Catalyst-on-a-Tape - ACS Publications - American Chemical Society

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10875-10888

Recycling and Delivery of Homogeneous Fluorous Rhodium Catalysts Using Poly(tetrafluoroethylene): “Catalyst-on-a-Tape” Long V. Dinh,§ Markus Jurisch,†,§ Tobias Fiedler,† and John A. Gladysz*,†,§,‡ †

Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States Institut für Organische Chemie and Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, Henkestraße 42, 91054 Erlangen, Germany

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S Supporting Information *

ABSTRACT: The red−orange fluorous rhodium(I) complexes ClRh(P((CH2)mRfn)3)3 (m/n = 2/6 (1a), 2/8, 3/6, 3/ 10; Rfn = (CF2)n−1CF3) are essentially insoluble in organic solvents at 20 °C but have measurable solubilities in dibutyl ether at 55−65 °C. Under these conditions, they are effective catalyst precursors for the hydrosilylation of cyclohexanone by PhMe2SiH. Upon cooling, the catalyst rest states precipitate, giving colorless solutions of C6H11OSiPhMe2. When this sequence is conducted in the presence of poly(tetrafluoroethylene) (PTFE; Teflon) tape, the catalysts precipitate onto the tape but desorb when used in subsequent cycles. The catalyst precursors can also be precoated onto the tape, allowing quantities to be delivered by length instead of mass. Rate measurements (1a) show an induction period in the first cycle, excellent retention of activity in the second and third cycles, and significant activity loss in the fourth. Rhodium leaching is 0.57% and 5.3% for the first two cycles (atomic absorption spectroscopy inductively coupled plasma analysis); (CF2)5CF3 leaching is 11.4% over the first three cycles (19F NMR). Reactions with added mercury show that metallic rhodium is not responsible for catalysis. Identical protocols are applied to 2-octanone, acetophenone, and benzophenone, albeit with some activity loss in the third cycle. Other forms of PTFE can be similarly employed (e.g., Gore-Tex membrane). However, fluorous/organic liquid/ liquid biphase conditions can give better retention of catalyst activity. Nonetheless, the diverse morphologies of PTFE that are commercially available suggest avenues for further optimization. KEYWORDS: Rhodium, Hydrosilylation, Catalysis, Fluorous, Phosphine, Poly(tetrafluoroethylene), Teflon, Gore-Tex, Adsorption, Recycling

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INTRODUCTION

In particular, the person to whom this article is dedicated, István Horvath, reported a ground breaking approach to catalyst recycling in 1994.8−10 This involved appending fluorous phase labels of the formula (CH2)mRfnoften termed “ponytails”to established homogeneous catalysts. As initially reduced to practice, the technique utilized fluorous/organic liquid/liquid biphase conditions as sketched in Figure 1. Fluorous solvents such as perfluoroalkanes and perfluoroethers are usually immiscible with organic solvents at room temperature.10 However, they often become miscible upon warming. This allows one phase, homogeneous reaction conditions. When biphasic conditions are reestablished upon cooling, appropriately designed catalysts partition essentially exclusively into the fluorous phase, and lipophilic products similarly partition into the organic phase. Unfortunately, the relatively high costs of fluorous solvents, as well as more recently recognized environmental factors,11

Functionalized and/or surface modified polymers have countless technological applications and are common items of commerce. Virtually every naturally occurring and synthetic polymer has been brought into play. However, methodologies that can be applied to fluoropolymers based upon tetrafluoroethylene (PTFE), such as Teflon, are limited.1−3 This is in part due to the very low chemical reactivity of (CF2)x segments and the dual lipophobic/hydrophobic character of the highly nonpolar surface. In this context, a study demonstrating the feasibility of PTFE surface modification by a surfactant polymer with pendant perfluorodecyl or Rf10 groups (Rfn = (CF2)n−1CF3) captured our attention.4 Given the absence of any covalent bonding, and the very modest (although measurable) attractive enthalpic interactions between (CF2)x segments,5 this was regarded as a surprising result. It furthermore suggested a new approach to polymer based strategies for the recovery of homogeneous molecular catalystsan area of intense interest and sustained effort for decades.6,7 © 2017 American Chemical Society

Received: August 15, 2017 Revised: September 12, 2017 Published: September 25, 2017 10875

DOI: 10.1021/acssuschemeng.7b02770 ACS Sustainable Chem. Eng. 2017, 5, 10875−10888

Research Article

ACS Sustainable Chemistry & Engineering

that clarify certain mechanistic details. Additional applications of fluoropolymers in catalysis developed after our initial communication are treated in the discussion section.

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RESULTS Catalyst Portfolio. As sketched in Figure 2, the catalyst precursors ClRh(P((CH2)mRfn)3)3 were prepared by stirring toluene solutions of the dimeric rhodium complex [Rh(Cl)(COD)]2 and CF3C6F11 (perfluoro(methylcyclohexane)) solutions of the fluorous phosphines P((CH2)mRfn)3 (3 equiv/ Rh). The fluorous phases were separated and taken to dryness, giving the rhodium complexes in ca. 90% yields. The syntheses of 1a,b (m/n = 2/6 and 2/8) were reported earlier,32 and those of 1c,d (m/n = 3/6 and 3/10; 63.9% and 68.0% weight% fluorine) are described in the Experimental Section. For baseline studies, the somewhat more expensive solvent CF3C6F11 is frequently used because (unlike many commercial fluorous solvents) it is not a mixture of isomers. However, for future preparative work, cheaper solvents such as perfluorohexanes (FC-72) should suffice. In practice, the purities of the rhodium complexes are functions of the reactant stoichiometries. For example, if the reactions are deficient in phosphine,31P{1H} NMR spectra show some of what are presumed to be the chloride bridged dimers [ClRh(P((CH2)mRfn)3)2]2 or other partially converted species.36 These can in turn be “titrated out” by adding small amounts of CF3C6F11 solutions of the phosphine. The samples of 1a,b employed were pure by 31P NMR or showed at most 1% of a contaminant. The samples of 1c,d contained up to 10% impurities, as documented in the Experimental Section. The synthesis of 1d required the phosphine P(CH2CH2CH2Rf10)3, which has only been described in a communication. 37 Although the lower homologue P(CH2CH2CH2Rf8)3 can be synthesized by a direct and atom economical synthesis from PH3 and H2CCHCH2Rf8,38,39 many researchers prefer to avoid the former species, which is

Figure 1. Original implementation of fluorous/organic liquid/liquid biphase catalysis. Rf = a fluorous phase label.

have limited commercial applications. However, in subsequent refinements, it was found that the fluorous solvent could be eliminated by exploiting the highly temperature-dependent solubilities of fluorous catalysts in common organic solvents.12−21 Appropriately designed fluorous molecules are soluble only at elevated temperatures, and essentially insoluble at low temperatures. Such “thermomorphic” character allows homogeneous catalysis at the high temperature limit, and catalyst recovery by simple liquid/solid phase separations at the low temperature limit. An obvious further refinement, especially in light of the PTFE/fluorous surfactant interaction noted above, would be to conduct such reactions in the presence of an insoluble fluorous support. In particular, efficient catalysts capable of high turnover numbers (TONs) may yield only small masses of precipitates, which can be challenging to manipulate in the absence of some type of “ballast”. Initial reports featured bulk PTFE shavings12,13 and fluorous silica gel,22−29 upon which the catalysts presumeably adsorbed. We sought to expand the range of fluorous supports that could be applied in such protocols and report here the development of surprisingly effective procedures involving readily available commercial PTFE in the form of Teflon (thread seal) tape and Gore or Gore-Tex products. These provide not only convenient means of catalyst recovery but also catalyst delivery. Ketone hydrosilylation was selected as the test reaction. In previous studies,30,31 the fluorous rhodium(I) complexes ClRh(P(CH 2 CH 2 R f6 ) 3 ) 3 and ClRh(P(CH 2 CH 2 R f 8 ) 3 ) 3 (1a,b)32 had been shown to be effective catalyst precursors under fluorous/organic liquid/liquid biphase conditions (Figure 1). These red−orange solids have highly biased fluorous/organic liquid/liquid partition coefficients (99.86:0.14 and 99.88:0.12, CF3C6F11/toluene) commensurate with the high weight percent of fluorine (66.3% and 68.3%). Although they have little or no solubility in organic solvents at room temperature, the solubilities increase markedly with temperature.32 At the same time, several parameters render these catalyst systems challenging for recovery via precipitation alone. For example, a variety of rest states are possible (e.g., various Rh(H)(SiR3) or Rh(OR′)(SiR3) species), each with unique solubility properties.33 Also, the first cycle exhibits an induction period,30,31 indicating some fundamental alteration of the catalyst precursor necessary for activity. In the narrative below, we present a full account of an indepth investigation, portions of which have been communicated.34 In particular, this work (a) expands the portfolio of catalysts to species with longer methylene spacers such as ClRh(P(CH2CH2CH2Rf6)3)3 and ClRh(P(CH2CH2CH2Rf10)3)3 (1c,d), (b) establishes the utility of additional PTFE materials,35 and (c) describes a number of experiments

Figure 2. Syntheses of fluorous rhodium catalysts 1a−d. 10876


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ACS Sustainable Chemistry & Engineering toxic and often inflames in air. Accordingly, for this study P(CH2CH2CH2Rf10)3 and P(CH2CH2CH2Rf6)3 were synthesized by the longer sequences depicted in Scheme 1. These involved Arbuzov reactions of the fluorous alkyl iodides ICH2CH2CH2Rfn and P(OEt)3, followed by LiAlH4 reduction to the primary phosphines PH2CH2CH2CH2Rfn, as described in the Experimental Section. Analogous reactions have been carried out starting with the “two methylene spacer” iodides ICH2CH2Rfn.40 The primary phosphines were in turn combined with AIBN (radical initiator) and excesses of the corresponding alkenes H2CCHCH2Rfn in the absence of solvent at 90 °C. Workups gave the target tertiary phosphines P(CH2CH2CH2Rf10)3 and P(CH2CH2CH2Rf6)3 in good overall yields. Baseline Experiments. Catalysis in Organic Solvents without PTFE Supports. Diethyl ether had been used as the organic solvent for hydrosilylations with 1a,b under fluorous/ organic liquid/liquid biphase conditions (Figure 1).30,31 However, it was replaced by dibutyl ether, which has an extended liquid range (bp 142 °C), for catalysis in the absence of fluorous solvents. This facilitates exploitation of the temperature dependent solubilities of fluorous compounds. As shown in Figure 3, a solution of cyclohexanone (2), PhMe2SiH (1.2 equiv), 1a (1.0 mol %), and an internal standard was warmed to 65 °C, achieving homogeneous conditions. After 8 h, the sample was cooled to room temperature. A brown−red catalyst residue slowly precipitated over several hours, and no color remained in the productcontaining supernatant. To speed this process, samples were transferred to a freezer (−30 °C); the supernatants were then removed by syringe. The residuewhich corresponds to the catalyst rest state and not 1awas washed twice with cold dibutyl ether. GC analysis showed a 98% yield of the silyl ether C6H11OSiPhMe2 (3). The residue was charged with fresh reactants and the cycle repeated three times, each giving a 98% yield of 3. When the catalyst 1b was employed in a similar sequence, identical yields of 3 were obtained. However, 1b did not completely dissolve in dibutyl ether at 55−65 °C at the concentrations and loadings (1.00−0.15 mol %) utilized in this study. The absolute solubilities of fluorous compounds always decrease as the Rfn segments are lengthened. Furthermore, additional catalyst or rest state precipitated during the reaction, presumably due to a decrease in polarity of the medium as the product ether forms. This phenomenon has also been reported for other types of ketone hydrosilylation catalysts.41 Interestingly, the reactions in Figure 3 could also be carried out in the absence of solvent, as the reactants and products are all liquids. A sequence conducted with 0.50 mol % of 1a at 70

Figure 3. Hydrosilylation of cyclohexanone (2) catalyzed by 1a,b; recycling of thermomorphic catalysts by liquid/solid phase separation.

°C gave 98%, 99%, and 95% yields of 3 over three cycles. However, we emphasize that the constancy of yield as a function of cycle in the preceding sets of experiments, while an auspicious sign, is not sufficient evidence for a high degree of catalyst recovery.33 Catalysis in Organic Solvents with 1a and a PTFE Tape Support. It was thought that fluorous supports should facilitate catalyst recovery in Figure 3, especially at lower loadings. As shown in Figure 4, a similar sequence was conducted with 0.15 mol % of 1a in the presence of PTFE tape (five pieces, length/width/thickness 30 mm × 12 mm × 0.0075 mm). With the decreased loading vs Figure 3, homogeneous conditions could be achieved at 55 °C. Photographs of each stage of the sequence are included in Figure 4. The white tape was first wetted by dibutyl ether, becoming translucent. During the reaction it became lightly colored, and then orange−red when the sample was cooled. It is noteworthy that the catalyst rest state phase separates onto the tape, as opposed to giving rise to another solid phase. The Teflon stir bar remained white, an aspect analyzed in the discussion section. GC analysis after 2.5 h indicated a 97% yield of the ether 3, as determined versus an internal standard. No byproducts were detected. An identical reaction was conducted, and the quantities of 2 and 3 assayed at 15 min intervals over four cycles. Data are summarized in Figure 5. Since all reactant and product quantities in this study are determined relative to a standard, the combined amounts do not always equal 100%, although they are usually close. As in earlier work using liquid/ liquid biphase conditions (Figure 1),30,31 the first cycle exhibited an induction period. Retention of activity was

Scheme 1. Syntheses of Fluorous Phosphines

Figure 4. Recycling of the thermomorphic catalyst 1a using PTFE (Teflon) tape. 10877


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Figure 6. Hydrosilylation of 2 under the conditions of Figure 4, but with precoated PTFE tape (panels A−E).

sequence analogous to the second and third stages of Figure 4, as represented by panels C (prior to heating), D (55 °C), and E (after cooling) of Figure 6. The yield and TON data are slightly lower than those in Figure 5, possibly due to incomplete coating of the catalyst charge onto the tape. Substrate Scope. Hydrosilylations of benzophenone (4), acetophenone (6), and 2-octanone (8) to the corresponding silyl ethers (5, 7, 9) were conducted under conditions identical to those for cyclohexanone (2) in Figures 4 and 5. As summarized in Figures 7−9, similar results were obtained, albeit with up to 20% activity loss in the third cycle. The aliphatic ketone 8 was consumed somewhat more slowly. With the exception of 7, yields dropped sharply to 20−24% in the fourth cycle. Although the basis for this somewhat diminished performance is not presently understood, it may be that substrates with phenyl or methyl ketone groups somehow promote catalyst deactivation. The supernatants obtained after the third cycles sometimes exhibited slight hints of color, whereas those obtained from 2 did not. Control Experiments: Catalysis by Metallic Rhodium? When catalysts are recycled as solid residues, it is important to exclude impurities that may “piggyback” in the residue as active species. The most probable alternatives to 1a−d would be rhodium metal particles. This was probed in two ways. First, the PTFE tape was removed after the first cycle of the reaction

Figure 5. Reaction profile for the hydrosilylation of 2 to 3 under the conditions of Figure 4.

excellent in the second and third cycles, but there was notable loss in the fourth. In view of the similar rates of the first three cycles, this can be attributed to catalyst deactivation, as opposed to leaching or some intrinsic problem with the recycling protocol. Leaching was probed in two ways. First, the dibutyl ether supernatants from the first three cycles in Figure 5 were combined, a fluorine-containing standard added, and a 19F NMR spectrum recorded. The chemical shifts of the (CF2)5CF3 signals are independent of the remote substituent, and integration indicated a “total perfluoroalkyl leaching” corresponding to 11.4% of the available groups in 1a. Since the free phosphine P(CH2CH2Rf6)3 shows a very slight affinity for organic phases (CF 3 C 6 F 11 /toluene partition coefficient 98.8:1.2),38 some perfluoroalkyl loss (which could also involve the oxide or other degradation product) is not surprising. Also, since the active and resting states of the catalyst likely involve less than three phosphine ligands (as well as new educt-derived ligands), their fluorophilicites should be diminished. Second, the supernatant from the first cycle was analyzed for rhodium by atomic absorption spectroscopy inductively coupled plasma analysis (AAS-ICP). An average of three determinations indicated leaching corresponding to 0.57% of the original charge. The second cycle gave an increased value of 5.3%. Coating of Catalysts onto PTFE Tape. An obvious refinement of Figure 4 would entail precoating the catalyst onto the PTFE tape. Furthermore, if the coating were uniform, this would allow low loadings to be delivered by length as opposed to mass measurements. In the case of highly active catalysts where only 1−2 mg are needed, direct weighing can be subject to considerable error. In contrast, a low catalyst loading can be spread over a considerable length of tape, which is easily cut to the nearest millimeter. Accordingly, two 50 mm × 12 mm × 0.0075 mm strips of tape were added to a solution of 1a (0.0015 g, 0.0045 mmol) in CF3C6F11 (1.0 mL). The solvent was removed under an inert gas stream to give a yellowish catalyst coated tape, shown in panels A and B of Figure 6. This was applied in a three cycle

Figure 7. Reaction profile for the hydrosilylation of benzophenone (4) under the conditions of Figure 4. 10878


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Figure 10. Reaction profile for the hydrosilylation of 2 under the conditions of Figure 4, but with modifications in the second cycle: (a) all reactants except PhMe2SiH are added; (b) the mixture is heated to 55 °C; (c) the tape is removed; (d) the PhMe2SiH is added.

decreased from 0.15 to 0.09 mol %, and reaction temperatures increased from 55 to 80 °C unless noted. The first objective was to compare the “three methylene spacer” catalyst 1c to the “two methylene spacer” analog 1a using PTFE supports as shown in Figure 12. Data for experiments with PTFE tape are summarized in Figure 13 (left, 1a; right, 1c). During the first cycle, both catalysts converted 2 to 3 at comparable rates. Hence, activity does not significantly depend upon phosphine ligand basicity, which is a strong function of the number of methylene groups.43 Also, the recycling efficiency appears comparable. The “three spacer” Rf10 homologue 1d only partially dissolved under the conditions of Figure 12. Reaction mixtures became homogeneous at 100 °C but all PTFE based recycling protocols attempted gave poor results, perhaps due to catalyst deactivation at the elevated temperature. Well-defined induction periods were generally not observed during the first cycle of these higher temperature reactions, perhaps because the initial sampling was not fast enough. However, in most cases initial rates are faster during the second cycle, as generally seen under the conditions of Figures 4 and 5. In the interest of thoroughness, the possibility of rhodium metal catalysis was also tested under the higher temperature conditions of Figure 12. Thus, an experiment with cyclohexanone, 1a, and (during the second cycle) elemental mercury was carried out, similar to that with acetophenone in Figure 11 (left). As shown in Figure 11 (right), activity was largely maintained, consistent with predominant molecular catalysis. Catalysis with Other Forms of PTFE. Gore or Gore-Tex materials are highly porous forms of PTFE, as further elaborated below. They are often referred to as ePTFE (e = expanded) in the literature.44,45 As such, it was of interest to compare their performance as solid supports, although (as was found out later) PTFE tape that is manufactured primarily for thread sealing can have comparable porosity. In a separate study involving a fluorous phosphine catalyst and ePTFE f ibers,46 recycling results were somewhat better than those with PTFE tape. The experiments represented in Figure 14 were conducted under the conditions of Figure 12, except with an ePTFE (Gore-Tex) membrane35 in place of PTFE tape. In contrast to the study with ePTFE fibers,46 the recycling efficiencies appeared rather comparable to those with PTFE tape (Figure 13). Liquid/Liquid Biphase Experiments. It was of interest to compare the retention of catalyst activities in the preceding experiments to those derived from fluorous/organic liquid/ liquid biphase recycling protocols. As originally implemented

Figure 8. Reaction profile for the hydrosilylation of acetophenone (6) under the conditions of Figure 4.

Figure 9. Reaction profile for the hydrosilylation of 2-octanone (8) under the conditions of Figure 4.

of 2, PhMe2SiH, and 1a in dibutyl ether (Figures 4 and 5). It was then rinsed with cold dibutyl ether and transferred to a new vessel (steps that should minimize adventitious solids). A second charge of 2 and dibutyl ether was added, but not the PhMe2SiH. The sample was warmed to 55 °C, the now off white tape “fished out”, and PhMe2SiH was added. As shown in Figure 10, the rate profile was similar to the first cycle (ca. 20% slower at higher conversions), consistent with predominant homogeneous catalysis by desorbed fluorous species. Second, elemental mercury inhibits catalysis by metal particles.42 Accordingly, the second cycle of a sequence carried out with acetophenone (6) was conducted in the presence of a 500-fold excess of mercury relative to 1a. However, as shown in Figure 11 (left), the rate profile was quite close to that of the first cycle in the absence of mercury. Hence, catalysis must result mainly from desorbed molecular fluorous species. Catalysts with CH2CH2CH2 Spacers (1c,d). For the experiments in this and later sections, catalyst loadings were 10879


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Figure 11. Reaction profiles for hydrosilylations carried out in the presence of mercury (500 equiv/1a) during the second cycle: 6 under the conditions of Figure 4 (left); 2 under the conditions of Figure 12 (right).

However, in many later applications, it was found that reactions were sufficiently fast using lower temperature biphasic conditions.30−32 Both variants have been employed for hydrosilylations of ketones by 1a,b (40 °C: CF3C6F11/hexane, monophasic; CF3C6F11/toluene, biphasic).30,31 Both show a slight loss of activity in the fourth cycle, with the monophasic conditions being faster. As shown in Figure 15, dibutyl ether solutions of cyclohexanone (2), PhMe2SiH, and an internal standard were combined with CF3C6F11 solutions of catalyst 1a. Since the objective was to compare activity as a function of cycle (and not absolute rates versus those in Figures 13 or 14), the catalyst loading was increased slightly to 0.12 mol % and the temperature decreased slightly to 75 °C, just below the boiling point of CF3C6F11 (76 °C). The upper dibutyl ether phase became bright yellow, and the lower CF3C6F11 phase was darker orange−yellow.

Figure 12. Hydrosilylation of 2 under conditions differing slightly from those in Figure 4 (see text) and in the presence of different types of PTFE materials.

by Horváth (Figure 1),10 the fluorous and organic phases were designed to become miscible under the reaction conditions.

Figure 13. Reaction profiles for the hydrosilylation of 2 with catalysts 1a (left) and 1c (right) under the conditions of Figure 12, using PTFE tape. 10880


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Figure 14. Reaction profiles for the hydrosilylation of 2 with catalysts 1a (left) and 1c (right) under the conditions of Figure 12, using an ePTFE membrane as the support.

Figure 15. Recycling of catalysts 1a−d using fluorous/organic liquid/ liquid biphasic conditions.

As shown in Figure 16, a rather astonishing seven cycles could be carried out without a substantial loss in rate. On the eighth cycle, the yield of silyl ether 3 was 84% after 90 min, as opposed to 95−99% for cycles 4−7. The only significant difference as compared to earlier studies under fluorous/ organic liquid/liquid biphase conditions noted above30,31 would be the dibutyl ether solvent and higher temperature. In contrast, parallel runs with 1b−d showed significant drops in activities in the fourth or fifth cycle, as illustrated in Figures S1−S3 in the Supporting Information. When the experiment in Figure 16 was repeated, but with the addition of some fluorous phosphine P(CH2CH2Rf6)3 at the start of cycles 2−5 (ca. 1.0 equiv/1a), rates slowed considerably.

Figure 16. Reaction profile for the hydrosilylation of 2 with catalyst 1a under the conditions of Figure 15.

rendering them impenetrable to liquid water but allowing water vapor and suitable solutes to permeate. In follow-up studies, we have found that other types of fluorous catalysts can also be recycled using PTFE or ePTFE materials. These include (1) the phosphines P(CH2CH2Rf8)312,13 and P(CH2CH2CH2Rf8)3,13,46 which serve as nucleophilic catalysts for conjugate additions and Morita−Baylis−Hillman and Rauhut−Currier condensations, and (2) the phosphonium salt [P(CH2CH2Rf6)3(CH2CH2Rf8)]+I−,47 which serves as a phase transfer catalyst for nucleophilic displacements in fluorous media. Both of these can be precoated on PTFE tape if desired. However, coating experiments do not always give the degree of uniformity apparent in Figure 6. For example, analogs of Grubbs’ second generation catalyst with fluorous phosphines, (H2IMes)(Cl)2Ru(P(CH2CH2Rfn)3)(=CHPh) (n = 6, 8, 10), have been synthesized.48 These adducts have a lower weight

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DISCUSSION Scope and Sustainability of PTFE based Catalyst Recycling. At the time the data in Figures 4 and 5 were communicated, they represented the first use of PTFE or Teflon tape in catalyst recycling. Since then, the protocol has been generalized in a number of directions. One has been the extension to ePTFE materials (vide supra),44,45 such as the Gore-Tex membrane used in this study, or Gore-Rastex fiber described elsewhere.46 These substances feature pore sizes of up to 0.20 μm, much smaller than the size of a water droplet, 10881


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ACS Sustainable Chemistry & Engineering percent fluorine content than the preceding catalysts and that with n = 8 did not homogeneously coat onto PTFE tape. Rather, samples were “blotched”, with a mixture of intensely red and much paler domains as depicted in Figure 17.49 These tapes initiated catalysis (CH2Cl2, room temperature) but were ineffective in recapturing the catalyst. More recently, a fluorous second generation Grubbs−Hoveyda catalyst with a single Rf10 phase label has been supported on PTFE powder.50 In this case, the catalyst could desorb in DMF at room temperature, giving ring closing olefin metathesis, and readsorb after reaction by the addition of water. In other contributions, Stuart and Hope have synthesized perfluoroalkylated styrene resins and used them and/or powdered PTFE as supports for hydrogenations and Suzuki− Miyaura reactions catalyzed by rhodium and palladium complexes with fluorous phosphines.51,52 Additional examples of catalyst recycling with fluorous silica gel since the early work mentioned above22−29 have also been reported.53−55 Other fluorous substances have been adsorbed onto PTFE, with the goal of useful surface modifications such as enhanced wettability or facile pyridylporphyrin binding.56,57 A reviewer requested some brief remarks on sustainability aspects of our methodologies. Except for the liquid/liquid biphase process in Figure 15, no fluorinated solvents are employed in any of the catalytic reactions. Although the rhodium catalysts are effective at very low loadings, their manufacture requires perfluoroalkyl building blocks, which as noted in the introduction have increasingly recognized environmental drawbacks.7,58 Minor perfluoroalkyl leaching can also be detected during recycling. These problems are prompting shifts to ethereal fluorous phase tags,59−61 which are so far regarded as environmentally benign. Unfortunately, phosphines based upon such building blocks remain to be synthesized. Importantly, there is increasing insight into how biodegradability can be engineered into fluorous materials.62−64 Physical Considerations. Although high porosity is a hallmark of ePTFE materials, other forms of PTFE can also have significant porosity. As alluded to above, in at least some cases PTFE tape manufactured for thread sealing has been found to have a maximum pore size (14.8 μm2; thickness 0.07 mm with pores 10.1% of the surface area) greater than that of Gore-Tex sheets (12.0 μm2; thickness 0.29 mm with pores 6.2% of the surface area).65 In contrast, bulk PTFE is a solid matrix with a regular surface. Although it is commonly regarded as nonporous, it remains permeable to some chemicals as described below. The permeability of solvents to PTFE tape, as evidenced by the translucent nature of the formerly white band in dibutyl ether in Figure 4 has been studied in detail by Dragojlovic,66−68

who has been interested in applications in phase vanishing reactions.69,70 He found that dichloromethane, ethyl acetate, and hexanes readily diffused through PTFE tape, as well as neat bromine, but not dimethyl phthalate (mp 2 °C). However, dimethyl phthalate did diffuse through PTFE tape when the solvents dichloromethane or ethyl acetate were stationed on the opposite side (but not hexane, in which dimethyl phthalate is insoluble). Dragojlovic detailed a number of related phenomena, suggesting a variety of mechanistic possibilities for the entrainment of fluorous compounds in PTFE pores. In a related effort, Weber has studied the transport of solvents and solutes through other types of fluoropolymers.71 Bulk PTFE is the outer coating of most magnetic stir bars, including those in Figure 4. Per the extensive experience of countless researchers, this material is not wetted by any common solvent. In view of the much higher porosity of PTFE tape, it is not surprising that the stir bar remains colorless in Figure 4. However, it is worth noting that Dragojlovic has established the (slow) permeation of bromine into bulk PTFE and stir bars.68 Indeed, this phenomenon cannot be extremely rare given the discolored stir bars that can be found in nearly every chemistry lab bench. Hence, it remains conceivable that bulk PTFE could in very special cases provide a locus for catalyst recycling. In retrospect, this project could have benefitted by a closer attention to physical characteristics of the PTFE materials employed. For example, membranes are available from common vendors with a gradation of pore sizes (e.g., 0.2, 0.5, 1.0, 5.0, 10.0 μm), all of which could be easily screened in the test reactions in Figures 4 or 12.72 However, this initial study was more concerned with developing practical test reactions, preparing a family of highly fluorophilic catalysts with varying solubilities and electronic properties, and excluding other possible sources of activity such as metal nanoparticles. Thus, the stage has been set for systematic optimization of the PTFE support. Ultimately, it may prove possible to use intrinsic as opposed to extrinsic PTFE objects for recycling, such as reactor liners or stirring assemblies.

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CONCLUSION This study has established, together with allied investigations, that diverse forms of PTFE have broad but not quite universal applicability for recycling fluorous catalysts. These protocols are generally fluorous solvent free, greatly reducing the environmental organofluorine footprint. Furthermore, fluorous materials not based upon environmentally persistent fluorinated building blocks are becoming increasingly available.59,73 The fluorous/organic solid/liquid biphase conditions do not yet match the retention of catalyst activity achievable with fluorous/organic liquid/liquid biphase conditions (Figures 1, 16).74 However, the incredibly diverse forms of PTFE as well as other fluoropolymers that are commercially available, particularly with respect to morphology and porosity, offer the possibility of extensive optimization and fine-tuning.

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EXPERIMENTAL SECTION

Reactions were conducted under inert atmospheres unless noted. Solvents were treated as follows: dibutyl ether (Acros, 99%), diethyl ether, and toluene, distilled from Na/benzophenone, with the dibutyl ether also freeze−pump−thaw degassed (3×); CF3C6H5 and CF3C6F11 (2 × ABCR), distilled from CaH2 and freeze−pump− thaw degassed (3×); C6F14 (ABCR, 95%), simple distillation; C6F6

Figure 17. Attempted coating of the fluorous Grubbs’ catalyst (H2IMes)(Cl)2Ru(P(CH2CH2Rf8)3)(=CHPh) onto PTFE tape. 10882


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36.2 (t, 2JCF = 22 Hz, CH2CF2). IR (oil film, cm−1): 1650 (w), 1436 (w), 1349 (w), 1235 (s), 1189 (s), 1144 (s). P(CH 2 CH 2 CH 2 R f6 ) 3 . 86 A Schlenk tube was charged with PH2CH2CH2CH2Rf6 (9.308 g, 23.62 mmol), H2CCHCH2Rf6 (25.50 g, 70.8 mmol), and AIBN (0.250 g, 1.52 mmol). The mixture was stirred for 2 h at 90 °C and became yellow. A second charge of AIBN (0.350 g, 2.13 mmol) was added. After an additional 18 h, the mixture was cooled and CF3C6H5 (20 mL) was added. The sample was filtered through a silica gel plug. The solvent was removed from the filtrate by distillation. Continued distillation (200 °C, 0.02 mbar) gave P(CH2CH2CH2Rf6)3 as a viscous colorless liquid (20.455 g, 18.36 mmol, 78%). Anal calcd (%) for C27H18F39P: C 29.08, H 1.62. Found C 29.24, H 2.00. NMR (δ/ppm, CF3C6F11, internal C6D6 lock): 1H (400 MHz) 2.22−2.09 (m, 6H, CH2CF2), 1.81−1.71 (m, 6H, PCH2), 1.36−1.32 (m, 6H, CH2CH2CH2). 13C{1H} (100 MHz)82 32.5−31.7 (m, CH2CF2), 26.7 (s, CH2CH2CH2), 13.7 (d, 1JCP = 17 Hz, PCH2). 31 1 P{ H} (162 MHz) −34.2 (s). IR (oil film, cm−1): 2957 (w), 2895 (w), 1366 (w), 1320 (w), 1235 (s), 1185 (s), 1143 (s), 1119 (s), 1073 (m), 1019 (w), 849 (w), 810 (w), 779 (w). MS:85 1115 (100) [M]+, 845 (40), 781 (7), 499 (15), 408 (40). P(O)(OEt)2CH2CH2CH2Rf10. The educts P(OEt)3 (13.90 g, 84.0 mmol, 14.4 mL) and ICH2CH2CH2Rf10 (19.311 g, 28.07 mmol)80,87 were combined in a procedure analogous to that for P(O)(OEt)2CH2CH2CH2Rf6. After removing the ethyl iodide coproduct, the mixture was allowed to cool to 110 °C. The remaining P(OEt)3 and volatiles were then distilled (110 °C, pressure slowly decreased to 42 mbar).81 A subsequent distillate fraction (135 °C, 0.02 mbar) gave P(O)(OEt)2CH2CH2CH2Rf10 as a colorless liquid (17.936 g, 24.80 mmol, 89%). Anal calcd (%) for C17H16F21O3P: C 29.23, H 2.29. Found C 29.20; H 2.40. NMR (δ/ppm, CDCl3): 1H (400 MHz) 4.19−4.09 (m, 4H, CH2CH3), 2.26−2.15 (m, 2H, CH2CF2), 2.01−1.95 (m, 2H, PCH2), 1.93−1.79 (m, 2H, CH2CH2CH2), 1.36 (t, 3JHH = 7 Hz, 6H, CH2CH3). 13C{1H} (100 MHz)82 61.5 (d, 2JCP = 6 Hz, CH2CH3), 31.5−30.9 (m, CH2CF2), 25.3 (d, 1JCP = 143 Hz, PCH2), 16.5 (d, 3JCP = 6 Hz, CH2CH3), 14.4 (d, 2JCP = 5 Hz, CH2CH2CH2). 31P{1H} (162 MHz) 31.3 (s). IR (oil film, cm−1): 2988 (w), 1393 (w), 1374 (w), 1343 (w), 1200 (s), 1150 (s), 1058 (s), 1027 (s), 961 (s), 899 (w), 833 (w). PH2CH2CH2CH2Rf10.88 The compounds LiAlH4 (1.70 g, 44.6 mmol), diethyl ether (50 mL), and P(O)(OEt)2CH2CH2CH2Rf10 (17.936 g, 24.80 mmol) in diethyl ether (10 mL) were combined at −5 °C in a procedure analogous to that for PH2CH2CH2CH2Rf6. After the diethyl ether was separated by distillation, the Vigreux column was removed and a subsequent distillate fraction (90 °C, 0.02 mbar) gave PH2CH2CH2CH2Rf10 as a colorless liquid (9.83 g, 16.6 mmol, 67%). Anal calcd (%) for C13H8F21P: C 26.26, H 1.35. Found C 26.30, H 0.95. NMR (δ/ppm, CF3C6F11, internal C6D6 lock): 1H (400 MHz) 2.58 (dt, 1JHP = 188 Hz, 3JHH = 8 Hz, 2H, PH2), 2.04−1.90 (m, 2H, CH2CF2), 1.72−1.65 and 1.30−1.25 (2m, 4H, PH2CH2CH2). 13C{1H} (100 MHz)82 32.5−31.1 (m, CH2CF2), 23.9 (s, CH2CH2CH2), 13.1 (d, 1JCP = 10 Hz, PH2CH2). 31P{1H} (162 MHz) −145.6 (s). H2CCHCH2Rf10.89 The educts Rf10I (13.40 g, 20.7 mmol) and Bu3SnCH2CHCH2 (10.30 g, 31.1 mmol) were combined in a procedure analogous to that for H2CCHCH2Rf6. A similar workup (15 mL CF3C6F11; 2 × 20 mL CH2Cl2; distillation at 65 °C and 0.02 mbar) gave H2CCHCH2Rf10 as a colorless liquid (9.282 g, 16.58 mmol, 80%). Anal calcd (%) for C13H5F21: C 27.85, H 0.89. Found C 26.29, H 0.88.83 NMR (δ/ppm, CDCl3): 1H (400 MHz) 5.93−5.77 (m, 1H, H2C CH), 5.38−5.32 (m, 2H, H2CCH), 3.01−2.81 (m, 2H, CH2CF2). 13 C{1H} (100 MHz)82 125.5 (s, H2CCH), 122.8 (s, H2CCH), 36.2 (t, 2JCF = 23 Hz, CH2CF2). IR (oil film, cm−1): 1650 (w), 1436 (w), 1374 (w), 1198 (s), 1148 (s). P(CH 2 CH 2 CH 2R f10) 3 .88 A Schlenk tube was charged with PH2CH2 CH2CH2 Rf10 (9.83 g, 16.6 mmol), H2CCHCH 2 Rf10 (23.23 g, 41.48 mmol), and AIBN (0.300 g, 1.83 mmol). The mixture was stirred for 3 h at 90 °C and became red−brown. A second charge

(Aldrich, 99%), used as received; deuterated solvents (Cambridge Isotopes or Aldrich), simple distillation. Cyclohexanone (2; Fluka or Merck, ≥99%), benzophenone (4; Fluka, ≥99.5%), acetophenone (6; Fluka or Acros, ≥99%), 2-octanone (8; Fluka or Acros, ≥99%), PhMe2SiH (Aldrich, 99%), tridecane (Aldrich, ≥99%), and Rf6I and Rf10I (2 × ABCR, 99%) were freeze− pump−thaw degassed (3×). The conc. HCl (Staub) was aspirated with nitrogen. The [Rh(Cl)(COD)]2 (Strem, 98%), P(OEt)3 (Aldrich, 98%), LiAlH4 (Aldrich, 95%), Bu3SnCH2CHCH2 (Aldrich, 97%), AIBN (Aldrich, 98%), and mercury (Acros) were used as received. Catalysts 1a,b were prepared as described earlier.32 PTFE tape was purchased from Carl Roth (Germany).35 NMR spectra were recorded on standard 400 MHz instruments at ambient probe temperatures and referenced as follows: 1H, residual internal CHCl3 (δ 7.24) or C6D5H (δ 7.15); 13C, CDCl3 (δ 77.2) or C6D6 (δ 128.0); 31P, 85% H3PO4 (δ 0.0, internal capillary). The other instrumentation employed was routine. P(O)(OEt)2CH2CH2CH2Rf6.77 This sequence78 was carried out in air. A flask was charged with P(OEt)3 (43.16 g, 260.0 mmol, 44.5 mL) and ICH2CH2CH2Rf6 (63.48 g, 130.1 mmol)79,80 and fitted with a distillation head. The mixture was stirred at 160 °C overnight, during which time the ethyl iodide coproduct distilled off, and was allowed to cool to 130 °C. The remaining P(OEt)3 and volatiles were then distilled (130 °C, pressure slowly decreased to 60 mbar).81 A subsequent distillate fraction (100 °C, 0.02 mbar) gave P(O)(OEt)2CH2CH2CH2Rf6 as a colorless liquid (54.239 g, 108.90 mmol, 84%). Anal calcd (%) for C13H16F13O3P: C 31.33, H 3.21. Found C 31.27, H 3.31. NMR (δ/ppm, CDCl3): 1H (400 MHz) 4.21−4.09 (m, 4H, CH2CH3), 2.28−2.22 (m, 2H, CH2CF2), 2.00−1.95 (m, 2H, PCH2), 1.88−1.80 (m, 2H, CH2CH2CH2), 1.35 (t, 3JHH = 7 Hz, 6H, CH2CH3). 13C{1H} (100 MHz) 61.8 (d, 2JCP = 6 Hz, CH2CH3), 31.4−31.2 (m, CF2CH2), 25.1 (d, 1JCP = 144 Hz, CH2P), 16.3 (d, 3JCP = 6 Hz, CH2CH3), 14.2 (d, 2JCP = 5 Hz, CH2CH2CH2). 31P{1H} (162 MHz) 31.3 (s). IR (oil film, cm−1): 2989 (w), 2927 (w), 2852 (w), 2107 (w), 1447 (w), 1366 (w), 1237 (s), 1192 (s), 1167 (s), 1144 (s), 1023 (s), 965 (s). PH2CH2CH2CH2Rf6. A flask was charged with LiAlH4 (2.43 g, 63.9 mmol) and diethyl ether (80 mL), fitted with a dropping funnel and a condenser, and cooled to 0 °C. A solution of P(O)(OEt)2CH2CH2CH2Rf6 (16.00 g, 32.0 mmol) in diethyl ether (5 mL) was slowly added. The outgoing gas was passed through aqueous NaOCl (10%). The mixture was stirred for 2 d at room temperature and cooled to 0 °C. Aqueous HCl (80 mL, 3 M) was carefully added dropwise with vigorous stirring. The ethereal layer was removed by cannula. The aqueous phase was extracted with additional diethyl ether (2 × 30 mL). The combined organic phases were dried (MgSO4). The diethyl ether was removed by distillation using a Vigreux column. A subsequent fraction (85 °C, 44 mbar) gave PH2CH2CH2CH2Rf6 as a colorless liquid (9.308 g, 23.62 mmol, 74%). Anal calcd (%) for C9H8F13P: C 27.41, H 2.03. Found C 26.72, H 2.27.83 NMR (δ/ppm, CF3C6F11, internal C6D6 lock): 1H (400 MHz) 2.48 (dt, 1JHP = 184 Hz, 3JHH = 7 Hz, PH2), 1.99−1.89 (m, 2H, CH2CF2), 1.69−1.61 and 1.27−1.20 (2 m, 4H, PH2CH2CH2); 13C{1H} (100 MHz) 31.7−31.3 (m, CH2CF2), 23.8 (s, CH2CH2CH2), 13.1 (d, 1JCP = 10 Hz, PH2CH2); 31P{1H} (162 MHz) − 144.2 (s). H2CCHCH2Rf6.84 A quartz Schlenk tube was charged with Rf6I (17.51 g, 39.3 mmol) and Bu3SnCH2CHCH2 (19.56 g, 58.9 mmol), fitted with a condenser, and externally irradiated with a mediumpressure mercury lamp (Heraeus Noblelight TQ150 QuecksilberMitteldruckstrahler). After 2 h, CF3C6F11 (20 mL) was added. The mixture was washed with CH2Cl2 (2 × 20 mL). The CF3C6F11 was removed by rotary evaporation. Distillation of the residue (70 °C, 40 mbar) gave H2CCHCH2Rf6 as a colorless liquid (6.02 g, 16.7 mmol, 42%). Anal calcd (%) for C9H5F13: C 30.00, H 1.39. Found C 30.04, H 1.57. NMR (δ/ppm, CDCl3): 1H (400 MHz) 6.06−5.77 (m, 1H, H2C CH), 5.53−5.33 (m, 2H, H2CCH), 2.93−2.82 (m, 2H, CH2CF2); 13 C{1H} (100 MHz) 125.5 (m, H2CCH), 122.9 (s, H2CCH), 10883


Research Article

ACS Sustainable Chemistry & Engineering of AIBN (0.300 g, 1.83 mmol) was added. After an additional 15 h, the mixture was cooled. A brown solid precipitated, which was washed with toluene (3 × 30 mL) and CF3C6H5 (2 × 20 mL). Then C6F14 (80 mL) was added, and the sample was filtered through a silica gel plug. All volatiles were removed from the filtrate by distillation (70 °C, 0.03 mbar) to give P(CH2CH2CH2Rf10)3 as a white waxy solid (24.60 g, 14.35 mmol, 86%), mp (capillary) 109 °C. Anal calcd (%) for C33H18F63P: C 27.30, H 1.05. Found C 27.39, H 1.14. NMR (δ/ppm, CF3C6F11, internal C6D6 lock): 1H (400 MHz) 2.30−1.92 (m, 6H, CH2CF2), 1.85−1.55 (m, 6H, PCH2), 1.40−1.10 (m, 6H, CH2CH2CH2). 13C{1H} (100 MHz)82 32.5−31.9 (m, CH2CF2), 26.9 (d, 2JCP = 15 Hz CH2CH2CH2), 17.0 (d, 1JCP = 17 Hz, PCH2). 31P{1H} (162 MHz) −34.2 (s). IR (powder film, cm−1): 2949 (w), 1374 (w), 1343 (w), 1197 (s), 1146 (s), 1077 (m), 984 (w), 899 (m). ClRh(P(CH2CH2CH2Rf6)3)3 (1c). A flask was charged with P(CH2CH2CH2Rf6)3 (0.163 g, 0.146 mmol), [Rh(Cl)(COD)]2 (0.014 g, 0.028 mmol), CF3C6F11 (2 mL), and toluene (2 mL). The biphasic mixture was stirred overnight. The phases were separated. The dark red CF3C6F11 phase was washed with toluene (2 mL). The CF3C6F11 was removed by oil pump vacuum to give 1c as a viscous brown oil (0.172 g, 0.049 mmol, 88%). NMR (δ/ppm, CF3C6F11, internal C6D6 lock): 1H (400 MHz) 2.17−1.93 (br m, 18H), 1.90−1.67 (br m, 27H), 1.54−1.45 (br m, 9H). 31P{1H} (162 MHz) 28.7 (dt, 1JPRh = 184 Hz, 2JPP = 41 Hz, P trans to Cl), 17.2 (dd, J = 126, 45 Hz, impurity, 9%), 14.8 (dd, 1JPRh = 134 Hz, 2JPP = 40 Hz, 2P cis to Cl), 10.5 (dt, J = 178, 45 Hz, impurity, 9%), −14.1 (s, impurity, 1%), −17.8 (s, impurity, 1%), −21.6 (s, impurity, 1%). IR (oil film, cm−1): 2961 (w), 1440 (w), 1366 (w), 1235 (s), 1185 (s), 1143 (s), 1119 (s), 1069 (s), 810 (m). MS:85 2366 (10) [1c − P(CH2CH2CH2Rf6)3]+, 1197 (8), 1131 (60) [OP(CH2CH2CH2Rf6)3]+, 1115 (100) [P(CH2CH2CH2Rf6)3]+. ClRh(P(CH2CH2CH2Rf10)3)3 (1d). Toluene (3 mL), CF3C6F11 (3 mL), [Rh(Cl)(COD)] 2 (0.0123 g, 0.025 mmol), and P(CH2CH2CH2Rf10)3 (0.250 g, 0.146 mmol) were combined in a procedure analogous to that for 1c. A similar workup (3 mL toluene) gave 1d as a bright yellow solid (0.246 g, 0.047 mmol, 93%), mp (capillary) 110−111 °C. NMR (δ/ppm, CF3C6F11, internal C6D6 lock): 1H (400 MHz) 2.17−1.96 (br m, 18H), 1.94−1.71 (br m, 27H), 1.62−1.42 (br m, 9H). 31P{1H} (162 MHz) 42.4 (s, impurity, 4%), 28.3 (dt, 1JPRh = 184 Hz, 2JPP = 41 Hz, P trans to Cl), 14.1 (dd, 1JPRh = 134 Hz, 2JPP = 40 Hz, 2P cis to Cl), −7.7 (s, impurity, 2%), −12.2 (s, impurity, 4%), −22.0 (s, impurity, 2%). IR (powder film, cm−1): 1444 (w), 1374 (w), 1347 (s), 1200 (s), 1146 (s), 899 (w). MS:85 3566 (5) [1d − PCH2CH2CH2)3Rf10]+, 1866 (40), 1821 (20), 1731 (100) [OP(CH2CH2CH2Rf10)3]+, 1715 (30) [P(CH2CH2CH2Rf10)3]+. Catalyst Recycling by Precipitation (Figure 3). In a glovebox, a 10 mL round-bottom flask was charged with 2 (0.2605 g, 2.654 mmol), tridecane (0.2002 g, 1.086 mmol), PhMe2SiH (0.4301 g, 3.156 mmol), freshly made 1a (0.0891 g, 0.265 mmol, 1.0 mol %), dibutyl ether (5.0 mL), and a stir bar, capped with a septum, and heated (65 °C bath) with stirring. A yellow solution formed. After 8 h, the sample was cooled to room temperature. An aliquot (0.005 mL) was removed and diluted with dibutyl ether. GC analysis (0.001 mL autoinjection) showed a 98% yield (2.601 mmol) of C6H11OSiPhMe2 (3).30,31,90 The sample was cooled to −30 °C. After 4 h, the dibutyl ether was removed by syringe and the residue was washed with cold dibutyl ether (2 × 0.50 mL). Then another charge of 2 (0.2595 g, 2.640 mmol), tridecane (0.2005 g, 1.087 mmol), PhMe2SiH (0.4309 g, 3.156 mmol), and dibutyl ether (5.0 mL) was added. Data for subsequent cycles: Figure 3. Catalyst Recycling Using PTFE Tape. A (Figures 4 and 5). In a glovebox, a 10 mL round-bottom flask was charged with 2 (0.2597 g, 2.650 mmol), dibutyl ether (5.0 mL), tridecane (0.2002 g, 1.086 mmol), PhMe2SiH (0.4301 g, 3.156 mmol), five strips of PTFE tape (30 mm × 12 mm × 0.0075 mm), freshly made 1a (0.0130 g, 0.0039 mmol, 0.15 mol %), and a stir bar, capped with a septum, and heated (55 °C bath) with stirring. A yellow solution formed. An aliquot (0.005 mL) was removed every 15 min and diluted with dibutyl ether.

GC analysis (0.001 mL autoinjection) indicated that the maximum yield of 3 (98%, 2.60 mmol) was reached within 3 h. The sample was cooled to −30 °C. After 4 h, the dibutyl ether was removed by syringe and the coated tapes were washed with cold dibutyl ether (2 × 0.50 mL). Then another charge of 2 (0.2591 g, 2.640 mmol), tridecane (0.2005 g, 1.086 mmol), PhMe2SiH (0.4309 g, 3.156 mmol), and dibutyl ether (5.0 mL) was added. Data for subsequent cycles: Figure 5. B (Figure 7). Dibutyl ether (5.0 mL), tridecane (0.2002 g, 1.086 mmol), 4 (0.4722 g, 2.591 mmol), PhMe2SiH (0.4310 g, 3.160 mmol), five strips of PTFE tape (30 mm × 12 mm × 0.0075 mm), and freshly made 1a (0.0128 g, 0.0038 mmol, 0.15 mol %) were combined in a procedure analogous to A. Similar analyses indicated that the maximum yield of Ph2CHOSiPhMe2 (5; 87%, 2.25 mmol) was reached within 4 h. After an identical workup, another charge of 4 (0.4800 g, 2.634 mmol), tridecane (0.2001 g, 1.085 mmol), PhMe2SiH (0.4301 g, 3.156 mmol), and dibutyl ether (5.0 mL) was added. Data for subsequent cycles: Figure 7. NMR (5;91 δ/ppm, CDCl3) 1H 7.67−7.23 (m, 15H), 5.88 (s, 1H), 0.43 (s, 6H). 13C{1H} 145.6, 138.3, 134.2, 130.6, 128.7, 128.5, 127.6, 127.5, 77.4, −0.9 (10 × s). C (Figure 8). Dibutyl ether (5.0 mL), tridecane (0.2005 g, 1.087 mmol), 6 (0.3170 g, 2.650 mmol), PhMe2SiH (0.4305 g, 3.159 mmol), five strips of PTFE tape (30 mm × 12 mm × 0.0075 mm), and freshly made 1a (0.0131 g, 0.0039 mmol, 0.15 mol %), were combined in a procedure analogous to A. Similar analyses indicated that the maximum yield of PhMeCHOSiPhMe2 (7; 87%, 2.31 mmol)92 was reached within 3.5 h. After an identical workup, another charge of 6 (0.3170 g, 2.650 mmol), tridecane (0.2001 g, 1.085 mmol), PhMe2SiH (0.4301 g, 3.155 mmol), and dibutyl ether (5.0 mL) was added. Data for subsequent cycles: Figure 8. NMR (7;92 δ/ppm, CDCl3): 1H 7.70−7.69 (m, 3H), 7.49−7.32 (m, 7H), 4.99 (q, 3JHH = 6.3 Hz, 1H, OCHCH3), 1.56 (d, 3JHH = 6.3 Hz, 3H, OCHCH3), 0.48 (s, 3H, SiCH3), 0.43 (s, 3H, SiC′H3); 13C{1H} 146.2, 138.1, 133.5, 129.5, 128.1, 127.7, 126.9, 125.4, 71.1, 26.8, −0.9, −1.4 (12 × s). D (Figure 9). Dibutyl ether (5.0 mL), tridecane (0.2010 g, 1.090 mmol), 8 (0.3398 g, 2.650 mmol), PhMe2SiH (0.4301 g, 3.156 mmol), five strips of PTFE tape (30 mm × 12 mm × 0.0075 mm), and freshly made 1a (0.0130 g, 0.0039 mmol, 0.15 mol %) were combined in a procedure analogous to A. Similar analyses indicated that the maximum yield of C6H13(Me)CHOSiPhMe2 (9; 76%, 2.01 mmol)92 was reached within 4 h. After an identical workup, another charge of 8 (0.3390 g, 2.644 mmol), tridecane (0.2010 g, 1.090 mmol), PhMe2SiH (0.4301 g, 3.156 mmol), and dibutyl ether (5.0 mL) was added. Data for subsequent cycles: Figure 9. NMR (9;92 δ/ppm, CDCl3): 1H 7.65−7.60 (m, 2H), 7.42−7.38 (m, 3H), 3.77−3.85 (m, 1H, OCHCH3), 1.26−1.48 (m, 10H, (CH2)5), 1.14 (d, 3JHH = 6.1 Hz, OCHCH3), 0.91 (t, 3JHH = 6.9 Hz, CH2CH3), 0.42 (s, 6H, Si(CH3)2). 13C{1H} 138.6, 133.5, 129.4, 128.0, 69.0, 39.5, 31.6, 29.3, 25.8, 23.7, 22.6, 14.1, −1.4, −1.6 (14 × s). Coating of Catalyst on PTFE Tape (Figure 6). A 10 mL roundbottom flask was charged with 1a (0.0150 g, 0.0045 mmol) and CF3C6F11 (1.0 mL). Two strips of PTFE tape (50 mm × 12 mm × 0.0075 mm) were added to the yellow solution, taking care to avoid folding the tape. Solvent was then allowed to evaporate under an inert gas stream. This gave yellowish coated PTFE strips. Catalyst Recycling Using Precoated PTFE Tape (Figure 6). In a glovebox, a 10 mL round-bottom flask was charged with two strips of PTFE tape (50 mm × 12 mm × 0.0075 mm) that had been coated with 1a (0.0150 g, 0.0045 mmol assuming complete adsorption), tridecane (0.2001 g, 1.085 mmol), 2 (0.2944 g 3.000 mmol), PhMe2SiH (0.4906 g, 3.600 mmol), and dibutyl ether (5.0 mL). The reaction, analyses, workup, and recycling were carried out as in procedure A for 2 above. The maximum yield of 3 (94%, 2.82 mmol) was reached within 3 h (second charge: 2 (0.2950 g 3.006 mmol), PhMe2SiH (0.4900 g, 3.598 mmol), tridecane (0.2004 g, 1.087 mmol), dibutyl ether (5.0 mL)). Data: Figure 6. “Fish Out” Experiment: Removal of PTFE Tape After Reaction is Warmed (Figure 10). In a glovebox, a 10 mL round10884


Research Article

ACS Sustainable Chemistry & Engineering bottom flask was charged with 2 (0.2601 g, 2.650 mmol), dibutyl ether (5.0 mL), tridecane (0.2001 g, 1.085 mmol), PhMe2SiH (0.4301 g, 3.156 mmol), two strips of PTFE tape (50 × 12 × 0.0075 mm), and freshly made 1a (0.0130 g, 0.0039 mmol, 0.15 mol %), capped with a septum and heated (55 °C bath) with stirring. A yellow solution formed. An aliquot (0.005 mL) was removed every 15 min and diluted with dibutyl ether. GC analysis (0.001 mL autoinjection) indicated that the maximum yield of 3 was reached within 3 h. The sample was cooled to −30 °C. After 4 h, the dibutyl ether was removed by syringe. The tape was removed, washed with cold dibutyl ether (2 × 0.50 mL), and transferred to a new round-bottom flask. Then 2 (0.2620 g, 2.670 mmol), tridecane (0.1970 g, 1.068 mmol), and dibutyl ether (5.0 mL) were added. The sample was warmed to 55 °C to allow the catalyst to desorb into solution. The tape was then removed, and PhMe2SiH (0.4301 g, 3.156 mmol) was added. The reaction was monitored by GC. Data: Figure 10. Reactions in the Presence of Mercury (Figure 11). A In a glovebox, a 10 mL round-bottom flask was charged with 6 (0.3180 g, 2.647 mmol), dibutyl ether (5.0 mL), tridecane (0.2003 g, 1.086 mmol), PhMe2SiH (0.431 g, 3.16 mmol), five strips of PTFE tape (30 mm × 12 mm × 0.0075 mm), and freshly made 1a (0.0131 g, 0.0039 mmol, 0.15 mol %), capped with a septum, and heated (55 °C bath) with stirring. A yellow solution formed. An aliquot (0.005 mL) was removed every 15 min and diluted with dibutyl ether. GC analysis (0.001 mL autoinjection) indicated that the maximum yield of 7 was reached within 3.5 h. The sample was cooled to −30 °C. After 4 h, the dibutyl ether was removed by syringe and the coated tapes were washed with cold dibutyl ether (2 × 0.50 mL). Then mercury (0.4001 g, 500:1 Hg/1a) was added, along with another charge of 6 (0.3190 g, 2.655 mmol), tridecane (0.2001 g, 1.085 mmol), PhMe2SiH (0.4301 g, 3.156 mmol) and dibutyl ether (5.0 mL). The mixture was vigorously stirred, maintained at 55 °C, and monitored by GC every 15 min. Data: Figure 11 (left). B A Schlenk tube was charged with 2 (0.260 g, 2.65 mmol), dibutyl ether (3 mL), tridecane (0.200 g, 1.09 mmol), four strips of Gore-Tex membrane (50 mm × 15 mm × 0.01 mm),35 and 1a (0.0097 g, 0.0023 mmol). The sample was stirred at 80 °C. An aliquot was taken for GC analysis (0.010 mL). Then PhMe2SiH (0.430 g, 3.16 mmol) was added, and aliquots (0.010 mL) were periodically analyzed by GC. When the run was complete, the sample was cooled (−33 °C). After at least 2 h, the dibutyl ether was removed by syringe and the membrane was washed with cold dibutyl ether. Then mercury (0.231 g, 1.15 mmol, 500:1 Hg/1a) was added, along with another charge of 2 (0.260 g, 2.65 mmol), tridecane (0.200 g, 1.09 mmol), and dibutyl ether (3 mL). The mixture was stirred at 80 °C. An aliquot was taken for GC analysis (0.010 mL). Then PhMe2SiH (0.430 g, 3.16 mmol) was added, and aliquots (0.010 mL) were periodically analyzed by GC. Data: Figure 11 (right). Leaching Measurements. A Procedure A (Catalyst Recycling Using PTFE Tape) was repeated with 2 (0.2600 g, 2.650 mmol), dibutyl ether (5.0 mL), tridecane (0.2002 g, 1.086 mmol), PhMe2SiH (0.4304 g, 3.157 mmol), five strips of PTFE tape (30 mm × 12 mm × 0.0075 mm), and 1a (0.0129 g, 0.0039 mmol, 0.15 mol %). After an identical workup, the solvent was removed from the combined dibutyl ether extracts by oil pump vacuum. Two identical cycles were carried out. The dibutyl ether extracts for these cycles were similarly taken to dryness and combined with that from the first cycle. A solution of C6F6 (0.0491 g, 0.264 mmol) in CF3C6H5 (0.7100 g) was prepared. Then 0.2250 g of the C6F6/CF3C6H5 solution (0.0781 mmol of C6F6) was added. A 19F NMR spectrum was recorded. The total intensities of all (CF2)5CF3-based CF3 triplets (−82.22 ppm) were integrated against the C6F6 signal (−163.28 ppm). B Procedure A was repeated with identical quantities of 2, tridecane, PhMe2SiH, PTFE tape, 1a, and dibutyl ether. After an identical workup, the solvent was removed from the combined dibutyl ether extracts by oil pump vacuum. Fresh dibutyl ether was added to give an analytical sample with a mass of 0.845 g, which was sealed in an ampule. Analysis by AAS-ICP indicated 2.7 ppm rhodium (2.6 × 10−6 mol Rh/1000 g sample; average of three determinations), corresponding to a loss of 2.197 × 10−8 mol of rhodium or 0.57% of total rhodium. An analogous analysis following a

second cycle indicated 25.6 ppm of rhodium in a 0.835 g sample, corresponding to a loss of 2.058 × 10−7 mol of rhodium or 5.3%. Additional Experiments with Fluorous Polymer Supports (Figures 12−14). A Schlenk tube was charged with 1a−d (0.0023 mmol, 0.0080−0.0126 g), 2 (0.260 g, 2.65 mmol), tridecane (0.200 g, 1.09 mmol), dibutyl ether (3.0 mL), and four strips of PTFE tape (50 mm × 12 mm × 0.0075 mm) or Gore-Tex membrane (50 mm × 15 mm × 0.01 mm).35 The sample was stirred at 80 °C or (with 1d) 100 °C. An aliquot for GC analysis was taken (0.010 mL). Then PhMe2SiH (0.430 g, 3.16 mmol) was added, and additional aliquots (0.010 mL) were periodically analyzed by GC. After completing a run, the mixture was cooled (−33 °C). After at least 2 h, the supernatant was separated from the catalyst coated polymer by syringe. The polymer was washed with cold dibutyl ether and charged with fresh reactants. Data: Figures 13 and 14. Catalysis under Liquid/Liquid Biphasic Conditions (Figures 15 and 16). A Schlenk tube was charged with 1a (0.0108 g, 0.0032 mmol), 1b (0.0140 g, 0.0033 mmol), 1c (0.0111 g, 0.0032 mmol), or 1d (0.0169 g, 0.0032 mmol), 2 (0.260 g, 2.65 mmol), tridecane (0.200 g, 1.09 mmol), dibutyl ether (3 mL), and CF3C6F11 (3 mL). The sample was stirred at 75 °C. An aliquot was taken for GC analysis from the dibutyl ether phase (0.010 mL). Then PhMe2SiH (0.430 g, 3.16 mmol) was added, and aliquots (0.010 mL) were periodically analyzed by GC. After completing a run, the mixture was cooled (−33 °C). After at least 2 h, the organic phase was removed by syringe. The fluorous phase was washed with cold dibutyl ether and charged with fresh reactants. Data: Figure 16 and text.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02770. Data analogous to that in Figure 16 for catalysts 1b−d (conditions of Figure 15) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tobias Fiedler: 0000-0001-7169-305X John A. Gladysz: 0000-0002-7012-4872 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Bundesministerium für Bildung und Forschung (BMBF) and the Deutsche Forschungsgemeinschaft (DFG, GL 300/3-3) for support at the Friedrich-AlexanderUniversität Erlangen-Nürnberg, the Welch Foundation for support at Texas A&M University (Grant A-1656), and Prof. Dr. A. Behr and Dr. R. Roll (University of Dortmund) for rhodium measurements.

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DEDICATION Dedicated to a long time friend and collaborator, István Horváth, on the occasion of his 65th birthday. ‡

REFERENCES

(1) Kang, E. T.; Zhang, Y. Surface Modification of Fluoropolymers via Molecular Design. Adv. Mater. 2000, 12, 1481−1494. (2) Combellas, C.; Kanoufi, F.; Nunige, S. Surface Modification of Halogenated Polymers. 10. Redox Catalysis Induction of the Polymerization of Vinylic Monomers. Application to the Localized

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tion to Pd-Mediated Suzuki and Sonogashira Couplings. Angew. Chem., Int. Ed. 2002, 41, 4500−4503. (23) Tzschucke, C. C.; Bannwarth, W. Fluorous-Silica-Supported Perfluoro-Tagged Palladium Complexes Catalyze Suzuki Couplings in Water. Helv. Chim. Acta 2004, 87, 2882−2889. (24) Tzschucke, C. C.; Andrushko, V.; Bannwarth, W. Assessment of the Reusability of Pd Complexes Supported on Fluorous Silica Gel as Catalysts for Suzuki Couplings. Eur. J. Org. Chem. 2005, 5248−5261. (25) Biffis, A.; Zecca, M.; Basato, M. A green protocol for the silylation of alcohols using bonded fluorous phase catalysis. Green Chem. 2003, 5, 170−173. (26) Biffis, A.; Braga, M.; Basato, M. Solventless Silane Alcoholysis Catalyzed by Recoverable Dirhodium(II) Perfluorocarboxylates. Adv. Synth. Catal. 2004, 346, 451−458. (27) Yamazaki, O.; Hao, X.; Yoshida, A.; Nishikido, J. Fluorous reverse-phase silica gel-supported Lewis acids as recyclable catalysts in water. Tetrahedron Lett. 2003, 44, 8791−8795. (28) Jenkins, P. M.; Steele, A. M.; Tsang, S. C. Thermoselective phase property of silica tethered with fluorinated chains for controlled ’release’ and ’capture’ of catalytic fluorous tin species. Catal. Commun. 2003, 4, 45−50. (29) Ablan, C. D.; Hallett, J. P.; West, K. N.; Jones, R. S.; Eckert, C. A.; Liotta, C. L.; Jessop, P. G. Use and recovery of a homogeneous catalyst with carbon dioxide as a solubility switch. Chem. Commun. 2003, 2972−2973. (30) Dinh, L. V.; Gladysz, J. A. Transition metal catalysis in fluorous media: extension of a new immobilization principle to biphasic and monophasic rhodium-catalyzed hydrosilylations of ketones and enones. Tetrahedron Lett. 1999, 40, 8995−8998. (31) Dinh, L. V.; Gladysz, J. A. Monophasic and biphasic hydrosilylations of enones and ketones using a fluorous rhodium catalyst that is easily recycled under fluorous−organic liquid−liquid biphasic conditions. New J. Chem. 2005, 29, 173−181. (32) Juliette, J. J. J.; Rutherford, D.; Horváth, I. T.; Gladysz, J. A. Transition Metal Catalysis in Fluorous Media: Practical Application of a New Immobilization Principle to Rhodium-Catalyzed Hydroborations of Alkenes and Alkynes. J. Am. Chem. Soc. 1999, 121, 2696−2704. (33) For further discussion of rest states and catalyst recycling, see Chapter 1 of ref 6 and Gladysz, J. A. Recoverable catalysts. Ultimate goals, criteria of evaluation, and the green chemistry interface. Pure Appl. Chem. 2001, 73, 1319−1324. (34) Dinh, L. V.; Gladysz, J. A. ″Catalyst-on-a-Tape″Teflon: A New Delivery and Recovery Method for Homogeneous Fluorous Catalysts. Angew. Chem., Int. Ed. 2005, 44, 4095−4097. (35) Technical specifications for the Gore-Tex ePTFE membrane employed in this study: https://www.gore-tex.com/technology/goretex-membrane (accessed July 30, 2017). (36) For the isolation of 1:1 fluorous phosphine/rhodium adducts of the formula Rh(PR(CH2CH2Rfn)2)(Cl)(COD) (n = 6, 8), see Klose, A.; Gladysz, J. A. Synthesis, properties, and reactions of enantiomerically pure, chiral fluorous phosphines of the formula (menthyl)P(CH2CH2(CF2)n−1CF3)2 (n = 6, 8). Tetrahedron: Asymmetry 1999, 10, 2665−2674. (37) Gimbert, C.; Vallribera, A.; Gladysz, J. A.; Jurisch, M. Thermomorphic fluorous phosphines as organocatalysts for Michael addition reactions. Tetrahedron Lett. 2010, 51, 4662−4665. (38) Alvey, L. J.; Rutherford, D.; Juliette, J. J. J.; Gladysz, J. A. Transition metal catalysis in fluorous media: application of a new immobilization principle to rhodium-catalyzed hydrogenation of alkenes. J. Org. Chem. 1998, 63, 6302−6308. (39) Alvey, L. J.; Meier, R.; Soós, T.; Bernatis, P.; Gladysz, J. A. Syntheses and Carbonyliridium Complexes of Unsymmetrically Substituted Fluorous Trialkylphosphanes: Precision Tuning of Electronic Properties, Including Insulation of the Perfluoroalkyl Groups. Eur. J. Inorg. Chem. 2000, 1975−1983. (40) Emnet, C.; Gladysz, J. A. Efficient Syntheses of Fluorous Primary Phosphines that do not Require PH3. Synthesis 2005, 1012− 1018.

Graft Copolymerization of Poly(tetrafluoroethylene) Surfaces by Vinylic Monomers. Chem. Mater. 2007, 19, 3830−3839. (3) Vollmer, C.; Schröder, M.; Thomann, Y.; Thomann, R.; Janiak, C. Turning Teflon-coated magnetic stirring bars to catalyst systems with metal nanoparticle trace deposits − A caveat and a chance. Appl. Catal., A 2012, 425−426, 178−183. (4) Wang, S.; Marchant, R. E. Fluorocarbon Surfactant Polymers: Effect of Perfluorocarbon Branch Density on Surface Active Properties. Macromolecules 2004, 37, 3353−3359 and references therein. (5) Baker, R. J.; Colavita, P. E.; Murphy, D. M.; Platts, J. A.; Wallis, J. D. Fluorine−Fluorine Interactions in the Solid State: An Experimental and Theoretical Study. J. Phys. Chem. A 2012, 116, 1435−1444. (6) Recoverable and Recyclable Catalysts, Benanglia, M., Ed.; WileyBlackwell: Oxford, England, 2009. (7) Molnár, Á .; Papp, A. Catalyst recyclinga survey of recent progress and current status. Coord. Chem. Rev. 2017, 349, 1−65. (8) Horváth, I. T.; Rábai, J. Facile Catalyst Separation Without Water: Fluorous Biphase Hydroformylation of Olefins. Science 1994, 266, 72−75. (9) Horváth, I. T. Fluorous Biphase Chemistry. Acc. Chem. Res. 1998, 31, 641−650. (10) Handbook of Fluorous Chemistry; Gladysz, J. A., Curran, D. P., Horváth, I. T., Eds.; Wiley/VCH: Weinheim, 2004. (11) Toxicological Effects of Perfluoroalkyl and Polyfluoroalkyl Substances; DeWitt, J. C., Ed.; Springer: Heidelberg, 2015. (12) Wende, M.; Meier, R.; Gladysz, J. A. Fluorous Catalysis without Fluorous Solvents: A Friendlier Catalyst Recovery/Recycling Protocol Based upon Thermomorphic Properties and Liquid/Solid Phase Separation. J. Am. Chem. Soc. 2001, 123, 11490−11491. (13) Wende, M.; Gladysz, J. A. Fluorous Catalysis under Homogeneous Conditions without Fluorous Solvents: A ″Greener″ Catalyst Recycling Protocol Based upon Temperature-Dependent Solubilities and Liquid/Solid Phase Separation. J. Am. Chem. Soc. 2003, 125, 5861−5872. (14) van Vliet, M. C. A.; Arends, I. W. C. E.; Sheldon, R. A. Perfluoroheptadecan-9-one: a selective and reusable catalyst for epoxidations with hydrogen peroxide. Chem. Commun. 1999, 263− 264. (15) Ishihara, K.; Kondo, S.; Yamamoto, H. 3,5-Bis(perfluorodecyl)phenylboronic Acid as an Easily Recyclable Direct Amide Condensation Catalyst. Synlett 2001, 1371−1374. (16) Mikami, K.; Mikami, Y.; Matsuzawa, H.; Matsumoto, Y.; Nishikido, J.; Yamamoto, F.; Nakajima, H. Lanthanide catalysts with tris(perfluorooctanesulfonyl)methide and bis(perfluorooctanesulfonyl)amide ponytails: recyclable Lewis acid catalysts in fluorous phases or as solids. Tetrahedron 2002, 58, 4015−4021. (17) Maayan, G.; Fish, R. H.; Neumann, R. Polyfluorinated Quaternary Ammonium Salts of Polyoxometalate Anions: Fluorous Biphasic Oxidation Catalysis with and without Fluorous Solvents. Org. Lett. 2003, 5, 3547−3550. (18) For a review of this literature since the early reports in references12−17 and additional background, see Gladysz, J. A. In Homogeneous Catalysis; Handbook of Green Chemistry; Anastas, P., Crabtree, R. H., Eds.; Wiley/VCH: Weinheim, 2009; Vol. 1; pp 17− 38. (19) Vuluga, D.; Legros, J.; Crousse, B.; Bonnet-Delpon, D. Fluorous 4-N,N-Dimethylaminopyridine (DMAP) Salts as Simple Recyclable Acylation Catalysts. Chem. - Eur. J. 2010, 16, 1776−1779. (20) Miura, T.; Nakashima, K.; Tada, N.; Itoh, A. An effective and catalytic oxidation using recyclable fluorous IBX. Chem. Commun. 2011, 47, 1875−1877. (21) Li, C.-K.; Ghalwadkar, A.; Lu, N. Recoverable cationic Pdcatalyzed Heck reaction under thermomorphic mode. J. Organomet. Chem. 2011, 696, 3637−3642. (22) Tzschucke, C. C.; Markert, C.; Glatz, H.; Bannwarth, W. Fluorous Biphasic Catalysis without Perfluorinated Solvents: Applica10886


Research Article

ACS Sustainable Chemistry & Engineering (41) Dioumaev, V. K.; Bullock, R. M. A recyclable catalyst that precipitates at the end of the reaction. Nature 2003, 424, 530−532. (42) Widegren, J. A.; Finke, R. G. A review of the problem of distinguishing true homogeneous catalysis from soluble or other metalparticle heterogeneous catalysis under reducing conditions. J. Mol. Catal. A: Chem. 2003, 198, 317−341. (43) Jiao, H.; Le Stang, S.; Soós, T.; Meier, R.; Kowski, K.; Rademacher, P.; Jafarpour, L.; Hamard, J.-B.; Nolan, S. P.; Gladysz, J. A. How To Insulate a Reactive Site from a Perfluoroalkyl Group: Photoelectron Spectroscopy, Calorimetric, and Computational Studies of Long-Range Electronic Effects in Fluorous Phosphines P((CH2)m(CF2)7CF3)3. J. Am. Chem. Soc. 2002, 124, 1516−1523. (44) Ebnesajjad, S. Expanded PTFE Applications Handbook; Elsevier: Amsterdam, 2017. (45) For a popular account, see the Chemical Heritage Foundation Historical Biographies series. https://www.chemheritage.org/ historical-profile/robert-w-gore (accessed September 06, 2017). (46) Seidel, F. O.; Gladysz, J. A. Catalysis of Intramolecular Morita− Baylis−Hillman and Rauhut−Currier Reactions by Fluorous Phosphines; Facile Recovery by Liquid/Solid Organic/Fluorous Biphase Protocols Involving Precipitation, Teflon® Tape, and Gore-Rastex® Fiber. Adv. Synth. Catal. 2008, 350, 2443−2449. (47) Mandal, D.; Jurisch, M.; Consorti, C. S.; Gladysz, J. A. Ionic Transformations in Extremely Nonpolar Fluorous Media: Easily Recoverable Phase-Transfer Catalysts for Halide-Substitution Reactions. Chem. - Asian J. 2008, 3, 1772−1782. (48) da Costa, R. C.; Gladysz, J. A. Syntheses and Reactivity of Analogues of Grubbs’ Second Generation Metathesis Catalyst with Fluorous Phosphines: A New Phase-Transfer Strategy for Catalyst Activation. Adv. Synth. Catal. 2007, 349, 243−254. (49) Corrêa da Costa, R. Fluorous Metathesis and Pincer Catalysts: New Design Strategies for Activation and Recycling of Catalysts. Doctoral Dissertation, Universität Erlangen-Nürnberg, 2006; Chapter 3. (50) Kobayashi, Y.; Inukai, S.; Kondo, N.; Watanabe, T.; Sugiyama, Y.; Hamamoto, H.; Shioiri, T.; Matsugi, M. A medium fluorous Grubbs−Hoveyda 2nd generation catalyst for phase transfer catalysis of ring closing metathesis reactions. Tetrahedron Lett. 2015, 56, 1363− 1366. (51) Hope, E. G.; Sherrington, J.; Stuart, A. M. Supported Fluorous Phase Catalysis on PTFE, Fluoroalkylated Micro- and Mesoporous Silica. Adv. Synth. Catal. 2006, 348, 1635−1639. (52) Audic, N.; Dyer, P. W.; Hope, E. G.; Stuart, A. M.; Suhard, S. Insoluble Perfluoroalkylated Polymers: New Solid Supports for Supported Fluorous Phase Catalysis. Adv. Synth. Catal. 2010, 352, 2241−2250. (53) Bernini, R.; Cacchi, S.; Fabrizi, G.; Niembro, S.; Prastaro, A.; Shafir, A.; Vallribera, A. Perfluoro-Tagged Gold Nanoparticles Immobilized on Fluorous Silica Gel: A Reusable Catalyst for the Benign Oxidation and Oxidative Esterification of Alcohols. ChemSusChem 2009, 2, 1036−1040. (54) Wang, L.; Yi, W.-B.; Cai, C. Fluorous silica gel-supported perfluoro-tagged palladium nanoparticles: an efficient and reusable catalyst for direct C-2 arylation of indoles. Chem. Commun. 2011, 47, 806−808. (55) Hensle, E. M.; Bannwarth, W. Perfluorinated Bis(dihydrooxazole) Complexes Immobilized on Fluorous ReversedPhase Silica Gel as Recyclable Catalysts for Enantioselective Diels-Alder Reactions. Helv. Chim. Acta 2012, 95, 2072−2083. (56) Motreff, A.; Belin, C.; Correa da Costa, R.; El Bakkari, M.; Vincent, J.-M. Self-adaptive hydrophilic and coordinating Teflon surfaces through a straightforward physisorption process. Chem. Commun. 2010, 46, 6261−6263. (57) For the adsorption of related species on fluorous silica gel, see Motreff, A.; Raffy, G.; Del Guerzo, A. D.; Belin, C.; Dussauze, M.; Rodriguez, V.; Vincent, J.-M. Chemisorption of fluorous copper(II)carboxylate complexes on SiO2 surfaces: versatile binding layers applied to the preparation of porphyrin monolayers. Chem. Commun. 2010, 46, 2617−2619.

(58) For additional recent lead references, see Schaider, L. A.; Balan, S. A.; Blum, A.; Andrews, D. Q.; Strynar, M. J.; Dickinson, M. E.; Lunderberg, D. M.; Lang, J. R.; Peaslee, G. F. Fluorinated Compounds in U.S. Fast Food Packaging. Environ. Sci. Technol. Lett. 2017, 4, 105− 101. (59) Lo, A. S. W.; Horváth, I. T. Fluorous Ethers. Green Chem. 2015, 17, 4701−4714. (60) Chu, Q.; Henry, C.; Curran, D. P. Second-Generation Tags for Fluorous Chemistry Exemplified with a New Fluorous Mitsunobu Reagent. Org. Lett. 2008, 10, 2453−2456. (61) Mukherjee, T.; Biswas, S.; Ehnbom, A.; Ghosh, S.; El-Zoghbi, I.; Bhuvanesh, N.; Bazzi, H. S.; Gladysz, J. A. Syntheses, Structures, and Stabilities of Aliphatic and Aromatic Fluorous Iodine(I) and Iodine(III) Compounds; the Role of Iodine Atom Lewis Basicity. Beilstein J. Org. Chem. 2017, in press. (62) Frömel, T.; Knepper, T. P. Biodegradation of Fluorinated Alkyl Substances. Rev. Environ. Contam. Toxicol. 2010, 208, 161−177. (63) Wu, C.-L.; Chiu, S.-H.; Lee, H.-T.; Suen, M.-C. Synthesis and properties of biodegradable polycaprolacton/polyurethanes using fluoro chain extenders. Polym. Adv. Technol. 2016, 27, 665−676. (64) Bourgeois, A.; Bergendahl, J.; Rangwala, A. Biodegradability of fluorinated fire-fighting foams in water. Chemosphere 2015, 131, 104− 109. (65) Atherton, S.; Brennan, J. C.; Morris, R. H.; Smith, J. D. E.; Hamlett, C. A. E.; McHale, G.; Shirtcliffe, N. J.; Newton, M. I. Plastron Respiration Using Commercial Fabrics. Materials 2014, 7, 484−495 (see section 3.1.1. and Table 1). (66) Van Zee, N. J.; Dragojlovic, V. Phase-Vanishing Reactions with PTFE (Teflon) as a Phase Screen. Org. Lett. 2009, 11, 3190−3193. (67) Pels, K.; Dragojlovic, V. Solvent-free phase-vanishing reactions with PTFE (Teflon®) as a phase screen. Beilstein J. Org. Chem. 2009, DOI: 10.3762/bjoc.5.75. (68) Parsons, B. A.; Smith, O. L.; Chae, M.; Dragojlovic, V. Properties of PTFE tape as a semipermeable membrane in fluorous reactions. Beilstein J. Org. Chem. 2015, 11, 980−993. (69) Van Zee, N. J.; Dragojlovic, V. Design of Phase-Vanishing Reactions. Chem. - Eur. J. 2010, 16, 7950−7958. (70) Matsubara, H.; Tsukida, M.; Ishihara, D.; Kuniyoshi, K.; Ryu, I. A Photoirradiative Phase-Vanishing Method: Efficient Generation of HBr from Alkanes and Molecular Bromine and Its Use for Subsequent Radical Addition to Terminal Alkenes. Synlett 2010, 2014−2018 and earlier work by this group cited therein. (71) Zhao, H.; Zhang, J.; Wu, N.; Zhang, X.; Crowley, K.; Weber, S. G. Transport of Organic Solutes through Amorphous Teflon AF Films. J. Am. Chem. Soc. 2005, 127, 15112−15119. (72) For representative offerings from Sigma-Aldrich, see http:// www.sigmaaldrich.com/labware/labware-products.html?TablePage= 21720976 (accessed July 25, 2017). (73) Lo, S.-W.; Law, E.; Lui, M. Y.; Wei, X.-G.; Lau, K.-C.; Ma, C. Y.; Murphy, M. B.; Horváth, I. T. Development of sustainable fluorous chemistry: the synthesis and characterization of fluorous ethers with nonafluoro-tert-butoxy groups. Org. Chem. Front. 2014, 1, 1180−1187. (74) Also, other immobilized tris(phosphine) rhodium(I) catalyst precursors have been shown to be effective for a greater number of cycles: see refs 75 and 76. (75) Merckle, C.; Blümel, J. Improved Rhodium Hydrogenation Catalysts Immobilized on Oxidic Supports. Adv. Synth. Catal. 2003, 345, 584−588. (76) Guenther, J.; Reibenspies, J.; Blümel, J. Synthesis, Immobilization, MAS and HR-MAS NMR of a New Chelate Phosphine Linker System, and Catalysis by Rhodium Adducts Thereof. Adv. Synth. Catal. 2011, 353, 443−460. (77) An alternative synthesis has been reported, but without experimental details or characteriztion: Ding, Y.; Zhao, G.; Huang, W. Hydroperfluoroalkylation of alkenes using RfI/YbCl3 (cat.)/Zn system. Tetrahedron Lett. 1993, 34, 1321−1322. (78) This synthetic procedure is based upon those used for related compounds earlier.40 10887


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ACS Sustainable Chemistry & Engineering (79) Brace, N. O.; Marshall, L. W.; Pinson, C. J.; van Wingerden, G. Effect of a perfluoroalkyl group on the elimination and substitution reactions of two homologous series of perfluoroalkyl-substituted iodoalkanes. J. Org. Chem. 1984, 49, 2361−2368. (80) For an optimized large scale synthesis of the precursor alcohol, see Rábai, J.; Szíjjárto, C.; Ivanko, P.; Szabó, D. 3-(Perfluoroalkyl)propanols: Valuable Building Blocks for Fluorous Chemistry. Synthesis 2007, 2581−2584. (81) It is important to distill off all of the P(OEt)3, since it would be converted to PH3 in the next step with LiAlH4. (82) The somewhat complex 13C{1H} NMR signals associated with the fluorinated carbons (most of which do not vary significantly from compound to compound) are not reported. Typical patterns are described in Gladysz, J. A.; Jurisch, M. Structural, Physical, and Chemical Properties of Fluorous Compounds. Top. Curr. Chem. 2011, 308, 1−24. (83) These microanalytical data feature a value outside of normally accepted ranges but are presented nonetheless as the best fit obtained to date. (84) An alternative synthesis has been reported.79 (85) FAB, 3-NBA: m/z for the most intense peak of the isotope envelope. (86) An alternative synthesis has been reported: Vlád, G.; Richter, F.; Horváth, I. T. Modular Synthesis of Fluorous Trialkylphosphines. Org. Lett. 2004, 6, 4559−4961. (87) Bayardon, J.; Sinou, D. Enantiopure Fluorous Bis(oxazolines): Synthesis and Applications in Catalytic Asymmetric Reactions. J. Org. Chem. 2004, 69, 3121−3128 (see the Supporting Information). (88) The synthesis of PH2CH2CH2CH2Rf10 has not been previously detailed, but its conversion to P(CH2CH2CH2Rf10)3 on a much smaller scale was communicated earlier.37 (89) An alternative synthesis has been reported: Haas, A.; Koehler, J. Darstellung von poly-fluororganotrichlorsilanen. J. Fluorine Chem. 1981, 17, 531−537. (90) Ojima, I.; Donovan, R. J.; Clos, N. Rh4(CO)12, Co2Rh2(CO)12, and Co3Rh(CO)12 as Effective Catalysts for Hydrosilylation of Isoprene, Cyclohexanone, and Cyclohexenone. Organometallics 1991, 10, 2606−2610. (91) Data for the corresponding trimethylsilyl ether: Nishiyama, Y.; Kajimoto, H.; Kotani, K.; Nishida, T.; Sonoda, N. Reaction of Carbonyl Compounds with Trialkylsilyl Phenylselenide and Tributylstannyl Hydride under Radical Conditions. J. Org. Chem. 2002, 67, 5696−5700. (92) Fujita, M.; Hiyama, T. Fluoride Ion Catalyzed Reduction of Aldehydes and Ketones with Hydrosilanes. Synthetic and Mechanistic Aspects and an Application to the Threo-Directed Reduction of αSubstituted Alkanones. J. Org. Chem. 1988, 53, 5405−5415.

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