) RCY = 79%, TE = 96%

Scheme 16.

Substituted organoiodides and the appropriate amines were used for synthesis of compounds 7, 8a – 8e (Table 5). The labelling of compound 8f was performed using 4-bromophenyl- (phenyl)methanone. The lower decay-corrected radiochemical yield of 8f (45%) could be due to the lower reactivity of the organobromide compared to the organoiodide (Table 5).

Table 5. Radiochemical yields and trapping efficiencies for the 11C-labelled amides shown in Figure 10a.

Compound RCY TE 7 87% 99% 8a 78% 99% 8b 71% 99% 8c 94% 98% 8d 79% 99% 8e 61% 79% 8f 45% 94%

3) Primary amines. Primary amines such as glycine, 2-pyridine-2-ylethanamine, methanamine. N-(4-Amino[carbonyl-11C]benzoyl)glycine (9) is an amide of glycine. Glycine, however, is nearly insoluble in most organic solvents, although the tetrabutylammonium salt of glycine is partly soluble in DMSO. The results are presented in Scheme 17.

I O NH 11 2 CO, Pd(PPh3)4, * N COOH + - + COO N Bu4 DMSO / dioxane, H H2N H2N 150 ° C, 5 min (9) RCY = 26%, TE = 93%

Scheme 17.

Compounds 10 - 13 were basically synthesized using the corresponding organoiodide and the appropriate amines as described above (Table 6).

Table 6. Radiochemical yields and trapping efficiencies the 11C-labelled amides shown in Figure 10a.

Compound RCY TE 10 71% 96% 11 75% 99% 12 44% 94% 13 83% 99%

When the previous method5c was applied for the labelling synthesis of compounds 14 - 18 the analytical radiochemical yields were in the range of 10 - 15%, as exemplified for 14 (Scheme 18). However, using PMP the decay-corrected isolated radiochemical yield increased considerably (52%, Scheme 18).

O I 11 CO, Pd(PPh3)4, H2N * N + H N THF, 150 ° C, 5 min N (14) RCY (Anal.) = 15%, TE = 98% RCY = 52% (PMP), TE = 95%

Scheme 18.

The corresponding organoiodides and PMP were used for the synthesis of 15 – 19. Isoindolin- 1-[carbonyl-11C]one (20) was synthesized using 1-(2-bromophenyl)methanamine in the presence of PMP. The results are presented in Table 7.

Table 7. Radiochemical yields and trapping efficiencies for the 11C-labelled amides shown in Fig 10b.

Compound RCY TE 15 85% 99% 16 32% 86% 17a 80% 99% 17b 56% 99% 18 38% 99% 19 61% 99% 20 65% 98%

Furthermore, phenylhydrazine and 1-(2-bromophenyl)hydrazine HCl-salt were used in the synthesis of compounds 21 – 22, respectively. Since the ring closure reaction is much more favoured than the corresponding synthesis of the open ring compound, the latter compound was obtained in relative high yield (Scheme 19).

27 11 H CH I CO, Pd(PPh3)4, 3 + NH N N 2 N * THF, PMP, 150° C, 5 min H H O (21) RCY = 16%, TE = 75% O Br 11 CO, Pd(PPh3)4, * NH .HCl N N 2 THF, PMP, 150 ° C, 5 min N H H H (22) RCY = 75%, TE = 80%

Scheme 19.

4) Aniline derivatives. Aniline derivatives such as 2-chloro-4-nitroaniline and N-(2-bromophenyl)pyridine-2-amine, aniline are usually associated with relatively poor radiochemical yields when using the previously published method.5c When compounds 23a, 24, 26a and 27 were synthesized using this method5c, the analytical radiochemical yields were 27, 31, 20 and 10%, respectively. The radiochemical yields were only slightly increased using PMP. However, when LiM was used, the radiochemical yields were increased significantly (Scheme 20).

R 1 NO2 R 11 1 CO, Pd(PPh3)4, O Cl + THF, LiM, 150 ° C, 5 min * X Cl N NO2 R R2 H 2 NH2

X = Br, R1 = Cl, R2 = OH (23a) RCY = 72%, TE = 99% X = I, R = R = H (23b) RCY = 74%, TE = 99% 1 2 Scheme 20.

Compounds 24 – 27 were synthesized using the corresponding organoiodides. In the synthesis of 25, PMP gave better results than LiM (Table 8). This may be due to the ionisation of the indole nitrogen when LiM is used. The synthesis of 7-pyridin-2-yl-7-azabicyclo[4.2.0]-octa- 1,3,5-trien-8-[carbonyl-11C]one (28) was performed using N-(2-bromophenyl)pyridin-2-amine (Table 8).

28 Table 8. Radiochemical yields and trapping efficiencies for the 11C-labelled amides shown in Fig 10b.

Compound RCY TE 24 63% 99% 25 54% 99% 26a 80% 96% 26b 85% 98% 26c 65% 99% 27 48% 99% 28 19% 89%

Without LiM, the analytical radiochemical yields of compounds 26a and 26c were in the range of 10 - 20%. The isolated radiochemical yield of 28 was decreased from 71% to 19% when LiM was used. The relative high radiochemical yield obtained without LiM is probably related to the favourable ring closure reaction, while the negative effect of LiM is unknown. This method was used in the synthesis of compound 29 using 5H-dibenzo[b,f]azepine presented in Scheme 21. Even in this case the decay-corrected radiochemical yield was increased from <0.1 (obtained using previous method5c) to 25%.

I 11 CO, Pd(PPh3)4, N + O * N THF, LiM, 150 ° C, 5 min H

(29) RCY = 25%, TE = 87%

Scheme 21.

In an attempt to synthesize the amide 1-[carbonyl-11C]benzoyl-3-methyl-1H-indole (30) using the corresponding 3-methyl-1H-indole and LiM, the trapping efficiency was nearly quantitative but the analytical radiochemical yield was less than 0.1%. Therefore, the corresponding organotin-amine was generated in situ by the reaction between the corresponding lithium amine and trimethyltin chloride. The reaction was performed at room temperature. The resulting organotin-amine was reacted with a 11C-palladium-acyl-complex, i.e. the reaction was performed in two steps presented in Scheme 22.

29 O 11 I CO, Pd(PPh3)4, * Pd(PPh ) I THF, 100 ° C, 4 min 3 2

6 min at 80 ° C N O *

1) BuLi, THF, rt, 2 min (30) RCY = 32%, N 2) Me3SnCl, 1 min at N TE = 97% H 70 ° C, 15 min at rt Sn

Scheme 22.

In attempts to synthesize 30 in a one-pot reaction almost 90% of the radioactivity remained in the stainless steel micro-autoclave.

The synthesis of compound 31 will be discussed in section 6.

4.4. 11C-Carboxylic acids (Paper V) The synthesis of 11C-carboxylic acids via the corresponding 11C-nitriles followed by acidic or alkaline hydrolysis, have been published previously.21 11C-Carboxylic acid has also been synthesized previously using a Grignard reagents such as phenylmagnesium chloride and [11C]carbon dioxide.22 The application of this method is limited due to the nucleophilic and basic properties of Grignard reagents. Therefore, relative few substituted 11C-labelled aromatic carboxylic acids has been synthesized with this approach. Another inherent drawback is the isotopic dilution from atmospheric carbon dioxide (3.4 × 104 ppm), which will affect the specific radioactivity.

In view of the problems mentioned above, we were interested to develop a mild one-pot labelling synthesis. Nineteen substituted 11C-carboxylic acids were synthesized. In the synthesis of [carbonyl-11C]benzoic acid using iodobenzene, palladium(0), [11C]carbon monoxide and either water or an aqueous solution of sodium hydroxide (1M), the analytical radiochemical yield was less than 5%. Increasing the temperature from 150ºC to 180ºC did not change the results. The failure in the synthesis of [carbonyl-11C]benzoic acid may be due to the low solubility of NaOH(aq) in the reaction mixture. Therefore, we used aqueous solutions of QOH or PQOH as hydroxide donors to develop an efficient method for incorporation of [11C]carbon monoxide in aryl halides / triflates or benzyl halides. The target compounds are presented in Figure 11.

30 O O O O O OH R * OH * * OH * OH F R O (1) R = H (11) R' R = R' = Cl (2) R = Cl (12a) (13) ' (3) R = OCH3 (12b) R = Br, R = H

(4) R = NO2 (5) R = OH O O (6) R = CN N * OH * OH (7) R = CH3 N (8) R = COCH N (15) 3 (14) (9) R = COOH O (10)R = COPh O O * OH * S * OH OH (18) (16) (17)

Figure 11. Target compounds (* = 11C)

A series of experiments were made in order to compare the reactivities of QOH and PQOH in the synthesis of [carbonyl-11C]benzoic acid (1) and 4-chloro[carbonyl-11C]benzoic acid (2) using the corresponding aryl iodides (at 180ºC) and triflates (at 190ºC) as outlined in Scheme 23. O X 11CO, Pd(PPh ) , Reag., 3 4 * OH R THF, 180/190 ° C, 5 min R (1) R = H, X = I, Reag. = QOH, RCY = 59%, TE = 91% (1) R = H, X = I, Reag. = PQOH, RCY = 78%, TE = 88% (1) R = H, X = OTf, Reag. = PQOH, RCY = 85%, TE = 79% (2) R = Cl, X = I, Reag. = QOH, RCY = 35%, TE = 98% (1) R = H, X = I, Reag. = PQOH, RCY = 77%, TE = 76% (1) R = H, X = OTf, Reag. = PQOH, RCY = 75%, TE = 81%

Scheme 23.

The results showed that PQOH is more efficient than QOH. Furthermore, the decay-corrected radiochemical yields using triflates were in the same order as when the corresponding aryl iodides were used.

The scope and limitations of this labelling method was further explored for compounds 3 - 10 (Table 9).

31 Table 9. Radiochemical yields and trapping efficiencies for the 11C-labelled carboxylic acids are shown in Figure 11. O X 11 CO, Pd(PPh3)4, Reag., * OH R THF, 180 ° C, 5 min R

Compound R X Reag. RCY TE 3 OCH3 I PQOH 76% 91% 4 NO2 I PQOH 74% 71% QOH 46% 68% 5 OH I PQOH 21% 94% QOH 40% 55% 6 CN Br QOH 31% 79% 7 CH3 I PQOH 70% 85% 8 COCH3 I PQOH 73% 79% QOH 26% 93% 9 COOH I QOH 33% 92% 10 COPh Br PQOH 25% 67% QOH 10% 95%

In nearly all cases PQOH gave higher radiochemical yields. The low decay-corrected radiochemical yield of 5 (21%) may be a result of the higher solubility of PQOH in comparison to QOH. Some additional examples of 11C-carboxylic acids are presented in Table 10.

Table 10. Radiochemical yields and trapping efficiencies for the 11C-labelled carboxylic acids are shown in Figure 11.

Compound Reag. RCY TE 11 PQOH 24% 86% 12a PQOH 50% 60% 12b PQOH 42% 89% 13 PQOH 66% 93% QOH 23% 96% 14 PQOH 75% 82% QOH 4% 88% 15 PQOH 65% 77% QOH 20% 96% 16 PQOH 68% 57%

As previously mentioned, organoiodides are sometimes more easily accessible than the corresponding iodide. Therefore, our previous investigation with the use of triflates was further explored in the synthesis of 2-[ carbonyl-11C]naphthoic acid as shown in Scheme 24.

32 O X 11CO, Pd(PPh ) , Reag., 3 4 * OH THF, 180/190 ° C, 5 min (17) X = Br, Reag. = PQOH, RCY = 7%, TE = 80% (14) X = OTf, Reag. = PQOH, RCY = 39%, TE = 72%

Scheme 24.

4.5. 11C-Carboxylic esters (Paper VI) 11C-carboxylic esters can be synthesized by treatment of the corresponding [11C]cyanide with 23 alcohol saturated with HCl(g). Sasaki reported another possible synthetic procedure for the synthesis of 11C-acetyl esters.24 The synthesis was performed by treating [11C]acetyl chloride (produced via [11C]carbon dioxide)19a and alcohol in the present of a bases such as DBU or triethylamine or acylation catalysis (DMAP). The use of DMAP in palladium-promoted synthesis of 11C-esters with [11C]carbon monoxide was recently described.25 Due to the relative weak nucleophilic property of alcohols, a high concentration of the appropriate alcohols (such as methanol) were needed in order to obtain good radiochemical yields.26

With the intention of improving the synthesis of 11C-esters an efficient one-pot synthesis of [carbonyl-11C]esters using [11C]carbon monoxide in palladium-mediated reaction was developed. This approach was based on the reaction between a palladium-acyl-complex 11 (R COPd(PPh3)2X) and an alcohol in the presence of MgBr2 and PMP (Scheme 25).

11 ' O CO, Pd(PPh3)4, ROH, MgBr2, R X R' ° R * O PMP, CH2Cl2, 150 C, 5 min

Scheme 25.

In this method, a hindered strong base (PMP, pKa = 11.25) was used to promote conversion of the alcohol to the corresponding magnesium alkoxide, which serves as the nucleophile.27 Here, the synthesis of [carbonyl-11C]esters, using [11C]carbon monoxide, palladium(0), freshly prepared MgBr2 in THF, PMP and alcohols was performed in CH2Cl2. The selected target molecules are shown in Figure 12.

O O O * O * R Z * O O O Z = C, R = H (1) O (2) Z = N, R = H (6) N (3) Z = C, R = OCH3 (4) Z = C, R = NO O 2 F (7) (5) Z = C, R = COCH 3 Figure 12. Target compounds (* = 11C)

33 A series of 11C-esters was selected to investigate the labelling method. Good trapping efficiencies and decay-corrected radiochemical yields for methyl [carbonyl-11C]benzoate (1) and methyl [carbonyl-11C]nicotinate (2) were obtained (Scheme 26).

O I 11 CO, Pd(PPh3)4, MeOMgBr, * O ° Z PMP, CH2Cl2, 150 C, 5 min Z (1) Z = C, RCY = 54%, TE = 93% (2) Z = N, RCY = 69%, TE = 87%

Scheme 26.

The synthesis of methyl 4-methoxy [carbonyl-11C]benzoate (3), methyl 4-nitro[carbonyl- 11C]benzoate (4), methyl 4-acetyl[carbonyl-11C]benzoate (5) and methyl 1-[carbonyl- 11C]naphthoate (6) using the corresponding iodides gave radiochemical yields in the range of 38 – 73% as presented in Table 11. In order to further investigate the possibilities of this 11C-labelling method, the synthesis of (2,3-dimethoxyphenyl){1-[(4-fluorophenyl)acetyl]piperidin-4-yl}methyl[carbonyl-11C]- benzoate (7) using iodobenzene and (2,3-dimethoxyphenyl){1-[(4-fluorophenyl)acetyl]- piperidin-4-yl}methanol was performed. However this result is based on a single experiment (Table 11).

Table 11. Radiochemical yields and trapping efficiencies for the 11C-labelled esters shown in Fig 12.

Compound RCY TE 3 59% 97% 4 73% 89% 5 63% 94% 6 38% 87% 7 54% 85%

4.6. 11C-Carbothioate (Paper VII) Thiol esters, e.g. acetyl coenzyme A, are of importance since they occur in living cells. The 11C-labelling of these compounds has not been reported previously. The synthesis of 11C- carbothioates was performed using the corresponding aryliodide PMP, the appropriate benzenethiol, [11C]carbon monoxide and palladium(0) as presented in Scheme 27. R SH O 11 I CO, Pd(PPh3)4, + * S THF, PMP, 150° C, 5 min R (1) R = Cl, RCY = 84%, TE = 98% (2) R = H, RCY = 75%, TE = 99%

Scheme 27.

34 When the labelling was performed without PMP, the radiochemical yields decreased slightly (10%). The result indicates that this method can be generally employed to label these types of 11C- carbothioates.

4.7. 11C-Ketones (Paper VIII) The first report describing, the palladium-mediated synthesis of 11C-ketones using [11C]carbon monoxide was published by Andersson in 1995.28 According to this report the isolated radiochemical yields were less than 60% (TE <10%). The labelling method was further investigated using a recirculating system to improve the trapping efficiency.29 Nader has reported another promising palladium-promoted carbonylative cross-coupling reaction for the synthesis of symmetrical and unsymmetrical biaryl 11C-ketones using [11C]carbon monoxide, phenylboronic acid and iodobenzene.30 Barletta has reported a palladium-mediated carbonylation reaction using the micro-autoclave based technique for handling [11C]carbon monoxide.31

In a project based on a combinatorial chemistry approach [5-(azetidin-2-ylmethoxy)pyridin-3- yl](phenyl) [11C-carbonyl]methanone (1) and 1-[5-(azetidin-2-ylmethoxy)pyridin-3-yl] [11C- carbonyl]ethanone (2) were required for a biological study. In an attempt to synthesize 1 using 3-(azetidin-2-ylmethoxy)-5-bromopyridine, (Paper IX) tetrakis(triphenylphosphine)palladium, tributylphenyltin and [11C]carbon monoxide, the trapping efficiency was 79% but the analytical radiochemical yield was only 5% (Scheme 28). In order to increase the radiochemical yield, the use of Pd(AsPh3)4 was investigated. In this experiment, the trapping efficiency decreased to 48% while the analytical radiochemical yield increased to 14% (Scheme 28). Finally, the use of Pd((o-tol)3P)4 was investigated using a large excess of ((o-tol)3P) and Pd2(dba)3. This resulted in an increased trapping efficiency and the decay-corrected isolated radiochemical yields increased to 81% and 59%, respectively, as presented in Scheme 28.

11CO, Pd (dba) , L, O N O Br 2 3 O N * H PhSnBu3, DMSO, H N 100 ° C, 5 min N

(1) L = PPh3, RCY = 5%, TE = 79%

L = AsPh3, RCY = 14%, TE = 48% L = (otol) P, RCY = 59%, TE = 81% 3 Scheme 28.

The reaction between tris(dibenzylideneacetone)palladium(0) (Pd2(dba)3) and tri-o- 32 tolylphosphine ((o-tol)3P) generates the activate Pd(0) species [(o-tol)3P-Pd-P((o-tol)3]. The bulkiness of tri-o-tolylphosphine (cone angle = 194°) is assumed to contribute to the activity of the Pd(0) species formed.32 In order to investigate the impact of the excess of ligand, a stoichiometric amount of ligand i.e. 1:4 was used. In this case the radiochemical yield decreased to 31%. Therefore, the assumption that an excess of ligand [(o-tol)3P] may stabilise the activated Pd(0) complex was made. This procedure was utilized for the synthesis of 2 using tetramethyltin instead of tributyphenyltin, as presented in Scheme 29.

11 O CO, Pd2(dba)3, (otol)3P, N O Br N O ° * H SnMe4, DMSO, 100 C, H N 5 min N (2) RCY = 50%, TE = 80%

Scheme 29.

This provides a new concept for the synthesis of 11C-ketones. Therefore, it was of interest to further explore the role of using the excess of (o-tol)3P. The following ketones (Figure 13) were selected and successfully labelled: [11C-carbonyl]-benzophenone (3), phenyl(pyridin-3-yl)methan[11C-carbonyl]one (4), 1-phenylethan[11C-carbonyl]one (5) and 1-pyridin-3-ylethan[11C-carbonyl]one (6).

O O O O * * * * N N (3) (4) (5) (6)

Figure 13. Target compounds (* = 11C)

The one-pot labelling synthesis of compounds 3-6 was performed using the previously described method (Table 12).

Table 12. Radiochemical yields and trapping efficiencies for the 11C-labelled ketones shown in Fig 13.

Compound RCY TE 3 85% 99% 4 50% 99% 5 71% 99% 6 80% 98%

36 5. [11C]Carbon monoxide in selenium-mediated carbonylation reactions The 11C-labelling of carbamoyl compounds were performed as described in Section 4, but using selenium instead of palladium.

5.1. 11C-Carbamoyl compounds (Paper X) Although, selenium has low solubility in most organic solvents, it rapidly dissolves when tetrabutylammonium fluoride is added. This synthetic approach has been used in the synthesis of 11C-carbamoyl compounds as exemplified in Scheme 30.

O 11 + - HO OH CO, Bu N F .Se, O 4 O * Ph Ph DMSO, 150 ° C, 5 min Ph Ph RCY = 38%, TE = 52%

Scheme 30

37 6. The impact of tetrabutylammonium iodide on 11C-carbonylation reactions Organoiodides are commercially less available than the corresponding chlorides or alcohols are. An alternative is the use of the appropriate chlorides or triflates, which are easily prepared from the corresponding alcohols. In a recent paper Rahman showed that the use of triflates is possible for palladium-promoted 11C-amide synthesis such as the synthesis of N- phenyl[carbonyl-11C]benzamide (RCY = 6%).33

It was discovered that the radiochemical yield was poor in some cases when organobromides are used (Scheme 31). Therefore, the possibilities of improving the radiochemical yield using 11 these less reactive precursors were investigated. It was assumed that R COPd(PPh3)2Br is more stable than the corresponding iodide-Pd-complex or that the formation of a 11CO- coordinated cationic organo-palladium is involved. In this case an in situ ion exchange could solve the problem. Therefore, the influence of addition of iodide ion in the reaction mixture was investigated. Would the presence of iodide increase the reactivity of the Pd-complex? Our first experiment was performed by addition of a solution of tetrabutylammonium iodide (QI) in DMSO to the reaction mixture containing palladium(0) and 2-bromo-5-nitrofuran dissolved in THF. After 5-15 min the hydrazine was added and the reaction procedure shown in Scheme 31 was followed.

O 11 Br CO, Pd(PPh ) , NH -NH , NH2 O 3 4 2 2 O * N O N O N 2 THF, 150 ° C, 5 min 2 H RCY = 7%, TE = 95% O NH2 O Br 11CO, Pd(PPh ) , NH -NH , O * N 3 4 2 2 O N O2N 2 H THF/DMSO, QI, 150 ° C, 5 min RCY = 38%, TE = 96%

Scheme 31.

The approach was further investigated in the synthesis of N-methyl-9H-β-carboline-3- [carbonyl-11C]carboxamide using 3-chloro-9H-β-carboline and QI as presented in Scheme 32.

O 11 Cl CO, Pd(PPh3)4, NH2CH3, * N H N THF/DMSO, QI, 150° C, 5 min N N N H H RCY = 78%, TE = 98%

Scheme 32.

In the 11C-labelling of N-phenyl[carbonyl-11C]benzamide, the use of phenyl trifluoromethanesulfonate and aniline was studied (Scheme 33). The labelled compound was produced in a 6%

38 yield after pre-treating the aniline with lithium bis(trimethylsilyl)amide when the reaction was performed at 150ºC. However, by increasing the reaction temperature to 190ºC the radiochemical yield was increased to 32%. We also investigated the impact of using tetrabutylammonium iodide with the corresponding triflate in the carbonylation reaction. The radiochemical yield of target compound increased to 65% when performing the reaction at 190°C using phenyl trifluoromethanesulfonate treated with QI and combining with the activated aniline (Scheme 33).

OTf NH 2 11 O CO, Pd(PPh3)4, LiM, * + ° N THF/DMSO, QI, 190 C, 5 min H RCY = 55%, TE = 94%

Scheme 33.

These experiments clarified that both QI and the activation of the less reactive amine have an important influence on the radiochemical yield.

How could the activation of the palladium-acyl-complex using QI be explained? Could formation of an 11CO-coordinated cationic organo-palladium species (1) or generation of an organo-palladium-complex (2) be involved (Scheme 34)?34

+ R' R' II II * Pd X Pd CO X L n Ln (1)

II + PdLn + R-X R-Pd Ln X (2)

X = I, Br, Cl, OTf Scheme 34.

If neither an 11CO-coordinated cationic organo-palladium intermediate (1) nor complex (2) is involved, how can we then explain the improvement of carbonylation reactions using QI?

39 7. Conclusion This thesis describes various palladium- / selenium-promoted carbonylation reactions using [11C]carbon monoxide at low concentrations in combination with different nucleophiles. These reactions yield a variety of compounds with different functionalities of interest in the life sciences.

The low concentration of [11C]carbon monoxide is furthermore an important factor for the production of labelled compounds with high specific radioactivities.

This work focuses on the improvement of radiochemical yields by increasing the reactivities of the nucleophiles in various ways.

A need remains for increasing current understanding of the various mechanisms, especially the impact of low concentrations of [11C]carbon monoxide in the palladium- / selenium- promoted reactions.

40 8. Acknowledgments To all the following I wish to express my sincere gratitude:

My supervisor Professor Bengt Långström for introducing me to labelling chemistry, for his concern and never-ending enthusiasm.

The chairman of the Department Dr. Anita Hussénius for her support and encouragement throughout these years.

I am grateful to Dr. Andres Földesi for learned discussions. I was your student for just a few months but a friend forever. You are a very good person, thank you Andres.

Drs. Robert Moulder and Pernilla Kovisto are gratefully acknowledged for helping me to solve any problem with LC-MS analysis and Joachim Schultz for keeping the Cyclotron in working form.

Professor Bengt Långström, Drs. Gunnar Antoni, Tor Kihlberg, Andres Földesi and Hisashi Doi are gratefully acknowledged for constructive criticism on this thesis.

I would like to thank Professor Wyn Brown for linguistic correction.

I am grateful to the people at the Uppsala University PET Centre and at the Department of Organic Chemistry who have contributed to this work in different ways. Special thanks to Leif Jansson, Gunnar Svensson, Ingvar Wik, Tomas Kronberg for valuable technical assistance and Eva Pylvänen for administrative assistance.

My warmest thanks to my parents, Farah, Sofia, Lena and all my relatives for making my life always so special.

Finally, my special thanks to my dear wife Zohreh for her enormous patience, encouragement and never-ending love during these years.

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