Aerobic oxidation of alkenes mediated by porphyrin rhodium(III) complexes in water

Abstract

Selective oxidation of alkenes in water to ketones using molecular oxygen as the oxidant is mediated by rhodium porphyrin complexes through observed β-hydroxy alkyl complexes.

---

Oxidation of olefins to produce epoxides, ketones, and aldehydes provides valuable routes for oxidative functionalization of C=C bond.^1–5 An important trend in the oxidation of hydrocarbons is to use dioxygen from air as the terminal oxidant.^6–12 Tetra (p-sulfonato phenyl) porphyrin rhodium complexes ((TSPP)Rh^III) in water are observed to mediate the selective oxidation of alkenes to ketones by atmospheric molecular oxygen through the intermediacy of β-hydroxy alkyl complexes (< 333K, 1 atm air).

Formation of β-hydroxy alkyl rhodium porphyrin complexes in water

Porphyrin rhodium(III) bisaquo complex [(TSPP)Rh^III(D₂O)₂]⁻³ (1) occurs in an equilibrium distribution with the mono and bishydroxo complexes, [(TSPP)Rh^III(D₂O)(OD)]⁻⁴ (2), [(TSPP)Rh^III(OD)₂]⁻⁵ (3) in water.¹³Ethene and larger terminal alkenes react regioselectively with (TSPP)Rh^III in D₂O solutions to form β-hydroxy alkyl complexes where rhodium is bonded to a terminal CH₂ unit ((TSPP)Rh-CH₂CH(OD)R(D₂O)) (eqn (1)).¹⁴

[(TSPP)Rh-OD(D₂O)]⁻⁴ + CH₂=CHR ⇌ [(TSPP)Rh-CH₂CH(OD)R(D₂O)]⁻⁴ (1)

The reactions to produce β-hydroxy alkyl complexes ((TSPP)Rh-CH₂CH(OD)R) (4) reach completion within hours in a borate buffer (pH = 9)(Table 1)(ESI).†

Table 1 Formation of β-hydroxy alkyl rhodium porphyrin complexes in water at 298K

Entry	Substrate	Time/h	Product	Yield
a The overall percentage of the formed (TSPP)Rh-CH2C(O)R in the process of the reaction. The concentration of (TSPP)Rh^III is 1.9 × 10^−3 M and the yield of the reaction was measured by ^1H NMR spectroscopy.
1		3		92 (3)^a
2		6.7		80 (6)^a
3		<5 min		>95
4		<5 min		>95
5		82		>95

---

Formation of the β-hydroxy ethyl octaethylporphyrin rhodium complex ((OEP)Rh-CH₂CH₂OH) was previously reported in hydrocarbon media through hydroxide nucleophilic attack on an ethene π complex ((OEP)Rh(CH₂=CH₂))⁺.¹⁵ The concentration of π complexes (TSPP)Rh(CH₂=CHR) in water at 298K was too small for observation by ¹H NMR.

Formation of ketones through β-hydrogen elimination.

The β-hydroxy alkyl complexes in the absence of air spontaneously transform to (TSPP)Rh^I and ketones over a period of hours at 333K in water (eqn (2), Table 2)(ESI).†

[(TSPP)Rh-CH₂CH(OD)R(D₂O)]⁻⁴⇌ [(TSPP)Rh(H)(D₂O)]⁻⁴ + CH₂DC(O)R (2)

Table 2 Stoichiometric conversion of β-hydroxy alkyl rhodium porphyrin complexes in water to ketones

Entry	Substrate	Time/h	Solvent	Product
a 353K. b When the reactions were run for a longer period of time, final products turned to be CD3C(O)CD2R. The reactions were run in borate buffer with pH = 9 and the product ketones were formed quantitatively as measured by ^1H NMR.
1		9	H2O
			D2O	^b
2		9	H2O
			D2O	^b
3		9	D2O	^b
4		16.5	H2O
		3^a	D2O

---

The resulting ketones were extracted by CDCl₃ and determined by the ¹H NMR and GC-MS compared with commercial samples of ketones(ESI).†

Thermal reaction of (TSPP)Rh-CH₂CH(OD)(CH₂)₂CH₃ in D₂O results in the formation of pentanone with deuterium incorporated into the methyl group (CH₂(D)C(O)CH₂CH₂CH₃) (J¹D–¹H = 2.2 Hz) (Fig. 1). The position for D atom incorporation into pentanone indicates that the ketone is formed through β-C–H elimination to form an enol containing an O–D unit (Scheme 1). This aspect of the mechanism differs from the Wacker process where all of the hydrogens come from the alkene. Substantially faster β-hydrogen elimination rate of Rh-CH₂CH(OD)R in water (333K, Table 2) compared to the process in benzene probably results from the efficacy of ionic pathways in water.

Fig. 1 ¹H NMR (400 MHz; CDCl₃) spectrum of pentanone produced from reaction of (TSPP)Rh-CH₂CH(OH)CH₂CH₂CH₃ in water. (A) (D₂O) CH₂(D)C(O)CH₂CH₂CH₃ in CDCl₃. (B) (H₂O) CH₃C(O)CH₂CH₂CH₃.

Scheme 1 Formation of ketones by thermal dissociation of (TSPP)Rh-CH₂CH(OD)R in water.

Aerobic oxidation of (TSPP)Rh^I to (TSPP)Rh^III in water

The brownish aqueous solutions of [(TSPP)Rh^I]⁻⁵ species formed in reaction 2 undergo an immediate colour change to a clear dark red solution when exposed to air, and the distinctive pyrrole ¹H NMR position for (TSPP)Rh^I species at 8.31 ppm is replaced by a pyrrole resonance at 9.04 ppm associated with (TSPP)Rh^III species.¹³ The sequence of reactions for dioxygen to convert (TSPP)Rh^I to (TSPP)Rh^III is depicted by eqn (3–7), and the overall catalytic reactions are shown in Scheme 2.

[(TSPP)Rh^I(D₂O)]⁻⁵ + O₂⇌ [(TSPP)Rh^III(O₂)(D₂O)]⁻⁵ (3)

[(TSPP)Rh^III(O₂)(D₂O)]⁻⁵ + D₂O ⇌ [(TSPP)Rh^III(D₂O)₂]⁻³ + O₂²⁻ (4)

O₂²⁻ + 2D₂O ⇌ D₂O₂ + 2OD⁻ (5)

D₂O₂⇌ D₂O + 1/2O₂ (6)

[(TSPP)Rh^I(D₂O)]⁻⁵ + 1/2O₂ + 2D₂O ⇌ [(TSPP)Rh^III(D₂O)₂]⁻³ +2OD⁻ (7)

Scheme 2 Proposed mechanism for aerobic oxidations of alkenes mediated by porphyrin rhodium(III) in water.

Rapid air oxidation of Rh(I) to Rh(III) completes the cycle which provides the possibility for catalytic oxidation of olefins. However, (TSPP)Rh^III reacts with α-C–H groups of ketones to form β-carbonyl organometallic derivatives (eqn (8)).

[(TSPP)Rh^III(D₂O)(OD)]⁻⁴ + CH₃C(O)R ⇌ [(TSPP)Rh-CH₂C(O)R(D₂O)]⁻⁴ + HOD (8)

The ketone products react with the Rh(III) oxidation catalyst resulting in a product inhibited catalytic oxidation of olefins with only small numbers of turnovers.

The oxidation reactions are envisioned to proceed through a four-step cycle involving: (a) displacement of one water molecule on (TSPP)Rh^III(H₂O)₂ by alkene to form (TSPP)Rh^III(CH₂=CHR) π complexes; (b) nucleophilic attack on the π complexes by hydroxide to produce β-hydroxy alkyl rhodium complexes; (c) release of ketones through β-hydrogen elimination; and (d) regeneration of (TSPP)Rh^III by molecular oxygen (Scheme 2).

This work was supported by a starter grant from Peking University, NSFC (grants 20841002 and 20801002), and the U.S. Department of Energy, Division of Chemical Science, Office of Science (grants DE-FG02–86ER13615 and DE-FG02–09ER16000).

Notes and references

1. (a) B. de Bruin, P. H. M. Budzelaar and A. W. Gal, Angew. Chem., Int. Ed., 2004, 43, 4142–4157; (b) S. Thewissen, M. D. M. Reijnders, J. M. M. Smits and B. de Bruin, Organometallics, 2005, 24, 5964–5972.
2. (a) R. S. Pryadun and J. D. Atwood, Organometallics, 2007, 26, 4830–4834; (b) R. Pryadun, D. Sukumaran, R. Bogadi and J. D. Atwood, J. Am. Chem. Soc., 2004, 126, 12414–12420.
3. (a) C. Liu, C. F. Bender, X. Han and R. A. Widenhoefer, Chem. Commun., 2007, 3607–3618; (b) A. R. Chianese, S. J. Lee and M. R. Gagne, Angew. Chem., Int. Ed., 2007, 46, 4042–4059.
4. S. S. Stahl, Angew. Chem., Int. Ed., 2004, 43, 3400–3420.
5. P. Zhao, C. D. Incarvito and J. E. Hartwig, J. Am. Chem. Soc., 2006, 128, 9642–9643.
6. T. Punniyamurthy, S. Velusamy and J. Iqbal, Chem. Rev., 2005, 105, 2329–2363.
7. (a) S. S. Stahl, Science, 2005, 309, 1824; (b) M. M. Konnick and S. S. Stahl, J. Am. Chem. Soc., 2008, 130, 5753–5762; (c) B. A. Steinhoff and S. S. Stahl, J. Am. Chem. Soc., 2006, 128, 4348–4355; (d) J. L. Brice, J. E. Harang, V. I. Timokhin, N. R. Anastasi and S. S. Stahl, J. Am. Chem. Soc., 2005, 127, 2868–2869.
8. T. J. Williams, A. J. M. Caffyn, N. Hazari, P. F. Oblad, J. A. Labinger and J. E. Bercaw, J. Am. Chem. Soc., 2008, 130, 2418–2419.
9. G. Brink, I. W. C. E. Arends, G. Papadogianakis and R. A. Sheldon, Chem. Commun., 1998, 2359–2360.
10. J. R. Khusnutdinova, L. L. Newman, P. Y. Zavalij, Y. Lam and Andrei N. Vedernikov, J. Am. Chem. Soc., 2008, 130, 2174–2175.
11. C. N. Cornell and M. S. Sigman, Inorg. Chem., 2007, 46, 1903–1909.
12. G. Jiang, J. Chen, H.-Y. Thu, J.-S. Huang, N. Zhu and C.-H. Che, Angew. Chem., Int. Ed., 2008, 47, 6638–6642.
13. X. Fu and B. B. Wayland, J. Am. Chem. Soc., 2004, 126, 2623–2631.
14. X. Fu, S. Li and B. B. Wayland, J. Am. Chem. Soc., 2006, 128, 8947–8954.
15. B. B. Wayland, S. L. VanVoorhees and K. J. Del Rossi, J. Am. Chem. Soc., 1987, 109, 6513–6515.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and scans of NMR spectra. See DOI: 10.1039/b904414k

---