Article pubs.acs.org/Organometallics
N-Vinyl Ferrocenophane Pyrrole: Synthesis and Physical and Chemical Properties Olivier Galangau,† Cécile Dumas-Verdes,† Elena Yu. Schmidt,‡ Boris A. Trofimov,‡ and Gilles Clavier*,† †
PPSM, ENS Cachan, CNRS, UniverSud, 61 av. President Wilson, F-94230 Cachan, France A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1 Favosrky Str., Irkutsk 664033, Russian Federation
‡
S Supporting Information *
ABSTRACT: A new synthesis of [3]ferrocenophan-1-one has been developed, and a Trofimov reaction has been applied to its ketoxime. The novel subsequent N-vinyl pyrrole ferrocenophane displays reversible electrochemical behavior centered on the ferrocene, without any participation of the pyrrole. Unexpected byproduct was also isolated, in which the α-pyrrolic position is methylated.
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achieve redox-controlled switching of their electronic properties. Conjugation should be first avoided to limit modification of the ferrocene electronic properties through orbital delocalization. Second, the distance between the two moieties should be as short as possible; thus, oxidation of ferrocene could affect the pyrrole’s properties. A fused pyrrole ferrocenophane could perfectly suit these criteria (Scheme 1)
INTRODUCTION
Pyrrole is one of the most widespread heterocyclic rings in nature and the key fragment of many natural molecules such as chlorophyll and heme derivatives. HIV fusion inhibitors and antitubercular drugs are examples of compounds containing pyrrole, among many other bioactive derivatives.1 In addition to these examples, the hydrogen donor ability of pyrroles can be exploited to chelate anions.2,3 Because of their electron-rich nature, numerous pyrroles have been introduced in a wide range of devices using semiconducting materials.4 These widespread applications have triggered the development of a wide scope of pyrrole synthetic methods,5 including Paal− Knorr reactions,6 Piloty−Robinson synthesis,7 metal-catalyzed reactions,8 retro aza Diels−Alder reactions,9 and Trofimov reactions.10 Pyrroles substituted by aromatic cycles such as benzothiophene, [2.2]paracyclophane, and mesityl groups have been successfully obtained with this last reaction.11 Surprisingly, few articles report the synthesis of pyrrole coupled with an electrochemically active group, 12 while developing organic molecules whose properties can be turned on or off via a redox process is an intense field of research. For instance, a controlled redox state can tune the stability of molecular complexes or the photoinduced electron transfer deactivation of fluorescence.13,14 In these examples molecules incorporate both pyrrole and a switchable entity, as separated moieties. Ferrocene, as a well-known reversible redox and electron donor group, appeared to be a simple model of a switchable molecule. Therefore, we attempted to synthesize and characterize a new pyrrole coupled with ferrocene as a new building block that could be used for the design of more complex switchable molecular structures such as BODIPY fluorophores15 or dipyrromethanes and other pyrrole-based structures for anion chelation.16 We reasoned that molecules containing both pyrrole and ferrocene should respect certain structural criteria in order to © 2011 American Chemical Society
Scheme 1. Retrosynthetic Scheme Proposed for the Synthesis of Pyrrole-Ferrocenophane
due to the orthogonal orientation of the moieties. Furthermore, α and β free pyrrole positions allow this compound to be an intermediate for the further development of electrochemically switchable organometallic compound based materials. Herein, we describe first a novel synthetic pathway to [3]ferrocenophan-1-one and then its conversion to a new N-vinyl ferrocenophane pyrrole derivative which is spectroscopically characterized. The electrochemical properties of the compounds will also be discussed.
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RESULTS AND DISCUSSION On the basis of previously published syntheses of ferrocenophane ketoxime17 and of pyrrole ferrocene through Trofimov reactions,18 it was anticipated that the target compound could be obtained using this procedure (Scheme 1) as an unprotected or N-vinylated pyrrole. Received: September 6, 2011 Published: November 16, 2011 6476
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Scheme 2. Synthetic Pathway to Compound 4
TLC monitoring suggested that free pyrrole is synthesized during the reaction but could not be isolated after chromatography. N-Vinylpyrroles are commonly known as byproducts of the Trofimov reaction. They are the result of the deprotonated pyrrole acting as a nucleophilic reagent and its subsequent reaction with acetylene. Given the apparent instability of the unprotected pyrrole, we decided to prolong the reaction time to 6 h in order to improve the yield of 5. We obtained the N-vinylpyrrole in 5% yield, together with byproducts which were identified as the initial ferrocenophanone 3 resulting from deoximation10 and the unexpected product 6 (8% yield). This compound was identified as the N-vinylpyrrole ferrocenophane substituted by a methyl group on the α-pyrrolic position (Figure 1). To the best of our knowledge, this type of product is new in the field of Trofimov pyrrole synthesis. 1H NMR spectra of the aromatic zone of 5 shows two doublet signals centered at 6.15 ppm (β position) and 7.04 ppm (α position), while for compound 6, only one singlet can be observed at 5.99 ppm. The aliphatic area of the spectra of 6 displays a new singlet at 2.28 ppm integrating for three protons. Comparison of the 13C NMR spectra of 5 and 6 clearly shows the new methyl peak of compound 6 at 13.6 ppm. In the aromatic region (Figure 1), the β-C−H pyrrolic signal of 5 positioned at 109.7 ppm is unchanged in 6. However, the CH signal at 116.2 ppm in 5 disappears and a new quaternary carbon peak at 113.8 ppm appears in 6. Mass spectroscopy also confirmed the presence of an additional methyl group. These combined results support the fact that the α-pyrrolic position was alkylated in compound 6. On the basis of O-vinyl synthesis, the pK value calculated in DMSO, and sulfoxide chemistry literature, we propose a synthetic mechanism of this unusual product (see the Supporting Information). Even though the O-vinyloxime derivative is another known byproduct of the Trofimov reaction, it was not isolated after chromatography. It was decided to verify if its synthesis is limiting the whole reaction by using activated acetylene (Scheme 4), as reported for other substrates.20 The E/Z mixture of O-vinyloxime intermediate 7 was isolated reproducibly in high yield (94%). Pyrrole formation in toluene, performed in a sealed tube, did not afford the pyrrole but instead the starting ferrocenophanone 3. Therefore, such synthetic results support the idea that the formation of the pyrrole could be the limiting step. Despite several examples of Trofimov reactions having been performed on cyclic compounds,10 in our case the rather short length of the carbon bridge could explain the low yield of N-vinylpyrrole. The electrochemical behavior of 5 and 7 has been investigated and compared to that of ferrocene, 3, and
Synthesis of [3]ferrocenophan-1-one has been reported as the result of a Friedel−Crafts reaction followed by a Michael annulation between ferrocene and acryloyl chloride at −78 °C in dichloromethane (DCM).15 Unfortunately, we could not reproduce this reaction because of difficulties in maintaining the low temperature. Indeed, it has been observed that temperatures higher than −78 °C yield lower amounts of the expected product. On the basis of this failure, we decided to change the synthetic method to get the ferrocenophanone. Oxime 4 was synthesized via a synthetic pathway transposed from the synthesis of [3]ruthenocenophanone (Scheme 2). 19 Ferrocenecarboxaldehyde underwent a malonic reaction followed by decarboxylation, leading to the corresponding acrylic acid 1 in good yield (83%). It was then reduced using standard hydrogenation conditions to give as the only product the corresponding ferrocenylpropionic acid 2 in 93% yield. Intramolecular Friedel−Crafts cyclization of 2 was first performed using trifluoroacetic anhydride in refluxing cyclohexane. Solubility issues were encountered, leading to 3 in moderate yield (40%). On a switch to refluxing DCM, highdilution conditions afforded the corresponding [3]ferrocenophan-1-one 3 in good to excellent yields (60−80%). 4 was quantitatively obtained as a 1:1 mixture of Z and E isomers by condensation of 3 with hydroxylamine hydrochloride in ethanol at room temperature. Finally, a one-pot, two-step version of the Trofimov reaction designed to avoid deoximation and maximize pyrrole formation was performed (Scheme 3).11a According to the accepted Scheme 3. Synthesis of Compound 5
mechanism for this reaction, deprotonation first occurred under “superbasic” conditions (CsOH/DMSO), leading to a nonisolated oximate cesium salt. After removal of methanol by distillation under reduced pressure, the O-nucleophile reacts with acetylene to afford the intermediate O-vinyloxime ferrocenophane. Pyrrole is then obtained after thermal cyclization, including [1,3] prototropic exchange, followed by sigmatropic [3,3] rearrangement and Paal−Knorr condensation. In a first run, we isolated N-vinylpyrrole 5 in 23% yield after 5 h of reaction. 6477
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Figure 1. Aromatic part of the 13C NMR spectra of 5 (dashed line) and 6 (bold line) in CDCl3 (left) and chemical structure of compound 6 (right).
Scheme 4. Retrosynthetic Scheme To Obtain Dicarboxy Pyrrole Ferrocenophane
Table 1. Oxidation Potentials of Ferrocene, 5, 7, and 3 in DCM vs Ag +/Ag ferrocene E 1/2ox (V) (ΔE 1/2)(b) E 1/2red (V) iox/ired a
0.35 0.10 1.1
5
3a
7
0.28 (−0.07) 0.16 1.0
0.49 (0.14) 0.23 1.5
Data from ref 21 recorded in acetonitrile; E 1/2ox(ferrocene) = 0.44 V.
(b)
[3]-ferrocenophanea
0.66 (0.22)
0.37 (−0.07)
ΔE 1/2 = E 1/2ox(compound) − E 1/2ox(ferrocene).
Both compounds present a broad absorption band centered around 425 nm. Extinction coefficients are low: 180 and 490 L
[3]ferrocenophane (Table 1 and Figure S1 in the Supporting Information). Both compounds display one oxidation wave around 0.3 V and good reversibility, even after 10 cycles. One could expect that N-vinylpyrrole should be oxidized at higher potentials. However, up to 2 V, 5 did not exhibit any other evolution, which suggests that, surprisingly, the pyrrole subunit does not undergo any electrochemical process. With regard to the oxidative half-waves, 5 is more readily oxidized than the ferrocene-like [3]ferrocenophane. Hence, the two constitutive units of 5 are independent groups. In compound 7, the oxidation half-wave is shifted to higher potentials. A similar, albeit larger, shift has been found for the [3]ferrocenophan-1-one, reflecting the weaker electron withdrawing effect of the oxime compared to that of the carbonyl. The ratio iox/ired for 5 is close to 1, meaning that only one electron is exchanged between the oxidized form and the reduced form, which allows us to identify the ferrocene moiety as the only electroactive subunit. Compound 7 displays a ratio close to 1.5, which is a sign of slow degradation of its radical cation under these conditions. Spectrophotometric measurements have been performed on compounds 5 and 7. UV−vis absorption spectra are presented in Figure S2 (Supporting Information), and the results are summarized in Table 2.
Table 2. Maximum Absorption Wavelengths and Molar Extinction Coefficients Recorded in DCM λ absmax,d−d* (nm) ε (L mol−1 cm−1) a
5
7
429 180
423 490
ferrocenea 441 148
Data from ref 20.
mol−1 cm−1 for compounds 5 and 7, respectively. They agree with a d−d* transition involved in the absorption process of ferrocene.22 7 shows a shoulder at 340 nm, usually attributed to the ferrocene MLCT, while in the case of 5, no such band could be clearly detected. In the case of 5, the pyrrole moiety absorption is probably positioned at lower wavelengths. Indeed, π−π* absorption for N-vinylpyrrole was reported at ca. 250 nm.23 Hence, no dramatic change is observed between 5 and 7. Therefore, the presence of the pyrrole does not affect the ferrocene properties; it clearly shows that the pyrrole moiety is not interacting with the ferrocene portion. The optimized geometry of 5 (Figure 2) was obtained from DFT calculations. The angle between the pyrrole and 6478
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(light path 10 mm) and a double-beam UV−vis spectrophotometer. Fluorescence spectra were recorded in a quartz cell at a 90° angle in solution with an OD below 0.1 at the excitation wavelength. The working electrode in cyclic voltammetry experiments was a platinum electrode, the counter electrode was a Pt wire, and the reference electrode was Ag+ (0.01 M in acetonitrile)/Ag. All the investigated solutions were deaerated by nitrogen bubbling for at least 5 min prior to electrochemical measurements. The concentration of the supporting electrolyte (n-Bu)4NPF6 was 0.1 M. The scan rate was set to 100 mV s−1. Quantum chemical calculations were performed at the B3LYP/Lanl2dz level of theory without symmetry constraint. Compound 1. A solution of ferrocenecarboxaldehyde (6.0 g, 28 mmol), malonic acid (2.9 g, 28 mmol), and piperidine (2,4 g, 28 mmol) in 100 mL of pyridine was placed into a three-necked round-bottom flask equipped with a condenser. Argon was bubbled through the mixture over a period of 15 min before heating at reflux for 2 h. The crude mixture was then warmed to room temperature, diluted with a 2 M NaOH aqueous solution (500 mL), and stirred overnight. Chlorhydric acid (38% w/w) was added until formation of a precipitate. Chloroform was then added, and the biphasic mixture was poured into a separatory funnel. The organic phases were collected. Water phases were extracted with chloroform several times, until the water solution was pale yellow. The organic phases were gathered, dried over magnesium sulfate, and filtered. Solvents were removed under reduced pressure to give 1 as a red powder (5.9 g, 83%). 1H NMR (400 MHz, CDCl3): δ 4.18 (s, 5H); 4.45 (broad signal, 2H); 4.53 (broad signal, 2H); 6.04 (d, 1H, 3Jtrans = 15.6 Hz); 7.70 (d, 1H, 3 Jtrans = 15.6 Hz). 13C NMR (100 MHz, CDCl3) δ 68.9; 69.8; 76.7; 78.1; 113.7; 148.6; 172.1. FTIR: ν OH 2926−2582 cm−1; ν CO 1659 cm−1; ν CC 1607 cm−1. HRMS (ESI): m/z M calcd for C13H12O256Fe+ 256.0187, found 256.0183. Mp: 208 °C. Compound 2. A solution of 1 (2.0 g, 7.8 mmol) and Pd/C (10% weight; 0.08 g, 0.1 mmol) in 250 mL of absolute ethanol was placed into a large pressurized reactor. Vigorous mechanical stirring was carried out, and the mixture was degassed several times with nitrogen. The solution was placed under 2.5 bar of hydrogen. After 3 h of reaction at room temperature under these conditions, TLC monitoring indicated that all the starting material was consumed and only one product was formed. The crude product was concentrated under reduced pressure and filtered through a short plug of silica. A yellow solution was collected to afford, after solvent evaporation, the corresponding propionic acid 2 (1.9 g, 93%). 1H NMR (400 MHz, CDCl3): δ 2.60−2.67 (m, 4H); 4.10−4.17 (m, 9H). 13C NMR (100 MHz, CDCl3): δ 24.6; 35.4; 67.7; 68.2; 68.9; 87.5; 179.3. FTIR: ν OH 2635−3090 cm−1; ν CO 1700 cm−1. HRMS (ESI): m/z M calcd for C13H14O256Fe+ 258.0270, found 258.0285. Mp: 150 °C. Compound 3. A solution of 2 (100 mg, 0.4 mmol) and trifluoroacetic anhydride (814 mg, 4 mmol) in dry DCM (80 mL) was placed into a three-necked round-bottom flask. A condenser was attached to the flask, and argon was bubbled through the solution for 5 min. The mixture was then refluxed until TLC monitoring showed complete consumption of the starting material. The crude product was concentrated under reduced pressure and purified by column chromatography (SiO2, DCM) to afford, after solvent evaporation, yellow crystals of 3 (56 mg, 60%). 1H NMR (400 MHz, CDCl3): δ 2.94 (m, 4H); 4.02 (dd, 2H, J a = J b =1.83 Hz); 4.37 (dd, 2H, J a = J b =1.83 Hz); 4.60 (dd, 2H, J a = J b =1.83 Hz); 4.83 (dd, 2H, J a = J b =1.83 Hz). 13 C NMR (100 MHz, CDCl3): δ 31.8; 44.2; 69.4; 70.4; 71.2; 72.8; 74.1; 88.1; 211.9. IR: ν CO 1656 cm−1. HRMS (ESI): m/z [M + H]+ calcd for C13H13O56Fe+ 24.0316, found 241.0316. Mp: 163 °C. Compound 4. To a solution of 3 (2.4 g, 10 mmol) in absolute ethanol (150 mL) were added hydroxylamine hydrochloride (1.4 g, 20 mmol) and potassium carbonate (5.8 g, 40 mmol). The mixture was stirred overnight at room temperature. When TLC monitoring showed complete consumption of the starting ketone, the mixture was filtered on cotton and the solvent was removed under reduced pressure. 4 (yellow powder) was isolated (2.3 g, quantitative) as a 1:1 mixture of Z and E isomers. 1H NMR (400 MHz, CDCl3): δ 2.45 (m, 4H); 2.82 (m, 2H); 2.99 (m, 2H); 4.05 (m, 4H); 4.21 (dd, 2H, J a =1.60 Hz, J b =1.83 Hz); 4.26 (m, 2H, J a =1.83 Hz, J b =1.83 Hz); 4.31 (dd, 2H,
Figure 2. Optimized geometry of compound 5: (left) front view; (right) side view.
cyclopentadienyl planes is calculated to be ca. 100°. This result suggests as well that the pyrrole is completely disconnected from the ferrocene. The two cyclopentadienyl planes are not coplanar but make a 14° angle, probably due to the short length of the bridge. A similar angle (9°) has been found in the crystal structure of 3.24 Compound 5 comprises one aromatic double CC bond which is shorter than the simple C−C bond in compound 3. Hence, this bond must enhance the bridge constraint and explain the larger calculated deviation from coplanarity for the two cyclopentadienyls. As shown in Table S3 (see the Supporting Information), the conjugation with the vinylic group induced a HOMO stabilization of 0.1 eV in comparison to the corresponding unprotected pyrrole (see the Supporting Information). This could partially explain why pyrrole in compound 5 did not undergo any electrochemical process. Calculations also indicate (see Figure S3, Supporting Information) that the pyrrole moiety is partially conjugated to the ferrocene. Such results support the idea that the appearance of a positive charge on the ferrocene could affect the pyrrole oxidation.
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CONCLUSION
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EXPERIMENTAL SECTION
A new N-vinylpyrrole fused to a ferrocenophane has been synthesized via five reaction steps, including a Trofimov reaction, from ferrocenecarboxaldehyde. Assuming that the bridge constraint limits the cyclization from proceeding, N-vinyl ferrocenophane pyrrole was obtained in moderate yield. On the basis of this hypothesis, future work will aim at elongating this bridge, which should increase the chance for this substrate to undergo a more efficient Trofimov reaction. Unprecedented methylation at the α position of the pyrrole was detected for the first time. Spectroscopy and calculations demonstrate that the pyrrole is electronically disconnected from the ferrocene, which allows the possibility that this kind of compound could be of great interest in the synthesis of pyrrole-centered electrochemically controllable organic materials. Further efforts are being made to better understand the unexpected electrochemical stability of N-vinylpyrrole at high potentials.
All chemicals were commercially available and used as received. Solvents for reaction and spectroscopy were of spectroscopic grade. NMR data were recorded on a 400 MHz spectrometer, and chemical shifts were recorded in parts per million (ppm). All chemical shifts were referenced to the internal reference TMS signal at 0.0 ppm. Twodimensional NMR experiments were carried out to confirm the NMR peak assignments (COSY 90). Melting points, taken on an electrothermal melting point apparatus, are uncorrected. Fourier transform infrared spectra were recorded on powders with an ATRequipped apparatus. Mass spectra were performed on an ESI mode MS instrument. Hydrogenation reactions were performed in a hydrogenpressurized reactor. UV/vis spectra were recorded in a quartz cell 6479
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J a =1.83 Hz, J b =1.83 Hz); 4.35 (dd, 2H, J a =1.37 Hz, J b =1.83 Hz); 4.49 (dd, 2H, J a =1.37 Hz, J b =1.83 Hz); 4.52 (dd, 2H, J a =1.37 Hz, J b =1.83 Hz); 8.33 (broad singlet, 1H, OH); 8.73 (broad singlet, 1H, OH). IR: ν O−H 3000 cm−1. HRMS (ESI): m/z [M + H]+ calcd for C13H14NO56Fe 256.0425, found 256.0357. Mp: >260 °C. Compounds 5 and 6. A suspension of CsF (268 mg, 1.8 mmol), LiOH (42.2 mg, 1.8 mmol), and 4 (450 mg, 1.8 mmol) in a dry MeOH (57 mg, 1.8 mmol) and dry DMSO (5 mL) solvent mixture was introduced into a Schlenk flask. The mixture was heated at 80 °C, and methanol was distilled under reduced pressure. Acetylene was then bubbled vigorously into the mixture, and the crude product was brought to 115 °C (bath temperature) for 5 h. Excess acetylene was trapped with a KOH/DMSO mixture. The black crude mixture was cooled to room temperature and diluted with water (50 mL). The aqueous phase was extracted with diethyl ether (3 × 25 mL). Organic layers were reunited, washed with brine, and dried over anhydrous magnesium sulfate. The yellow solution was filtered, and solvents were removed by evaporation under reduced pressure. The crude solid was purified by column chromatography (basic alumina, DCM/PE, 1/1 v/v, to DCM/AcOEt, 1/1 v/v). The first fraction was collected, affording, after solvent evaporation, compound 5. The second fraction consisted of 6 when formed. Compound 5 was obtained in the best case in 23% yield (111 mg). When 6 was isolated (183 mg, 8%), 5 was obtained in only 5% yield (113 mg). Compounds 5 and 6 are unstable in air over a period of 1 week, as seen by TLC analysis, and should be stored under argon at −18 °C or used quickly after synthesis. Characterization data for compound 5 are as follows. 1H NMR (400 MHz, CDCl3): δ 3.31 (s, 2H); 4.09 (t, J = 1.83 Hz, 2H); 4.23 (t, J = 1.83 Hz, 2H); 4.25 (t, J = 1.83 Hz, 2H); 4.34 (t, J = 1.83 Hz, 2H); 4.45 (d, 1H, J = 8.70 Hz); 4.96 (d, 1H, J = 16.03 Hz); 6.15 (d, 1H, J = 2.75 Hz); 6.76 (dd, 1H, Jcis = 8.70 Hz, Jtrans = 16.03 Hz); 7.04 (d, 1H, J = 2.75 Hz). 13C NMR (100 MHz, CDCl3): δ 24.1; 66.7; 70.1; 70.9; 74.0; 75.2; 92.1; 96.0; 109.9; 116.4; 124.7; 131.8; 133.0. IR: ν CC 1635 cm−1; ν C−H,Ar 3094 cm−1 ; ν C−H 2894 cm−1. HRMS (ESI): m/z M calcd for C17H15N56Fe 289.0554, found 289.0555. Mp: 88 °C dec. Characterization data for compound 6 are as follows. 1H NMR (400 MHz, CDCl3): δ 2.28 (s, 3H); 3.30 (s, 2H); 4.09−4.25 (m, 8H); 5.07 (d, J = 8.70 Hz, 1H); 5.17 (d, J = 15.57 Hz, 1H); 5.99 (s, 1H); 6.74 (dd, J a = 8.93 Hz, J b = 15.80 Hz). 13C NMR (100 MHz, CDCl3): δ 13.6; 23.0; 68.3; 69.1; 70.3; 73.3; 80.8; 87.2; 107.9; 109.7; 113.8; 128.5; 131.2; 136.9. IR: ν CC 1635 cm−1; ν C−H,Ar 3072 cm−1; ν C−H 2920 cm−1. HRMS (ESI): m/z M calcd for C18H17N56Fe 303.0710, found 303.0707. Mp: 143−144 °C. Compound 7. A round-bottom flask, previously oven-dried and placed under an argon atmosphere, was filled with a solution of ketoxime 4 (130 mg, 0.51 mmol) and DABCO (5.7 mg, 0.05 mmol) in dry DCM (7 mL) and cooled to 0 °C in an ice/water bath. Dimethyl acetylenedicarboxylate (63 μL, 0.51 mmol) dissolved in 2 mL of DCM was added dropwise over a period of 5 min. The mixture was warmed to room temperature and then stirred under argon overnight. At the end of the reaction, TLC monitoring showed only one product. The crude product was concentrated under vacuum and purified by column chromatography (SiO2, chloroform). 7 was obtained in the form of a yellow oil (190 mg, 94%) as mixture of diastereoisomers. 13C NMR (100 MHz, CDCl3): δ 27.2; 28.5; 33.8; 39.1; 51.5; 52.5; 68.7; 68.9; 69.5; 70.3; 70.4; 70.6; 70.7; 76.7; 85.9; 88.5; 105.1; 105.4; 153.7; 153.9; 162.8; 163.7; 164.6; 165.6. IR: ν CC 1433 −1649 cm−1; ν CN 1673 cm−1; ν CO,ester 1721 cm−1; ν CH, Ar and Al 2951 cm − 1 . HRMS (ESI): m/z [M + Na + ] calcd for C19H19NO556FeNa 420.0510, found 420.0509.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected].
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ACKNOWLEDGMENTS We thank the CNRS and the Ministry of French Research for funding the project. We also thank Professors J. Zakrzewski and M. Gormen for their fruitful advice and C. Bonnafé for his help.
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REFERENCES
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ASSOCIATED CONTENT * Supporting Information Figures, tables, and text giving cyclic voltammetry and UV−vis spectra of compounds 5 and 7, 1H and 13C NMR spectra of all compounds, the DEPT 135 spectrum of compound 6, and Cartesian coordinates of compound 5. This material is available free of charge via the Internet at http://pubs.acs.org. S
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