Rhodium-Catalyzed Hydroformylation of 1,3-Butadiene to Adipic

Article pubs.acs.org/Organometallics

Rhodium-Catalyzed Hydroformylation of 1,3-Butadiene to Adipic Aldehyde: Revealing Selectivity and Rate-Determining Steps Sebastian Schmidt,† Eszter Baráth,‡,§ Christoph Larcher,†,∥ Tobias Rosendahl,†,# and Peter Hofmann*,†,‡ †

Organisch-Chemisches Institut, University of Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany Catalysis Research Laboratory (CaRLa), University of Heidelberg, Im Neuenheimer Feld 584, D-69120 Heidelberg, Germany

‡

S Supporting Information *

ABSTRACT: The reaction mechanism of Rh-catalyzed hydroformylation of 1,3-butadiene with triptycene-derived bisphosphite ligands L was studied by in situ NMR and IR experiments, kinetic measurements, and deuterioformylation. Under CO pressure the η3-crotyl complex (κ2-L)Rh(η3crotyl)(CO) is a stable intermediate that slowly liberates 3pentenal when hydrogen is added. It also represents the most stable intermediate under real hydroformylation conditions and exists in high concentrations during the catalysis. The rate law of the reaction is found to be second-order in syngas pressure and independent of the butadiene concentration. This agrees with the total barrier between the η3-crotyl complex and one of the transition states of hydrogen addition or reductive elimination being rate determining. The olefin insertion step is found to be partly reversible depending on the pressure, which means that n/iso regioselectivity is not controlled by this step alone.

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INTRODUCTION The selective bis-hydroformylation of 1,3-butadiene to adipic aldehyde (eq 1) would be a highly attractive, atom-efficient,

single-step process that could be used for the industrial largescale production of several important downstream products such as adipic acid, hexamethylenediamine, or 1,6-hexanediole.1,2 These are, among other uses, key monomers for the production of polyamides (e.g., Nylon-6.6), polyesters, or polyurethanes. In contrast to the hydroformylation of nonconjugated terminal olefins, 1,3-butadiene is generally converted with low regioselectivity and turnover frequency.3 The reasons for this observation have so far been speculative, and detailed information on the reaction mechanism and the cause for the abnormal reaction behavior has been scarce. We recently published a detailed quantum chemical study4 of the reaction mechanism employing DFT and one of our bisphosphite ligands with a triptycene-derived rigid backbone (L1 in Figure 1). This class of P-based bidentate ligands (phosphines, phosphonites, phosphites, phosphoramidites), which had originally been developed for highly selective and highly active Rh-hydroformylation catalysts for terminal olefins and for Nicatalyzed hydrocyanation,2,5,6 showed so far the highest selectivities in butadiene hydroformylation of up to 50% adipic aldehyde.2 © 2015 American Chemical Society

Figure 1. Ligands with a triptycene-derived rigid backbone and biphenyl side groups used in this study.

Our detailed DFT calculations for the monohydroformylation of butadiene highlighted two dominant reaction pathways (Scheme 1), which lead to the primarily formed unsaturated monoaldehydes 4-pentenal and E/Z-3-pentenal. We named Received: October 6, 2014 Published: February 23, 2015 841

DOI: 10.1021/om501015z Organometallics 2015, 34, 841−847

Article

Organometallics

which would lead to a complicated dependency of the regioselectivity on several reaction steps. In order to test and validate the predictions from DFT with a different technique, we have performed experimental mechanistic studies including in situ IR and NMR spectroscopy, kinetic measurements, and deuterioformylation experiments. We employed ligands L2−L4 in these experiments, which are very similar. We did not observe any major differences in reactivity for these ligands. In the following the experiments with the most comprehensible spectra are given. Analogous experiments with other ligands can be found in the Supporting Information.

Scheme 1. Simplified Mechanism of the Monohydroformylation of 1,3-Butadiene and the Free Energy Profile for the Two Dominant Pathwaysa

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RESULTS AND DISCUSSION In Situ Spectroscopy. The CO-free crotyl complexes (L)Rh(η3-crotyl) have been suggested to be important intermediates in the catalytic cycle.10−12 We therefore synthesized and characterized this type of complex with several of our bisphosphite ligands including L2−L4 in a previous work.13 In the present work we studied such complexes under hydroformylation-like conditions as a possible entry into the catalytic cycle. Starting with a solution of the known compound (L3)Rh(η3-crotyl) in toluene (7.5 mmol/L), the reaction was followed by in situ IR spectroscopy in a titanium autoclave at 60 °C. The carbonyl region of the IR spectrum at different stages of the experiment is depicted in Figure 2. From earlier investigations13 we knew that (L)Rh(η3-crotyl) systems coordinate CO already at 1 bar CO pressure. a

Reproduced from ref 4. SMD/M06/def2-TZVP//SMD/M06/def2SV(P), 110 °C, solvent: toluene, CO 0.2 mol/L, H2 0.05 mol/L, ligand L1.

these two pathways n- and iso2-pathway, descriptive for the n/ iso olefin insertion and the subsequent isomerization after isoinsertion. 4-Pentenal is known to be selectively converted into adipic aldehyde in a second hydroformylation step under the reaction conditions with our bisphosphite ligands, while 3pentenal leads primarily to branched dialdehydes and pentanal.7 A simplified representation of the proposed mechanism and the corresponding free energy profiles are given in Scheme 1. The n-pathway is closely related to the well-known Wilkinson mechanism8 for n-selective hydroformylation of 1-alkenes, whereas the iso2-pathway includes the formation of an η3crotyl complex and leads to a formal 1,4-addition of the hydrogen atom and carbonyl group. On the basis of the computed free energy profiles for these pathways we were able to make several predictions regarding the mechanistic scenario: (a) The most stable intermediate and hence the resting state of the catalyst is the CO-coordinate crotyl complex (L)Rh(η3-crotyl)(CO) (7a), which should be present in highest concentrations under hydroformylation conditions; (b) the reaction rate should be influenced the most by the total barrier (energetic span9) between the aforementioned crotyl complex and the transition state of either the oxidative addition of molecular H2 (13) or the reductive aldehyde elimination (15); (c) since one molecule each of H2 and CO are consumed in these series of steps, the total reaction rate should depend on both hydrogen and CO partial pressures in first-order and should be independent of butadiene concentration; (d) the olefin insertion step (5) may be partly reversible under experimental hydroformylation conditions,

Figure 2. In situ IR spectra of the reaction of (L3)Rh(η3-crotyl) with CO and H2. (a) Background spectrum: solution of (L3)Rh(η3-crotyl) in toluene under 1 atm of argon. (b) After addition of 22 bar of CO. (c) After 1 h under 44 bar of CO/H2.

In a first step, the autoclave was pressurized with 10 bar of CO, which led to the immediate appearance of a strong band at 1993 cm−1 and a much smaller band at 2016 cm−1. These bands are in agreement with calculated vibrational frequencies of the two most stable isomers of the complex (L3)Rh(η3-crotyl)(CO), which are computed as 1994 and 2002 cm−1 at the BP86/def2-SV(P) level of theory. These two (computed) isomers bear an η3-coordinated crotyl moiety with the methyl group pointing toward the CO ligand, just like the analogous complex that was recently13 characterized structurally. They differ only with respect to the orientation of the ligand backbone (see Supporting Information). Increasing the CO pressure to 22 bar after several minutes did not lead to significant changes in the IR spectrum. CO insertion was not 842

DOI: 10.1021/om501015z Organometallics 2015, 34, 841−847

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Organometallics

complex (L4)Rh(η3-crotyl)(CO) was formed already at 2 bar of CO (dd at 160.9 ppm with JP−P = 170.3 Hz, JP−Rh = 254.6 Hz and dd at 163.9 ppm with JP−P = 170.3 Hz, JP−Rh = 257.1 Hz). In the case of ligand L4 only one isomer is formed. Upon heating to 50 °C (spectrum d), a slow reaction could be observed and a second species, complex (L4)RhH(CO)2, and similar amounts of butadiene were formed. This indicates that the backward reaction of the olefin insertion step, namely, β-Helimination and liberation of butadiene, can take place under these conditions. However, when hydrogen was added (40 bar of CO/H2, approximately 1:1, spectrum f) and the mixture was heated to 50 °C (spectrum g), both butadiene and the crotyl complex (L4)Rh(η3-crotyl)(CO) disappeared. The only remaining Rh-containing species was the hydrido dicarbonyl complex (L4)RhH(CO)2 (d at 165.3 ppm with JP−Rh = 219.0 Hz), which is the expected product of the hydroformylation reaction in the catalytic cycle. Indeed, the formation of aldehydes could be detected in the 1H NMR spectrum in equimolar amounts. In contrast to the IR spectra the isomeric forms of this hydrido complex cannot be observed separately because isomerization is too fast on the NMR time scale (see ref 16 for details). During this experiment no significant amounts of dinuclear species were observed. A similar experiment was carried out with (L2)Rh(η3-crotyl), during which again dinuclear species could be detected (see Supporting Information). In order to verify the postulated coordination of CO to the crotyl complex (L4)Rh(η3-crotyl), we reacted this compound with 2 bar of isotopically labeled 13CO in an NMR tube. The recorded spectra were exactly the same as with unlabeled CO, except that one of the double doublets in the 31P{1H} NMR spectrum exhibits an additional doublet splitting with a coupling constant of 16 Hz. In the 13C{1H} NMR spectrum the coordinated CO appears at 198 ppm as a doublet of doublets with a C−Rh coupling constant of 66 Hz and another coupling constant of 16 Hz, which corresponds to the observed C−P coupling in the 31P{1H} spectrum. In summary, the observations from IR and NMR experiments with the crotyl complexes can be explained with the simplified mechanistic picture in Scheme 2. Under CO pressure, the crotyl carbonyl complex 7a is formed immediately from the crotyl complex 6a. In the absence of hydrogen, formation of the aldehyde is not possible. In the

observed at this pressure: no acyl bands were detected. After the addition of 22 bar of H2 a signal of the formed aldehyde at 1727 cm−1 slowly appeared together with four intensive bands in the carbonyl region at 1999, 2017, 2040, and 2072 cm−1, which are well known14−16 from the active precatalyst (L3)RhH(CO)2 formed from, for example, (acac)Rh(CO)2 in the usual catalyst preformation step of hydroformylation. This typical spectrum results from nine overlapping bands, which can be assigned to two ee-coordinated isomers and one ae isomer. Each isomer exhibits three coupled CO and Rh−H stretching vibrations. A simulation of the spectrum with DFT calculations is given in the Supporting Information. For more details on the ae−ee isomerism see ref 16. In a subsequent GC analysis of the reaction mixture it was possible to determine the constitution of the aldehyde: 3-pentenal was formed as the main product together with small amounts of pentanal, which is known to be formed from 3-pentenal via hydrogenation under the reaction conditions. A similar experiment was carried out with (L2)Rh(η3-crotyl) (see Supporting Information). In this case also dinuclear species with bridging carbonyl ligands were observed, indicated by bands in the region around 1800 cm−1. However, under real hydroformylation conditions these dinuclear species could not be observed (vide infra, Figure 4) and thus should play only a minor role in the catalysis.

Scheme 2. Spectroscopically Observed Intermediates under Pseudo-hydroformylation Conditions

Figure 3. 31P{1H} NMR spectra of (L4)Rh(η3-crotyl) in C6D6 (202.5 MHz) taken subsequently under different conditions (see text).

An analogous experiment was performed in a high-pressure sapphire NMR tube, and a series of NMR spectra were taken over the course of a day (Figure 3). A solution of (L4)Rh(η3crotyl) in deuterated benzene (30 mmol/L) was filled into a sapphire NMR tube (spectrum a, dd at 170.4 ppm with JP−P = 78.8 Hz, JP−Rh = 319.7 Hz and dd at 172.2 ppm with JP−P = 78.0 Hz, JP−Rh = 330.4 Hz), pressurized with initially 2 bar of CO (spectrum b) and subsequently with 20 bar of CO (spectrum c). Consistently with the IR experiment the crotyl carbonyl 843

DOI: 10.1021/om501015z Organometallics 2015, 34, 841−847

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Organometallics forward direction the reaction can proceed only to an η1-alkenyl dicarbonyl complex 7iso2 or to an acyl complex like 10iso2. However, none of these complexes could be observed, which means that they are higher in free energy than 7a. Instead, a partial back-reaction (β-H-elimination) to complex 1 takes place. This hydrido complex is in equilibrium with dinuclear species D with bridging CO ligands. We could not determine the exact constitution of these dinuclear species, but previous studies15,17,18 suggest a constitution as is shown in Scheme 2. These complexes can be cleaved easily with hydrogen and consequently should be present in only minor concentrations when hydrogen and olefins are present. Under H2/CO pressure the reaction proceeds along the catalytic cycle, and aldehyde (3pentenal) and 1 are formed. After having confirmed the reactivity of the crotyl complex, we turned our attention toward real catalytic conditions. We therefore carried out a hydroformylation experiment in our in situ IR autoclave in order to find out which of the intermediates is the kinetically most stable one (most abundant reaction intermediate). This intermediate plays an important role regarding the kinetics of the catalysis, because its relative stability controls the overall reaction rate.9 Similar experiments have been carried out earlier19,20 with dienes and enals in order to study their ability to poison catalysts for 1-alkene hydroformylation: Upon addition of the substrate (e.g., 1,3pentadiene and 3-buten-2-one), new carbonyl bands were observed. For the latter, η1-oxygen-bound rhodium enolate complexes could be identified as the dormant species,19 which have also been proposed21 as important intermediates in enone hydrogenation. We first carried out the preformation of the catalyst with Rh(acac)(CO)2 (20 mmol/L in toluene), ligand L2, and syngas, which is a common method to prepare the catalyst in situ. Within 1 h the hydrido dicarbonyl complex (L2)RhH(CO)2 with its typical spectrum was formed (spectrum c in Figure 4). After preformation, the temperature was increased to 100 °C and butadiene was added together with 20 bar of syngas (substrate to rhodium ratio 750). During the whole reaction, which was stopped after 90 min at 50% conversion, only one band, at 1993 cm−1 with a very broad shoulder, could be observed in the carbonyl region of the IR spectrum. This band can be assigned to the crotyl complex (L2)Rh(η3-crotyl)(CO) (spectra d and e in Figure 4). This observation leads to the conclusion that complex 7a is the kinetically most stable intermediate (most abundant reaction intermediate) and thus has a high influence on the turnover frequency of the catalysis, which is in accordance with our computational results4 and also provides an explanation for why butadiene slows down most hydroformylation catalysts. Interestingly, η3-coordination is preferred in the present study in contrast to the η1-enolate species mentioned above. However, besides the electronic properties of the allyl moiety the ligand could also play an important role (bisphosphite vs PPh3). Kinetic Experiments. During the aforementioned hydroformylation experiment the reaction rate stayed constant until the reaction was terminated at approximately 50% conversion, which is a general observation also in other hydroformylation runs whenever the syngas pressure was kept constant. Clearly, the reaction rate does not depend on the butadiene concentration, at least for relatively high concentrations above 1 mol/L (saturation regime). The reaction rate was also found

Figure 4. In situ IR spectra of the hydroformylation of butadiene with L2. (a) Rh(acac)(CO)2 and L2 dissolved in toluene at 90 °C as background; (b) immediately after adding 20 bar of CO/H2; (c) after 1 h preformation of the catalyst; (d) after addition of butadiene at 100 °C; (e) after 90 min of hydroformylation.

as independent from substrate concentration for the hydroformylation of isoprene.22 Furthermore, our calculations4 had suggested that the turnover frequency depends on the free energy difference between the crotyl complex (7a in Scheme 1) and the transition states for oxidative H2-addition or reductive elimination of the aldehyde (13iso2 and 15iso2 in Scheme 1). In both cases one would expect a first-order dependence on both hydrogen and CO partial pressure. We thus performed another hydroformylation experiment with a high excess of butadiene during which we increased the syngas pressure stepwise from 20 to 80 bar and measured the aldehyde formation in situ by following the aldehyde band at 1730 cm−1. Note that the total aldehyde concentration is measured here, but since the n/iso selectivity changes only slightly2 in this pressure region, this approach gives a reasonable approximation for the reaction rate of the iso2 cycle and the overall relative reaction rate. As in previous kinetic studies22 of the hydroformylation of isoprene, an increasing reaction rate was observed with increasing pressure. A plot of the relative rate against the square of the total syngas pressure (Figure 5) revealed the second-order dependence on the total pressure, which most likely means a first-order rate law in CO and H2 partial pressure, respectively. Deuterioformylation. If the overall reaction rate is determined in the second half of the catalytic cycle, the question arises, which steps control the regioselectivity? The connectivity of the final products is set solely during the olefin insertion step because isomerization of the primarily formed unsaturated aldehydes was not observed. However, as has already been shown for 1-hexene and styrene, the olefin insertion step may be reversible, which complicates the situation.23−26 The degree of reversibility of this step can be determined by deuterioformylation23−27 experiments: A hydroformylation experiment is run with D2/CO instead of H2/CO, the reaction is stopped at low conversion (60 bar) βelimination of the n-alkenyl complex can be effectively suppressed, and consequently the n-selectivity can be increased. In contrast, butadiene iso-insertion retains a constant degree of reversibility independent of the syngas pressure, although the computed reaction profiles suggest a much lower tendency for β-elimination compared to n-insertion. A possible explanation could be provided by a barrier for the rearrangement of the η1iso-butenyl complex to the η3-crotyl complex, which would exhibit the same pressure dependence as the transition state for olefin insertion. However, further investigations from both

EXPERIMENTAL SECTION

General Experimental Details. All reactions and manipulations were performed under an inert atmosphere of dry argon using standard Schlenk and high-vacuum-line techniques or in an MBraun inert-atmosphere (Ar) glovebox if not otherwise noted. Gases were employed in high purity (>3.0) without further treatment. Solvents were dried using an MBraun solvent purification system and molecular sieves (4 Å) and degassed by several freeze−pump−thaw cycles. In situ high-pressure IR spectra were recorded on a Mettler-Toledo ReactIR 4000 instrument in a 20 mL titanium autoclave with an ATR SiComp sentinel probe at the bottom side of the autoclave and a mechanical stirrer. All NMR chemical shifts are reported as δ in parts per million (ppm) relative to tetramethylsilane and 85% H3PO4. The bisphosphite ligands were synthesized according to previously published procedures.2 The known crotyl complexes (L)Rh(η3-crotyl) were synthesized according to a previously published route.13 In Situ IR Experiments. A 65 mg (0.075 mmol) amount of (L3)Rh(η3-crotyl) was dissolved in 1 mL of toluene and added to 9 mL of toluene (60 °C) in the ReactIR autoclave. After injection of this solution the measurement was started with 32 scans at 2 min intervals. After 2 min the autoclave was pressurized with 10 bar of CO. The pressure was increased to 22 bar after another 6 min. After 50 min at this pressure 22 bar of H2 was added, so that the reaction ran at 44 bar (H2:CO approximately 1:1) and 60 °C for 2 h. After cooling to 22 °C and releasing the pressure, the reaction mixture was analyzed by GC. In Situ NMR Experiments. A 15 mg (0.015 mmol) amount of (L4)Rh(η3-crotyl) was dissolved in 0.5 mL of C6D6 in a sapphire NMR tube. 1H and 31P{1H} NMR spectra were recorded after every step of the following treatment: The tube was pressurized with 2 bar of CO and rigorously shaken, pressurized with 20 bar of CO and shaken, heated to 50 °C, cooled to 22 °C, pressurized with 20 bar of H2, resulting in a total pressure of 40 bar of H2/CO, heated to 50 °C, and cooled to 22 °C again. A 8 mg (0.008 mmol) sample of (L4)Rh(η3-crotyl) was dissolved in 0.4 mL of C6D6 in a middle-pressure NMR tube. The tube was pressurized with 2 bar of 13CO and rigorously shaken. Kinetic Experiments. A 7.7 mg (0.03 mmol) portion of Rh(acac)(CO)2 and 46.7 mg (0.06 mmol)28 of L2 were dissolved in 6 mL of toluene, added to the ReactIR autoclave, and heated to 80 °C. At this temperature the background spectrum was recorded and the measurement started (resolution 4 cm−1, 64 scans, 1 min interval). Catalyst preformation was carried out under 10 bar of H2/CO (1:1) for approximately 1 h until the typical bands for (L2)RhH(CO)2 were clearly visible and constant. To start the reaction, 5.7 g of butadiene was added via a 20 mL steel cylinder, into which the butadiene had been condensated, together with 20 bar of H2/CO. The reaction was run at 80 °C and a high stirring rate. The pressure was increased from 20 to 80 bar in steps of 10 bar. At each pressure the reaction rate was 846

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Organometallics measured for a period of approximately 10 min after an equilibration time of 2−3 min. A calibration curve (see Supporting Information) was used to relate the absorption of the band at 1730 cm−1 to the aldehyde concentration. The reaction rate was determined by linear regression of the aldehyde concentration for every pressure region. Deuterioformylation Experiments. These were carried out according to the procedure of the kinetic experiment, except that D2/ CO (1:1) was used instead of syngas. The pressure was held constant for each experiment (20, 40, and 60 bar, respectively). The reaction was stopped after about 50−200 min, when the butadiene band at 1015 cm−1 had decreased by 4−7%. The reaction mixture was immediately recondensated into a cold trap (−196 °C) under vacuum in order to separate it from the catalyst. Handling of the reaction mixture for analysis was carried out below −15 °C at all times. 2H NMR spectra of the reaction mixtures were recorded with 80 scans and a 20 s D1 relaxation delay. Quantification of signals that were partly overlapping was done with the deconvolution tool of the Bruker TopSpin29 software.

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M.; Hofmann, P.; Tensfeldt, M.; Goethlich, A. WO 01/85739, 2001. (c) Ahlers, W.; Paciello, R.; Vogt, D.; Hofmann, P. WO 02/083695, 2002. (7) (a) Rosendahl, T. Dissertation, University of Heidelberg, 2007. (b) Schmidt, S. Dissertation, University of Heidelberg, 2014. (8) Evans, D.; Osborn, J. A.; Wilkinson, G. J. Chem. Soc. A 1968, 3133−3142. (9) Kozuch, S.; Shaik, S. Acc. Chem. Res. 2011, 44, 101−110. (10) Roobeek, C. F.; van Leeuwen, P. W. N. M. J. Mol. Catal. 1985, 31, 345−353. (11) (a) Fell, B.; Rupilius, W. Tetrahedron Lett. 1969, 2721−2723. (b) Fell, B.; Boll, W. Chem.-Ztg. 1975, 99, 452−458. (c) Fell, B.; Boll, W.; Hagen, J. Chem.-Ztg. 1975, 99, 485−492. (d) Fell, B.; Bahrmann, H. J. Mol. Catal. 1977, 2, 211−218. (e) Fell, B.; Bahrmann, H. J. Mol. Catal. 1980, 8, 329−337. (12) Ohgomori, Y.; Suzuki, N.; Sumitani, N. J. Mol. Catal. 1998, 133, 289−291. (13) Schmidt, S.; Baráth, E.; Prommnitz, T.; Rosendahl, T.; Rominger, F.; Hofmann, P. Organometallics 2014, 33, 6018−6022. (14) Buisman, G. J. H.; Vos, E. J.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. J. Chem. Soc., Dalton Trans. 1995, 409−417. (15) Castellanos-Páez, A.; Castillón, S.; Claver, C.; van Leeuwen, P. W. N. M.; de Lange, W. G. J. Organometallics 1998, 17, 2543−2552. (16) Schmidt, S.; Abkai, G.; Rosendahl, T.; Rominger, F.; Hofmann, P. Organometallics 2013, 32, 1044−1052. (17) Evans, D.; Yagupsky, G.; Wilkinson, G. J. Chem. Soc. A 1968, 2660−2665. (18) Haji, S.; Erkey, C. Tetrahedron 2002, 58, 3929−3941. (19) Walczuk, E. B.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Angew. Chem., Int. Ed. 2003, 42, 4665−4669. (20) Liu, G.; Garland, M. J. Organomet. Chem. 2000, 608, 76−85. (21) Scheuermann, C. J.; Jaekel, C. Adv. Synth. Catal. 2008, 350, 2708−2714. (22) Barros, H. J. V.; Guimaraes, C. C.; dos Santos, E. N.; Gusevskaya, E. V. Organometallics 2007, 26, 2211−2218. (23) Lazzaroni, R.; Settambolo, R.; Raffaelli, A.; Pucci, S.; Vitulli, G. J. Organomet. Chem. 1988, 339, 357−365. (24) Horiuchi, T.; Shirakawa, E.; Nozaki, K.; Takaya, H. Organometallics 1997, 16, 2981−2986. (25) van der Slot, S. C.; Duran, J.; Luten, J.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Organometallics 2002, 21, 3873−3883. (26) Watkins, A. L.; Landis, C. R. J. Am. Chem. Soc. 2010, 132, 10306−10317. (27) Casey, C. P.; Petrovich, L. M. J. Am. Chem. Soc. 1995, 117, 6007−6014. (28) We did not find any effect of the ligand-to-rhodium ratio as long as the ligand was used in slight excess. (29) TopSpin 3.0; Bruker BioSpin GmbH, 2010.

ASSOCIATED CONTENT

S Supporting Information *

Additional information and spectra for the in situ experiments. Calibration curve and additional information for the kinetic experiments. Evaluated 2H NMR data. Computational details and Cartesian coordinates of the computed structures. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses §

Department of Technical Chemistry II, TU München, Lichtenbergstraße 4, D-85748 Garching, Germany. # BASF SE, GVX/H, D-67056 Ludwigshafen, Germany. ∥ Saltigo GmbH, Q18, D-51369 Leverkusen, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support of this work by the Fonds der Chemischen Industrie (fellowship to S.S.) and the Deutsche Forschungsgemeinschaft (SFB 623, TP D4). We thank BASF SE for supplying materials and performing one step (high-pressure Diels−Alder reaction of ethylene and anthracene derivatives) of the ligand synthesis. E.B. and P.H. worked at CaRLa (Catalysis Research Laboratory) of Heidelberg University, being cofinanced by the University of Heidelberg, the State of Baden-Wuerttemberg, and BASF SE.

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REFERENCES

(1) Franke, R.; Selent, D.; Börner, A. Chem. Rev. 2012, 112, 5675− 5732. (2) Smith, S. E.; Rosendahl, T.; Hofmann, P. Organometallics 2011, 30, 3643−3651. (3) van Leeuwen, P. W. N. M.; Claver, C. Rhodium Catalyzed Hydroformylation; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2000. (4) Schmidt, S.; Deglmann, P.; Hofmann, P. ACS Catal. 2014, 4, 3593−3604. (5) Tauchert, M. E.; Warth, D. C. M.; Braun, S. M.; Gruber, I.; Ziesak, A.; Rominger, F.; Hofmann, P. Organometallics 2011, 30, 2790−2809. (6) (a) Ahlers, W.; Röper, M.; Hofmann, P.; Warth, D. C. M.; Paciello, R. WO 01/58589, 2001. (b) Ahlers, W.; Paciello, R.; Röper, 847

DOI: 10.1021/om501015z Organometallics 2015, 34, 841−847