Oxidatively-Induced Reductive Elimination of Dioxygen from an η 2 -Peroxopalladium(II) Complex Promoted by Electron-Deficient Alkenes Brian V. Popp and Shannon S. Stahl* Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706 Table of Contents Pages General Considerations Dioxygen Gas Trapping Experiment Details Dioxygen Evolution and Trapping Data S2 S3 S4 1 H-NMR Spectroscopic Kinetic Study Details S4 1 H NMR Spectral Timecourse of the Dioxygen Substitution Reaction S5 Kinetic Fitting of 1 H NMR Spectroscopic Kinetic Study UV-visible Spectroscopic Kinetic Study Details Eyring Plots for Dioxygen Substitution Activation Parameters for Dioxygen Substitution S5 S6 S7 S7 S1
General Considerations. All syntheses and subsequent handling of palladium complexes was performed under a nitrogen atmosphere using either a glovebox (MBraun) or standard Schlenk techniques. Dichloromethane (Fisher) was purified by passing it over a column of activated alumina (A-2, Purifry). Dichloromethane-d 2 was obtained from Cambridge Isotope Laboratories, Inc. and was dried over CaH 2 and vacuum distilled into a Schlenk tube for storage in the glovebox. (bc)pd(dba) and (bc)pd(o 2 ), 1, were synthesized according to previously published methods 1 and (bc)pd(ns X ) complexes, 2 X, have been previously characterized. 2 All β-nitrostyrene derivatives (ns X ) were obtained from Aldrich or Fluka and were used as received. NMR spectroscopic data were obtained at 25.0 C using a Varian INOVA 500 MHz or a Bruker AC 300 MHz spectrometer. S2
Dioxygen Gas Trapping Experiment. This experiment was designed to quantify and characterize the dioxygen evolved during the reaction of (bc)pd(o 2 ) (1) with alkene (ns NO 2 ) via a chemical trap. The reaction apparatus used for this reaction is depicted in Figure S1. The experiment was carried out under an N 2 atmosphere inside of a glovebox in order to prevent exposure of the solutions to exogenous dioxygen. In reaction A, 8(2) µmol of 1 and 240 µmol of ns NO 2 were dissolved in 1 ml of CD 2Cl 2. In reaction B, 26(2) µmol of (bc)pd(dba) was dissolved in 1 ml of CD 2 Cl 2. The reactions were vigorously stirred for six hours in the sealed reaction apparatus, and subsequently, the reaction mixtures were transferred under N 2 to NMR tubes equipped with Teflon screw caps. 1 H NMR spectroscopic analysis using 1,3,5-tri-tertbutylbenzene as an internal standard revealed quantitative conversion of 1 to 2 NO 2 and (bc)pd(dba) to 1 within experimental error (Table S1). 6 cm Reaction A Reaction B Figure S1. Dioxygen Evolution and Trapping Reaction Apparatus. S3
Table S1. Dioxygen Evolution and Trapping Reaction Analysis. Reaction A a Reaction B a 1 2 NO 2 (bc)pd(dba) 1 Before Rxn 8(2) 0 26(2) 0 After Rxn 0 8(2) 16(2) 10(2) a Values reported in µmol. 1 H-NMR Spectoscopic Kinetic Studies. All data were acquired using a Varian Inova 500 MHz spectrometer. Experimental samples were prepared from stock solutions prepared in dried and degassed dichloromethane-d 2. A representative experiment for the substitution reaction between 1 and ns CF 3 follows. Using gas tight syringes, the stock solutions were transferred to an NMR tube with a J-Young valve to obtain a final sample concentration of 1.5 mm for 1 and 22.5 mm for ns CF 3. Immediately upon addition of the reagents, the tube was immersed in liquid N 2 and the head space of the tube was evacuated to ensure complete removal of gaseous O 2. Samples were thawed prior to insertion into the NMR probe and kinetic data acquisition was initiated after an equilibration time of approximately 5 minutes. The bathocuproine methylgroups for 1 and 2 CF 3 were integrated over the kinetic time-course (Figure S2). The kinetic decay of 1 and the concomitant rise of 2 CF 3 were found to fit a simple exponential equation (Figure S3). Absolute concentrations of 1, 2 CF 3 and nscf 3 were obtained by using 1,3,5-tri-tert-butylbenzene as an internal standard. S4
1 H NMR Spectral Timecourse of the Dioxygen Substitution Reaction Figure S2. 1 H NMR Spectra of the Reaction Timecourse for Dioxygen Substitution with ns CF 3. [Pd] (M) 0.0008 0.0007 k = 0.132 M -1 sec -1 for 0.0006 0.0005 0.0004 0.0003 0.0002 k = 0.178 M -1 sec -1 for 0.0001 0 0 200 400 600 800 1000 1200 time (sec) Figure S3. Kinetic Analysis of the Reaction Timecourse Monitored by 1 H-NMR Spectroscopy. S5
UV-visible Spectroscopic Kinetic Studies. All UV-visible kinetics data were acquired on a PC-controlled Cary 3E spectrophotometer using WinUV 2.01 software. A Cary 1x1 peltier temperature controller was used to regulate the sample temperature between 10 C and 40 C. Solutions of known concentration were used to determine extinction coefficients for the reactants and products by the method of Beer s Law. A representative experiment for the substitution reaction between 1 and ns NO 2 follows. Stock solutions of (bc)pd(o 2) (10.2 mm) and ns NO 2 (75.9 mm) in dichloromethane were prepared in the glovebox and removed from the glovebox in a schlenk tube equipped with a 4 mm Kontes Teflon valve. The (bc)pd(o 2 ) stock solution was maintained at -78 C to minimize decomposition of the palladium complex. A gas-tight UVvisible cell equipped with a side-arm reservoir was used to allow both stock solutions to be added to the cell under a nitrogen atmosphere prior to initiating the reaction. The palladium stock solution (150µl) and dichloromethane (2.35 ml) were added via gas-tight syringe to the UV-visible cell, and the ns NO 2 solution (500 µl) was added to the side-arm. The total volume of the reaction was maintained at 3 ml for all experiments. After obtaining the initial absorbance reading of the (bc)pd(o 2 ) solutions, the contents of the cell and side-arm were mixed rapidly and single-wavelength (425 nm) data collection was initiated. The reactions were typically monitored for >4 half-lives. A text file of the reaction time-course was imported into Microsoft Excel for fitting. The pseudo-first order integrated rate law was used to fit the data. Three parameters (k obs, A 0, A inf ) were varied in order to minimize the sum of the squared deviations between the experimental data and the non-linear least squares fit (eq 1). A representative data set and fit is shown in Figure 1A. A t = (A " + (A 0 # A " )exp(#k obs * t)) (1) S6
Eyring Plots for Dioxygen Substitution -5.5-6.5 NO2 CF3 Br F ln(k for /T) -7.5-8.5-9.5 0.0031 0.0032 0.0033 0.0035 0.0036 1/T (K -1 ) Figure S4. Eyring plots showing the temperature dependent behavior of the exchange of olefin for dioxygen between 1 and ns X between 10 40ºC measured by UV-visible spectroscopy. Activation parameters were determined by a non-linear least squares solution of the Erying equation. Table S2. Activation Parameters for the Dioxygen Exchange Reaction between 1 and ns X. Entry Olefin k for (M -1 sec -1 ) a H (kcal/mol) S (e.u.) G (kcal/mol) b 1 ns NO 2 0.410 7.3(7) -36(2) 18.0±1.3 2 ns CF 3 0.159 9.4(8) -31(2) 18.6±1.4 3 ns Br 0.050 10.0(5) -31(2) 19.2±1.1 4 ns F 0.043 10.3(4) -30(1) 19.2±0.7 a. Observed at 298 K. b. Calculated at 298 K. 1 Stahl, S. S.; Thorman, J. L.; Nelson, R. C.; Kozee, M. A. J. Am. Chem. Soc. 2001, 123, 7188-7189. 2 (a) Stahl, S. S.; Thorman, J. L.; de Silva, N.; Guzei, I. A.; Clark, R. W. J. Am. Chem. Soc. 2003, 125, 12-13. (b) Popp, B. V.; Thorman, J. L.; Morales, C. M.; Landis, C. R.; Stahl, S. S. J. Am. Chem. Soc. 2004, 126, 14832-14842. S7