Supporting Information

Transcription

1 Supporting Information An Electric Field Induced Change in the Selectivity of a Metal Oxide Catalyzed Epoxide Rearrangement Craig F. Gorin, Eugene S. Beh and Matthew W. Kanan* *To whom correspondence should be addressed. mkanan@stanford.edu Index Experimental methods Fig. S1. Ellipsometry of an Al 2 O 3 -coated Si electrode; wafer mapping Fig. S2. XPS spectrum of a 45 Å Al 2 O 3 layer coated with octylphosphonic acid Fig. S3. Assembly of a parallel plate cell Fig. S4. Double-step chronocoulometry charge vs. time trace Fig. S5. Bode plots for the parallel plate cell Fig. S6. Current density vs. time trace for a parallel plate cell reaction Fig. S7. Leakage current density vs. voltage for a parallel plate cell Fig. S8. Voltage-dependence of the rearrangement of cis-stilbene oxide Fig. S9. Comparison of acid-catalyzed reactions to parallel plate cell reactions Fig. S10. Parallel plate cell reactions with 2-(4-chlorophenyl)-3-phenyloxirane Table S1. Double layer capacitances extracted from EIS Table S2. Acid-catalyzed epoxide rearrangement References page S2 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S9 S1

2 Experimental Methods Materials and general methods for chemical synthesis. All chemicals were purchased from Sigma-Aldrich and used as received unless otherwise specified. Tetrabutylammonium hexafluorophosphate was recrystallized from H 2 O/EtOH. 2-(4-chlorophenyl)-3-phenyloxirane, 1 2-(4-chlorophenyl)-2-phenylacetaldehyde 2 and 2-(4-chlorophenyl)-1-phenylethanone 3 were prepared according to the literature methods. Hafnium oxide was purchased from VWR International. Cu mesh and the vises for parallel plate cell reactions were purchased from McMaster-Carr. HPLC grade dichloromethane (CH 2 Cl 2 ) and acetonitrile (CH 3 CN) purchased from Fisher Scientific were used for parallel plate cell reactions. All other solvents used for synthesis were ACS grade. Catalyst powder experiments. 100 mg of activated Al 2 O 3 or HfO 2 powder was added to 10 ml of a solution of 0.5 mm cis-stilbene oxide 1 and 0.5 mm TBAPF 6 in CH 2 Cl 2 or CH 3 CN in a vial. The resulting suspension was stirred at room temperature for 16 hours, filtered and analyzed by HPLC. The diphenylacetaldehyde 2 to 1,2-diphenylethanone 3 ratio in CH 2 Cl 2 was 1:4.1 at 90% conversion, and 1:2.2 at 78% conversion in CH 3 CN. Oxide-coated Si electrode preparation and characterization. Procedures were performed at the Stanford Nanofabrication Facility. 4" 0.5 mm I Prime Si (100) wafers were purchased from Silicon Quest International. The wafers were cleaned by soaking in a 4:1 H 2 SO 4 :H 2 O 2 piranha solution at 90 C for 10 min followed by a 5:1:1 H 2 O:H 2 O 2 :HCl solution at 70 C for 10 min. Native SiO 2 was removed by soaking in a 2% HF solution for 30 s. Wafers were thoroughly rinsed several times in 18 M deionized water after each soak. Oxide layers were deposited within 1 hour of treatment with HF by using a Savannah S200 Atomic Layer Deposition System (Cambridge Nanotech). Aluminum oxide (Al 2 O 3 ) was deposited with trimethylaluminum and S2

3 H 2 O precursors at 200 C, and hafnium oxide (HfO 2 ) was deposited with tetrakis- (dimethylamido)hafnium and H 2 O at 200 C. The oxide-coated wafers were then annealed in forming gas (10% H 2 in N 2 ) at 400 C for 3 h in a Tylan tube furnace. Oxide thickness was measured before and after annealing by using a J. A. Woollam M2000 Spectroscopic Ellipsometer. Wafer mapping indicated a relatively uniform coverage of 48 Å, which includes a few Å of SiO 2 (Figure S1). The same thickness was observed before and after the annealing step. Attachment of alkylphosphonic acid blocking layers to oxide-coated Si electrodes. Al 2 O 3 - coated Si electrodes ~1 cm 1.5 cm in size were cut from the wafers described above by using a diamond scribe. The pieces were cleaned by sonicating for 15 min in methanol (Sigma-Aldrich, TraceSelect), then soaked overnight in a solution of octylphosphonic or octadecylphosphonic acid (1 mm in tetrahydrofuran (THF)). The electrodes were subsequently removed from the solution, allowed to dry at room temperature and then baked at 140 C in air for 24 hours. The resulting functionalized electrodes were rinsed with THF and methanol, and sonicated in THF and methanol to remove unattached alkylphosphonic acid molecules. The electrodes were stored at room temperature until used in parallel plate cell reactions. X-ray photoelectron spectroscopy. X-ray photoelectron spectra were obtained by using a PHI VersaProbe Scanning XPS Microprobe in the Stanford Nanocharacterization Laboratory. Curve fitting of high-resolution spectra was done with PHI Multipak software. An XPS spectrum obtained for a piece of the alkylphosphonic acid-functionalized electrode is consistent with monolayer coverage (Figure S2). XPS after use of the electrode in a parallel plate cell reaction at +4.5 V (negative charge on the counter electrode) showed little change in coverage. Assembly of parallel plate cells. Parallel plate cells were assembled from two oxide-coated electrodes, a perfluorinated gasket (Kalrez 6375) with an opening in the center and an electrolyte S3

4 solution. The cell used for most experiments (the "standard cell") was comprised of one electrode with a 45 Å Al 2 O 3 layer (the "catalyst electrode") and a second electrode with a 45 Å Al 2 O 3 layer coated with octylphosphonic acid (the "counter electrode"). The electrolyte used for most experiments consisted of 0.5 mm TBAPF 6 and 5 mm cis-stilbene oxide in CH 3 CN or CH 2 Cl 2. Electrical contact was made to the back (non-coated side) of each electrode with strips of copper mesh and glass slides were used as outer insulating layers. The sides of the Si electrodes in contact with the strips of copper mesh were scratched with a diamond scribe to improve electrical contact. To set up a reaction (Figure S3), all components were assembled dry and a small opening was made between the electrodes and the gasket by clipping one side of the glass slides together with a binder clip. An excess of electrolyte solution was pipetted in and the opening between the glass slides was quickly closed with a second binder clip. The assembly was transferred into a vise and clamped tight. After removal of the binder clips, the space between the glass slides was rinsed with CH 2 Cl 2, blown dry with air and the entire vise placed under vacuum for 3 min to remove remaining external solvent. The cell was then immediately connected to a Keithley 2400 sourcemeter and the appropriate voltage was applied. Voltages were applied to the catalyst electrode relative to the counter electrode. For 0 V experiments, the electrodes were shorted with a wire connecting the two strips of copper mesh. HPLC analysis. Reactions were analyzed on an Agilent 1200 HPLC equipped with a Diode Array Detector and an Agilent Eclipse XDB-C18 reverse phase column. Analysis was carried out at 40 C with 45 % H 2 O and 55 % CH 3 CN as the mobile phase at a total flow rate of 1.5 ml/min. HPLC grade solvents were obtained from Fisher Chemical. Peak areas were calculated after fitting to Gaussian curves in Matlab and adjusted for extinction coefficients to provide conversion and product ratios. S4

5 Double-step chronocoulometry. Chronocoulometry measurements were performed with a CH Instruments 760D BiPotentiostat. The working electrode lead was connected to the catalyst electrode and both the counter and reference electrode leads were connected to the counter electrode. Voltage was stepped between 0 V and selected positive or negative potentials with a pulse width of 5 s and a sampling interval of s. Total charge was taken to be the difference between the charge prior to the discharge step and the intercept of the discharge trace at the start of the step. Experiments were performed in 0.5 mm TBAPF 6 in CH 3 CN or CH 2 Cl 2 with the standard cell (see above) and with a cell in which the counter electrode was coated with an octadecylphosphonic acid monolayer. A sample trace of q vs t stepping to 3 V in a standard cell with 0.5 mm TBAPF 6 in CH 3 CN is shown in Figure S4. Electrochemical impedance spectroscopy. Impedance measurements were performed with a CH Instruments 760D BiPotentiostat. The working electrode lead was connected to the catalyst electrode and both the counter and reference electrode leads were connected to the counter electrode. AC impedance was measured at DC biases of 1 V, 3 V and 4.5 V at frequencies between 10 khz and 0.1 Hz, with an AC oscillation amplitude of 5 mv. The dependence of the complex impedance on the AC frequency for all DC biases was characteristic of a series connection of resistors and capacitors. 4 Double layer capacitance was taken to be 1/2 fz" at a frequency of 20 Hz, where f is the frequency and Z'' is the imaginary part of the impedance. Experiments were performed in 0.5 mm TBAPF 6 in both CH 3 CN and CH 2 Cl 2 with the standard cell (see above) and with a cell in which the counter electrode was coated with an octadecylphosphonic acid monolayer. Selected Bode plots are shown in Figure S5 and extracted capacitance values are shown in Table S1. S5

6 Current density vs. time traces. When V 5 V is applied to the standard cell with 0.5 mm TBAPF 6 in CH 3 CN, an initial charging current is observed that decays to a steady-state value of typically 2 10 na/cm 2 (Figure S6). A smaller discharge current that decays over several minutes is observed upon stepping to 0 V after an initial charging at a nonzero voltage. Although larger currents are generally observed at higher voltages (see experiment below), significant variability (up to several na/cm 2 ) is observed for multiple experiments at the same voltage. We attribute this variability to differences in the density of defects in the oxide layer either due to small variations in the Si surface prior to ALD or to defects that form in the sonication of the electrodes prior to use. At V > 5 V, the leakage current is observed to slowly increase over time instead of decreasing to a steady-state value. This behavior is ascribed to dielectric breakdown of the Al 2 O 3 layer, exposing more of the underlying Si electrode to the electrolyte and consequently increasing the electrolytic processes. Consistent with the V-dependence of the product ratio seen in Figure 3 (main text), the 2:3 ratio increases; however, many additional products are observed on the HPLC chromatogram. Voltage dependence of leakage current for a single parallel plate cell: Leakage currents as a function of applied potential were measured for a single parallel plate cell by using the Keithley sourcemeter. A parallel plate cell with 45 Å Al 2 O 3 on both the working and counter electrodes was filled with a 0.5 mm solution of TBAPF 6 in CH 3 CN. The voltage was then stepped from 0 V to selected voltages between 5 V to 5 V. The cell was allowed to equilibrate for 2 min at which time the current was flat, with noise of < 1 na. A plot of the plateau leakage current density vs. voltage is shown in Figure S7. We note, however, that independently assembled cells at the same voltage exhibit leakage current variability of 1 4 na/cm 2. We attribute this variability to slight S6

7 differences in defects in different wafer pieces. However, no clear correlation between leakage current at the same voltage and selectivity was observed higher leakage current did not consistently correspond to a higher selectivity. Voltage dependence of conversion. In addition to the selectivity changes, the conversion of 1 is increased for V > 3 V in the parallel plate cell. In CH 3 CN, 2 4-fold changes are observed at these voltages relative to 0 V, while in CH 2 Cl fold changes are observed (Figure S7). No clear trend in the conversions are observed for V > 3 V. Parallel plate experiments with two alkylphosphonic acid-functionalized electrodes. Parallel plate cell reactions were performed at 0 V and ±4.5 V for 16 h with 5 mm epoxide 1 and 0.5 mm TBAPF 6 in CH 3 CN in cells in which both Al 2 O 3 layers were coated with octylphosphonic acid monolayers. HPLC analysis indicated very low conversion (<1%) under these conditions. Reuse of a catalyst at 0 V following an experiment at nonzero voltage. Reactions were performed at +4.5 V for 16 h in standard parallel plate cells with 0.5 mm TBAPF 6 and 5 mm 1 in CH 3 CN. The cells were subsequently disassembled and the electrodes were rinsed with copious amounts of CH 3 CN. The cells were then reassembled with the same electrodes and a fresh CH 3 CN solution containing 5 mm 1 and 0.5 mm TBAPF 6. The cells were left at 0 V for 16 h and the reaction mixtures were subsequently analyzed as described above. The observed ratio of 2:3 for these experiments was 0.31 ± 0.02 : 1.00; in comparison, the ratio for new electrodes used at 0 V was 0.27 ± 0.01 : Acid-catalyzed rearrangement of cis- and trans-stilbene oxide compared to parallel plate cell reactions. A solution of 5 mm cis- or trans-stilbene oxide, 0.5 mm TBAPF 6, and HCl at various concentrations (diluted from 4M HCl in dioxane) in CH 3 CN or CH 2 Cl 2 was stirred in a S7

8 vial for 16 h and subsequently analyzed by using HPLC. The results are summarized in Table S2. Parallel plate cell reactions were performed in the standard cell at +4.5 V for 16 h with a solution of 5 mm cis- or trans-stilbene oxide and 0.5 mm TBAPF 6 in CH 3 CN. HPLC traces comparing the acid-catalyzed reactions to the corresponding parallel plate cell reactions are shown in Figure S8. At 10 μm HCl, very little conversion is observed for either cis- or trans-stilbene oxide. The addition of 100 μm of a strong acid catalyzes the conversion of both cis- and trans-stilbene oxide in 0.5 mm TBAPF 6 in CH 3 CN to products 2 and 3 and other unidentified products. In contrast, cis-stilbene oxide reacts readily in the parallel plate cell to produce 2 and 3 in a V- dependent ratio (Figure 3 and Figure S7), but parallel plate cell reactions with trans-stilbene oxide used as the substrate exhibit very low (< 2%) conversion at 0 V and +4.5 V. This low reactivity likely reflects the inability of trans-stilbene oxide to bind strongly to the Al 2 O 3 surface. Together, these results indicate that H + does not significantly contribute to the selectivity changes observed at V > 3 in the parallel plate cell. First, it is unlikely that any significant concentration of H + accumulates in the cell during a parallel plate cell reaction because any H + produced by an oxidation event at the positive electrode is likely to be consumed by a reduction at the negative electrode. (Attaining a concentration of 100 μm would require 10 na/cm 2 (the upper limit of currents observed) for the full 16 h of a reaction that produced H + at the positive electrode but did not reduce any H + at the negative electrode.) Second, if sufficient quantities of H + were produced to effect epoxide rearrangement, conversion of both cis- and trans-stilbene oxide are expected, in contrast to what is seen in the parallel plate cell at V > 3. Parallel plate experiments in which the counter electrode is blocked with octadecylphosphonic acid. Parallel plate cell reactions were performed in cells with 45 Å Al 2 O 3 S8

9 on the catalyst electrode and 45 Å Al 2 O 3 coated with an octadecylphosphonic acid monolayer on the counter electrode. The electrolyte contained 5 mm epoxide 1 and 0.5 mm TBAPF 6 in CH 3 CN and reactions were performed at 0 V or ±4.5 V for 16 h. Product ratios of 2:3 were 0.42 ± 0.03 : 1.00 at 0 V, 0.48 ± 0.11 : 1.00 at V and 0.64 ± 0.20 : 1.00 at 4.5 V. Parallel plate experiments with HfO 2 coated Si electrode. Parallel plate cell reactions were performed in a cell with 45 Å of HfO 2 on the catalyst electrode and 45 Å of Al 2 O 3 coated with octylphosphonic acid on the counter electrode. Reactions were performed in a solution of 5 mm epoxide 1 and 0.5 mm TBAPF 6 in CH 3 CN at 0 V and ± 4.5 V for 16 h. Product ratios of 2:3 were 0.78 ± 0.02 : 1.00 at 0 V, 1.93 ± 0.48 : 1.00 at V and 5.19 ± 0.37 : 1.00 at 4.5 V. Parallel plate experiments with an alternative epoxide substrate. Parallel plate cell reactions were performed in the standard cell with 5 mm 2-(4-chlorophenyl)-3-phenyloxirane (4) and 0.5 mm TBAPF 6 in CH 3 CN at 0 V and ± 4.5 V for 16 h. Product ratio of aldehyde 5 to combined ketones 6 and 7 was 0.11 ± 0.03 : 1.00 at 0 V, 3.35 ± 1.68 : 1.00 at V and 4.21 ± 1.86 : 1.00 at 4.5 V (Fig. S8). References 1. Belger, C.; Neisius, N. M.; Plietker, B. Chem. Eur. J. 2010, 16, Anderson, A. M.; Blazek, J. M.; Garg, P.; Payne, B. J.; Mohan, R. S. Tet. Lett. 2000, 41, 1527.; Ranu, B. C.; Jana, U. J. Org. Chem. 1998, 63, Sato, K.; Naruse, K.; Enokiya, M.; Fujisawa, T. Chem. Lett. 1981, Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 2001; Ch. 10. S9

10 Figure S1 Point Thickness (Å) Point Thickness (Å) average: 47.9 Å standard deviation: 0.23 Å Figure S1: Characterization of an Al 2 O 3 -coated Si electrode by ellipsometry. Wafer mapping of a 4" Si electrode after annealing in forming gas following ALD of Al 2 O 3. S10

11 Figure S2 Figure S2: Survey XPS spectrum of a Si electrode coated with 45 Å of Al 2 O 3 to which an octylphosphonic acid layer has been attached. S11

12 Figure S3 Figure S3: Assembly of a parallel-plate cell. (a) Cell components, from top to bottom: glass slide, copper mesh, Al 2 O 3 -coated silicon wafer, Kalrez 6375 gasket, Al 2 O 3 -coated silicon wafer, copper mesh, glass slide. (b) The assembly is clipped on one side to create a small gap on the other. (c) An electrolyte solution is pipetted inside the cell. (d) The other side of the assembly is clipped shut and quickly clamped tightly in a vise. (e) An assembled parallel plate cell reaction attached to a sourcemeter. S12

13 Figure S4 Figure S4: Double-step chronocoulometry trace of the standard cell at 3V. The electrode size is approximately 1.5 cm 2. S13

14 Figure S5 Figure S5. Selected Bode plots for standard parallel plate cells in 0.5 mm TBAPF 6 in CH 3 CN (A) and 0.5 mm TBAPF 6 in CH 2 Cl 2 (B). Plots in (C) show a comparison of the standard cell (data from (A)) to a cell with an octadecylphosphonic acid on the counter electrode V. S14

15 Figure S6 Figure S6: Current density vs. time for a standard parallel plate cell at +4.5 V (positive current). After 2 h, the cell was stepped to 0 V and the discharge current was recorded (negative current). S15

16 Figure S7 na / cm 2 Voltage / V Figure S7: Leakage current density vs. voltage for a standard parallel plate cell with a 0.5 mm TBAPF 6 solution in CH 3 CN. Leakage current was allowed to equilibrate for 2 min at each voltage. S16

17 Figure S8 Figure S8: Voltage-dependence of the rearrangement of cis-stilbene oxide catalyzed by a 45 Å Al 2 O 3 layer in the parallel plate cell. Experiments were performed in 0.5 mm TBAPF 6 in CH 3 CN (top) and CH 2 Cl 2 (bottom). The 2:3 product ratio (left axes) as a function of voltage is shown for experiments in which the counter electrode is coated with an octylphosphonic acid monolayer (black squares) or an octadecylphosphonic acid monolayer (red triangles). The gray columns depict the % conversion (right axes) of stilbene oxide. S17

18 Figure S9 1 A B Figure S9: Comparison of acid-catalyzed stilbene oxide rearrangements to reactions in the parallel plate cell at +4.5 V. (A) HPLC traces at 210 nm for the reaction of cis-stilbene oxide (1) in the presence of 100 μm HCl in a vial (blue) or at +4.5 V in the parallel plate cell with no acid (red). (B) Corresponding traces for the reaction of trans-stilbene oxide (1 ) under the same two conditions. Aldehyde product 2 and ketone product 3 are indicated. S18

19 Figure S10 Figure S10: Rearrangement of 2-(4-chlorophenyl)-3-phenyloxirane S19

20 Table S1 solvent 0 V 1 V 3 V 4.5 V A CH 3 CN B CH 2 Cl C CH 3 CN Table S1. Parallel plate cell double layer capacitances (μf/cm 2 ) extracted from the AC impedance at 20 Hz at various DC voltages. The cell is comprised of a catalyst electrode coated with 45 Å of Al 2 O 3 and a counter electrode with 45 Å of Al 2 O 3 coated with octylphosphonic acid (A and B) or octadecylphosphonic acid (C). Measurements were recorded in 0.5 mm TBAPF 6 in the indicated solvents. S20

21 Table S2 Solvent substrate 1M HCl 10M HCl 100M HCl CH 3 CN cis (<1%) 6.5 (1%) 4.9 (31%) CH 2 Cl 2 cis (<1%) 0.6 (4%) 3.2 (53%) CH 3 CN trans (<1%) 0.7 (1%) 12.5 (56%) Table S2. Ratios of products 2 to 3 produced by H + -catalyzed rearrangement of cis- or transstilbene oxide at different concentrations of HCl. Conversion of the stilbene oxide is shown in parentheses (not including the production of other unidentified products). S21