Deoxygenation of Epoxides with Carbon Monoxide†

Abstract The use of carbon monoxide as a direct reducing agent for the deoxygenation of terminal and internal epoxides to the respective olefins is presented. This reaction is homogeneously catalyzed by a carbonyl pincer‐iridium(I) complex in combination with a Lewis acid co‐catalyst to achieve a pre‐activation of the epoxide substrate, as well as the elimination of CO2 from a γ‐2‐iridabutyrolactone intermediate. Especially terminal alkyl epoxides react smoothly and without significant isomerization to the internal olefins under CO atmosphere in benzene or toluene at 80–120 °C. Detailed investigations reveal a substrate‐dependent change in the mechanism for the epoxide C−O bond activation between an oxidative addition under retention of the configuration and an SN2 reaction that leads to an inversion of the configuration.


General information
Unless otherwise stated, all reactions were carried out under an argon atmosphere in dried and degassed solvents using Schlenk technique. Toluene, pentane, were purchased from Sigma Aldrich and dried using an MBraun SPS-800 solvent purification system. All lithium salts used were obtained from commercial suppliers, dried in vacuum and used without further purification. Chemicals from commercial suppliers were degassed through freeze-pump-thaw cycles prior to use. Carbon monoxide was purchased from Westfalen with a purity of 99.97 %. All epoxides were purchased from commercial suppliers, except epoxides 3l, 3m, 3o-3t and 3za which were synthesized from the respective aldehydes [1] , 3k, 3v [2] , 3x [3] and 3y [4] from the olefins, and 3z which was synthesized from the corresponding acid [5] according to literate procedures. High pressure NMR scale experiments were performed in Wilmad Heavy/Medium Wall Precision Pressure/Vacuum Valve NMR Sample Tubes. 1 H and 13 C NMR spectra were recorded using a Bruker AVANCE II+ 400 spectrometer, a Bruker AVANCE AVII+ 500 or a Bruker Avance III HDX 600. Chemical shifts δ (ppm) are reported relative to the solvent's residual proton and carbon signal respectively: THF-d8: 3.58 ppm ( 1 H NMR) and 67.57 ppm ( 13 C NMR); C6D6: 7.16 ppm ( 1 H NMR) and 128.39 ppm ( 13 C NMR); DMSO-d 6 : 2.50 ppm ( 1 H NMR) and 39.51 ppm ( 13 C NMR), CDCl 3 : 7.27 ppm ( 1 H NMR) and 77.0 ppm ( 13 C NMR), toluene-d8: 6.97 ppm ( 1 H NMR) and 125.96 ppm ( 13 C NMR). Coupling constants (J) are expressed in Hz. Signals were assigned as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet) and variations thereof. Assignment of the peaks was made using 2D NMR correlation spectra. 1 H NMR spectra of catalytic experiments were recorded without CO atmosphere, after confirming no difference in accuracy, and under CO atmosphere in presence of volatile compounds (indicated with "*" in the manuscript). An increased delay time d1 of 60 s ensured reliable integration values. The mass spectra were recorded on a Bruker Esquire 3000 Plus ion trap mass spectrometer. IR spectra were recorded with a Bruker Vertex 70 or with a Mettler Toledo ReactIR 15. X-ray structure analysis: Crystallographic data collection was carried out on a Bruker APEX Duo CCD with an Incoatec IµS Microsource with a Quazar MX mirror using Mo K α radiation (λ = 0.71073 Å) and a graphite monochromator.

S3
Corrections for absorption effects were applied using SADABS. [6] All structures were solved by direct methods using SHELXS and refined using SHELXL. [ Numbering Scheme: For the assignment of the peaks, the following numbering scheme was used.

Reaction of 5a with LiNTf 2
LiNTf2 (2.8 mg, 10 µmol) was added to a solution of the Intermediate 5a (7.4 mg, 10 µmol) in C6D6 and the reaction was observed via 1 H NMR spectroscopy.

Selectivity and isomerization of 4j and 4z
In the case of 4j no cis/trans isomerization is observed at a reaction temperature of 80 °C. At 120 °C slow isomerization is observed. After 24 h 6% of the total amount of olefin 4j are the trans-isomer starting from cis-3j and 4% are the cis-isomer when starting form trans-3j. After full conversion of cis-3j (after 72 h) the amount of trans-4j increased to 22 % due to the ability of 2 to catalyze olefin-isomerization (vide infra for 1-hexene). Trans-3j was not fully converted after 72 h the isomerized product amounts to 4 % of the total amount of the olefin.

Reaction Monitoring
A catalytic experiment was set up according to the general procedure with propylene oxide as substrate.
After pressurizing with carbon monoxide, the thick-wall NMR tube is placed into the spectrometer and heated to 60 °C. The temperature was maintained for 23 hours and 1 H NMR spectra were recorded every 11 minutes during this time.
* only the amount of dissolved propene was recorded. Figure S2. Monitoring of the deoxygenation reaction of propylene oxide by 1

Isomerization of α-Hexane
In further experiments the isomerization of the olefinic product was investigated. The results indicate that the isomerization of the olefin occurs under the same conditions as the deoxygenation. Furthermore, the isomerization requires both in catalytic amounts, the iridium complex and the Lewis acid.  S10 Figure S5. 1 H NMR (C 6 D 6 (#), 400 MHz) spectrum of 5a (see general information for the peak assignment). Figure S6. 13 C NMR (C 6 D 6 (#), 100 MHz) spectrum of 5a (see general information for the peak assignment). S11 Figure S7. 1 H NMR (toluene-d8 (#), 600 MHz) spectrum of cis-5z (see general information for the peak assignment). Figure S8. 13 C NMR (toluene-d8 (#), 150 MHz) spectrum of cis-5z (see general information for the peak assignment). S12 Figure S9. 1