**2. Radical chain reactions of BD with oxygen**

Olefins readily interact with radical species. The most susceptible to radical attack is allyl position to produce allyl oxygenates [28, 29]. In the absence of an allylic carbon atom, one of the double bonds of BD is involved in the oxidation. Neat or dissolved in a nonpolar solvent, BD interacts with oxygen at moderate temperature according to radical chain mechanism to form oligomeric butadiene polyperoxide, C4 H6 O2 [30]. The reaction is accelerated by increasing the temperature or adding free radical initiators and inhibited by adding acids. From the NMR analysis, molecular structure of the polyperoxide formed at 50°C in the presence of 37 Torr of oxygen was composed of equal amounts of 1,4- and 1,2-butadiene units separated by peroxide units [31]. The structure of the polyperoxide (the ratio of 1,4- to 1,2-butadiene units) does not depend on the reaction temperature, whereas the content of bound oxygen in the polyperoxide varies with oxygen pressure. The ratio of peroxide to hydrocarbon units is below 1 at a low oxygen partial pressure. Thermal decomposition as well as hydrogenation of polyperoxide leads to the formation of 3-butene-1,2-diol and 2-butene-1,4-diol or corresponding saturated diols, preferably 1,4-derivatives (**Scheme 1**) [30, 32–35].

but-2-ene. Both BD and C4

100 Alkenes


O5

phase oxidation [1, 2]. The oxidation on oxide catalysts in gas phase results in the formation of maleic anhydride together with crotonaldehyde and 2,5-dihydrofuran. Centy and Trifiro suggested a simple consecutive pathway for BD oxidation over V-P-oxide catalysts [3, 4], whereas Honicke et al. proposed multiple pathways from BD to crotonaldehyde, 2,5-dihydrofuran, 2-butene-1,4-dial, 2(5H)-furanone and furan, and finally to maleic anhydride over V2

catalysts [5]. Schroeder specified the oxidation pathway on V-Mo-oxide catalysts, including 3,4-epoxy-1-butene as a primary oxidation product [6]. Epoxidation of BD occurs over Ag catalysts [7–10] used in industry for the production of ethylene oxide and intensively investigated in the oxidation of other olefins (e.g., [11, 12]). 3,4-Epoxy-1-butene is further converted into 2,3-dihydrofuran followed by hydrolysis to form 4-hydroxybutyraldehyde. The secondary transformations occur directly under epoxidation conditions on Ag catalysts promoted with B-P [13], Mo [14], and Mo-P-Sb [15] or by subsequent treatments of 3,4-epoxy-1-butene. In the early 1980s, oxidation of n-butane has become the preferred method for manufacturing maleic anhydride [16, 17]. The invented synthesis of maleic anhydride from butane creates a competition for the gas-phase oxidation of BD since hydrogenation of maleic anhydride opens a possibility of producing various oxygenates, which produced from BD earlier. At the same time, the gas-phase oxidation of BD still suffers from formation of polymer resins, which leads to excessive consumption of raw materials and catalyst deactivation. This problem and large power consumption inherent to all gas-phase reactions are absent in the liquidphase oxidation since the low temperature and application of appropriate solvents prevent the formation of the resins. The liquid-phase low-temperature oxidative reactions, in particular the oxidation of olefins, were intensively studied at the end of the last century [18–23]. A renewed interest in this area is growing now [24–27] and can be expected to be strengthened in the nearest future as a response to modern requirements of green chemistry to minimize power and materials consumption. In addition, the liquid-phase reactions are well applicable for the oxidation of various olefins and BD because of high reactivity of these hydrocarbons that allows the oxidation at low temperature. At the same time, BD becomes more affordable

The title of this chapter concerns the application of green oxygen (air and hydrogen peroxide) in liquid-phase conditions. The liquid-phase oxidative reactions are an important part in chemistry of all olefins and, in particular, of the simplest representative of conjugated diolefins as they open many routes for the conversion of the hydrocarbons. We represent here an analysis of literature information concerning the oxidation of BD in liquids and references to the related reactions of olefins. In detail, we described the catalytic systems in the study of

Olefins readily interact with radical species. The most susceptible to radical attack is allyl position to produce allyl oxygenates [28, 29]. In the absence of an allylic carbon atom,

owing to permanent improvements in its manufacturing.

**2. Radical chain reactions of BD with oxygen**

which we acquired our own experience.

Decomposition of the polyperoxide forms not only 3-butene-1,2-diol and 2-butene-1,4-diol but also side products such as formaldehyde, acrolein (from 1,2-units), and resinous insoluble material (presumably resulting from the reaction of the 1,4-units with aldehydes) [31]. Therefore, the preferred formation of 1,4-oxygenates from the thermal decomposition of polyperoxide is not a strong support of predominance of 1,4-units in the polyperoxide structure.

The rate of decomposition of the polyperoxide increases with increasing temperature, addition of bases (amines) [36], or metal ions as radical initiators. Butadienyl polyperoxide is readily decomposed in the presence of metal ions of variable oxidation state. Therefore, the transition metal compounds participate as catalysts in the radical chain oxidation of BD with oxygen. The oxidation products are similar to those obtained under the decomposition of the polyperoxide. 3-Butene-1,2-diol and 2-butene-1,4-diol can be obtained with the selectivity sufficiently high for the chain radical process, especially if one considers the low stability of these products with respect to secondary oxidation. Thus, a mixture of 3-butene-1,2-diol and 2-butene-1,4-diol has been prepared by oxidative dihydroxylation of BD with oxygen in acetic acid solution of Pd(OAc)2 . From a practical point of view, the most valuable 2-butene-1,4-diol has been formed with selectivity of 25% [37].

We tested Pd and Au catalysts in the radical chain oxidation of BD in polar media. Both soluble palladium acetate and insoluble supported metals caused the formation of the products,

**Scheme 1.** Formation and reductive decomposition of the polyperoxide [30].

the most part of which appeared from decomposition of the intermediate butadienyl polyperoxide [32] (**Scheme 2**). The main products are 3-butene-1,2-diol and 2-butene-1,4-diol. 4-Hydroxybut-2-enal can be formed in the decomposition of polyperoxide and in oxidation of 2-butene-1,4-diol. Oxidative dehydration of 2-butene-1,4-diol produces furan. Both butanediols can be esterified to form corresponding diacetates, but only 2-butene-1,4-diol diacetate has been found in the reaction solution. Acrolein occurs from breaking C─C bond under decomposition of polyperoxide or, possibly, from secondary conversion of 3-butene-1,2-diol. C8 oxygenates originate from polyperoxide fragments containing less than 1:1 ratio of butadiene to oxygen units. In addition, there are impurities of C6 cyclic oxygenates occurring from cyclodimerization of BD (Diels-Alder reaction) followed by oxidation of 4-vinylcyclohexene. The amount of the products is given in **Table 1**.

In addition to the stable compounds, a large amount of peroxide compounds have been iodometrically detected in acetic acid and acetic acid/dioxane solutions (**Table 1**). Peroxide oxygen refers to butadienyl polyperoxide since the addition of Ph3 P reducer to the solution results in disappearance of the peroxide and formation of 2-butene-1,4-diol together with minor amounts of furan and 3-butene-1,2-diol. The polyperoxide exhibited sufficient stability in several oxidation tests but almost completely decomposed with a large amount of Pd/C catalysts. As a result, enhanced formation of 2-butene-1,4-diol and 4-hydroxybut-2-enal is achieved in this case (fifth row in **Table 1**).

The addition of Te to Pd/C catalyst lowers the production of all oxidation products. Bottom row in **Table 1** shows the inhibitory effect of Te on the chain radical oxidation reaction. At the same time, more noticeable becomes formation of the oxidation products non typical for the chain radical mechanism. These are crotonaldehyde and methyl vinyl ketone, which show the possibility of a nonradical heterolytic mechanism of oxidation on the PdTe/C catalyst.

**Catalyst (mg)**

Pd(OAc)2 2.5 Pd(OAc)2 2.5 0.5%Au/SiO2 120

0.5%Au/SiO2 120

5%Pd/C 3000 5%Pd0.5%Te/C

100 1C8 diols and acetates, and C6 cyclic oxygenates.

2Iodometric titration.

30.1mmol сrotonaldehyde and methyl vinyl ketone.

40.4 mmol сrotonaldehyde and methyl vinyl ketone.

**Table 1.**

GC detected products from oxidation of BD (70

mmol) by oxygen (O2/N2

 =

10/90, 60

atm) in a solvent (100mL).

Reactivity of a Simplest Conjugated Diolefin in Liquid-Phase Oxidation: Mechanisms and Products

http://dx.doi.org/10.5772/intechopen.71259

103

DMA/H

O 94/6 2

90

3

0.5

0.1

0

0.1

0.1

0.84

0.2

3000

100

DMA/H

O 94/6 2

90

3

10.3

0.4

0

4.8

2.2

0.73

0.8

 70

 70

70

70

HOAc/H

O 88/12

70

2

2.5

0.6

0.1

<0.1

1.3

0.2

8.5

2 HOAc/dioxane/H

HOAc/dioxane/H

HOAc/dioxane/H

2

O 44/50/6

80

6

8.5

5.3

0.4

1.3

10.7

7.1

7.0

2

O 44/50/6

80

4

4.7

3.2

<0.1

0.1

8.9

2.0

9.1

2

O 19/75/6

80

2

4.6

3.1

<0.1

0.4

7.8

0.1

9.4

**BD (mmol)**

**Solvent**

*T* **(°C)**

**Time (h)** **1 + 2**

**3**

**4**

**5**

**6**

**Others1**

**Peroxide2**

**Products (mmol)**

**Scheme 2.** GC-detected products of the radical chain oxidation of BD.


the most part of which appeared from decomposition of the intermediate butadienyl polyperoxide [32] (**Scheme 2**). The main products are 3-butene-1,2-diol and 2-butene-1,4-diol. 4-Hydroxybut-2-enal can be formed in the decomposition of polyperoxide and in oxidation of 2-butene-1,4-diol. Oxidative dehydration of 2-butene-1,4-diol produces furan. Both butanediols can be esterified to form corresponding diacetates, but only 2-butene-1,4-diol diacetate has been found in the reaction solution. Acrolein occurs from breaking C─C bond under decomposition of polyperoxide or, possibly, from secondary conversion of 3-butene-1,2-diol. C8 oxygenates originate from polyperoxide fragments containing less than 1:1 ratio of butadiene to oxygen units. In addition, there are impurities of C6 cyclic oxygenates occurring from cyclodimerization of BD (Diels-Alder reaction) followed by oxidation of 4-vinylcyclohexene.

In addition to the stable compounds, a large amount of peroxide compounds have been iodometrically detected in acetic acid and acetic acid/dioxane solutions (**Table 1**). Peroxide oxygen

in disappearance of the peroxide and formation of 2-butene-1,4-diol together with minor amounts of furan and 3-butene-1,2-diol. The polyperoxide exhibited sufficient stability in several oxidation tests but almost completely decomposed with a large amount of Pd/C catalysts. As a result, enhanced formation of 2-butene-1,4-diol and 4-hydroxybut-2-enal is achieved in

The addition of Te to Pd/C catalyst lowers the production of all oxidation products. Bottom row in **Table 1** shows the inhibitory effect of Te on the chain radical oxidation reaction. At the same time, more noticeable becomes formation of the oxidation products non typical for the chain radical mechanism. These are crotonaldehyde and methyl vinyl ketone, which show the possibility of a nonradical heterolytic mechanism of oxidation on the

P reducer to the solution results

The amount of the products is given in **Table 1**.

this case (fifth row in **Table 1**).

PdTe/C catalyst.

102 Alkenes

refers to butadienyl polyperoxide since the addition of Ph3

**Scheme 2.** GC-detected products of the radical chain oxidation of BD.

**Table 1.** GC detected products from oxidation of BD (70 mmol) by oxygen (O2/N2 = 10/90, 60 atm) in a solvent (100mL).

40.4 mmol сrotonaldehyde and methyl vinyl ketone.
