Evidence for an HX-Reductive-Elimination Pathway

Transcription

1 Supporting Information Reaction of Molecular Oxygen with a Pd II Hydride to Produce a Pd II Hydroperoxide: Experimental Evidence for an HX-Reductive-Elimination Pathway Michael M. Konnick and Shannon S. Stahl* Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin Synthesis and Characterization of Pd Hydride 1 Synthesis and Characterization of Pd Hydroperoxide 2 Characterization of Decomposition Product 6 S2 S2 S3 1 H NMR Spectra of 6 S4-S5 Exponential Timecourse for the Oxygenation of 1 at Low O 2 Pressure S5 S1

2 Synthesis and Characterization of Pd Hydrides, Pd(IMes) 2 (H)(OAr) (Ar = Ph, 1, p- CH 3 C 6 H 4, 1 CH3 ; p-o 2 NC 6 H 4, 1 NO2 ). Pd(IMes) 2 (52.0mg, 72.7µmol) was dissolved in 10 ml of diethylether. A solution of benzoic acid (8.9mg, 72.9µmol) in diethylether (10 ml) was added dropwise over the course of 5 minutes. After allowing the solution to stir for 48 hours, a faint yellow color (associated with trace Pd(IMes) 2 ) was evident, indicating that no unreacted acid remained in solution. The solution was filtered through celite and evaporated. Recrystallization from ether/pentane at -30 C yielded the colorless 1 (39 mg, 64%). X-ray quality crystals were obtained by slow diffusion of pentane into a saturated toluene solution at -30 C. 1 The other two derivatives were prepared similarly. 1: 1 H NMR (500 MHz, C 6 D 6 ): δ 8.09 (dd,2h), 7.33 (m,2h), 7.28 (m,1h), 6.80 (s,8h), 6.10 (s,4h), 2.34 (s,12h), 2.00 (s, 24H), (s, 1H); 13 C NMR (125 MHz, C 6 D 6 ): δ (N- C-N), (Ar-COO-), 141.4, 137.7, 137.4, 136.4, 131.0, 129.4, 128.7, (aryl C), (- HC=CH-), 21.7 (p-me), 18.8 (o-me). IR(ATR): 2036 cm -1 (Pd-H). 1 CH3 : 1 H NMR (500 MHz, C 6 D 6 ): δ 8.01 (dd,2h), 7.13 (dd,2h), 6.82 (s,8h), 6.10 (s,4h), 2.35 (s,12h), 2.25 (s, 3H), 2.00 (s, 24H), (s, 1H); 13 C NMR (125 MHz, C 6 D 6 ): δ (N-C-N), (Ar-COO-), 138.5, 137.9, 137.7, 137.5, 136.5, 131.1, 129.4, (aryl C), (-HC=CH-), 21.8 (p-me), 21.7 (p- MeArCOO-), 18.7 (o-me). 1 NO2 : 1 H NMR (500 MHz, C 6 D 6 ): δ 8.08 (dd,2h), 7.88 (dd,2h), 6.75 (s,8h), 6.16 (s,4h), 2.31 (s,12h), 1.92 (s, 24H), (s, 1H); 13 C NMR (125 MHz, C 6 D 6 ): δ (N-C-N), (Ar-COO-), 149.0, 146.4, 137.7, 137.5, 136.2, 131.3, 129.4, (aryl C), (-HC=CH-), 21.6 (p-me), 18.6 (o-me). Independent preparation of Pd(IMes) 2 (OOH)(OBz), 2. Independent preparation of 2 from Pd(IMes) 2 (O 2 ) was accomplished through a variation of the established literature procedure 2 : Pd(IMes) 2 (O 2 ), (58.0 mg, 77.6 µmol) was dissolved in 10 ml S2

3 of toluene. A solution of BzOH (9.5 mg, 77.8 µmol) in toluene (10 ml) was added to the Pd(IMes) 2 (O 2 ) solution in the dark, resulting in an immediate color change from brown to colorless. The solution was stirred for 15 min, filtered through celite, and the solvent was removed under vacuum. Purification was accomplished via recrystallization from ether/pentane at -30 C to yield the colorless 2 (38 mg, 56%). 1 H NMR (500 MHz, C 6 D 6 ): δ 7.78 (dd, 2H), 7.23 (m, 3H), 6.87 (broad s, 8H), 6.08 (s, 4H), 4.02(broad s, 1H), 2.30 (s,12h), 2.16 (broad s, 24H). 13 C NMR (125MHz, C 6 D 6 ): δ (N-C-N), (Ar-COO-), 139.1, 137.7, 137.4, 137.0, 130.8, 129.5, (aryl C), (-HC=CH-), 21.6 (p-me), 19.3 (o-me). (a missing benzoate aryl resonance is most likely hidden by the residual solvent triplet, as in the case for the corresponding hydride, 1). IR(ATR): 3502 cm -1 (PdOO-H). Characterization of Decomposition Product 6. N N BzO Pd O IMes 6 The product formed upon decompostion of Pd II -hydroperoxide 2 is tentatively assigned as 6 on the basis of 1 H NMR and mass spectral data (Figures S1-S3). Eight new resonances appear in the aliphatic region of the 1 H NMR spectrum ( ppm) during the decomposition of 2 (Figure S2), together with an additional resonance at 4.8 ppm. These peaks correspond to the methyl and methylene protons, respectively, of 6. New aryl/heterocycle resonances ( ppm) and a broad peak corresponding to H 2 O (0.52 ppm) are also observed (Figure S1). Variabletemperature 1 H NMR spectra of the resonance at ~4.8 ppm reveals a singlet at high temperature (>70 C) that splits into an AB quartet at lower temperatures (Figure S3). These data are consistent with assignment of this resonsance(s) to the diastereotopic methylene protons of S3

4 oxygenated IMes ligand. Mass spectral analysis (MALDI-TOF) of this complex revealed a peak at m/z = 728.2, which corresponds to the mass of 6 minus BzOH. Figure S1. Full 1 H NMR spectrum of the crude product 6 (C 6 D 6, 73 C) obtained following decomposition of the Pd hydroperoxide species 2. For an expansion of the aliphatic region, see Figure S2. S4

5 Figure S2. 1 H NMR spectrum of the aliphatic region for the crude product 6 (C 6 D 6, 73 C) obtained following decomposition of the Pd hydroperoxide species 2. Ar N N BzO Pd O H H IMes Figure S3. Variable-temperature 1 H NMR spectra of 6, focusing on the resonance at 4.8 ppm, which corresponds to the methylene protons of the oxygenated IMes methyl group. S5

6 0.8 [Pd-H (1)] (mm) Time (min) Figure S4. Exponential decay of hydride at low O 2 pressure in benzene. Conditions: [1] 0 = 1.0 mm, 0.32 atm O 2 (2.9 mm dissolved O 2 ), [TBAOBz] = 13 mm, 0.4 ml C 6 D 6, 65 C. 1 For X-ray crystallographic analysis, see: Konnick, M. M.; Gandhi, B. A.; Guzei, I. A.; Stahl, S. S. Angew. Chem., Int. Ed. 2006, 45, Konnick, M. M.; Guzei, I. A.; Stahl, S. S. J. Am. Chem. Soc. 2004, 126, S6