Reactivity and kinetic–mechanistic studies of regioselective reactions of rhodium porphyrins with unactivated olefins in water that form β-hydroxyalkyl complexes and conversion to ketones and epoxides

Abstract

This article reports on the selective oxidation of unactivated alkenes to ketones and epoxides through the intermediacy of β-hydroxyalkyl rhodium porphyrin complexes which are formed by reactions of terminal alkenes with tetra(p-sulfonatophenyl)porphyrin rhodium(III) complex. The β-hydroxyalkyl rhodium porphyrin complexes in water undergo β-C–H elimination to produce ketones in aqueous pH 9.0 solutions and O–H deprotonation in KOH/DMSO solutions resulting in the rapid and quantitative intramolecular nucleophilic displacement to form 1,2-epoxyalkanes.

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Introduction

The activation of alkenes by coordination to metal centers is one of the most widely exploited synthetic methodologies in the functionalization of organic molecules,^1-5 and has contributed significantly to advances in selective organic transformations.^6-10

Catalytic air oxidation of ethene to acetaldehyde by an aqueous Pd(II)/Cu(II) catalyst system is known as the Wacker process. Mechanistic studies for the Wacker reaction and closely related processes have shown that alkene binding with palladium(II) activates reactions of olefins with nucleophiles including water, alcohols, and amines to form β-substituted alkyl complexes.¹¹ Subsequent C–H and O–H elimination reactions of the β-substituted alkyl complexes produce organic carbonyls, epoxides, heterocyclic molecules, and hydrofunctionalized alkenes.^12-15 Several rhodium and iridium complexes have also been reported to mediate oxidation of olefins in hydrocarbon media,^16-19 but relatively little attention has been given to complexes of transition metals other than palladium(II). The importance of olefin transformations and objectives of green chemistry justify the continuing search for new classes of catalyst materials that give oxidations in water and use dioxygen as the oxidant. This article reports on the observed stepwise reactions of rhodium porphyrins with olefins in water to form β-hydroxyalkyl complexes that react on to give stoichiometric oxidation of alkenes to ketones in aqueous media and epoxides in DMSO. Several objectives in olefin activation and oxidation have been advanced in this study through observation of product inhibited catalytic oxidation of olefins to organic carbonyls in water using dioxygen as the oxidant.

Results and discussion

Formation of β-hydroxyalkyl rhodium complexes in water

Tetra(p-sulfonatophenyl)porphyrin rhodium(III) diaquo complex [(TSPP)Rh^III(D₂O)₂]³⁻ (1) is a convenient entry point for aqueous (D₂O) solution reactivity studies of rhodium porphyrins. The rhodium(III) diaquo complex occurs in D₂O solution as a pH dependent equilibrium distribution with the mono- and bis-hydroxo complexes ([(TSPP)Rh^III(OD)(D₂O)]⁴⁻ (2) and [(TSPP)Rh^III(OD)₂]⁵⁻ (3)) where K₁ = 1.4 ± 0.2 × 10⁻⁸ and K₂ = 2.8 ± 0.3 × 10⁻¹²(eqn (1) and (2)).²⁰

[(TSPP)Rh^III(D₂O)₂]³⁻⇌ [(TSPP)Rh^III(OD)(D₂O)]⁴⁻ + D⁺ (1)

[(TSPP)Rh^III(OD)(D₂O)]⁴⁻⇌ [(TSPP)Rh^III(OD)₂]⁵⁻ + D⁺ (2)

Reactions of (TSPP)Rh^III complexes with a series of olefins produce β-hydroxyalkyl complexes in water.‡

Reaction of (TSPP)Rh^III with 2-substituted olefins. Ethene and larger terminal alkene hydrocarbons (CH₂=CHR) react regioselectively with (TSPP)Rh^III in D₂O to form β-hydroxy alkyl rhodium complexes ([(TSPP)Rh–CH₂CH(OD)R(D₂O)]⁴⁻) where rhodium is attached to the terminal primary CH₂ unit (eqn (3)).²¹

[(TSPP)Rh–OD(D₂O)]⁴⁻ + CH₂=CHR ⇌ [(TSPP)Rh–CH₂CH(OD)R(D₂O)]⁴⁻ (3)

The β-hydroxyalkyl complexes are easily identified by the porphyrin ring current induced high-field ¹H NMR resonances of the organic group bonded to the rhodium center. The ¹H NMR resonances for the diastereotopic α-CH₂ group in (TSPP)Rh–CH₂CH(OD)CH₂CH₂CH₃ in D₂O are centered at −5.93 and −5.74 ppm. Isolating ((TSPP)Rh–CH₂CH(OH)CH₂CH₂CH₃) formed in water and dissolving in anhydrous DMSO-d₆ permitted observing the doublet hydroxyl proton resonance ((TSPP)RhCH₂CH(OH)CH₂CH₂CH₃) at 0.01 ppm (d, ³J_HH = 5.6 Hz). Reactions of (TSPP)Rh^III with a series of terminal alkenes in a borate buffer with pH 9.0 produce β-hydroxyalkyl complexes [(TSPP)Rh–CH₂CH(OD)R(D₂O)]⁴⁻ (4) that reach equilibrium in time periods of several minutes to hours depending inversely on the olefin solubility (Table 1). Highly soluble alkenes (Table 1, entries 4–6) cleanly react to completion within minutes but poorly soluble alkenes (Table 1, entries 1–3) react much slower which allows time to produce some β-carbonyl organometallic derivatives ([(TSPP)Rh–CH₂C(O)R)(D₂O)]⁴⁻) by β-C–H elimination of 4.²²

Table 1 Formation of β-hydroxyalkyl rhodium porphyrin complexes in borate buffer aqueous solution (pH = 9.0) at 298 K^a

Entry	Substrate	t	Product	Conv. (%)
a The initial concentration of (TSPP)Rh^III was 1.9 × 10^−3 M. The yield of the reaction was measured by ^1H NMR spectroscopy.b The reaction was run in a mixed-solvent composed of 1,4-dioxane-d8 and D2O (1/4 v/v).c The values in parentheses are the overall percentage of the formed (TSPP)Rh–CH2C(O)R as by-product.
1		3 h		92 (3)^c
2		6.7 h		80 (6)^c
3		1.5 h^b		84 (2)^c
4		<5 min		>95
5		<5 min		>95
6		<5 min		>95
7		82 h		>95
8		1.4 h^b		94
9		34 h^b		89
10		34 h^b		91

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Reaction of (TSPP)Rh^III with 2,2-disubstituted alkenes in water. Reaction of (TSPP)Rh^III with a 2,2-disubstituted alkene (2-methyl-1-butene) also forms a β-hydroxyalkyl complex [(TSPP)Rh–CH₂C(OD)(CH₃)(CH₂CH₃)(D₂O)]⁴⁻ (5) (eqn (4)).

[(TSPP)Rh–OD(D₂O)]⁴⁻ + CH₂=C(CH₃)CH₂CH₃⇌ [(TSPP)Rh–CH₂C(OD)(CH₃)(CH₂CH₃)(D₂O)]⁴⁻ (4)

The ¹H–¹H and ¹⁰³Rh–¹H coupling patterns that are partially obscured by line broadening in water became clearly resolved when 5 was isolated from water and redissolved in CD₃OD (Fig. 1(b)). The characteristic AB pattern for the α-CH₂ unit (δ(ppm): −5.55 (H_A), −5.48 (H_B), J_AB = 9.9 Hz, J_RhH = 2.9 Hz) in 5 in CD₃OD is substantially improved from the broad resonance observed in D₂O at −5.72 ppm (Fig. 1(a)). The hydroxyl proton resonance for (TSPP)Rh–CH₂C(OH)(CH₃)(CH₂CH₃) was also identified from the ¹H NMR spectrum (s, −0.16 ppm) in anhydrous DMSO-d₆ by preparation of the sample in H₂O and transferring into DMSO-d₆ (Fig. 1(c)).

Fig. 1 High-field ¹H NMR (400 MHz) for (TSPP)Rh–CH_2αC(OH_β)(CH_3γ)(CH_2δCH_3ε) (a) in D₂O, (b) in CD₃OD, and (c) in DMSO-d₆.

Kinetics of alkene reaction with (TSPP)Rh^III in water

Evaluation of rate and equilibrium constants (k_f, k_r, K_eq). Formations of (TSPP)Rh–CH₂CH(OD)R as a function of time from reactions of (TSPP)Rh^III species with pentene and hexene in D₂O at 298 K using a pH = 9.0 buffer are illustrated in Fig. 2.

Fig. 2 Formation of [(TSPP)Rh–CH₂CH(OD)R(D₂O)]⁴⁻ from [(TSPP)Rh^III(OD)(D₂O)]⁴⁻ with pentene and hexene at 298 K with pH = 9.0 buffer. The solid line is the nonlinear least-square best fit to the equation d[(TSPP)Rh–CH₂CH(OD)R]/dt = β−α[(TSPP)Rh–CH₂CH(OD)R], β = k_fc_(Rh^T)c₀/(([D⁺]/K₁) + (K₂/[D⁺]) + 1), α = (β/c_(Rh^T)) + k_r using OriginPro 7.5 software. The initial concentration of (TSPP)Rh^III, c_(Rh^T), equals 1.9 × 10⁻³ M. The saturated solubility of pentene in water, c₀, is 2.1 × 10⁻³ M, and hexane is 1.9 ×10⁻³ M.²³ (A) Reaction of pentene gives α = (5.5 ± 0.1) × 10⁻⁴, β = (8.5 ± 0.1) × 10⁻⁷, and k_f = (2.3 ± 0.1) × 10⁻¹ L mol⁻¹ s⁻¹, k_r = (10.0 ± 1.0) × 10⁻⁵ s⁻¹, and K_eq = (2.3 ± 0.1) × 10³ are derived. (B) Reaction of hexane gives α = (2.2 ± 0.1) × 10⁻⁴, β = (2.7 ± 0.1) × 10⁻⁷ and k_f = (8.0 ± 0.1) × 10⁻² L mol⁻¹ s⁻¹, k_r = (7.8 ± 1.0) × 10⁻⁵ s⁻¹, and K_eq = (1.0 ± 0.1) × 10³ are derived.

Expressions were derived for several plausible mechanisms. The kinetics for this reaction are potentially complicated by having three different aquo/hydroxo complexes, but the monohydroxo complex is the dominant species (>93%) at pH = 9.0 so that effectively all of the β-hydroxyalkyl products were formed through the monohydroxo complex ([(TSPP)Rh^III(D₂O)(OD)]⁴⁻ (2)). Thus, the rate of formation of [(TSPP)Rh–CH₂CH(OD)R(D₂O)]⁴⁻ is derived as d[Rh–CH₂CH(OD)R]/dt = k_f[2][alkene] −k_r[Rh–CH₂CH(OD)R]. The best fit from nonlinear least squares curve fitting is obtained for the mechanism where effectively all of the product forms from reaction of 2 with alkenes (see Experimental section for detailed derivation). The forward rate constant for the pentene reaction at 298 K (k_f = (2.3 ± 0.1) × 10⁻¹ L mol⁻¹ s⁻¹) and the reverse reaction rate constant (k_r = (10.0 ± 1.0) × 10⁻⁵ s⁻¹), along with the corresponding values for the hexene reaction (298 K) (k_f = (8.0 ± 0.1) × 10⁻² L mol⁻¹ s⁻¹, k_r = (7.8 ± 1.0) × 10⁻⁵ s⁻¹) are obtained from the kinetic simulation (Fig. 2). The equilibrium constant for pentene K_eq (K_eq = k_f/k_r = (2.3 ± 0.1) × 10³) that is derived from the kinetic analysis is close to the value obtained from direct equilibrium thermodynamic measurement by integration of the ¹H NMR spectrum (K_eq = (4.1 ± 0.6) × 10³).

Reaction (3) occurs through formation of π complexes ((TSPP)Rh(CH₂=CHR)) by alkene substitution for a coordinated water molecule followed by nucleophilic attack on the olefin π complex and a proton transfer.

The regioselectivity for the initial nucleophilic attack step is controlled by the sterically bulky porphyrin ligand which favors placing rhodium on the less hindered terminal primary carbon as the kinetic product. In the case of unactivated alkenes the kinetic and thermodynamic products have rhodium on the primary –CH₂ ((TSPP)Rh–CH₂CH(OD)R), but olefins with electron withdrawing groups (R = phenyl, CO₂Na) initially form (TSPP)Rh–CH₂CH(OD)R and then rearrange to (TSPP)Rh–CH(R)CH₂OD as the thermodynamic products.

Electron withdrawing groups better stabilize the negative charge and provide an electronic energy term that favors placing the metal on the same carbon as the electron withdrawing group which for the phenyl and CO₂Na derivatives more than compensates for the unfavorable steric effect.^21,24

An octaethylporphyrin rhodium ethene π complex ((OEP)Rh(CH₂=CH₂))⁺ has been directly observed in toluene by ¹H NMR at 243 K. The β-hydroxyalkyl complex of rhodium octaethylporphyrin ((OEP)Rh–CH₂CH₂OH) was formed in hydrocarbon media through hydroxide nucleophilic addition to ((OEP)Rh(CH₂=CH₂))⁺.²⁵ Olefin π complexes of (TSPP)Rh^III in water at 298 K have not yet been observed by ¹H NMR even when the conditions for π complex formation are optimized by using high concentrations of water-soluble olefins and slightly acidic aqueous media where the diaquo complex 1 dominates.

Thermal dissociation of β-hydroxyalkyl rhodium porphyrin complexes

β-Hydroxyalkyl rhodium porphyrin complexes are intermediate organometallic complexes for both β-C–H elimination reactions that produce ketones in pH 9.0 aqueous solution and intramolecular C–O elimination to form epoxides quantitatively in KOH/DMSO (Scheme 1).

Scheme 1 Thermal dissociation pathways of β-hydroxyalkyl rhodium porphyrin complexes.

β-Hydrogen elimination of β-hydroxyalkyl rhodium porphyrin complexes in water. β-Hydrogen elimination reactions are among the most important fundamental organometallic transformations in a variety of transition metal alkyl and alkoxide complexes. The hydrogen migration or elimination usually occurs in coordinately unsaturated complexes through use of a vacant cis coordination site. However, β-hydrogen elimination processes for alkyl rhodium porphyrin complexes must utilize a different mechanism because all of the cis coordination sites are occupied by the porphyrin pyrrole nitrogen donors.²⁶ The β-hydroxyalkyl complexes formed by reaction (3) in water spontaneously transform to (TSPP)Rh–H and ketones in the absence of air by an effective β-hydrogen elimination (eqn (5), Table 2).²²

[(TSPP)Rh–CH₂CH(OD)R(D₂O)]⁴⁻⇌ [(TSPP)Rh(H)(D₂O)]⁴⁻ + CH₂DC(O)R (5)

Table 2 Formation of ketones through β-hydrogen elimination of β-hydroxyalkyl rhodium porphyrin complexes in pH = 9.0 buffer^a

Entry	Substrate	t/h	Solvent	Product
a The reactions were performed in borate buffers with pH = 9.0 in the absence of air, and the ketone products were nearly quantitatively formed as measured by ^1H NMR spectroscopy although small amounts of (TSPP)Rh^III were also observed.b 333 K.c 353K.
1		9^b	H2O
			D2O
2		16.5^b	H2O
		3^c	D2O
3		9^c	H2O
			D2O
4		3^c	H2O
5		3^c	H2O

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Thermal reaction of (TSPP)Rh–CH₂CH(OD)(CH₂)₂CH₃ in D₂O results in the formation of (TSPP)Rh–H and a mono-deuterated 2-pentanone (CH₂(D)C(O)CH₂CH₂CH₃) where deuterium is selectively incorporated into the methyl group. The selective deuteration indicates that β-hydrogen elimination produces an enol containing an OD unit (Scheme 2). This mechanistic feature is different from the Pd(II) Wacker oxidation where deuterium from D₂O does not occur in the oxidation product. Isomerization of β-hydroxyalkyl to the α-hydroxy isomer that occurs prior to the product forming step in the Wacker reaction cannot occur for the rhodium porphyrin because the cis coordination sites are blocked.

Scheme 2 Formation of ketones through (i) thermal β-hydrogen elimination of 4 and (ii) keto–enol tautomerism in water.

The observed fast reactions of rhodium porphyrin hydrides with olefins in water compared to benzene is ascribed to water supporting ionic reaction pathways.²⁷ In aqueous solution the hydride complex [(TSPP)Rh^III(D)(D₂O)]⁴⁻ functions as a weak acid and partially dissociates into D⁺ and [(TSPP)Rh^I(D₂O)]⁵⁻. The rhodium(I) porphyrin complex in water is proposed to activate olefins which subsequently protonate to form rhodium alkyl complexes. The β-C–H elimination or migration process is the microscopic reverse of the addition of the rhodium hydride to olefins in water which is proposed to occur by dissociation of Rh–H into H⁺ and Rh(I)^- and subsequent stepwise addition.²⁷ The complex (TSPP)Rh–CH₂C(OD)(CH₃)(CH₂CH₃) 5 lacks a β-hydrogen atom and thus is not capable of producing carbonyl compounds.

Aerobic oxidation of (TSPP)Rh–H to (TSPP)Rh^III in water.

The [(TSPP)Rh(H)(D₂O)]⁴⁻ species formed in reaction (5) in a rapid dissociation equilibrium with [(TSPP)Rh^I(D₂O)]⁵⁻ and H⁺ undergo an immediate color change to a clear dark red solution when exposed to air. The distinctive pyrrole hydrogen resonance associated with (TSPP)Rh^III species is observed by ¹H NMR. The sequence of reactions with dioxygen to convert (TSPP)Rh–H/(TSPP)Rh^I to (TSPP)Rh^III is depicted by Scheme 3.

Scheme 3 Aerobic oxidations of (TSPP)Rh–H/(TSPP)Rh^I to (TSPP)Rh^III in water.

Rapid air oxidation of Rh(I) to Rh(III) completes the cycle which provides the potential for catalytic oxidation of olefins.

Formation of β-carbonyl alkyl rhodium porphyrin complexes in water

Aqueous buffer solutions of (TSPP)Rh^III species react with ketones and aldehydes to produce β-carbonyl derivatives [(TSPP)Rh–CH₂C(O)R(D₂O)]⁴⁻ (6), which has a precedent in reaction of [(OEP)Rh^III]⁺ with ketones.²⁸ This electrophilic C–H activation by the cationic metal center is thought to proceed through a stepwise mechanism similar to that of reaction (3), in which the enol intermediate reacts with (TSPP)Rh^III to give (TSPP)Rh–CH₂C(OD)₂R, followed by the dehydration step of the unstable gem-diol complex.

Reaction of the acetaldehyde methyl C–H unit occurs with high regioselectivity to form (TSPP)Rh–CH₂CHO (eqn (6)). No evidence is found for reaction of the weaker aldehydic C–H bond, which is analogous to reactions of (por)Rh^II complexes with CH₃CHO in benzene.²⁹

[(TSPP)Rh–OD(D₂O)]⁴⁻ + CH₃CHO ⇌ [(TSPP)Rh–CH₂CHO(D₂O)]⁴⁻ + HOD (6)

The compound formed from reaction of (TSPP)Rh^III with acetone (eqn (7)) has an identical ¹H NMR spectrum with the complex formed from reaction of (TSPP)Rh^I in water with ClCH₂C(O)CH₃ that produces (TSPP)Rh–CH₂C(O)CH₃.

[(TSPP)Rh–OD(D₂O)]⁴⁻ + CH₃C(O)CH₃⇌ [(TSPP)Rh–CH₂C(O)CH₃(D₂O)]⁴⁻ + HOD (7)

Addition of 300 equiv. of 2-pentanone to an aqueous solution of (TSPP)Rh^III in pH 9.0 D₂O buffer resulted in an equilibrium distribution of (TSPP)Rh–CH₂C(O)CH₂CH₂CH₃ and (TSPP)Rh^III which permitted evaluation of the equilibrium constant (K_eq(298 K) = 33). The relatively slow reaction rates of ketones compared to alkenes may result from the very low enol concentration. The enolization equilibrium constant K_E for acetone³⁰ is (4.69 ± 0.19) × 10⁻⁹ and the resulting low concentration of enol place limits on the rate of β-carbonyl formation. 2-Pentanone can enolize in two ways resulting in regio-isomeric enols, CH₂=C(OD)CH₂CH₂CH₃ (A) and CH₃C(OD)=CHCH₂CH₃ (B), but the reaction of (TSPP)Rh^III with 2-pentanone exclusively produces (TSPP)Rh–CH₂C(O)CH₂CH₂CH₃. The origin of this result may be the thermodynamic preference for primary alkyl rhodium complexes.

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

Formation of epoxides from β-hydroxyalkyl rhodium porphyrin complexes

The β-hydroxyalkyl rhodium porphyrin complexes, (TSPP)Rh–CH₂CH(OD)(CH₂)_nCH₃ (n = 2–4) (eqn (8)) undergo immediate (<5 min) epoxide forming elimination reactions in KOH/DMSO-d₆ (c_KOH = 3.3 mg mL⁻¹) at room temperature to give quantitative formation of (TSPP)Rh–D which is rapidly deprotonated to form (TSPP)Rh^I and 1,2-epoxyalkanes as observed by both ¹H NMR and GC-MS (Table 3).

Table 3 Formation of 1,2-epoxyalkanes through epoxide-forming elimination of β-hydroxyalkyl rhodium porphyrin complexes in KOH/DMSO

Entry	Substrate	t	Product
The reactions were performed in KOH/DMSO (cKOH = 3.3 mg mL^−1) and the product epoxides were formed quantitatively as measured by ^1H NMR spectroscopy. The resulting 1,2-epoxyalkanes were extracted into Et2O and examined by GC-MS.
1		<5 min
2		<5 min
3		<5 min
4		<5 min

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A brownish solution characteristic of (TSPP)Rh^I is observed to form immediately after (TSPP)Rh–CH₂CH(OD)(CH₂)_nCH₃ is dissolved in KOH/DMSO-d₆. C–O reductive elimination from the d⁶ metal center usually occurs through an S_N2 route and a direct C–O elimination pathway.³¹ Groves reported a facile C–O reductive elimination from β-hydroxyalkyl complexes of rhodium tetraphenyl porphyrin in KOtBu/C₆D₆ by a proposed S_N2 pathway.¹⁹

The origin of the selective formation of ketone in water and epoxide in KOH/DMSO may be that the nucleophilicity of the alkoxy group is dramatically reduced in water due to the solvation. β-Hydroxyalkyl rhodium porphyrin complexes thus proceed through the alternate usually higher energy β-H elimination pathway which produces ketones. The favored formation of three-membered oxygen heterocycles through intramolecular C–O bond formation is driven by the strong nucleophilicity of the alkoxy group formed in strongly basic conditions of the (KOH/DMSO) medium.

Conclusions

(TSPP)Rh^III reacts with terminal alkenes to form β-hydroxy alkyl rhodium porphyrin complexes with high conversion in pH 9.0 aqueous buffer solution at room temperature. Regioselectivity of the reaction is controlled by the sterically bulky porphyrin ligand and the less-substituted alkyl rhodium complexes are formed as the kinetic products. The equilibrium constant of reaction of (TSPP)Rh^III with pentene was evaluated as K = (2.3 ± 0.1) × 10³ from kinetic simulation. The β-hydroxyalkyl rhodium porphyrin complexes underwent the β-hydrogen elimination reaction to produce ketones in pH 9.0 aqueous solution and near quantitative intramolecular formation of epoxides in KOH/DMSO. Product inhibited catalytic oxidation of olefins to ketones by (TSPP)Rh^III is accomplished by using dioxygen as the oxidant.

Experimental

Typical procedure for preparation of (TSPP)Rh–CH₂CH(OD)R in water

Alkenes (0.01 mmol) and 1 (1.1 mg, 0.001 mmol) were dissolved in 0.4 mL borate buffer D₂O solution (pH = 9.0) in J. Young valve adapted NMR tubes at room temperature. The progress of the reaction was monitored by ¹H NMR spectroscopy.

Typical procedure for β-hydrogen elimination of (TSPP)Rh–CH₂CH(OD)R in water

The (TSPP)Rh–CH₂CH(OD)R complexes were prepared according to the procedure given above, which exclusively converted (TSPP)Rh^III into (TSPP)Rh–CH₂CH(OD)R. The excesses of alkenes and solvent D₂O were pumped out. Fresh D₂O was added into the NMR tube and subjected to three freeze–pump–thaw cycles. The initial ¹H NMR spectrum was recorded to show the formation of Rh–CH₂CH(OD)R and a clean range from 0 to 4 ppm. The sample (TSPP)Rh–CH₂CH(OD)R was heated in a water-bath at 60 °C (or 80 °C) for a period of hours, and the progress of the reaction was monitored by ¹H NMR spectroscopy. When the reactions reached completion after all (TSPP)Rh–CH₂CH(OD)R complexes were converted to ketones and (TSPP)Rh–D (which was rapidly deprotonated to form (TSPP)Rh^I), the product ketones were extracted by CDCl₃. A parallel sample of (TSPP)Rh–CH₂CH(OH)R dissolved in H₂O was also heated under the same reaction conditions, and extracted by CDCl₃. Both ¹H NMR and GC-MS were used to characterize the product ketones.

Kinetic simulation for reaction of (TSPP)Rh^III with pentene

We then have:

d[4]/dt = k₃[2]c₀−k₋₃[4]

[1] = [2][D⁺]/K₁,

[3] = [2]K₂/[D⁺],

[1] + [2] + [3] + [4] = c_(Rh^T)

c₀ = [CH₂=CHCH₂CH₂CH₃]

The concentration of 2 is related to 4,
As stated above:

d[4]/dt = k₃[2]c₀−k₋₃[4]

= β−α[4]

where
so

[4]_t = (β/α)[1 − exp(−αt)]

β = (8.5 ± 0.1) × 10⁻⁷, α = (5.5 ± 0.1) × 10⁻⁴ are obtained from simulation, and k₃ = (2.3 ± 0.1) × 10⁻¹ L mol⁻¹ s⁻¹, k_-3 = (10.0 ± 1.0) × 10⁻⁵ s⁻¹, and K₃ = (2.3 ± 0.1) × 10³ are derived when c_(Rh^T) = 1.9 × 10⁻³ M and c₀ = [CH₂=CHCH₂CH₂CH₃] = 2.1 × 10⁻³ M are used.

Acknowledgements

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 Sciences, Office of Science DE-FG02–09ER16000.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental procedures and NMR spectra. See DOI: 10.1039/b912219b

‡ Tetra(p-sulfonatophenyl)porphyrin rhodium(III) monohydroxide complex ([(TSPP)Rh^III(D₂O)(OD)]⁴⁻) and its alkyl derivatives [(TSPP)Rh^III–R(D₂O)]⁴⁻ are 4− charged complexes and abbreviated as (TSPP)Rh–OD and (TSPP)Rh–R, respectively.

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