Selective and Clean Cyclohexene-Promoted Oxidation and Photooxidation by Air

doi:10.4236/gsc.2011.12005

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Green and Sustainable Chemistry, 2011, 1, 26-30

doi:10.4236/gsc.2011.12005 Published Online May 2011 (http://www.SciRP.org/journal/gsc)

Selective and Clean Cyclohexene-Promoted

Oxidation and Photooxidation by Air

Grigoriy Sereda, Vikul Rajpara

Department of C hemi st ry , University of South Dakota, Vermillion, USA

E-mail: [email protected]

Received March 17, 2011; revised April 17, 2011; accepted April 24, 2011

Abstract

A simple and environmentally friendly selective procedure for cyclohexene-promoted photooxidation of

p-xylene, ethylbenzene, and cumene by air in the presence of a pristine or oxidized carbonaceous material is

reported. Depending on the catalyst and conditions, the reaction yields either of the following industrially

important products: 4-methylbenzyl hydroperoxide, 4-methylbenzoic acid, 1-phenylethyl hydroperoxide,

2-phenyl-2-propanol, acetophenone with high selectivity and practical extent of conversion. Exposure the

reaction mixture to ambient light further increased the yields. Improved performance of oxidized graphite

has demonstrated the potential of surface modification for the design of novel carbonaceous catalysts.

Keywords: Oxidation, Synthesis, Catalysis, Hydrocarbons

1. Introduction

Alkylbenzenes industrially produced by the petroleum

processing, are known as key commodities for the or-

ganic synthesis. Products of their benzylic oxidation

have countless practical applications. To name just a few,

4-methylbenzyl hydroperoxide 1 was found to act as

oxygen donor in cytochrome c [1] and inhibit alcohol

dihydrogenase [2]. 4-Methylbenzoic acid 2 is a common

plasticizer [3], steel anti-corrosion agent [4], and a sig-

nificant component of writing ink [5]. 1-Phenylethyl

hydroperoxide 3 is a good oxygen donor for alkene ep-

oxidation [6]. 2-Phenyl-2-propanol 4 is a component of

optical lenses [7] and a catalyst for styrene polymeriza-

tion [8]. Acetophenone 5 is commonly used as a fra-

grance [9] and anticorrosion agent [10]. Synthesis of

these compounds usually involves toxic tungsten and

palladium-based catalysts [11,12]. Here we report a series

of simple, environmentally friendly, and selective proce-

dures for the synthesis of compounds 1-5 by oxidation of

alkylbenzenes with air in the presence of a carbonaceous

catalyst, ambient light, and the cyclohexene promoter.

Recently we reported [13] that oxidation of p-xylene 6

by air on carbonaceous materials can be significantly

affected by ambient light that may lead to either activa-

tion or passivation of the catalyst along with significant

shifts in the product composition. Due to the adsorption

of the key intermediate 1 (Figure 1), carbonaceous ma-

terials with the elevated defects-to-basal plane ratio and

highly sorptive surface (carbon black (CB) and oxidized

graphite (OG)), provided higher yields of the hydroper-

oxide 1 comparing with graphite [13]. However, notice-

able loss of their catalytic activity upon exposure to light

significantly curtailed application of these materials for

organic synthesis.

The herein reported procedures have taken advantage

of the free-radical promoting activity of cyclohexene that

we have shown before on the example of the graphite

catalyst [14]. Graphite-catalyzed oxidation of p-xylene 6

led to hydroperoxide 1 as the major product along with

the acid 2, alcohol 7, aldehyde 8, and ester 9 (Figure 1).

However, the relatively low yield and selectivity led us

to the exploration of alternative carbonaceous catalysts.

2. Results and Discussion

We found that in the presence of cyclohexene, carbon

Figure 1. Oxidation and photooxidation of p-xylene.

G. SEREDA ET AL.

black has demonstrated excellent selectivity toward hy-

droperoxide 1 (Table 1, Entry 2), however, significant

photopassivation of the catalyst made the extent of con-

version unpractical. In contrast, catalytic performance of

oxidized graphite struck the best balance between the

extent of conversion and selectivity despite noticeable

photopassivation (Table 1, Entries 3, 4). Adsorption of

the intermediate hydroperoxide 1 by the polar surface of

oxidized graphite was apparently counteracted by the

ability of cyclohexene to generate free radicals due to its

allylic oxidation. Ultrapure graphite comparable with

oxidized graphite by the cost, presents an alternative ef-

ficient catalyst for the synthesis of hydroperoxide 1 (Ta-

ble 1, Entries 5, 6), which has a potential for scaling the

process up to the industrial level.

As we reported earlier [13], graphite nanofibers (GNF)

belong to the group of carbonaceous materials activated

by ambient light. In the presence of cyclohexene, GNF

kept their photocatalitic properties toward oxidation of

p-xylene (Table 1, Entries 7, 8) and produced acid 2

with high conversion and selectivity similar to that we

observed in the absence of cyclohexene [13].

Addition of cyclohexene significantly increases selec-

tivity of Single Wall Carbon Nanotubes (SWCNT) to-

wards the acid 2 that surpass that of GNF. However, due

to the low cost, graphite nanofibers seem to be the most

practical photocatalyst for the preparation of 4-methyl-

benzoic acid 2. Addition of cyclohexene did not change

the unique ability of Multi Wall Carbon Nanotubes

(MWCNT) to catalyze formation of “coupled” products:

di(4-methylbenzyl) ether 10 and 1,4-dimethyl-2-

(4-methylbenzyl)benzene 11. However, both the in-

creased conversion and complete suppression of uniden-

tified phenolic products [13] make MWCNT a promising

material for the design of new carbonaceous catalysts for

organic synthesis.

In order to demonstrate reproducibility of the experi-

ments performed under the ambient light, we repeated

several experiments (Table 1, Entries 8, 13, 14) two

more times and arrived to very similar yield and product

distribution.

It is also worth mentioning that without a carbona-

ceous catalyst, cyclohexene itself did not catalyze oxida-

tion of p-xylene [14].

Interestingly, the presence of cyclohexene does not

affect the relationship between photosensitivity of car-

bonaceous materials toward oxidation of p-xylene, and

their structure, discussed in detail earlier [13]. Employ-

ing cyclohexene as an activator led us to the most prac-

tical procedures for preparation of hydroperoxide 1 par-

tially due to the complete suppression of formation of

unidentified phenolic products.

Fortunately, deactivation of carbon black by light is

very sensitive to the substrate of oxidation and does not

hold for the practically important reaction of oxidation of

ethylbenzene 12 (Figure 2).

In the absence of cyclohexene, photopassivation of

carbon black reverses (Table 2, Entries 1, 2), which makes

it a practical catalyst for the synthesis of 1-phenylethyl

Figure 2. Oxidation and photooxidation of ethylbenzene.

Table 1. Oxidation of p-xylene in the presence of cyclohexene.

Molar ratio

Entry Catalyst Surface area,

m2/g (average

pore size, nm)Light 1 7 8 2 9 10 11

Total yield

(g)

1 No 0.8 1.5 1 2 0.25 0 0 0.46

2 CB 80

(289) Yes 33 2 1 0.3 0 0 0 0.05

3 No 2.1 1.65 1 1 0 0 0 0.42

4 OG 13

Yes 8 1.1 1 0.8 0 0 0 0.21

5 No 2.2 1.8 1 0.72 0 0 0 0.23

6 UPG 30

Yes 12.5 1.5 1 1 0 0 0

0.23

7 No 0 0.6 1 1.1 0.1 0 0 0.21

8 GNF 19

(1.4) Yes 0 1 1 2.7 0.31 0 0 0.57

9 No 0 0.85 1 1.25 0.09 0 0 0.17

10 SWCNT 281 Yes 0 0.32 1 3 0 0 0

0.54

11 No 0 0.7 1 0.7 0.7 0.14 0.34 0.11

12 MWCNT 99

(261) Yes 0.3 0.2 1 0.5 0.7 0.14 0.5 0.13

1314 No 0 1 1 0.7 0.7 0 0 0.23

1414 Graphite 5.9

(19.6) Yes 4.5 1.5 1 1.5 0 0 0 0.23

28 G. SEREDA ET AL.

Table 2. Oxidation of ethylbenzene.

Entry Catalyst Light Cyclohexene

present 1-Phenylethyl hydroperoxide 3 1-Phenylethanol 13 Acetophenone 5 Total

yield (g)

1 No No 2.9 2.2 1 0.38

2 CB Yes No 4.5 0 1

0.72

3 No Yes 1 1 1 0.51

4 CB Yes Yes 10 0.3 1 0.42

5 No Yes 2.1 1.65 1 0.51

6 OG Yes Yes 8.0 1.1 1

0.60

Table 3. Oxidation of cumene.

Entry Catalyst Light 2-Phenyl-2-propyl hydroperoxide 152-Phenyl-2-propanol 4Acetophenone 5 Total yield (g)

1 No 0 2.9 1 0.30

2 OG Yes 0 2.2 1 2.0

3 No 0 0.55 1 1.25

4 CB Yes 0 0.32

1 1.24

514 No 0.3 1.8 1 0.97

614 None Yes 0 0.7 1 1.83*

*0.1 equiv. of unidentified phenolic products are formed.

hydroperoxide 3 (Table 1, Entry 2). In the presence of

cyclohexene, both carbon and oxidized graphite provided

practical conversion and selectivity towards 3 (Table 2,

Entries 4, 6). Contrary to carbon black, graphite [14]

completely suppress oxidation of ethylbenzene 12, which

produces hydroperoxide 3 with low selectivity along

with alcohol 13 and acetophenone 5 (Table 2).

While cumene 14 undergoes oxidation (Figure 3) with

and without the presence of graphite [14], its cyclohex-

ene-promoted oxidation catalyzed by carbon black pro-

vides by far best selectivity toward alcohol 4 (Table 3,

Entry 2). Oxidized graphite has shown excellent photo-

catalytic performance for the preparation of another

practically important product - acetophenone 5 (Table 3,

Entry 4). Low cost of carbon black and oxidized graphite

presents a potential for their industrial application.

Presence of ambient light is not necessary for the

reactions to proceed, however it improved selectivity and

usually overall yields. For carbon black (Table 1 , Entries

1, 2) the observed photopassivation rendered the reaction

unpractical. However, oxidized graphite was less prone

to photopassivation, which was conteracted by the sig-

nificantly improved selectivity (Table 1, Entries 3, 4).

3. Experimental Part

Carbon Black (CB), Graphite Nanofibers (GNF), Ul-

trapure Graphite (UPG), Multi Wall Carbon

Figure 3. Oxidation and photooxidation of cumene.

Nanotubes (MWCNT), and Single Wall Carbon Nano-

tubes (SWCNT) were purchased from Aldrich. Oxidized

graphite (OG) was prepared by oxidation for 10 h with

aqueous nitric and sulfuric acids [15]. The reactions of

benzylic oxidation were performed by passing air at the

rate of 1 mL/min through 37 mg of a catalyst, suspended

in 5 mL of p-xylene, and 0.3 mL of cyclohexene under

reflux for 24 h. After addition of 10 ml of hexane, the

catalyst was removed by filtration. The filtrate was con-

centrated in vacuum and the residue was analyzed by

non-overlapping 1H NMR signals, characteristic for the

hydroperoxide 1 [16] (singlet at 4.95 ppm), alcohol 7 [17]

(singlet at 4.60 ppm), aldehyde 8 [18] (singlet at 9.95

ppm), acid 2 [19] (doublet at 8.0 ppm), ester 9 [20]

(singlet at 5.30 ppm), ether 10 [21] (singlet at 4.50 ppm),

and hydrocarbon 11 [22] (singlet at 3.85 ppm).

Oxidation of ethylbenzene 12 and cumene 14 was

performed by the same procedure. The reaction product

was analyzed by non-overlapping 1H NMR signals,

characteristic for the hydroperoxide 3 [23] (quadruplet at

5.05 ppm), alcohol 13 [24] (quadruplet at 4.90 ppm), and

acetophenone 5 [25] (singlet at 2.60 ppm). The methyl

groups of the hydroperoxide 15 [26] and alcohol 4 [27]

show up in the 1H NMR as singlets at 1.55 - 1.60 ppm.

We determined the molar ratio of 15 and 4 by integration

of the inverse-gated broad band decoupled 13C NMR

spectrum as we described earlier [14]. The molar ratios

of products are presented in Tables 2 and 3. Conversion

of the oxidation was characterized by the mass of the

product mixture, given in Tables 2 and 3 (Total Yield).

Although the synthetic outcome of the reported reactions

was characterized by NMR, we previously demonstrated

[14] that all components of the reaction mixtures can be

efficiently separated on a silica gel chromatography

column eluted by hexane-ethyl acetate.

G. SEREDA ET AL.

Surface analysis of the catalysts was performed by the

nitrogen adsorption/desorption measurements. The sam-

ples were outgassed for 1 h at 100˚C and analyzed at 77 K

using a Quantachrome Nova 2200e gas adsorption ana-

lyzer.

4. Conclusions

In conclusion, we introduced carbon black, ultrapure

graphite, and graphite nanofibers as selective, ambient

light-activated catalysts for selective oxidation of

p-xylene, ethylbenzene, and cumene by air. While the

presence of ambient light is not necessary, it significantly

improves the practical outcome in terms of selectivity

and usually overall yield. Utilization of cyclohexene as a

promoter has allowed us to circumvent previously re-

ported photopassivation of carbon black and oxidized

graphite. Depending on the catalyst and conditions, this

environmentally friendly reaction yields either of the

following industrially important products: 4-methlbenzyl

hydroperoxide 1, 4-methylbenzoic acid 2, 1-phenylethyl

hydroperoxide 3, 2-phenyl-2-propanol 4, acetophenone 5

with high selectivity and practical extents of conversion

without any use of toxic metal co-catalysts. Improved

performance of oxidized graphite has demonstrated the

potential of surface modification for the design of novel

carbonaceous catalysts. Furthermore, addition of cyclo-

hexene significantly improves the unique catalytic per-

formance of carbon nanotubes towards oxidation and

oxidative coupling that makes them perspective materials

for additional research.

5. Acknowledgements

This work has been supported by the Director, Office of

Science, Office of Biological & Environmental Research,

Biological Systems Science Division, of the U.S. De-

partment of Energy under Contract No. DE-FG02-

08ER64624, and NSF (EPSCoR Grants No. 0554609,

0903804). We also thank the group of Prof. Ranjit

Koodali (USD) for assistance in performing surface

analyses.

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