Article pubs.acs.org/Organometallics
Efficient Thiol−Yne Click Chemistry of Redox-Active Ethynylferrocene ́ ́ Alejandra Enrıquez, Ana Ma. González-Vadillo,* Ignacio Martınez-Montero, Sonia Bruña, Laura Leemans, and Isabel Cuadrado* Departamento de Quı ́mica Inorgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain S Supporting Information *
ABSTRACT: The application of ethynylferrocene, FcCCH (1), as a highly efficient electroactive precursor for the thiol−yne click reaction is presented. For this purpose, a wide range of functionalized thiols, namely 2-mercaptoethanol, 1-thioglycerol, 3-mercaptopropionic acid, 4-aminothiophenol, and benzene-1,3dithiol as well as tetrathiol pentaerythritol tetrakis(3-mercaptopropionate), were investigated. This facile thiol−ethynylferrocene radical reaction has resulted in the quantitative formation and isolation of the newly ferrocenyl−vinyl sulfides FcCH CHS(CH2)2OH (2Z and 2E), FcCHCHSCH2CH(OH)CH2OH (3Z and 3E), FcCHCHS(CH2)2COOH (4Z and 4E), FcC(CH2)S(1,4-C6H4)NH2 (5α), FcCHCHS(1,3C6H4)SCHCHFc (6), and [FcCHCHS(CH2)2COOCH2]4C (7). Thiol−ethynylferrocene reactions have been initiated either by heat, in toluene with AIBN, or by UV light irradiation in THF in the presence of DMPA as photoinitiator. The outcome of the hydrothiolation of ethynylferrocene strongly depends on the thiol structure and on the initiation method employed. A simple mixing of metallocene 1 with the thiol HS(CH2)2OH or HS(CH2)2COOH in a proper ratio, in THF at 20 °C, in a initiator-free thiol−yne reaction, causes hydrothiolation of 1 to occur, allowing for the formation of vinyl sulfides 2Z, 2E and 4Z, 4E in good isolated yields. In contrast to the bis-addition typically observed for thiol−yne reactions, no double hydrothiolation to FcCCH has been observed for any of the thiols under any conditions studied. Electrochemical studies showed that tetrametallic compound 7, containing four sulfur-bridged ferrocenyl−vinyl moieties, behaves as a tetrapodal adsorbate molecule, exhibiting excellent chemisorption properties, and spontaneously forms robustly adsorbed 7 films onto Au or Pt electrode surfaces.
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product isolation.18 This particular reaction involving ethynylferrocene has been successfully applied, over the past few years, to the synthesis of ferrocenyl-containing polymers17 and dendrimers7b as well as ferrocene-based redox sensors for anions and metal cations.7b,14 In a different context, by virtue of its valuable redox properties, ethynylferrocene has also been used in the past few years for the modification of a wide variety of surfaces, including carbon nanotubes,19 carbon nanofibers,20 glassy carbon,21 and conductive diamond22 as well as other metallic surfaces such as platinum and gold.23 The linkage of the electroactive metallocenyl moiety to the electrode surface can be achieved either by direct oxidation of the ethynyl group of 1 through an internal electron-transfer process21,23 or by covalent attachment of the FcCCH moiety to azide-modified surfaces via the CuAAC click reaction.19,20,22,24 To our surprise, although there have been numerous reports on the Cu(I)-catalyzed azide−ethynylferrocene click reactions, thiol−yne click chemistry involving redox-active FcCCH as an alkyne has not been investigated to date.25 The radical
INTRODUCTION Ethynylferrocene, FcCCH (1; Fc = Fe(η5-C5H4)(η5-C5H5)), represents a highly versatile class of electroactive organometallic compounds, which was first prepared in 1961 by Benkeser and Fitzgerald1 by bromination of vinylferrocene FcCHCH2 (VFc) and subsequent dehydrobromination. Shortly thereafter, Rosenblum and co-workers reported a further improvement to this synthesis by treating acetylferrocene with phosphorus(V) oxychloride in dimethylformamide.2 This functionalized metallocene is a useful starting material, not only for the synthesis of its homopolymer, poly(ethynylferrocene),3−6 but also for a wide variety of more complex organometallic derivatives, such as ferrocenyl dendrimers,7−9 dendronized polymers,10 and heterometallic molecules.11 Likewise, ethynylferrocene has been used as a starting compound for the preparation of bioorganometallic steroidal androgen derivatives.12 Interestingly, ethynylferrocene (1) readily undergoes copper(I)-catalyzed 1,3-dipolar cycloaddition reactions with azides (the CuAAC reaction), yielding ferrocene-containing 1,2,3triazole derivatives.13−17 Indeed, the CuAAC alkyne−azide reaction has recently emerged as an easy and popular synthetic method, as it falls into the category of click chemistry due to its high regioselectivity and efficiency, mild conditions, and simple © 2014 American Chemical Society
Received: November 4, 2014 Published: December 11, 2014 7307
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addition of a thiol to an unsaturated carbon−carbon bond, referred to as thiol−ene26,27 and thiol−yne chemistries,26,28 is currently attracting increasing interest as a valuable metal-free alternative for the heavily exploited CuAAC reaction. Nevertheless, the use of thiol−ene and thiol−yne reactions in the field of organotransition-metal chemistry has been almost unexplored. Thiol−yne chemistry, whose discovery dates back to the beginning of the 20th century,29,30 has experienced a renaissance over the past few years, enabling access to a wide variety of sulfide-based systems, including polymers and materials.26,28 This synthetic procedure allows simple addition of one thiol to an alkyne, thereby forming vinyl sulfide compounds. In addition, such vinyl sulfides are reactive species that can undergo a subsequent addition of a second thiyl R−S• radical, thus leading to the formation of dithioether products. The first step, although regioselective, is usually not stereoselective, since the vinyl sulfide derivatives are often formed in mixtures of both E and Z stereoisomers. Much of the research of the past few years has focused on the consecutive double hydrothiolation of alkynes, thus producing asymmetric dithioether branching points, which allows a high degree of functionalization useful in macromolecular chemistry, but less effort has been directed toward the simple addition of thiols. Thiol−yne reactions, following the same pattern as thiol− ene chemistry, can proceed via a free-radical-based mechanism, typically initiated by photochemical conditions.31 This type of reaction can also be carried out in the presence of different initiators, including peroxides,32 thermally activated radical initiators such as 2,2′-azobis(isobutyronitrile) (AIBN),33 2,2dimethoxy-2-phenylacetophenone (DMPA) as a UV-activated radical initiator,34 and even metal-based catalysts.25,35 Likewise, catalyst-free thiol−yne click polymerization without any external stimulus has been recently reported.36 Due to its facile nature and efficient characteristics, thiol−yne chemistry has been used to obtain highly cross linked polymer networks,37 block and graft copolymers,38 and functionalized nanoparticles.39 The radical thiol−yne reaction has also proved to be a powerful synthetic tool for the synthesis of dendrimers40 and hyperbranched polymers.41 Furthermore, the thiol−yne procedure has been extended to biochemistry, where it has been used in the glycosylation of peptides42 and the development of platinum drug delivery carriers.43 In this work, we focus our attention on developing a new family of sulfur-containing ferrocenyl compounds, obtained through the radical-mediated thiol−yne coupling procedure, using ethynylferrocene (1) as the redox-active precursor. This goal was motivated by our recent work concerning the covalent linkage of vinylferrocene (FcCHCH2, VFc) to different mono- and polythiols and to a S−H-polyfunctionalized polysiloxane using radical-mediated thiol−ene chemistry.44 On the other hand, the thiol−ene radical reaction has recently been used for the controlled functionalization of vinylcontaining polyferrocenylsilane homopolymers and block copolymers.45 Therefore, we reasoned that the reactivity of ethynylferrocene could be similarly exploited and would allow easy access to structurally new sulfur-rich ferrocenyl molecules. The general reactions that occur in thiol−vinylferrocene and thiol−ethynylferrocene chemistries are compared in Scheme 1. Theoretically, as it occurs in radical thiol−vinylferrocene hydrothiolation, the addition of thiols to FcCCH could proceed through both Markovnikov and anti-Markovnikov
Scheme 1. General Representations of Thiol−Vinylferrocene and Thiol−Ethynylferrocene Chemistries
addition routes, to yield regioisomers with branched and linear structures, respectively. In an effort to explore new synthetic transformations using simple functionalized ferrocenes, and as a natural extension of our research on ferrocene-containing molecules,44,46−49 herein we report full details of the first successful application of ethynylferrocene (1) in thiol−yne chemistry. Results obtained using FcCCH (1) and some thiols are compared with those found for the analogous coupling of FcCHCH2 (VFc) in thiol−ene reactions. The solution electrochemical behavior of the novel sulfur-rich ferrocenyl compounds and the outstanding ability of a tetrakis(ferrocenyl−vinyl sulfide) molecule to be immobilized onto gold and platinum electrodes have been investigated.
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RESULTS AND DISCUSSION Thiol−Yne Reactions of Ethynylferrocene with Mono-, Di-, and Tetrathiols. With the aim of demonstrating the compatibility and efficiency of the thiol−ethynylferrocene chemistry with different functional groups, a series of model reactions between 1 and several commercially available small monothiols were first investigated. Accordingly, 2-mercaptoethanol (I), 1-thioglycerol (II), 3-mercaptopropionic acid (III), and 4-aminothiophenol (IV) were chosen as the source of the different thiyl radicals (Figure 1). The thiol−ethynylferrocene reaction was investigated under different and complementary
Figure 1. Chemical structures of thiols and thermal and photochemical initiators employed in thiol−ethynylferrocene (1) radical reactions. 7308
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All reactions were easily monitored by 1H NMR by the progressive disappearance of the characteristic peaks of ethynylferrocene (1), mainly the singlet at δ 2.71 ppm. From these reactions, after purification using column chromatography, the desired ferrocenyl−vinyl sulfides FcCHCHS(CH2)2OH (2), FcCHCHSCH(OH)CH2OH (3), FcCH CHS(CH2)2COOH (4) (as separable E and Z stereoisomers), and FcC(CH2)S(1,4-C6H4)NH2 (5α) (Scheme 2) were isolated. These novel ferrocenyl−vinyl sulfides bear different terminal reactive functionalities, useful for a variety of further synthetic transformations. For instance, compound 3, derived from 1-thioglycerol (II), contains two readily functionalizable hydroxyl groups, useful for an ensuing dendrimer construction. Regarding the thermally initiated thiol−1 reaction, all processes were carried out in a 1/thiol molar ratio of 1/1.2, using toluene as solvent and in the presence of AIBN as thermally activated initiator. All solutions were then heated to 80−85 °C over different time periods, from 1 h (for monothiol II) up to 16 h (for monothiol III) (Table 1), and monitored by 1 H NMR. In contrast to the photochemical reaction with aromatic thiol IV, a higher yield of hydrothiolated compound 5 was obtained after only 6 h of reaction. Under these thermal conditions, the thiol−yne reaction of 1 and IV again afforded the α isomer. It is interesting to note that not all thiols were equally reactive toward thiol−ethynylferrocene chemistry, suggesting that the thiol structure is a critical factor regarding the efficiency of the thiol−yne reaction. Among the studied monothiols, derivatives I and II, bearing alcohol functionalities, exhibit the highest reactivity. Similar results were found in thiol−ene reactions with vinylferrocene, which afforded the corresponding ferrocenyl−thioether products in similar yields.44 Times of reaction in both photochemical and thermal routes differ
conditions (see Table 1): in THF with DMPA and UV light and in toluene solution using AIBN as thermal initiator. Table 1. Summary of the Reaction Conditions and Yields for the Thiol−Ethynylferrocene Reactions Δb
hνa thiol
product
time (h)
yield (%)c
I II III IV V VI
2 3 4 5α 6 7
8 2 6 20 4 3
81 93 65 25 88 83
E/Zd 38/62 65/35 30/70 38/62 50/50
time (h)
yield (%)c
4 1 16 6 5 4
73 94 39 71 82 90
E/Zd 36/64 65/35 38/62 40/60 61/39
a
Photochemical reaction (hν) at room temperature in THF with DMPA as photoinitiator. bThermal reaction (Δ) in toluene (∼80−85 °C) with AIBN as initiator. cAfter complete purification and isolation of the vinyl sulfide compounds by column chromatography. dRatio of cis and trans double bond conformations in the mixture of isomers.
We began our studies by examining the photochemically initiated reaction of ethynylferrocene (1) with monothiols I− IV. These reactions were carried out in a molar ratio of 1:1.2 (1/thiol), using THF as solvent and in the presence of DMPA as photoinitiator. The reactions were usually complete in a few hours, ranging from 2 h for monothiol II up to 8 h with monothiol I. In clear contrast, unlike the case for the aliphatic thiols, the hydrothiolation of 1 with the bulkier aromatic thiol IV having an amino functionality requires longer reaction times and produces the thiol−ethynyl product 5 in low yield under photochemical conditions. Extended reaction times did not significantly alter the reaction yield. This aromatic aminofunctionalized thiol IV behaves in a different fashion, since it affords the Markovnikov addition product 5α (Scheme 2).
Scheme 2. Synthesis of Ferrocenyl-Containing Vinyl Sulfides through Thiol−Yne Chemistry
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source, solvent, and thiol/ethynylferrocene (1) ratio. However, even when the conditions were forced to a 1/10 ethynylferrocene/thiol ratio, extending reaction times (48 h) and heating up to 110 °C, the monoadducts were always formed in a very selective fashion and no trace of the bis-addition products was observed. These results are noteworthy and reveal that the ethynylferrocene is much more reactive than the ferrocenyl− vinyl sulfide formed after the first addition. This could be explained by the increased steric hindrance that a second thiol addition would present to the metallocene sandwich substituent.50 Furthermore, it is noteworthy to mention that, according to 1 H NMR spectroscopy, the first aliquot, taken right after the addition of the specific thiol to a solution of FcCCH (1) in THF in the absence of initiator and without any external source (UV light or heat), already showed the formation of the desired vinyl sulfides. These results suggest that the alkyne−thiol reaction is extremely fast and lead us to believe that it does not require UV light and photoinitiator to be achieved. To verify this hypothesis, we performed two explorative reactions in the absence of initiator, in which mixtures of 1 and monothiol I or III, in a molar ratio of 1/1.2, were allowed to react at room temperature, with no UV light or thermal sources. After the same amount of time used in the photochemical reaction, analyses by 1H NMR showed the formation of the targeted ferrocenyl−vinyl sulfides 2 and 4, which were isolated in 75 and 77% yields, respectively. On the basis of these preliminary results, further studies of thiol−1 reactions effected under such initiator-free mild conditions are in progress in our laboratory and will be published in due course. On the other hand, to further investigate the high reactivity of ethynylferrocene, we carried out a comparative study between FcCCH (1) and FcCHCH2 (VFc), under identical hydrothiolation conditions. To this end, monothiols I and II were selected, and their hydrothiolation reactions were studied for a 1/1.2 molar ratio of 1/thiol or VFc/thiol, respectively. Both metallocenes were dissolved in the minimal amount of THF, and the mixtures were subjected to irradiation with a UV lamp, at room temperature, using DMPA as photoinitiator. When 1 was used as starting metallocene, 1H NMR spectroscopy showed the formation of the corresponding vinyl sulfide product within a few minutes (Figure S40A, Supporting Information). However, the thiol−ene reaction with vinylferrocene proved to be considerably slower (Figure S40B, Supporting Information), starting when thiol−yne reactions with 1 had already been completed. In addition, yields resulting from reactions with FcCCH (1) were always higher (81 and 93% using thiols I and II, respectively), versus 77 and 79% in the case of vinylferrocene. Interestingly, unlike the case for ethynylferrocene (1), reaction of vinylferrocene with amino-functionalized thiol IV, under either thermal or photochemical conditions, did not give any thiol−ene ferrocenyl−thioether product, proving again that FcCCH (1) undergoes notably easier hydrothiolation. On the basis of the results for the monothiols, the thiol− ethynylferrocene chemistry was successfully extended to a dithiol, namely 1,3-benzenedithiol (V), and to a tetrafunctionalized thiol, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) (VI). In both cases, the thiol−yne reactions with FcCCH (1) were carried out under the same conditions as for the monothiols, except for an ethynylferrocene/thiol ratio of 2.2/1 for thiol V and 4.2/1 for VI. These hydrothiolation reactions were monitored by 1H NMR by the disappearance of
appreciably for all monothiols, indicating that one route is clearly more favorable than the other. On the basis of previously reported thiol−yne mechanisms,26,28 Scheme 3 shows an idealized thiol−ethynylferrocene Scheme 3. Idealized General Mechanism for the Radical Hydrothiolation of Ethynylferrocene (1)
radical reaction that could operate under thiol−yne conditions. In the first cycle, thiyl radicals are generated from a specific thiol by thermo- or photoinitiation. This thiyl radical is then added to the −CC− triple bond of 1, thus forming a vinyl sulfide radical, which abstracts an hydrogen atom from another thiol, producing the ferrocenyl-containing vinyl sulfide compound and another thiyl radical that, under certain conditions, would allow the cycle to continue toward a second hydrothiolation. Consequently, after complete reaction, each FcCCH (1) moiety can be combined with two thiols to generate a monoferrocenyl−dithioether product. Theoretically, as shown in Scheme 1, the first step of the radical-mediated reaction between alkynes and thiols can progress through Markovnikov and anti-Markovnikov routes, yielding different regioisomers depending on the α or β addition, respectively. Most of the thiols selected proceeded through addition of the hydrogen to the more substituted carbon of the triple bond but, unexpectedly, as noted above, the reaction of ethynylferrocene with 4-aminothiophenol (IV) did proceed as the Markovnikov rule predicts, an α-addition, giving a more branched type of vinyl sulfide. Although a detailed analysis of this reaction goes beyond the object of this research, it can be indicated that the factor that results in the formation of the 5α product, during both the thermal and photochemical thiol−yne reactions, seems to be related to the nature of the thiol. Moreover, it is worth mentioning that for those thiols whose addition to 1 follows an anti-Markovnikov route and yields the β-addition isomer, the E/Z ratio of the ferrocenyl− vinyl sulfides, calculated from their integrals in the 1H NMR spectrum of the reaction mixture does not follow a pattern and differs notably depending on the thiol (see Table 1). Unlike other thiol−yne reactions,26,28 these specific hydrothiolations involving FcCCH (1) only follow the first step of a general thiol−yne mechanism (see cycle 1 in Scheme 3), whereas the consecutive addition of a second thiol is not observed for a 1/2 ethynylferrocene/thiol stoichiometric reaction. In order to try bis-addition of thiols to 1, we followed different methodologies, varying the reaction time, radical 7310
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the thiol proton at δ 3.43 ppm for V and at δ 1.65 ppm for tetrathiol VI. The diferrocenyl compound FcCHCHS(1,3C6H4)SCHCHFc (6) was formed after 4 h of exposition to UV light, in the presence of DMPA, and isolated by column chromatography in high yield (88%). Likewise, the reaction between FcCCH (1) and PETMP occurred easily after only 3 h of irradiation. After the reaction mixture was washed with nhexane several times, the desired star-shaped tetrametallic compound [FcCHCHS(CH2)2COOCH2]4C (7) was obtained as a stable orange oil in 83% isolated yield (Scheme 2). Both reactions were also achieved via thermally initiated thiol− yne reactions after 5 and 4 h, respectively, in the presence of AIBN (see Table 1). Structural characterization of the mono-, di-, and tetraferrocenyl−vinyl sulfides was achieved by IR, 1H and 13C NMR, and MALDI-TOF mass spectrometry. The 1H NMR spectra of all vinyl sulfides show that the characteristic peak for the ethynylferrocene terminal proton (−CCH) at δ 2.71 ppm has disappeared and new peaks corresponding to double bond protons in the range of δ 5.5−6.5 ppm are clearly observed. For compound 2 bearing an alcohol (triplet at δ 2.01 ppm), the ethylene protons of the −CH2CH2OH unit are visible at approximately δ 2.90 and 3.80 ppm for both isomers (Figures S1 and S7, Supporting Information). The same occurs with compound 4 (Figures S19 and S23, Supporting Information), which, in addition to the ethylene protons, shows in the trans isomer 4E a signal at δ 10.53 ppm for the −COOH group. Whereas this signal is not observed in the 1H NMR spectrum of 4Z, the carbonyl functionality can be easily detected by IR and 13 C NMR spectra (Figures S21 and S20, Supporting Information). The 1H NMR spectrum of 3 (Figures S12 and S16, Supporting Information) also displays characteristic signals of the ABX system at around δ 2.80, 2.90, and 3.90 ppm, along with both OH broad singlets. For compound 5α (Figure S26, Supporting Information), two signals are observed at δ 6.65 and 7.28 ppm that account for the AA′BB′ system in the parasubstituted aromatic ring and just one signal is observed in the double bond region of the spectrum (at δ 6.15 ppm), which confirms the formation of the α isomer. Furthermore, the amino protons show a broad singlet at δ 3.73 ppm that disappears with the addition of D2O. In the case of diferrocenyl compound 6 (Figure S31, Supporting Information), signals attributable to the meta-substituted aromatic ring can be seen at δ 7.10−7.45 ppm as a complex system. This bimetallic compound exhibits a particular behavior in 1H NMR, as the trans double bond part of the cis,trans- and the trans,trans isomers does not show resonance at the same shift; therefore, instead of showing a single AB system including all trans double bond protons, it displays two AB systems with 3J = 15.3 Hz, one for each isomer. Nonetheless, this effect is not observed for the cis part, where all isomers (cis,trans and cis,cis isomers) show just one AB system with 3J = 10.2 Hz. In the tetraferrocenyl compound 7, all cis and trans double bond conformations due to all 16 possible isomers overlap in the same signals (δ 5.92− 6.35 ppm) and the four CH2 units connected to the −O(C)O linkages are shifted downfield, to δ 4.16 ppm (see Figure S36, Supporting Information). Figure 2 shows the 1H NMR spectra of 2Z and 2E. The assignments of the different signals have been established on the basis of the through-space couplings observed in the NOE effect spectra (Figures S2 and S8, Supporting Information). As is expected, the vicinal coupling constants (3J) between the two vinyl protons in the cis and trans isomers are different, as they
Figure 2. Comparison of 13C (75 MHz) and 1H NMR (300 MHz) spectra of cis- and trans-FcCHCHS(CH2)2OH (2Z and 2E), recorded in CDCl3.
depend upon the stereochemistry of the double bond. For hydrogens in cis positions in compounds 2Z−4Z, this 3J coupling constant lies around 10 Hz, while for trans hydrogens in 2E−4E, 3J is typically around 15 Hz. Furthermore, the peaks assignable to the AB system of the −CHCH− bond in both isomers are slightly different, the AB system of the trans isomer being found at δ 6.18 and 6.41 ppm for 2E and at δ 5.94 and 6.21 ppm for 2Z. Likewise, the ferrocenyl protons are also affected by the different conformations. Thus, the signal corresponding to one of the pair of protons from the substituted cyclopentadienyl ring (H3) is shifted downfield in the cis isomer. This behavior can also be assessed in the 13C NMR spectra (Figure 2), where the same ferrocenyl carbon (C3) also shifts in both isomers. This may be explained by the rotation of the Fc−CH single bond, which at some point forces the sulfur atom to be closer to the cyclopentadienyl ring, therefore deshielding its protons, which translates into a higher chemical shift (δ 4.24, 4.55 ppm for C5H4 in 2Z and δ 4.22, 4.30 ppm for C5H4 in 2E). Furthermore, the ipso ferrocenyl carbon is also affected by the diastereoisomerism. For example, in compound 2Z, this signal appears at δ 81.3 ppm, while it appears in 2E at δ 82.9 ppm. This effect is attributed to a through-space interaction that appears significant in the trans isomer, since in this configuration both the ipso ferrocenyl carbon and the sulfur atom are held in an eclipsed form.51 The IR and MALDI-TOF mass spectra of compounds 2−7 provided further evidence of the formation of the targeted ferrocenyl vinyl thioethers. Specifically, the MALDI-TOF mass spectra display the corresponding molecular ions M+ at m/z 288.1 (2), 318.1 (3), 316.1 (4), 335.1 (5), 562.1 (6), and 1328.2 (7), with excellent agreement between the experimental and calculated isotopic patterns (see the Supporting Information). Electrochemical Studies. The redox properties of the ferrocenyl-containing vinyl sulfides 2−7 were examined by cyclic voltammetry (CV) and square wave voltammetry (SWV) using dichloromethane as a non-nucleophilic solvent and tetran-butylammonium hexafluorophosphate (n-Bu4NPF6) as the supporting electrolyte. The half-wave potentials (E1/2) of the 7311
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chromatography provided pure E and Z isomers. As exemplified in Figure 3B, which shows the CV responses exhibited by the pure Z and E forms of ferrocenyl-containing vinyl sulfide 3, the electrochemistry allows differentiation of these isomers.53 Although it is small, a measurable difference between the redox potential values of both isomeric forms can be observed and, specifically, the 3Z isomer oxidizes at a slightly higher potential. Despite their greater number of electroactive units, the CVs and SWVs of 6 and 7 (Figures S49 and S50, Supporting Information) also showed a single reversible oxidation process, at E1/2 = +0.50 V (6) and E1/2 = +0.46 V (for 7). These voltammetric data reveal that the two (for 6) and four (for 7) ferrocenyl units behave as independent redox moieties because their reversible oxidations to the positively charged ferrocenium form take place at roughly the same potential. This result is completely within expectations, as in both molecules the sulfurbridged vinylferrocenyl moieties are spatially well separated and their degree of electronic coupling is negligible. Sulfur-rich tetraferrocenyl 7 is a particularly interesting molecule, since it contains four vinyl sulfide groups potentially suitable for chemisorption on metallic surfaces and may eventually lead to the formation of films. Owing to a surface chelate effect, oligodentate adsorbate molecules can bind particularly strongly to some metallic substrates. In this context, it is well-known that thiols, thioethers, and disulfides are frequently used as binding units of oligodentate precursor molecules to fabricate self-assembled monolayers (SAMs) on coinage metals.54 In fact, an outstanding electrochemical characteristic of sulfur-rich tetraferrocenyl 7 is its great ability to undergo efficient and spontaneous chemisorption onto gold or platinum electrodes, simply by dip-coating. In order to test the ability of 7 to be immobilized on metal surfaces, freshly polished and electrochemically cleaned Au or Pt electrodes were exposed to unstirred solutions of 7 in CH2Cl2 for a variable period of time (2−10 h). Subsequently, the electrodes were rinsed with CH2Cl2 to remove physisorbed molecules and dried in air. The resulting chemically modified electrodes were immersed in ferrocenyl−vinyl sulfide free CH2Cl2 solutions, containing only supporting electrolyte, in order to assess the extent of adsorption by cyclic voltammetry. Figure 4A,C shows CV responses of two electrodes (Au and Pt) modified with thin films of 7 formed by chemisorption. In both cases, there is a linear dependence of the anodic peak current ipa versus the scan rate (for values up to v = 250 mV s−1), suggesting a nondiffusive redox process to and from the electrode surface, which is typical for surface-confined reversible redox species.52,55 Likewise, for the immobilized 7 films, the peak to peak separation values (ΔEp) vary from 5 mV (at a scan rate of 20 mV s−1) to 15 mV (at 200 mV s−1), which suggests that the rate of electron transfer is rapid on the time scale. The surface coverage (Γ) of electrochemically active ferrocenyl groups in the film was determined by integrating the anodic peak, yielding results of Γ = 6.7 × 10−11 mol Fe/cm2 for the modified electrodes shown in Figure 4A and Γ = 7.1 × 10−11 for that of Figure 4C. The electrochemical characteristics of these modified surfaces confirm that molecules of 7 are immobilized on the electrode. The attachment of 7 seems to occur in a SAM-like manner, through the −CHCHS− vinyl sulfide groups. Consequently, as is schematically represented at the top of Figure 4, compound 7 acts as a tetrapodal molecule
electrochemical processes are summarized in Table 2, together with those of the parent ethynylferrocene (1) measured in the Table 2. Electrochemical Data for Ferrocenyl-Functionalized Vinyl Sulfide Compounds
a
compound
E1/2 (CV)a
Epa
Epc
ΔEa
1 2Z 2E 3Z 3E 4Z 4E 5α 6 7
640 474 435 497 462 479 473 488 498 464
682 525 485 548 503 519 525 540 540 509
597 427 388 452 421 438 425 439 455 418
85 98 97 96 82 81 100 101 85 91
The E1/2 and peak potential separation values, ΔE, are given in mV.
same medium. The anodic electrochemical behavior of monometallic compounds 2−5 is dominated by the reversible wave corresponding to the one-electron Fe2+/Fe3+ oxidation of the ferrocenyl moiety (Figures S41−S48, Supporting Information). The voltammetric features, ipc/ipa being essentially equal to 1, the peak-to-peak separation values (ΔEp) being about 75− 100 mV at slow scan rates, and the potential of the forward peak Epa being independent of the scan rate, show that oxidation of 2−5 is chemically and electrochemically reversible.52,53 Interestingly, considerable variations in the E1/2 data summarized in Table 2 can be observed. First, the oxidation of ethynylferrocene 1 occurs at a relatively high E1/2 value (+0.64 V vs SCE) (Figure 3A), a potential that is 180 mV more
Figure 3. Comparative CV responses, on a Pt-disk electrode, in CH2Cl2 containing 0.1/n-Bu4NPF6: (A) ethynylferrocene (1) and ferrocenyl−vinyl sulfide 2Z; (B) pure isolated 3Z and 3E isomers. CVs were obtained at a scan rate of v = 100 mV s−1.
positive than that of unsubstituted ferrocene, thus highlighting the electron-withdrawing effect that the ethynyl group directly attached to the cyclopentadienyl ring has on the ferrocenyl moiety. Likewise, as a representative example, Figure 3A compares the CV response of 2Z with that of the synthetic precursor 1. It becomes obvious that replacement of the −C CH group of 1 by the −CHCH−S− substituent in 2 produces a marked effect on the half-wave potential, which shifts toward less anodic values. In comparison with 1, the oxidation of the ferrocenyl moiety in 2−5 occurs at a potential of about 170 mV more negative, indicating that the oxidation of the iron center in these ferrocenyl−vinyl sulfides is thermodynamically more favorable. As mentioned above, for vinyl sulfide compounds 2−4, careful separation of the isomeric mixtures through column 7312
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Figure 4. (A, C) CV responses of Pt-disk (A) and Au-disk (C) electrodes modified with a film of 7, measured in 0.1 M n-Bu4NPF6/ CH2Cl2 at a scan rate of v = 100 mV s−1, and (B, D) variations of the ip anodic current versus scan rate v of the electrodes modified with films of 7, shown in A (B) and C (D), respectively. Figure 5. (A) Multiple CV scans of tetrametallic 7 measured in solution of CH2Cl2 with 0.1 M n-Bu4NPF6. CV responses measured in 7-free CH2Cl2/n-Bu4NPF6, at different scan rates, of Pt-disk electrodes with a film of 7, modified with a single scan (B) and with 50 scans (D). (C) Linear dependence of the anodic peak current (ipa) versus v for the modified electrode shown in (B). (E) Dependence of ipa against v1/2 for the modified electrode shown in (D).
in which the four sulfur atoms, linked to the ferrocenyl-vinyl moieties, bind strongly to the metallic substrates. Interestingly we found that, while chemisorption of tetrametallic 7 onto Pt or Au electrodes does not require scanning of the potential, the amount of adsorbed material is considerably enhanced by scanning to positive potentials, through the ferrocene wave. Figure 5A shows the solution CV behavior of ferrocenyl−vinyl sulfide 7 after multiple repeated scans, which is noteworthy. Upon continuous scanning, there is a significant increase in the anodic and cathodic peak currents with each successive scan, which indicates that an electroactive film is growing onto the electrode surface. Thus, a significant advantage of such dual redox behavior is that the 7 film thickness can be controlled either by regulating the time of immersion of the electrode by dip-coating or, alternatively, by varying the number of scans in the CV. Representative examples of the voltammetric responses obtained for different films of 7 are shown in Figure 5B,D. These electroactive films have been prepared by cyclically scanning the potential at different amounts of time, and consequently, they have different film thicknesses and different surface coverages Γ (mol/cm2) of the ferrocenyl sites. A comparison of Figure 5B,D shows that the film thickness has a considerable effect on the electrochemical behavior, since the peak shapes of the ferrocenyl−vinyl sulfide based films are clearly different. At a relatively low coverage, obtained after a single CV scan (Figure 5B), the film in contact with 0.1 M CH2Cl2/n-Bu4NPF6 exhibited a well-resolved oxidation− reduction wave at E1/2 = +0.45 V (vs SCE), which is nearly identical with the E1/2 value of 7 in solution. The estimated surface coverage for this modified electrode was Γ = 5.8 × 10−10 mol/cm2 and suggests the formation of approximately a monolayer.56 For this wave, a perfectly linear relationship of
peak current with potential sweep rate (v) was observed (Figure 5C), and the potential difference between the cathodic and anodic peaks (ΔEp) is 10 mV at a scan rate of 50 mV s−1, which indicates that the rate of electron transfer is rapid on the time scale. In addition, the full width at half-maximum (Efwhm) of the surface voltammetric wave is 65 mV, measured at a scan rate of 50 mV s−1, which indicates that there are no significant nearneighbor interactions present between the ferrocenyl electroactive sites in the film, as this Efwhm value is only slightly smaller than that ideally expected (Efwhm = 90.6 mV) for a one-electron transfer or for reactions of multiple, independent redox centers with identical formal potentials. These voltammetric features unequivocally indicate the surface-confined nature of the electroactive ferrocenyl−vinyl sulfide based film.52,55 As the film thickness increases, that is, at multilayer coverage obtained after 50 CV scans (Figure 5D, Γ = 9.2 × 10−9 mol/ cm2), significant differences in peak shape and peak-to-peak splitting (ΔEp) appear. Specifically, the surface redox waves of the film in Figure 5D are broader and show diffusion-like tailing. The oxidation and reduction peaks are both sensitive to the scan rate, becoming more distorted at faster scan rates. The Epa values are clearly sensitive to film thickness, as they shift to positive potentials quickly. This fact is consistent with the viewpoint that electrolyte permeability and diffusion control the oxidation process. In addition, the peak-to-peak separation of 7313
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the film is larger (for instance, ΔEp = 120 at a scan rate of 200 mV s−1) and the peak currents are directly proportional to the square root of the scan rate, v1/2 (see Figure 5E) (rather than to the scan rate v, as experienced at low coverage), which suggests that in this case the charge transport through the film is limited by the electrolyte diffusion into the polymer film. Likewise, the diffusional behavior for the film of the modified electrode shown in Figure 5D is also proved by the slope of log ip vs log v, which is approximately 0.60 (Figure S52, Supporting Information), in contrast to the slope value of 0.92 (Figure S51, Supporting Information) calculated for the film of Figure 5B, which exhibits a surface-bound redox behavior. These CV features indicate that increasing the 7 film thickness hinders diffusion and electron transport and the film electrode process becomes quasi-reversible. A key concern in any chemically modified electrode is the electrochemical stability of the surface-confined species. The electrodes modified with adsorbed films of 7 have proved to be extremely durable. The stability of the 7 films was demonstrated by its nearly quantitative persistence after multiple consecutive scans (up to 100 scans) in pure CH2Cl2/n-Bu4NPF6 solution. Likewise, after storage of the modified electrodes for several weeks under ambient conditions, the redox responses were practically unchanged, without loss of electroactive material, which indicates the good stability of the ferrocenyl−vinyl sulfide based 7 films. Finally, it is worth mentioning that the ability of 7 to modify metallic electrode surfaces sharply contrasts with the redox behavior that we observed previously for the closely structurally related compound 8 shown in Scheme 4, recently prepared in
sulfide products. With respect to vinylferrocene, ethynylferrocene 1 undergoes notably easier hydrothiolation. In addition, a noteworthy aspect of the redox behavior of tetrametallic ferrocenyl−vinyl sulfide 7 is its ability to modify electrode surfaces by chemisorption, resulting in detectable electroactive films that remain persistently attached to the surface. To the best of our knowledge, this is the first example of electrode surfaces modified with films of redox-active ferrocenylcontaining vinyl sulfide species. The facile and efficient thiol−ethynylferrocene synthetic methodology can be further developed to prepare more complex sulfur-rich macromolecular polyferrocenyl structures, which are currently being investigated in our laboratory.
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EXPERIMENTAL SECTION
Materials. Tetrahydrofuran (THF) and toluene were dried over sodium/benzophenone and sodium, respectively, under argon before use. Hexane, ethyl acetate, diethyl ether, and dichloromethane were dried by standard procedures over the appropriate drying agents and distilled under argon, immediately prior to use. Silica gel (70−230 mesh) (Sigma-Aldrich) was used for column chromatographic purifications. AIBN (98%, Sigma-Aldrich) was recrystallized twice from methanol and stored at −20 °C. The photoinitiator DMPA, as well as the thiols 2-mecaptoethanol, 1-thioglycerol, and pentaerythritol tetrakis(3-mercaptopropionate), were purchased from Sigma-Aldrich and used without further purification. 3-Mercaptopropionic acid was acquired from Merck, and 4-aminothiophenol and benzene-1,3-dithiol were obtained from Alfa Aesar. Ethynylferrocene (1) was synthesized by adapting a previously published procedure from acetylferrocene in a two-step process in which (2-formyl-1-chlorovinyl)ferrocene is formed as an intermediate.57 Phosphorus oxychloride, acetylferrocene, and sodium acetate trihydrate, used in the synthesis of 1, were purchased from Sigma-Aldrich and Merck. Equipment. Infrared spectra were recorded on a PerkinElmer 100 FT-IR spectrometer. Elemental analyses were performed in a LECO CHNS-932 elemental analyzer, equipped with a MX5METTLER TOLEDO microbalance. All NMR spectra were recorded on BrukerAMX-300 and Bruker DRX-500 spectrometers. Chemical shifts were reported in parts per million (δ) with reference to CDCl3 residual solvent resonances for 1H (δ 7.26 ppm) and 13C (δ 77.0 ppm). The matrix-assisted laser desorption/time-of-flight (MALDI-TOF) mass spectra were recorded using a Ultraflex III (Bruker) mass spectrometer equipped with a nitrogen laser emitting at 337 nm. Ultraviolet radiation for photochemical reactions was provided by a 150 W medium-pressure mercury lamp (Heraeus Noblelight TQ 150, emission between 248 and 579 nm, with maximum energy at 365 nm). The lamp was contained in a quartz tube and placed in a borosilicate cooling jacket. All irradiation reactions were carried out in 50 or 100 mL two-necked round-bottomed flasks at a distance of 10 cm from the radiation source. Electrochemical Measurements. Cyclic voltammetric (CV) and square wave voltammetric (SWV) experiments were recorded on a Bioanalytical Systems BASCV-50W potentiostat. CH2Cl2 and CH3CN (SDS, spectrograde) for electrochemical measurements were freshly distilled from calcium hydride under argon. The supporting electrolyte used was tetra-n-butylammonium hexafluorophosphate (Fluka), which was purified by recrystallization from ethanol and dried under vacuum at 60 °C. The supporting electrolyte concentration was 0.1 M. A conventional three-electrode cell connected to an atmosphere of prepurified nitrogen was used. All CV experiments were performed using either a platinum- or gold-disk working electrode (A = 0.020 cm2) (Bioanalytical Systems). All potentials were referenced to the SCE electrode. Under our conditions, the ferrocene redox couple [FeCp2]0/+ is +0.462 and the decamethylferrocene redox couple [FeCp*2]0/+ is −0.056 V vs SCE in CH2Cl2/0.1 M n-Bu4NPF6. A coiled platinum wire was used as a counter electrode. Solutions were, typically, 10−3 M in the redox-active species and were purged with nitrogen and kept under an inert atmosphere throughout the
Scheme 4. Tetraferrocenyl Compound Obtained from the Thiol−Ene Radical Reaction of Thiol VI and FcCHCH2 (VFc)
our group via a thiol−ene reaction between tetrathiol (VI) and vinylferrocene.44 This tetraferrocenyl molecule, which has the same tetradirectional framework, with thioether bonds (instead of vinyl sulfide binding units) is not able to modify electrode surfaces under the same experimental conditions we have used with vinyl sulfide 7. This finding suggests that the interaction of the ferrocenyl−thioether bonds with the metallic surfaces is weaker in comparison to those of their related ferrocenyl−vinyl sulfide counterpart.
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CONCLUSIONS In summary, this work significantly expands both the scope of thiol−yne chemistry and the synthetic utility of ethynylferrocene since, for the first time, we have demonstrated the efficiency of a radical thiol−yne addition using FcCCH (1) as a redox-active alkyne precursor. Both thermally and UV photochemically initiated thiol−ethynylferrocene reactions tolerate a broad range of chemical functionalities and afforded the hydrothiolated ferrocenyl−vinyl sulfide products in relatively high yields. This study also showed that, under both thermal and photochemical reaction conditions, ethynylferrocene (1) is able to selectively add only one thiol, leading to the selective formation of the corresponding ferrocenyl−vinyl 7314
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acetate/ether (3/1) as eluent. The trans isomer was being eluted first and, later, the cis isomer. Compounds 3Z and 3E were then isolated as air-stable, orange solids. Yield: 0.280 g (93% with an E/Z ratio of 65/ 35). Thermal Initiation. Compounds 3Z and 3E were obtained from 1 (200 mg, 0.95 mmol) in 3 mL of toluene, 3-mercapto-1,2-propanediol (II; 98 μL, 1.15 mmol), and AIBN (16 mg, 0.097 mmol), using the same thermally initiated procedure as detailed for the preparation of model compounds 2Z and 2E. The reaction mixture was heated to 80−85 °C for 1 h. After filtration, all volatiles were removed under vacuum. An orange oily residue was obtained and purified in the same way as in the photochemical reaction. Yield: 0.285 g (94% with an E/Z ratio of 65/35). Data for 3Z are as follows. Anal. Calcd for cisC15H18O2SFe: C, 56.60; H, 5.70; S, 10.05. Found: C, 56.83; H, 5.35; S, 10.20. 1H NMR (CDCl3, 300 MHz): δ 1.98 (br s, 1H, OH1), 2.62 (br s, 1H, OH2), 2.85, 2.93 (AB of ABX system, JAB = 13.8 Hz, JAX = 7.5 Hz, JBX = 5.1 Hz, 2H, SCH2), 3.57−3.83 (m, 2H, CH2OH1), 3.89 (X of ABX system, 1H, −CHOH2), 4.13 (s, 5H, C5H5), 4.24, 4.56 (m, 4H, C5H4), 5.94 (A of the AB system, JAB = 10.2 Hz, 1H, CHS), 6.21 (B of AB system, JAB = 10.2 Hz, 1H, FcCH). 13C NMR (CDCl3, 75 MHz): δ 38.5 (SCH2), 65.0 (CH2OH), 68.8, 69.1 (C5H4), 69.2 (C5H5), 71.0 (CHOH), 81.1 (ipso-Fc), 121.8 (CHS), 125.6 (FcCH). IR (neat, cm−1): ν(OH) 3349, ν(CC) 1641, 1601, 1103, ν(C−OH) 1068, 1026. MS (MALDI-TOF): m/z 318.1 [M+]. Data for 3E are as follows. Anal. Calcd for trans-C15H18O2SFe: C, 56.60; H, 5.70; S, 10.05. Found: C, 56.38; H, 5.96; S, 9.83. 1H NMR (CDCl3, 300 MHz): δ 2.03 (br s, 1H, OH1), 2.68 (br s, 1H, OH2), 2.79, 2.89 (AB of ABX system, JAB = 14.1 Hz, JAX = 7.8 Hz, JBX = 4.8 Hz, 2H, SCH2), 3.55−3.83 (m, 2H, CH2OH), 3.89 (X of ABX system, 1H, CHOH), 4.12 (s, 5H, C5H5), 4.21, 4.29 (pt, 4H, C5H4), 6.19 (A of AB system, JAB = 15.3 Hz, 1H, CHS), 6.42 (B of AB system, JAB = 15.3 Hz, 1H, FcCH).13C NMR (CDCl3, 75 MHz): δ 36.9 (SCH2), 65.2 (CH2OH), 66.4, 68.8 (C5H4), 69.2 (C5H5), 70.5 (CHOH), 82.7 (ipso-Fc), 118.6 (CHS), 129.7 (FcCH). IR (neat, cm−1): ν(OH) 3358, ν(CC) 1634, 1597, 1103, ν(C−OH) 1066, 1024. MS (MALDI-TOF): m/z 318.1 [M+]. Synthesis of cis- and trans-FcCHCHS(CH2)2COOH (4Z and 4E). Photoinitiation. A mixture of ethynylferrocene (1; 200 mg, 0.95 mmol) in 3 mL of THF, 3-mercaptopropionic acid (III; 99 μL, 1.14 mmol), and DMPA (24 mg, 0.095 mmol) was irradiated for 6 h. The orange oily residue obtained was purified by column chromatography using hexane/ether (2/1) as eluent and increasing the polarity of the eluent up to hexane/ether (1/3), the trans isomer being eluted first and, later, the cis isomer. Yield: 0.196 g (65% with an E/Z ratio of 30/ 70). Thermal Initiation. The reaction mixture containing 1 (200 mg, 0.95 mmol) in 3 mL of toluene, 3-mercaptopropionic acid (III; 99 μL, 1.14 mmol), and AIBN (16 mg, 0.097 mmol) was heated to 80−85 °C for 16 h. An orange oily residue was obtained, which was purified in the same way as in the photochemical reaction. Yield: 0.117 g (39% with an E/Z ratio of 38/62). Data for 4Z are as follows. Anal. Calcd for cis-C15H16O2SFe: C, 56.96; H, 5.10; S, 10.12. Found: C, 57.14; H, 5.35; S, 10.33. 1H NMR (CDCl3, 300 MHz): δ 2.76 (t, 3J = 7.2 Hz, 2H, SCH2), 3.01 (m, 3J = 7.2 Hz, 2H, CH2COOH), 4.12 (s, 5H, C5H5), 4.22, 4.52 (pt, 4H, C5H4), 5.94 (A of AB system, JAB = 10.2 Hz, 1H, CHS), 6.21 (B of AB system, JAB = 10.2 Hz, 1H, FcCH). 13C NMR (CDCl3, 75 MHz): δ 29.9 (SCH2), 35.2 (CH2COOH), 68.7, 69.1 (C5H4), 69.2 (C5H5), 81.4 (ipso-Fc), 122.1 (CHS), 125.4 (FcCH), 176.3 (COOH). IR (neat, cm−1): ν(OH) 3437, ν(CO) 1710, ν(CC) 1101, ν(C−OH) 1043. MS (MALDI-TOF): m/z 316.1 [M+], 242.3 [M+ − CH2CH2COOH]. Data for 4E are as follows. Anal. Calcd for trans-C15H16O2SFe: C, 56.96; H, 5.10; S, 10.12. Found: C, 56.74; H, 4.93; S, 9.98. 1H NMR (CDCl3, 300 MHz): δ 2.74 (t, 3J = 7.2 Hz, 2H, SCH2), 2.96 (t, 3J = 7.2 Hz, 2H, CH2COOH), 4.12 (s, 5H, C5H5), 4.21, 4.30 (pt, 4H, C5H4), 6.19 (A of AB system, JAB = 15.3 Hz, 1H, CHS), 6.38 (B of AB system, JAB = 15.3 Hz, 1H, FcCH), 10.53 (br s, 1H, COOH). 13C NMR (CDCl3, 75 MHz): δ 27.8 (SCH2), 34.6 (CH2COOH), 66.4, 68.9 (C5H4), 69.3 (C5H5), 83.0 (ipso-Fc), 118.6 (CHS), 129.7 (FcCH), 178.0 (COOH). IR (neat, cm−1): ν(OH) 3080, ν(CO) 1706, ν(CC) 1103, ν(C−
measurements. No IR compensation was used. Square wave voltammetry was performed using frequencies of 10 Hz. For the electrodes modified with films of tetrametallic 7, the surface coverages Γ (mol/cm2) of the surface-confined ferrocenyl sites were calculated from the charge, Q, under the voltammetric current peaks, using the equation Γ = Q/nFA, where Q is the integrated charge of the redox peak, n is the number of electrons involved in the redox reaction (n = 1 for the Fc/Fc+ couple), F is the Faraday constant, and A is the geometric area of the electrode.52,55 General Procedure for Thiol−Yne Model Reactions of Ethynylferrocene. All UV light and thermally initiated model reactions of ethynylferrocene (1) with thiols were performed under an inert atmosphere (Ar) using Schlenk techniques. Representative details of the thiol−yne reactions are summarized in Table 1. Typical experimental procedures were as follows and correspond to the photochemically and thermally initiated hydrothiolation of 1 with 2mercaptoethanol (I). Synthesis of cis- and trans-FcCHCHS(CH2)2OH (2Z and 2E). Photoinitiation. In a 25 mL, two-necked flask equipped with a gas inlet, an Allihn condenser, and a magnetic stir bar, ethynylferrocene (1; 200 mg, 0.95 mmol) was dissolved in 3 mL of dry THF under argon at room temperature. To this stirred system were added 2mercaptoethanol (I; 80 μL, 1.14 mmol) and DMPA (24 mg, 0.095 mmol). The orange solution was irradiated with UV light (365 nm) for 8 h in time intervals of 2 h in order to avoid potential heating. After the volatiles were removed under vacuum, an orange oily residue was obtained and purified by column chromatography on silica gel. First, 1 was eluted with hexane, and subsequently, an orange band containing first the trans isomer and later the cis isomer was eluted with diethyl ether/hexane (3/1). The desired compounds 2Z and 2E were isolated as air-stable orange solids. Yield: 0.221 g (81% with an E/Z ratio of 38/62). Thermal Initiation. In a 25 mL, two-necked flask equipped with a gas inlet, an Allihn condenser, and a magnetic stir bar, metallocene 1 (200 mg, 0.95 mmol) was dissolved in 3 mL of dry toluene under argon at room temperature. Afterward, 2-mercaptoethanol (I; 80 μL, 1.14 mmol) and AIBN (16 mg, 0.097 mmol) were added. The reaction was allowed to proceed at 80−85 °C, and after 4 h, 1H NMR spectroscopy showed the complete disappearance of ethynylferrocene. The reaction was then stopped, the mixture was cooled to room temperature, and the volatiles were removed under reduced pressure, affording an orange-brown oily residue which was purified in the same way as in the photochemical reaction. Yield: 0.200 g (73% with an E/Z ratio of 36/64). Data for 2Z are as follows. Anal. Calcd for cisC14H16OSFe: C, 58.33; H, 5.60; S, 11.10. Found: C, 58.18; H, 5.48; S, 10.97. 1H NMR (CDCl3, 300 MHz): δ 2.01 (t, 3J = 6 Hz, 1H, OH), 2.94 (t, 3J = 6 Hz, 2H, SCH2), 3.83 (q, 3J = 6 Hz, 2H, CH2OH), 4.13 (s, 5H, C5H5), 4.24, 4.55 (pt, 4H, C5H4), 5.94 (A of AB system, JAB = 10.2 Hz, 1H, CHS), 6.21 (B of AB system, JAB = 10.2 Hz, 1H, FcCH). 13 C NMR (CDCl3, 75 MHz): δ 38.0 (SCH2), 61.4 (CH2OH), 68.7, 69.1 (C5H4), 69.2 (C5H5), 81.3 (ipso-Fc), 121.7 (CH−S), 125.5 (Fc CH). IR (neat, cm−1): ν(OH) 3409, ν(CC) 1634, 1105, ν(C−OH) 1043. MS (MALDI-TOF): m/z 288.1 [M +], 242.3 [M + − CH2CH2OH]. Data for 2E are as follows. Anal. Calcd for transC14H16OSFe: C, 58.33; H, 5.60; S, 11.10. Found: C, 58.55; H, 5.51; S, 11.22. 1H NMR (CDCl3, 300 MHz): δ 2.01 (t, 3J = 6 Hz, 1H, OH), 2.90 (t, 3J = 6 Hz, 2H, SCH2), 3.82 (q, 3J = 6 Hz, 2H, CH2OH), 4.12 (s, 5H, C5H5), 4.22, 4.30 (pt, 4H, C5H4), 6.18 (A of AB system, JAB = 15.3 Hz, 1H, CHS), 6.41 (B of AB system, JAB = 15.3 Hz, 1H, FcCH). 13 C NMR (CDCl3, 75 MHz): δ 36.5 (SCH2), 60.9 (CH2OH), 66.3, 68.8 (C5H4), 69.2 (C5H5), 82.9 (ipso-Fc), 118.5 (CHS), 129.6 (FcCH). IR (neat, cm−1): ν(OH) 3205, 3265, ν(CC) 1592, 1103, ν(C−OH) 1064, 1013. MS (MALDI-TOF): m/z 288.1 [M+], 242.3 [M+ − CH2CH2OH]. Synthesis of cis- and trans-FcCHCHSCH2CH(OH)CH2OH (3Z and 3E). Photoinitiation. Compounds 3Z and 3E were prepared starting from 1 (200 mg, 0.95 mmol) in 3 mL of dry THF, 3mercapto-1,2-propanediol (II; 98 μL, 1.15 mmol), and DMPA (24 mg, 0.095 mmol). The orange solution was irradiated for 2 h. The targeted vinyl sulfide was purified by column chromatography using ethyl 7315
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Organometallics
Article
OH) 1026. MS (MALDI-TOF): m/z 316.1 [M+], 242.3 [M+ − CH2CH2COOH]. Synthesis of FcC(CH2)S(1,4-C6H4)NH2 (5α). Photoinitiation. Compound 5α was prepared starting from ethynylferrocene (1; 250 mg, 1.19 mmol), 4-aminothiophenol (IV; 179 mg, 1.43 mmol), and DMPA (30 mg, 0.12 mmol). After the orange solution was irradiated for 20 h, an orange oily residue was obtained and purified by column chromatography using hexane/ether (1/1) as eluent. Solvent removal afforded the reaction product 5α as an air-stable, orange solid. Yield: 0.100 g (25%). Thermal Initiation. This compound was obtained from 1 (200 mg, 0.95 mmol) in 3 mL of toluene, 4-aminothiophenol (IV; 143 mg, 1.15 mmol), and AIBN (16 mg, 0.097 mmol). After 6 h at 80−85 °C, the reaction mixture was filtered and the solvent was removed under vacuum, yielding a brown-orange oil which was purified in the same way as in the photochemical reaction. Yield: 0.226 g (71%). Anal. Calcd for C18H17NSFe: C, 64.47; H, 5.11; S, 9.54. Found: C, 64.26; H, 5.37; S, 9.73. 1H NMR (CDCl3, 300 MHz): δ 3.73 (br s, 2H, NH2), 4.16 (s, 5H, C5H5), 4.25, 4.58 (pt, 4H, C5H4), 6.15 (s, 2H, FcC CH2), 6.65, 7.28 (AA′BB′ system, J = 8.4 Hz, 4H, SC6H4N). 13C NMR (CDCl3, 75 MHz): δ 68.7, 69.1 (C5H4), 69.2 (C5H5), 81.4 (ipso-Fc), 115.7, 124.1, 132.8, 146.1 (C6H4), 123.7, 124.9 (FcCCH). IR (KBr, cm−1): ν(NH) 3474, 3377, ν(CC) 1615, 1594, 1103. MS (MALDITOF): m/z 335.1 [M+]. Synthesis of cis,cis-, trans,trans-, and cis,trans-FcCHCHS(1,3-C6H4)SCHCHFc (6). Photoinitiation. Compound 6 was prepared from 1 (200 mg, 0.95 mmol) in 3 mL of THF, benzene1,3-dithiol (V; 50 μL, 0.43 mmol), and DMPA (24 mg, 0.095 mmol). The orange solution was irradiated for 4 h, to afford an orange oily residue that was purified by column chromatography using hexane/ ethyl acetate (15/1) as eluent. Yield: 0.213 g (88% with an E/Z ratio of 38/62 in the mixture of isomers). Thermal Initiation. The reaction of 1 (200 mg, 0.95 mmol) in 3 mL of toluene, dithiol V (50 μL, 0.43 mmol), and AIBN (16 mg, 0.097 mmol) was allowed to proceed at 80−85 °C for 5 h. The orange oily residue obtained was purified in the same way as in the photochemical reaction. Yield: 0.198 g (82% with an E/Z ratio of 40/60 in the mixture of isomers). Anal. Calcd for C30H26S2Fe2: C, 64.05; H, 4.66; S, 11.38. Found: C, 64.28; H, 4.86; S, 11.09. 1H NMR (CDCl3, 300 MHz): δ 4.07, 4.09 (s, 30H, C5H5), 4.20, 4.28, 4.31, 4.52, 4.54 (m, 24H, C5H4), 6.18 (A of Z-AB system, 3H, JAB = 10.2 Hz, Z(CHS)), 6.29−6.35 (B of Z-AB system, 3H, Z(FcCH) and A of E-AB system, 3H, E(CHS)), 6.59, 6.61 (B of E-AB system, 3H, JAB = 15.3 Hz, E(FcCH)), 7.10−7.45 (m, 12H, C6H4). 13C NMR (CDCl3, 75 MHz): δ 66.9, 67.0, 69.1, 69.2, 69.3 (C5H4), 69.4, 69.5 (C5H5), 80.7 (cis(ipsoFc)), 82.0 (trans(ipso-Fc)), 116.4, 116.6, 119.9, 128.2, 134.7, 134.9 (CHCHS), 126.1, 126.3, 126.7, 127.1, 128.1, 129.1, 129.6 (C6H4), 137.9, 138.0, 138.2 (ipso-C6H4). IR (neat, cm−1): ν(C−H) 3088, ν(CC) 1631, 1594, 1105. MS (MALDI-TOF): m/z 562.1 [M+], 242.3 [M+ − C6H4SCHCHFc]. [FcCHCHS(CH2)2COOCH2]4C (7). Photoinitiation. This tetrametallic compound was prepared starting from 1 (200 mg, 0.95 mmol) in 3 mL of dry THF, pentaerythritol tetrakis(3-mercaptopropionate) (VI; 87 μL, 0.23 mmol), and DMPA (24 mg, 0.095 mmol). The reaction mixture was irradiated with UV light for 3 h at room temperature to afford an orange oily residue which was purified by washing it several times with n-hexane. Compound 7 was isolated as an air-stable, orange oil. Yield: 0.253 g (83%, with an E/Z ratio of 50/50 in the mixture of isomers). Thermal Initiation. To prepare 7, a mixture containing 1 (200 mg, 0.95 mmol), tetrathiol VI (87 μL, 0.23 mmol), and AIBN (16 mg, 0.097 mmol) was allowed to react at 80−85 °C for 4 h. After cannula filtration to remove an insoluble precipitate, all volatiles were removed under vacuum. An orange oily residue was obtained and purified by washing it with n-hexane. Yield: 0.275 g (90% with an E/Z ratio of 61/ 39 in the mixture of isomers). Anal. Calcd for C65H68O8S4Fe4: C, 58.73; H, 5.16; S, 9.63. Found: C, 58.41; H, 5.34; S, 9.38. 1H NMR (CDCl3, 300 MHz): δ 2.69 (m, 8H, SCH2), 2.95 (m, 8H, CH2COO), 4.11 (s, 20H, C5H5), 4.15 (s, 8H, OCH2C), 4.21, 4.30, 4.50 (m, 16H, C5H4), 5.92 (A of Z-AB system, 2H, JAB = 10.2 Hz, Z(CHS)), 6.14−
6.21 (B of Z-AB system, 2H, Z(FcCH) and A of E-AB system, 2H, E(CHS)), 6.35 (B of the E-AB system, 2H, JAB = 15.3 Hz, E(FcCH). 13 C NMR (CDCl3, 75 MHz): δ 28.1, 29.8 (SCH2) 34.5, 35.3 (CH2COO), 42.3 (C(CH2)4), 62.3 (OCH2C), 66.4, 68.8, 68.9, 69.2 (C5H4), 69.3 (C5H5), 81.4 (Z(ipso-Fc), 83.0 (E(ipso-Fc)), 118.6, (E(CH-S)), 122.0 (Z(CH−S)), 125.3 (Z(FcCH)) 129.6 (E(FcCH)), 171.1, 171.2 (CO). IR (neat, cm−1): ν(CO) 1740, ν(CC) 1599, 1105,ν(C−O) 1235. MS (MALDI-TOF): m/z 1328.2 [M+].
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ASSOCIATED CONTENT
S Supporting Information *
Supplementary figures referred to in the text, structural characterization data for 2−7 (IR, NMR, and mass spectra), and additional CVs and SWVs for 1−7. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for A.M.G.-V.:
[email protected]. *E-mail for I.C.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to the Spanish Ministerio de Economı ́a y Competitividad (MINECO) (project CTQ2012-30728) for the generous support of this work.
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REFERENCES
(1) Benkeser, R. A.; Fitzgerald, W. P. J. Org. Chem. 1961, 26, 4179− 4180. (2) Rosenblum, M.; Brawn, N.; Papenmeier, J.; Applebaum, M. J. Organomet. Chem. 1966, 6, 173−180. (3) (a) Simionescu, C.; Lixandru, T.; Mazilu, I.; Tǎtaru, L. Makromol. Chem. 1971, 147 (1), 69−78. (b) Toshihide, N.; Toyoki, K.; Chuji, A. Makromol. Chem. 1972, 157 (1), 73−85. (4) Pittman, C. U.; Sasaki, Y.; Grube, P. L. J. Macromol. Sci. Chem., Part A 1974, 8, 923−934. (5) Buchmeiser, M.; Schrock, R. R. Macromolecules 1995, 28, 6642− 6649. (6) Tamura, K.; Akutagawa, N.; Satoh, M.; Wada, J.; Masuda, T. Macromol. Rapid Commun. 2008, 29, 1944−1948. (7) (a) Liang, L.; Ruiz, J.; Astruc, D. J. Inorg. Organomet. Polym. 2010, 20, 503−510. (b) Ornelas, C.; Ruiz, J.; Cloutet, E.; Alves, S.; Astruc, D. Angew. Chem., Int. Ed. 2007, 46, 872−877. (8) Maiti, S. K.; Jardim, M. G.; Rodrigues, J.; Rissanen, K.; Campo, J.; Wenseleers, W. Organometallics 2013, 32, 406−414. (9) Nievas, A.; Medel, M.; Hernández, E.; Delgado, E.; Martı ́n, A.; Casado, C. M.; Alonso, B. Organometallics 2010, 29, 4291−4297. (10) Boisselier, E.; Shun, A.; Ruiz, J.; Cloutet, E.; Belin, C.; Astruc, D. New J. Chem. 2009, 33, 246−253. (11) See for example: (a) Strehler, F.; Hildebrandt, A.; Korb, M.; Rü ffer, T.; Lang, H. Organometallics 2014, 33, 4279−4289. (b) Packheiser, R.; Ecorchard, P.; Ruffer, T.; Lohan, M.; Brauer, B.; Justaud, F.; Lapinte, C.; Lang, H. Organometallics 2008, 27, 3444− 3457. (c) Dı ́ez, A.; Fernández, J.; Lalinde, E.; Moreno, M. T.; Sánchez, S. Inorg. Chem. 2010, 49, 11606−11618. (d) Badèche, S.; Daran, J.-C.; Ruiz, J.; Astruc, D. Inorg. Chem. 2008, 47, 4903−4908. (12) Top, S.; Thibaudeau, C.; Vessières, A.; Brulé, E.; Le Bideau, F.; Joerger, J.-M.; Plamont, M.; Samreth, S.; Edgar, A.; Marrot, J.; Herson, P.; Jaouen, G. Organometallics 2009, 28, 1414−1424. (13) Uppal, B. S.; Zahid, A.; Elliott, P. I. P. Eur. J. Inorg. Chem. 2013, 2571−2579. (14) (a) Romero, T.; Orenes, R. A.; Tárraga, A.; Molina, P. Organometallics 2013, 32, 5740−5753. (b) Otón, F.; González, M. C.; 7316
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Organometallics
Article
(41) (a) Konkolewicz, D.; Gray-Weale, A.; Perrier, S. J. Am. Chem. Soc. 2009, 131, 18075−18077. (b) Barbey, R.; Perrier, S. Macromolecules 2014, 47, 6697−6705. (42) (a) Minozzi, M.; Monesi, A.; Nanni, D.; Spagnolo, P.; Marchetti, P.; Massi, A. J. Org. Chem. 2011, 76, 450−459. (b) Conte, M. L.; Pacifico, S.; Chambery, A.; Marra, A.; Dondoni, A. J. Org. Chem. 2010, 75, 4644−4647. (43) Huynh, V. T.; Chen, G.; de Souza, P.; Stenzel, M. H. Biomacromolecules 2011, 12, 1738−1751. (44) Martı ́nez-Montero, I.; Bruña, S.; González-Vadillo, A. M.; Cuadrado, I. Macromolecules 2014, 47, 1301−1315. (45) (a) Chabanne, L.; Pfirrmann, S.; Lunn, D. J.; Manners, I. Polym. Chem. 2013, 4, 2353−2360. (b) Lunn, D. J.; Boott, C. E.; Bass, K. E.; Shuttleworth, T. A.; McCreanor, N. G.; Papadouli, S.; Manners, I. Macromol. Chem. Phys. 2013, 214, 2813−2820. (46) Cuadrado, I. In Silicon-Containing Dendritic Polymers; Dvornic, P., Owen, M. J., Eds.; Springer: Berlin, 2009; pp 141−196. (47) (a) Bruña, S.; Nieto, D.; González-Vadillo, A. M.; Perles, J.; Cuadrado, I. Organometallics 2012, 31, 3248−3258. (b) Bruña, S.; González-Vadillo, A. M.; Nieto, D.; Pastor, C. J.; Cuadrado, I. Macromolecules 2012, 45, 781−793. (c) Bruña, S.; González-Vadillo, A. M.; Nieto, D.; Pastor, C. J.; Cuadrado, I. Organometallics 2010, 29, 2796−2807. (d) Bruña, S.; Perles, J.; Nieto, D.; González-Vadillo, A. M.; Cuadrado, I. J. Organomet. Chem. 2014, 751, 769−780. (e) Nieto, D.; Bruña, S.; Montero-Campillo, M. M.; González-Vadillo, A. M.; Méndez, J.; Mo, O.; Cuadrado, I. Chem. Commun. 2013, 49, 9785− 9787. (48) (a) Zamora, M.; Alonso, B.; Pastor, C.; Cuadrado, I. Organometallics 2007, 26, 5153−5164. (b) Alonso, B.; Cuadrado, I.; Morán, M.; Losada, J. J. Chem. Soc., Chem. Commun. 1994, 2575− 2576. (c) Cuadrado, I.; Casado, C. M.; Alonso, B.; Morán, M.; Losada, J.; Belsky, V. J. Am. Chem. Soc. 1997, 119, 7613−7614. (49) (a) Cuadrado, I.; Morán, M.; Casado, C. M.; Alonso, B.; Lobete, F.; Garcı ́a, B.; Ibisate, M.; Losada, J. Organometallics 1996, 15, 5278− 5280. (b) Casado, C. M.; González, B.; Cuadrado, I.; Alonso, B.; Morán, M.; Losada, J. Angew. Chem., Int. Ed. 2000, 39, 2135−2138. (50) For comparison, phenylacetylene (PhCCH) is usually able to selectively add only one thiol, forming a vinyl sulfide.34,38,40a (51) (a) Carbon-13 NMR Spectroscopy; Stothers, J., Ed.; Elsevier: Amsterdam, 2012; Vol. 24, pp 81−82. (b) Kleinpeter, E.; Seidl, P. R. J. Phys. Org. Chem. 2004, 17, 680−685. (52) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (53) Inorganic Electrochemistry. Theory, Practice and Application; Zanello, P., Nervi, C., Fabrizi de Biani, F., Eds.; Royal Society of Chemistry: Cambridge, U.K., 2013. (54) See for example: Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103−1170. (55) Murray, R. W. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; Wiley: New York, 1992; Techniques of Chemistry Series 22, pp 1−45. (56) Considering the ferrocene molecules as spheres with a diameter of 6.6 Å, the theoretical maximum coverage for an idealized ferrocene monolayer can be estimated as Γ = 4.8 × 10−10 mol/cm2. See: Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301−4306. (57) Polin, J.; Schottenberger, H. Org. Synth. 1996, 73, 262−266.
Espinosa, A.; Tárraga, A.; Molina, P. Organometallics 2012, 31, 2085− 2096. (15) Cao, Q.-Y.; Pradhan, T.; Kim, S.; Kim, J. S. Org. Lett. 2011, 13, 4386−4389. (16) Badèche, S.; Daran, J. C.; Ruiz, J.; Astruc, D. Inorg. Chem. 2008, 47, 4903−4908. (17) Ornelas, C.; Diallo, A. K.; Ruiz, J.; Astruc, D. Adv. Synth. Catal. 2009, 351, 2147−2154. (18) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004−2021. (19) Coates, M.; Griveau, S.; Bedioui, F.; Nyokong, T. Electroanalysis 2012, 24, 1833−1838. (20) Landis, E. C.; Hamers, R. J. Chem. Mater. 2009, 21, 724−730. (21) Sheridan, M. V.; Lam, K.; Geiger, W. E. J. Am. Chem. Soc. 2013, 135, 2939−2942. (22) Ruther, R. E.; Cui, Q.; Hamers, R. J. J. Am. Chem. Soc. 2013, 135, 5751−5761. (23) Sheridan, M. V.; Lam, K.; Geiger, W. E. Angew. Chem., Int. Ed. 2013, 52, 12897−12900. (24) Pellow, M. A.; Stack, T. D. P.; Chidsey, C. E. D. Langmuir 2013, 29, 5383−5387. (25) To the best of our knowledge, only one Rh-catalyzed thiol−yne reaction has been reported to date involving a ferrocenyl-containing monomer, namely (E,E)-1,1′-bis[2-(4-ethynylphenyl)vinyl]ferrocene. See: Liu, J.; Lam, J. W. Y.; Jim, C. K. W.; Ng, J. C. Y.; Shi, J.; Su, H.; Yeung, K. F.; Hong, Y.; Faisal, M.; Yu, Y.; Wong, K. S.; Tang, B. Z. Macromolecules 2011, 44, 68−79. (26) Thiol-X Chemistries in Polymer and Materials Science; Lowe, A., Bowman, C., Eds.; RSC: Cambridge, U.K., 2013. (27) For comprehensive reviews on thiol−ene chemistry, see: (a) Hoyle, C. E.; Bowman, C. N. Angew. Chem., Int. Ed. 2010, 49, 1540−1573. (b) Lowe, A. B. Polym. Chem. 2010, 1, 17−36. (c) Hoyle, C. E.; Lowe, A. B.; Bowman, C. N. Chem. Soc. Rev. 2010, 39, 1355− 1387. (d) Kade, M. J.; Burke, D. J.; Hawker, C. J. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 743−750. (28) For excellent recent reviews on thiol−yne coupling reactions see: (a) Massi, A.; Nanni, D. Org. Biomol. Chem. 2012, 10, 3791−3807. (b) Lowe, A. B.; Hoyle, C. E.; Bowman, C. N. J. Mater. Chem. 2010, 20, 4745−4750. (c) Lowe, A. B. Polymer 2014, 55, 5517−5549. (29) Ruhemann, S. J. Chem. Soc. Trans. 1905, 87, 461−468. (30) (a) Kohler, E. P.; Potter, H. J. Am. Chem. Soc. 1935, 57, 1316− 1321. (b) Finzi, C.; Venturini, G.; Sartini, L. Gazz. Chim. Ital. 1930, 60, 798−811. (31) Fairbanks, B. D.; Sims, E. A.; Anseth, K. S.; Bowman, C. N. Macromolecules 2010, 43, 4113−4119. (32) Bader, H.; Cross, L. C.; Heilbron, I.; Jones, E. R. H. J. Chem. Soc. 1949, 619−623. (33) Kiskan, B.; Weber, J. ACS Macro Lett. 2012, 1, 37−40. (34) Pötzsch, R.; Komber, H.; Stahl, B. C.; Hawker, C. J.; Voit, B. I. Macromol. Rapid Commun. 2013, 34, 1772−1778. (35) For references of metal-catalyzed thiol−yne reactions, see for example: (a) Weiss, C. J.; Marks, T. J. J. Am. Chem. Soc. 2010, 132, 10533−10546. (b) Weiss, C. J.; Wobser, S. D.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 2062−2063. (c) Weiss, C. J.; Wobser, S. D.; Marks, T. J. Organometallics 2010, 29, 6308−6320. (d) Sarma, R.; Rajesh, N.; Prajapati, D. Chem. Commun. 2012, 48, 4014−4016. (36) Yao, B.; Mei, J.; Li, J.; Wang, J.; Wu, H.; Sun, J. Z.; Qin, A.; Zhong Tang, B. Macromolecules 2014, 47, 1325−1333. (37) Fairbanks, B. D.; Scott, T. F.; Kloxin, C. J.; Anseth, K. S.; Bowman, C. N. Macromolecules 2009, 42, 211−217. (38) Sprafke, J. K.; Spruell, J. M.; Mattson, K. M.; Montarnal, D.; McGrath, A. J.; Potzsch, R.; Miyajima, D.; Hu, J.; Latimer, A. A.; Voit, B. I.; Aida, T.; Hawker, C. J. J. Polym. Sci. Part A: Polym. Chem. 2014, DOI: 10.1002/pola.27345. (39) Demay-Drouhard, P.; Nehlig, E.; Hardouin, J.; Motte, L.; Guenin, E. Chem. Eur. J. 2013, 19, 8388−8392. (40) (a) Chen, G.; Kumar, J.; Gregory, A.; Stenzel, M. H. Chem. Commun. 2009, 6291−6291. (b) Chan, J. W.; Hoyle, C. E.; Lowe, A. B. J. Am. Chem. Soc. 2009, 131, 5751−5753. 7317
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