Aerobic oxidative carboxylation of olefins with metalloporphyrin catalysts

Abstract

Dioxo(tetraphenylporphyrinato)ruthenium(VI) and quaternary onium salt were successfully developed as catalysts to initiate a three component reaction of olefin, O₂, and CO₂ at ambient temperature under low pressure. The reaction can be carried out under solvent-free or solvent conditions. Styrene carbonate was obtained in 76% yield with 100% selectivity using 4 mol% of catalyst under optimized conditions.

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The catalytic transformation of carbon dioxide into useful organic compounds has attracted much attention during the last two decades due to economic and environmental benefits arising from the utilization of renewable sources and a growing concern about the greenhouse effect.^1,2 The preparation of cyclic carbonates (CC) via cycloaddition of CO₂ with epoxides is one of the methodologies for CO₂ fixation^3–11 in terms of their valuable uses such as organic synthetic intermediates, monomers, aprotic polar solvents, pharmaceutical/fine chemical intermediates.^12–20

The direct oxidative carboxylation of olefins seems to be an efficient method for the synthesis of cyclic carbonates (Scheme 1). Aresta et al. reported the one-pot synthesis of styrene carbonate from styrene, CO₂ and molecular oxygen as an oxidant with homogeneous rhodium complex catalysts^21,22 or metal oxide,²³ which, however, suffer from high pressure, low yields of the desired carbonates and by-products such as benzaldehyde, benzoic acid, acetophenone, phenylacetaldehyde and styrene oxide. Srivastava et al. used titanosilicate catalysts for the direct synthesis of cyclic carbonate from styrene in a single reactor; initially they conducted the epoxidation of olefin with H₂O₂ or tert-butyl hydroperoxide (TBHP) at 60 °C and then added CO₂ and a co-catalyst with an organic base at a temperature of 120 °C. The yield of carbonate was not high although the reaction conditions were optimized.²⁴ Arai et al. used TBHP as an oxidant to synthesize carbonates from styrene and CO₂. The yield of carbonate was higher than in the literature above, but it also suffers from high pressure, high temperature and by-products such as benzaldehyde and benzoic acid.^25–30 Li et al. developed a catalytic system of N-bromosuccinimide (NBS) together with 1,8-diazabicyclo[5.4.0]undecenc-7-ene (DBU) to synthesize carbonate with aqueous H₂O₂ as an oxidant under high pressure and temperature.³¹

Scheme 1 The synthesis of carbonate from olefins.

In the search for an alternative homogeneous catalyst for this aerobic oxidative carboxylation approach and expanding our own efforts toward the development of efficient catalysts for cycloaddition of CO₂ to epoxides,³ we initiated our investigations using metalloporphyrin catalysts following the principle that ruthenium porphyrin catalysts can catalyze the epoxidation of alkenes without reductive aldehyde additives.^32,33 Herein, we demonstrate a convenient route to synthesize cyclic carbonates by an aerobic oxidative carboxylation of olefins using O₂ as an oxidant and CO₂ as a carboxylation reagent with 100% selectivity and high yield under lower pressure at ambient temperature.

We have discovered that dioxo(tetraphenylporphyrinato) ruthenium(VI) [Ru(TPP)(O)₂](1)/quaternary onium salt can effectively catalyze the aerobic oxidative carboxylation of olefins at ambient temperature under 1.6 MPa pressure of oxygen and carbon dioxide. Styrene reacts with 2 equivalents of TBAB in the presence of 0.4 mol% of catalyst 1 to generate styrene carbonate (SC) in 19.7% yield within 48 h at 30 °C under solvent-free conditions. It is noteworthy that this three component reaction does not lead to any by-products. Moreover, with catalyst loading as low as 0.1 mol%, NMR monitoring of the reaction showed a slight conversion in the product after 48 h. These results outshine the use of heterogeneous catalysts of metal oxide or homogeneous rhodium complex catalysts which require higher temperature and pressure.

The analysis and identification of products were performed by GC-MS, ¹H NMR, and IR etc. As shown in Table 1, these attractive reactions can be carried out under solvent-free conditions or with solvent. The ruthenium catalyst 1 combined with tetrabutylammonium bromide (TBAB) was found to be the best catalyst for this three component reaction (Entries 1-10). Any additives diminished the reaction rate (Entries 7, 8–10). The cocatalyst of CrO₃/CrCl₃ induced many by-products of benzaldehyde and benzoic acid (Entry 10). In dichloromethane, the reaction was carried out very well to yield the desired SC in about 76% yield with 100% selectivity (Entry 13) using 2 equivalents of tetrabutylammonium iodide (TBAI) as cocatalyst. More or less cocatalyst loading decelerated the reaction rate under both solvent-free conditions and with solvent (Entries 1, 3 and 12-14). Surprisingly, this reaction can be achieved in air (Entry 20) and can also be accomplished in other solvents such as ethanol (Entry 21) and THF (Entry 22). The common biomimetic catalyst of Fe(TPP)Cl can lead to styrene carbonate formation in poor yield (3.6%) accompanied by benzoic acid (55.3%) and benzaldehyde (37.5%) as major products because of its good oxidative property and poor coupling property from epoxide to cyclic carbonate (Entry 23). Meanwhile, the in situ catalyst of Ru(Salen)(O) generated from Ru(Salen)(PPh₃)₂ by oxidation with m-CPBA can convert styrene to styrene carbonate in considerable yield (40.7%) with a little benzaldehyde (2.3%) as a by-product (Entry 24).

Table 1 Aerobic oxidative carboxylation of styrene^a

Entry	Co-catalyst	Styrene	Time/h	SC Yield (%)^h
a Catalyst 1 37.5 mg (0.05 mmol), cocatalyst 1-3 equivalents of catalyst, styrene 10-250 equivalents of catalyst, P(O2) 0.5 MPa, P(CO2) 1.1 MPa, CH2Cl2 5 ml, T 30 °C.b Neat.c P(air) 2.0 MPa.d Ethanol 5 ml.e THF 5 ml.f Catalyst Fe(TPP)Cl.g Catalyst Ru(Salen)(PPh3)2.h Determined by ^1H NMR
1	1TBAB	250^b	48	trace
2	2TBAB	250^b	48	19.7
3	3TBAB	250^b	48	<1
4	2TBAF	250^b	48	trace
5	2TBAC	250^b	48	2.2
6	2TBAI	250^b	48	1.5
7	2TBAB/imidazole	250^b	48	trace
8	2TBAB/V2O5	250^b	120	4.6
9	2 PTAT	250^b	48	<1
10	2CrO3/CrCl3	250^b	120	trace
11	2TBAI	50	48	23.9
12	1TBAI	25	48	9.8
13	2TBAI	25	48	76
14	3TBAI	25	48	7.1
15	2TBAI	10	48	79.3
16	2TBAF	25	48	14.7
17	2TBAC	25	48	8.3
18	2TBAB	25	48	17.5
19	2PTAT	25	48	50.5
20	2TBAI	25^c	72	7.5
21	2TBAI	25^d	60	89
22	2TBAI	25^e	48	14.5
23	2TBAI	25^f	48	3.6
24	m-CPBA/TBAI	25^g	48	40.7

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The best catalysts were thus found to be 1/TBAB under solvent-free conditions and 1/TBAI in dichloromethane for styrene and 3-chloropropylene (Table 2, entry 1). To expand the scope of the reaction, various olefins were used with this green reaction. The results are listed in Table 2 and reveal that the cocatalyst of phenyltrimethylammonium tribromide (PTAT) was more active than TBAI (Entry 2-5) for 1-hexene, 1-octene, cyclohexene, and cyclooctene etc. The best example is that 1-octene can be transformed to cyclic carbonate in 78.4% yield when the ratio of 1-octene to catalyst was 25 (Entry 3). Under these reaction conditions, some branched carbonates of cyclohexene and bromination addition species of other olefins were found as by-products. It is well known that the cyclohexene is easily converted to polycarbonate in the presence of catalyst.² For example, the vibration of the carbonyl group of polycarbonate formed from cyclohexene appeared at 1746 cm⁻¹ whereas the carbonyl group of cyclic carbonate appeared at 1793 cm⁻¹. A little cyclooctene oxide was also detected by GC-MS in the presence of PTAT as cocatalyst.

Table 2 Aerobic oxidative carboxylation of olefins^a

Entry	Olefin	Co-catalyst	Yield (%) ^bof CC
a Catalyst 1 37.5 mg (0.05 mmol), cocatalyst 0.1 mmol, olefin 1.25 mmol (25 equivalents). P(O2) 0.5 MPa, P(CO2) 1.1 MPa, CH2Cl2 5 ml, T 30 °C, 48 h.b Determined by ^1H NMR.c Combined bromination by-products indicated by GC-MS.d Some branched carbonate by-product can be found by IR.e Some epoxide by-product can be found by GC-MS.
1	3-Chloropropylene	TBAI	56.7
		PTAT	4.6
2	1-Hexene	TBAI	11.1
		PTAT	28.6^c
3	1-Octene	TBAI	<1
		PTAT	78.4^c
4	Cyclohexene	TBAI	6.8^d
		PTAT	30.5^c^,^d
5	Cyclooctene	TBAI	1.8
		PTAT	49.8^c^,^e

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Hence it is evident that the cocatalyst plays an important role in initiating the reaction. These findings impelled us to propose a mechanism for this attractive green reaction (Scheme 2). The formation of the target is rationalized by the transfer of an oxygen atom from the catalyst to form the epoxide intermediate which is attacked by active carbon dioxide. The intermediate then undergoes intramolecular cyclization to yield the cyclic carbonate and metalloporphyrin that reacts immediately with oxygen and olefin to form a new intermediate. The new intermediate is attacked by active carbon dioxide to regenerate the oxo-metalloporphyrin catalyst accompanied by another molecule of target compound, which completes the catalytic cycle.

Scheme 2 Proposed mechanism of aerobic oxidative carboxylation of olefins.

In summary, we have developed a one-pot, highly selective synthetic method for forming cyclic carbonate using Ru(TPP)(O)₂ as catalyst and quaternary onium salt as cocatalyst. The most important feature of this methodology can be attributed to the use of 4 mol% of catalyst providing a high yield of product. Thus, this simple methodology would be a novel protocol to directly approach the cyclic carbonate from various alkenes. To the best of our knowledge, this is the first example of a homogenous protocol towards the formation of styrene carbonate with 100% selectivity at ambient temperature under low pressure reported in the literature. The investigation of this reaction mechanism and optimization of catalyst systems to enhance their catalytic efficiency are under way in our laboratory.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (No. 20843005, 20973086).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and spectra of IR, ¹H NMR, GC-MS and GC trace. See DOI: 10.1039/b916042f

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