Article pubs.acs.org/Organometallics
Synthesis of Adipic Aldehyde by n‑Selective Hydroformylation of 4‑Pentenal Jaroslaw Mormul,† Michael Mulzer,† Tobias Rosendahl,‡ Frank Rominger,‡ Michael Limbach,† 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
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S Supporting Information *
ABSTRACT: Several phosphine and phosphite ligands were tested in the hydroformylation of 4-pentenal to adipic aldehyde, a versatile starting material for industrially very relevant compounds. By varying the ligand structure we were able to increase the selectivity toward adipic aldehyde to >95%. Additionally, two molecular structures of important catalytic intermediates [(bisphosphite)RhH(CO)2] and one structure of a previously unknown catalyst decomposition product were obtained.
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INTRODUCTION The hydroformylation of olefins toward aldehydes (the “oxo reaction”) is one of the most important homogeneous transition-metal-catalyzed reactions in industry.1 Discovered in 1938 by Otto Roelen, more than 10 million tons of “oxo products” were produced in 2002.2 In most cases the resulting aldehyde is not the desired product but is converted to more valuable compounds such as carboxylic acids, alcohols, amines, or other downstream products. An industrially very important product is polyamide nylon-6,6, which is made via cocondensation of adipic acid and hexamethylenediamine.3,4 Adipic acid is commonly manufactured by a multistep oxidation of cyclohexane (Scheme 1, A). The problems with this process
oxidized to the acid or reductively aminated to the amine (C). Several groups worked on the selective double hydroformylation of 1,3-butadiene toward adipic aldehyde but only reached selectivities below 30%.11−15 In 1998, the group of Ohgomori reported a selectivity of 37% with a catalyst system composed of rhodium and the DIOP ligand.16 In 2011, our group presented a new ligand system with which, to date, the highest selectivity of about 50% adipic aldehyde was obtained.17 However, from the viewpoint of an industrial application these values are still unsatisfactory. Another potential starting material for the synthesis of adipic aldehyde is 4-pentenal, which can be produced from readily available starting materials such as allyl alcohol and acetaldehyde (Scheme 2, A) or allyl alcohol and acetylene (B) with high atom-efficiency.18−21 Over the last years our group has worked on the development of highly n-selective and highly active, easily accessible modular ligand systems for the hydroformylation of terminal olefins. By now, our best system is tMe-Rucaphosphite (L1, Figure 1), a symmetric bisphosphite ligand with a triptycene-derived backbone and tetramethylbiphenol wingtips. This ligand also showed the highest selectivity (∼50%) in the double hydroformylation of 1,3-butadiene to adipic aldehyde and an overall aldehyde selectivity (for adipic aldehyde plus 4-pentenal plus Zand E-3-pentenal) of around 90%.17 L1 is similar to BIPHEPHOS (L2, Figure 1), a highly n-selective ligand for the hydroformylation of terminal and internal olefins.22−24 The aim of the work reported here was to synthesize modified congeners of ligands L1 and L2 (Figure 1) and to test
Scheme 1. Current (A, B) and Desired (C) Pathways toward Adipic Acid and Hexamethylenediamine
are that the conversion of the first step has to be kept below 10% to avoid overoxidation and that the second step needs nitric acid as the oxidant, which usually creates large amounts of N2O waste.5,6 Hexamethylenediamine is predominantly produced by Ni-catalyzed hydrocyanation of 1,3-butadiene (B, the “Du Pont process”).7 This is a rather complicated multistep process and requires the handling of HCN gas, which is extremely toxic and prone to autocatalytic polymerization.8−10 A promising starting material for both adipic acid and hexamethylenediamine is adipic aldehyde, which can either be © 2015 American Chemical Society
Received: June 22, 2015 Published: August 5, 2015 4102
DOI: 10.1021/acs.organomet.5b00538 Organometallics 2015, 34, 4102−4108
Article
Organometallics
and six commercially available ligands (L2−L6, L14). Because of the modular synthesis of these bisphosphite ligands, their preparation was straightforward with the exception of L10. Originally we wanted to prepare the symmetric BIPHEPHOS (L2) derivative L10* to examine the influence of the higher steric pressure on the selectivity (Scheme 3).
Scheme 2. Potential Pathways toward 4-Pentenal and Subsequent Conversion to Adipic Aldehyde
Downloaded by UNIV OF NEBRASKA-LINCOLN on September 15, 2015 | http://pubs.acs.org Publication Date (Web): August 5, 2015 | doi: 10.1021/acs.organomet.5b00538
Scheme 3. Formation of the Asymmetric Ligand L10 via Rearrangement
them in the Rh-catalyzed hydroformylation of 4-pentenal. To the best of our knowledge, the synthesis of adipic aldehyde on this route has not yet been described in the literature. Additionally, 4-pentenal is the mandatory intermediate on the way from 1,3-butadiene toward adipic aldehyde.25,26 Therefore, this work may also give further insight into which of the two consecutive steps in the double hydroformylation of 1,3butadiene determines the selectivity toward adipic aldehyde.
Symmetric bisphosphite ligands usually give a singlet around 140 ppm in their 31P NMR spectra in CDCl3. Surprisingly, after the reaction aimed toward L10* we detected a doublet of doublets in this region, which was indicative of an asymmetric compound like L10. To prove this assumption, single crystals of the product were grown via slow diffusion of pentane into a chloroform solution. The molecular structure of L10 is shown in Figure 2. Similar rearrangements of bisphosphite ligands
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RESULTS AND DISCUSSION For our screening of the Rh-catalyzed hydroformylation of 4pentenal we used eight ligands prepared by us (L1, L7−L13)
Figure 1. Ligands L1−L14 tested in the hydroformylation of 4-pentenal. 4103
DOI: 10.1021/acs.organomet.5b00538 Organometallics 2015, 34, 4102−4108
Article
Organometallics
bisphosphite ligand with full conversion and a selectivity above 95% was tMe-Rucaphosphite (L1), which was developed in our group. The main side product in all reactions was the branched isomer 2-methylpentanedial. It is interesting to compare L1 with the similar ligands L7 and L8. By the use of unsubstituted biphenols instead of tetramethylbiphenols as wingtips (L1 → L7) the selectivity drops from 96% to 80%. The introduction of an additional methylene bridge in the wingtips (L1 → L8) lowers the selectivity to 86%. Among the bisphosphite ligands with a substituted biphenol as a backbone, BIPHEPHOS (L2) leads to the highest selectivity of 92%. A similar selectivity (89%) was reached with the sterically demanding L9. The rearranged unsymmetric ligand L10 showed a much lower selectivity (81%), similar to the symmetric derivative L11 (80%). The introduction of a methylene bridge into the biphenol backbone (L11 → L12) had only a small effect on the selectivity (80 → 83%). Comparatively bad results were obtained with the BINOLbased L13 (77%) and the commercially available L14 (69%) with adjacent NH groups. Even though all tested bisphosphite ligands show a high selectivity toward adipic aldehyde (>77%), tMe-Rucaphosphite (L1) still represents the most efficient system. The corresponding molecular structure of the catalyst resting state [(ligand)RhH(CO)2] (2) in the crystal has already been reported and features an axial−equatorial (ae) coordination of the ligand (Scheme 4).29,30
Downloaded by UNIV OF NEBRASKA-LINCOLN on September 15, 2015 | http://pubs.acs.org Publication Date (Web): August 5, 2015 | doi: 10.1021/acs.organomet.5b00538
were reported previously and may actually represent a simple approach toward asymmetric ligands.27,28
Figure 2. Molecular structure of ligand L10 showing 20% probability ellipsoids. Hydrogen atoms have been omitted for clarity.
The ligand screening for the hydroformylation of 4-pentenal was done at 60 bar syngas pressure (CO/H2 1:1), 80 °C, and 2 h reaction time in a Chemspeed high-throughput robot system (see Experimental Section for details). The results are shown in Table 1. Although the monodentate ligands L3 and L4 both showed very good activity, the measured selectivity toward adipic aldehyde was quite low (75% and 71%). In contrast, the two tested bisphosphine ligands L5 and L6 showed very high selectivity (91% and 98%) but rather low activity. The best
Scheme 4. Reaction of L1 with [Rh(acac)(CO)2] under Syngas Pressure Giving the Catalyst Resting State 2
Table 1. Screening of Ligands L1−L14 in the Catalytic Hydroformylation of 4-Pentenala
entry
ligand
conversion [%]d
selectivity [%]d,e
1 2 3b 4b 5 6 7 8 9 10 11 12 13 14 15
L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L12 L13 L14 without ligand
>95 >95 >95 >95 51 46 >95 >95 >95 >95 >95 >95 >95 >95
