PROCESS FOR THE PHOTOCATALYTIC ALLYLIC OXIDATION OF OLEFINS USING CARBON DIOXIDE

Abstract

The present invention relates to a novel method for photocatalytic oxidation of allylic C—H bonds present in alkenes containing at least three carbon atoms. In this newly disclosed method, such alkenes, when reacted with carbon dioxide (CO.sub.2) in an organic solvent containing a catalyst comprising of a supported molecular complex of transition metal ions under conditions of ambient temperature and pressure using a readily available household LED lamp, yield oxygenated products. The developed method represents a unique way to use CO.sub.2 as an oxygen transfer agent to unsaturated organic compounds along with the formation of CO as a co-product using light as an energy source.

Claims

1. A process for photocatalytic allylic oxidation of olefins using carbon dioxide comprising the step of: a) oxidizing allylic compound with 1 atmospheric pressure of CO.sub.2 dissolved in a polar organic solvent in presence of a supported copper catalyst under light irradiation at temperature in the range 15-35° C. for 12-30 hours irradiation time to obtain corresponding oxidized products having conversions ranging from 40-80%, and selectivity >65% for the allylic hydroxy or carbonyl compound along with the formation of carbon monoxide as a co-product in the gaseous phase.

2. The process as claimed in claim 1, wherein light irradiation is done by using any light source having wavelength λ greater than 420 nm.

3. The process as claimed in claim 2, wherein light irradiation is done by using a household LED light of 15-30 W.

4. The process as claimed in claim 1, wherein allylic compound is an olefin having unhindered allylic C—H bond and olefin is selected from the group consisting of monoolefin both cyclic or chain or bicyclic or compound having unhindered allylic position.

5. The process as claimed in claim 1, wherein the process is carried out in an organic solvent selected form the group consisting of acetonitrile, dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N-methylpyrrollidone (NMP), dimethyl imidazoline (DMI), dimethyl sulfoxide and water.

6. The process as claimed in claim 5, wherein the organic solvent is acetonitrile.

7. The process as claimed in claim 1, wherein hybrid photocatalyst is a combination of copper complex with photoactive carbon support.

8. The process as claimed in claim 1, wherein copper complex is selected from the group consisting of copper (II) bipyridine, copper (II) phthalocyanine, copper (II) porphyrin and copper (II) Schiff base.

9. The process as claimed in claim 8, wherein copper complex is copper (II) bipyridine complex.

10. The process as claimed in claim 1, wherein photoactive carbon support is selected from the group consisting of graphene oxide or their functionalized variants.

11. The process as claimed in claim 1, wherein effective reaction time is ranging from 10 to 30 hrs.

12. The process as claimed in claim 1, wherein hybrid photocatalyst is recovered by simple filtration or centrifugation for recycling experiments.

13. The process as claimed in claim 1, wherein conversion of monoolefin is determined by GC-FID and selectivity of the oxidized product is determined by GC-MS.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1: Chemical synthesis, structure and formula of the substrates.

[0039] FIG. 2: Chemical synthesis and structure of the photocatalyst.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

[0040] The process comprises reacting an olefin having an unhindered allylic C—H bond with carbon dioxide in the presence of a photocatalyst consisting of molecular organic ligand assisted metal ions and a polar organic solvent at temperature ranging from 20 to 40° C. and at atmospheric pressure under the visible light irradiation to prepare the corresponding α,β-unsaturated hydroxyl or carbonyl compounds selectively.

[0041] As utilized herein including in the claims “allylic oxidation” means oxidation of an allylic compound by replacing the allylic hydrogens with oxygen derived from CO.sub.2.

[0042] As utilized herein including in the claims “reactants” collectively references both alkene and CO.sub.2 (oxidant). Solvents including both aqueous and organic solvents and the hybrid photocatalyst are a combination of molecular complex attached with a photoactive support.

[0043] Within this disclosure, “visible light” means light having a wavelength (λ) greater than 420 nm.

[0044] In a preferred embodiment of this invention, any olefin having an unhindered allylic C—H bond can be employed in the process described by this invention. Mono-olefins, whether cyclic or acyclic—whether linear or branched—are preferred, but bicyclic or inactivated olefins such as terpenes and olefins having pharmaceutical importance such as Δ5 steroids can also be employed. Most of the aforementioned olefin types are available commercially and used as received.

Consstituents

Olefins

[0045] The olefin used in the present invention a simple hydrocarbon containing only carbon and hydrogen atoms. Non-limiting examples of olefins which are suitable for the process of this invention include 1-hexene, 2-hexene, 1-heptene, 2-methylpentene, cyclohexene, cycloheptene and analogously, the various isomers of the mentioned olefins, as well as bicyclic olefins such as β-pinene, limonene and their substituted variants.

Oxidant (Carbon Dioxide)

[0046] In the present invention carbon dioxide is used to allylically oxidize olefins in the presence of a photocatalyst under visible illumination. Carbon dioxide, among the two oxygen atoms transferred one oxygen atom to the allylic C—H position and converted to carbon monoxide during the process. The reaction mixture containing substrate, solvent and photocatalyst was either saturated with CO.sub.2 or purged continuously with CO.sub.2 flow for effective oxidation.

Organic Solvents

[0047] Substrates (olefins and CO.sub.2) used in the present invention are preferably dissolved in organic solvents. Specifically polar organic solvents were used mainly due to the higher solubility of CO.sub.2 in polar solvents. Suitable organic solvents include specifically, but not limited to dimethylformamide (DMF), dimethylacetamide (DMA), acetonitrile (ACN), dimethyl sulfoxide (DMSO) and N-methyl pyrrolidone (NMP) or mixtures thereof.

Photocatalyst

[0048] Suitable photocatalyst effective for catalyzing the allylic oxidation in accordance with the present invention is a hybrid photocatalyst consisting of a chelated copper complex supported on a 2D carbon structure. Examples of suitable copper complexes include, specifically but not exclusively, copper (II) bipyridine, copper (II) phthalocyanine, copper (II) Schiff base supported on a photoactive support which consists of a functionalized carbon network that provide active sites for stable anchoring of the metal complex to prevent metal leaching during the photoreaction. The copper complexes and photoactive supports were prepared by following well-documented literature protocols. The supported hybrid catalysts are highly stable they remain intact during the oxidation process and can be easily recovered and reused.

EXAMPLES

[0049] Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

Example 1-19

[0050] Various olefins (1a-1h) were oxidized in accordance with the standard protocol set forth above and the results of these experiments are summarized in Table 1.

Example 1:—General Procedure for the Oxidation of Cyclohexene

[0051] Cyclohexene (1a) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. Further, the hybrid photocatalyst (1 to 10 mol % of the substrate) was added and the resulting mixture was saturated with CO.sub.2 by purging at 1 atm pressure. The reaction vessel was sealed and irradiated with 20 W LED light (Model No. HP-FL-20 W-F, Hope LED Opto-Electric CO., Ltd) for 24 h. The intensity of the LED light at the reaction flask was measured to be 86 W/m.sup.2 by intensity meter. The conversion of the olefin was examined by GC-FID based on the unreacted substrate. The selectivity of the α,β-unsaturated hydroxyl or carbonyl compounds was determined by GC-MS. The formation of CO along with the minute amount of hydrogen in the gaseous phase was confirmed by Residual Gas Analysis (RGA). Furthermore, after the reaction, the catalyst was recovered by filtration and any unreacted olefin and solvent were recovered by distillation under reduced pressure. The resulting residue was subjected to column chromatography to isolate the products. The conversion of olefin was consistently in the range of 40-80% and the selectivity towards the corresponding α,β-unsaturated hydroxyl and ketone remained >65%.

Example 2: Oxidation of Cyclohexene Using Molecular Copper Complex as Catalyst

[0052] Cyclohexene (1a) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. Further, the copper complex (1 to 10 mol % of the substrate) was added and the resulting mixture was saturated with CO.sub.2 by purging at 1 atm pressure. The reaction vessel was sealed and irradiated with 20 W LED light (Model No. HP-FL-20 W-F, Hope LED Opto-Electric CO., Ltd) for 24 h. The intensity of the LED light at the reaction flask was measured to be 86 W/m.sup.2 by intensity meter. The conversion of the olefin was examined by GC-FID based on the unreacted substrate. There was no reaction occurred using homogeneous complex under otherwise identical conditions.

Example 3: Oxidation of Cyclohexene Using Bare Graphene Oxide as Catalyst

[0053] Cyclohexene (1a) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. Further, the bare graphene oxide as photocatalyst (1 to 10 mol % of the substrate) was added and the resulting mixture was saturated with CO.sub.2 by purging at 1 atm pressure. The reaction vessel was sealed and irradiated with 20 W LED light (Model No. HP-FL-20 W-F, Hope LED Opto-Electric CO., Ltd) for 24 h. The intensity of the LED light at the reaction flask was measured to be 86 W/m.sup.2 by intensity meter. The conversion of the olefin was examined by GC-FID based on the unreacted substrate. The selectivity of the α,β-unsaturated hydroxyl or carbonyl compounds was determined by GC-MS. The conversion of olefin and the selectivity towards the corresponding α,β-unsaturated hydroxyl and ketone is given in the Table 1, entry 3.

Example 4: Oxidation of Cyclohexene in the Dark

[0054] Cyclohexene (1a) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. Further, the hybrid photocatalyst (1 to 10 mol % of the substrate) was added and the resulting mixture was saturated with CO.sub.2 by purging at 1 atm pressure. The reaction vessel was sealed and kept in the dark condition under continuous stirring. The conversion of the olefin was examined by GC-FID based on the unreacted substrate. There was no conversion observed that illustrates that visible illumination was essential for the oxidation.

Example 5: Oxidation of Cyclohexene in the Absence of Catalyst

[0055] Cyclohexene (1a) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. The reaction vessel was sealed and irradiated with 20 W LED light (Model No. HP-FL-20 W-F, Hope LED Opto-Electric CO., Ltd) for 24 h. The intensity of the LED light at the reaction flask was measured to be 86 W/m.sup.2 by intensity meter. There was no conversion observed as ascertained by GC-FID, which illustrated that presence of photocatalyst was essential for the oxidation with CO.sub.2 (Table 1, entry 5)

Example 6: Oxidation of Cyclohexene Using Recovered Catalyst (Recycling Experiment-1)

[0056] Cyclohexene (1a) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. Further, the recovered photocatalyst from experiment 1 was added and the resulting mixture was saturated with CO.sub.2 by purging at 1 atm pressure. The reaction vessel was sealed and irradiated with 20 W LED light (Model No. HP-FL-20 W-F, Hope LED Opto-Electric CO., Ltd) for 24 h. The conversion of the olefin and selectivity of the α,β-unsaturated hydroxyl or carbonyl compound as determined by GC-FID and GC-MS is mentioned in the Table 1 (entry 6).

Example 7: Oxidation of Cyclohexene Using Recovered Catalyst (Recycling Experiment-2)

[0057] Cyclohexene (1a) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. Further, the recovered photocatalyst from experiment 6, was added and the resulting mixture was saturated with CO.sub.2 by purging at 1 atm pressure. The reaction vessel was sealed and irradiated with 20 W LED light (Model No. HP-FL-20 W-F, Hope LED Opto-Electric CO., Ltd) for 24 h. The conversion of the olefin and selectivity of the α,β-unsaturated hydroxyl or carbonyl compound as determined by GC-FID and GC-MS is mentioned in the Table 1 (entry 7).

Example 8-13: Oxidation of Cyclohexene Using Different Solvents

[0058] Cyclohexene (1a) and polar organic solvent (as mentioned in Table 1) in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. Further, the hybrid photocatalyst was added and the resulting mixture was saturated with CO.sub.2 by purging at 1 atm pressure. The reaction vessel was sealed and irradiated with 20 W LED light (Model No. HP-FL-20 W-F, Hope LED Opto-Electric CO., Ltd) for 24 h. The conversion of the olefin and selectivity of the α,β-unsaturated hydroxyl or carbonyl compound as determined by GC-FID and GC-MS is mentioned in the Table 1 (entry 8-13).

Example 14-19: Oxidation of Different Olefins Under Optimized Conditions

[0059] Olefin (1b-1h) and polar organic solvent, preferably acetonitrile in (1:2 to 1:10 weight ratio with respect to the substrate) was taken in to a 60 ml vessel. Further, the hybrid photocatalyst (1 to 10 mol % of the substrate) was added and the resulting mixture was saturated with CO.sub.2 by purging at 1 atm pressure. The reaction vessel was sealed and irradiated with 20 W LED light (Model No. HP-FL-20 W-F, Hope LED Opto-Electric CO., Ltd) for 24 h. The intensity of the LED light at the reaction flask was measured to be 86 W/m.sup.2 by intensity meter. The conversion of the olefin was examined by GC-FID based on the unreacted substrate. The selectivity of the α,β-unsaturated hydroxyl or carbonyl compounds was determined by GC-MS. The results obtained for the conversion of olefin and the selectivity towards the corresponding α,β-unsaturated hydroxyl and ketone is summarized in Table 1, entry 14-19.

TABLE-US-00001 TABLE 1 Photocatalytic allylic oxidation of alkenes with CO.sub.2 Ex- Product am- Ole- Conv. Select. (%).sup.c ple fin Photocatalyst Light Solvent (%).sup.b I II III 1 1a Cu(bpy).sub.2/GO Yes CH.sub.3CN 65 35 60 5 2 1a Cu(bpy).sub.2 Yes CH.sub.3CN — — — — 3 1a GO Yes CH.sub.3CN 15 30 55 — 4 1a Cu(bpy).sub.2/GO No CH.sub.3CN — — — — 5 1a — Yes CH.sub.3CN — — — — 6 1a Cu(bpy).sub.2/GO* Yes CH.sub.3CN 65 35 60 5 7 1a Cu(bpy).sub.2/GO* Yes CH.sub.3CN 65 32 58 6 8 1a Cu(bpy).sub.2/GO Yes DMA 25 15 30 5 9 1a Cu(bpy).sub.2/GO Yes DMF 30 15 25 4 10 1a Cu(bpy).sub.2/GO Yes DMSO 20 — — 5 11 1a Cu(bpy).sub.2/GO Yes NMP 15 — — — 12 1a Cu(bpy).sub.2/GO Yes H.sub.2O 20 — — Trace 13 1b Cu(bpy).sub.2/GO Yes CH.sub.3CN 60 28 35 8 14 1c Cu(bpy).sub.2/GO Yes CH.sub.3CN 75 — 94 6 15 1d Cu(bpy).sub.2/GO Yes CH.sub.3CN 65 — 90 — 16 1e Cu(bpy).sub.2/GO Yes CH.sub.3CN 54 — 90 — 17 1f Cu(bpy).sub.2/GO Yes CH.sub.3CN 40 30 60 — 18 1g Cu(bpy).sub.2/GO Yes CH.sub.3CN 37 32 54 — 19 1h Cu(bpy).sub.2/GO Yes CH.sub.3CN 45 — 85 — *Using recovered photocatalyst; .sup.adetermined by GC-FID; .sup.bdetermined by GC-MS

Observations

[0060] The best results were obtained with the hybrid photocatalyst; whereas there was no reaction occurred in the presence of homogeneous copper complex as catalyst. Among the various organic solvents, acetonitrile exhibited best performance in terms of conversion of olefin and selectivity of the desired allylic oxidized compounds. The light irradiation was found to be essential and there was no reaction occurred under dark conditions in the absence of light. The use of bare support afforded a poor conversion with the selective formation of allylic hydroxyl compound. In addition, the use of hybrid photocatalyst offered facile recovery of the catalyst after the reaction by simple filtration and showed almost consistent efficiency at least for three recycles under similar conditions.

Advantages of the Invention

[0061] The various advantages of the present process are given below. [0062] The present invention discloses the first photocatalytic oxidation using CO.sub.2 as an oxidant under ambient temperature and pressure conditions. The use of CO.sub.2 as an oxidant offers several advantages as it is abundantly available, safe, and inexpensive; also, it provided carbon monoxide, an important building block as a co-product during the oxidation process. [0063] The present process serves monocyclic olefins (cyclohexene and cycloheptene) as substrates allylic oxidation products α,β-unsaturated hydroxyl, or carbonyl compounds obtained in higher yield. Unhindered chain olefins (1-hexene, 1-heptene and 1-octene) showed maximum conversion with the selective formation of the corresponding α,β-unsaturated ketones. [0064] The present invention provides a unique approach for the oxidation of olefins and many other organic substrates that are not exemplified here using CO.sub.2 as an oxidant.