**4. Heterolytic mechanism of 1,4-oxidative addition to BD**

Wacker-type oxidation of olefins and analogous Pd-catalyzed nonradical oxidation of BD produce usually carbonyl compounds, but special additives are required for obtaining dioxygenates. Nevertheless, the oxidative 1,2-addition to olefins is known to occur under the action of Pd2+ complex and oxoanion strong oxidants, such as periodate [48] or nitrate anions, in acetic acid solution to form glycol derivatives [49–51]. Mechanism of the oxidation is based on a nonradical inner sphere interaction of olefin with oxidant in Pd2+ complex. Similar interaction is probably realized in oxidation of BD in the presence of palladium as the catalyst of nonradical heterolytic olefin oxidation and Sb, Bi, Te, or Se promoters. Heterogeneous catalysts containing these active components have shown unique catalytic properties in oxidation of BD selectively to 2-butene-1,4-diol diacetate (**Scheme 5**) [52, 53].

XPS analysis of the Pd and PdTe catalysts indicates that Te-oxide is able to increase positive charge on Pd surface [46], thus being an oxidation promoter for palladium. The ionic state of surface palladium is responsible for heterolytic oxidation. Acetic acid is used as a solvent for this reaction. The mechanism of formation of 2-butene-1,4-diol diacetate is proposed by Takehira et al. for PdTe catalyst (**Scheme 6**) [54], and fundamentally identical one is proposed for the RhTe catalyst [55]. The details in intermediate structures explain the preferential formation of trans-2-butene-1,4-diol in the case of Pd-containing catalyst and cis-isomer in the case of Rh.

Exceptionally high selectivity of BD to 2-butene-1,4-diol diacetate conversion is explained by a concert interaction of BD with surface Pd and with acetate anions. Adsorbed on Pd, BD forms π-allyl-type intermediate that undergoes acetoxylation on the terminal carbon atom. Resulting monoacetoxyl reacts with the second acetate to give 2-butene-1,4-diol diacetates and 3-butene-1,2-diol diacetate in amounts proportional to the reactivity of carbon atoms 1 and 2 (**Scheme 6**). In fact, only 2-butene-1,4-diol diacetates are produced. Analogous mechanisms are realized in homogeneous oxidation of various dienes in the presence of Pd complexes and p-benzoquinone oxidizing agent, instead of Te. Oxidation of diene alcohols [56] and substituted conjugated diolefins [57] proceed effectively, but BD reacts with low yield and selectivity.

As noted earlier, Te-oxide is able to inhibit radical chain oxidation of BD, the selectivity of which is lower than the selectivity of the heterolytic process. Besides, Te operates as an inhibitor of radical polymerization of BD and oxidation products, thus preventing the formation of side high-boiling products. Acetic acid (possibly, other carboxylic acids) also contributes to the achievement of high selectivity in BD oxidation. Being not only solvent but also reagent (OAc− anions), it is involved in an intermediate interaction with olefin to form the surface Pd

**Scheme 5.** Oxidative 1,4-addition to BD.

**Scheme 4.** Tentative routes for nonradical oxidation of BD on PdTe/C catalyst.

and two oxidation states of tellurium Te0

surface Pd2+ ions and TeO2

generated from oxygen on Pd0

reagents due to the adsorption of O2

two active components.

of TeO2

106 Alkenes

According to XPS analysis, the 5%Pd2%Te/C catalyst contains both reduced Pd0

and Te4+ [46]. The Pd2+ to Pd0

species. The high reactivity of olefins with respect to peroxide

and BD. Thus, the PdTe/C catalyst opens the possibility

face becomes larger with an increase in tellurium content that indicates an oxidizing influence

 oxidant providing regeneration of Pd2+. However, there is a difference in products composition. Crotonaldehyde and furan are produced in above-mentioned oxidations of BD with homogeneous Pd2+ catalysts [41, 42], whereas methyl vinyl ketone is the second product formed in our oxidation on the PdTe catalyst. To explain this difference, one can consider an oxidation of BD by hydrogen peroxide as an alternative or parallel reaction (Route 2 in **Scheme 4**). Hydrogen peroxide is

compounds is known [47]. It is known that hydrogen peroxide does not accumulate during reaction. But it is found in trace amounts in the reaction solution and can form a reactive peroxide compound of Te4+ on the surface of the catalyst. In both mechanisms proposed,

of oxidation of BD by a nonradical heterolytic mechanism due to the combined effect of the

Te serves as a carrier of molecular or peroxide oxygen, and the surface of Pd2+/Pd0

. It can be expected that the oxidation state of the surface is enhanced under the reaction conditions. Nevertheless, dissolution of Pd and Te during reaction does not exceed 1% of the content of both components in the solid catalyst, the solution exhibiting no catalytic activity. Therefore, activity of the catalyst refers to the active components on the surface of carrier and is associated with their reversible redox transformations. Based on the known mechanisms of homogeneous oxidation of olefin, one can propose two possibilities for oxidation of BD by oxygen on the PdTe species, both assuming a nonradical heterolytic interaction. Perhaps the mechanism is in general similar to that postulated for the oxidation of BD and olefins in the presence of Pd2+ complexes, oxygen, and intermediate oxidant (**Scheme 4**, Route 1). It involves

and ionic Pd2+,

activates

ratio on the catalyst sur-

outer layer of supported particles. The characteristics of the PdTe/C catalysts were detailed by HAADF-STEM analysis of the surface and line EDX analysis of composition of the supported particles [60]. The results represent an unusual distribution of components on the surface, where Te does not form an individual crystalline phase but is located on the surface of Pd particles in a highly dispersed state. These data explain properties of the PdTe catalysts. In particular, the ability of Te to inhibit the radical reactions is in part due to the coverage of the

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

The primary products in BD oxidation on PdTe/C catalyst in methanol and further conversion of them under the oxidation conditions are shown in **Scheme 8**, and the amounts are given

As well as in DMA, nonradical heterolytic oxidation of BD in alcohol medium leads to the formation of crotonaldehyde (1) and methyl vinyl ketone (4). Besides, 1,4-dimethoxy-2-butene (6) is produced analogously to 2-butene-1,4-diol diacetate in acetic acid. The primary products undergo further transformations depending on the reaction conditions. Sulfuric acid promotes oxidation, especially toward 1,4-oxidative addition (comparison of first and second rows in **Table 4**). An increase in Te content lowers the reaction rate but increases proportion of products formed through 1,4-addition (third row in **Table 4**). Composition of oxidation

in the radical chain oxidation (compare data given in **Tables 1** and **4**). 3,4-Dimethoxy-1-butene and acrolein that indicate nonradical oxidation do not appear. Peroxide compounds were also not detected in the solution after the reaction. The chain process does not develop due to the presence of Te and low concentration of BD used to eliminate the formation of the radical chains. Moreover, the radical products do not appear even at increased concentration of BD (fourth row in **Table 4**). Similarly to acetic acid, methyl alcohol in a mixture with sulfuric acid converts the oxidation products to methyl esters. However, oxidation in the alcohol medium is slower than in acetic acid, and further improvement of the selectivity of the formation of

SO4

is differed from the one

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

109

palladium surface, which normally tends to initiate radical chains.

products obtained in the presence of the Pd0.5Te/C catalyst and H2

**Scheme 8.** Products of BD oxidation on PdTe/C catalyst in methyl alcohol.

1,4-addition products is required.

in **Table 4**.

**Scheme 6.** Mechanism of 1,4-oxidative addition to BD [54].

intermediate, and finally stabilizes the product as ester, preventing its secondary transformations. Based on the unique properties of the PdTe/C-HOAc catalytic systems, the industrial process for the production of 2-butene-1,4-diol diacetate has been developed by Mitsubishi Chemical. BD is oxidized to 2-butene-1,4-diol diacetate with selectivity of 98%. Possible further improvements of the process can be connected with the application of other platinum metals (Pt, Rh, and Ir) combined with various promotors.

If acetic acid is replaced by alcohol, 1,4-dialkoxylation of conjugated dienes was developed in Pd(OAc)2 solution. p-Benzoquinone was used as the oxidant and methanesulfonic acid as a promoter [58]. The oxidation is suggested to follow mechanism including the formation of the (π-allyl)palladium(benzoquinone) intermediate (**Scheme 7**).

In other case, dialkoxybutenes are prepared by reacting BD in the presence of carbon-supported Group VIII noble metals with Te or Se additives. Similar to diacetates, the formation of ethers in alcohol solvent increased the stability of dioxygenated products against secondary oxidation. However, the formation of 3,4-dimethoxy-1-butene and 1,4-dimethoxy-2-butene in comparable amounts is in contrast with **Scheme 6** and indicates a radical mechanism of BD oxidation, when 2-butene-1,4-diol and 3-butene-1,2-diol are formed as primary products and then converted to ethers in the alcohol medium [59].

We have prepared PdTe/C catalysts by hydrolytic deposition of palladium under the reductive conditions, followed by treatment with H<sup>6</sup> TeO<sup>6</sup> . The procedure is similar to one often described for the synthesis of PdTe catalysts. No evidences for the occurrence of binary Pd–Te phases have been provided by XRD, and XPS analysis evidences Pd0 , PdO, Te0 , and TeO2 [60]. The absence of the Pd-Te phase and the partially oxidized state of the active metals have also been reported by Takehira [54] for Pd-Te-C catalysts. As assumed, Te is located in the

**Scheme 7.** 1,4-Dialkoxylation of conjugated dienes [38].

outer layer of supported particles. The characteristics of the PdTe/C catalysts were detailed by HAADF-STEM analysis of the surface and line EDX analysis of composition of the supported particles [60]. The results represent an unusual distribution of components on the surface, where Te does not form an individual crystalline phase but is located on the surface of Pd particles in a highly dispersed state. These data explain properties of the PdTe catalysts. In particular, the ability of Te to inhibit the radical reactions is in part due to the coverage of the palladium surface, which normally tends to initiate radical chains.

The primary products in BD oxidation on PdTe/C catalyst in methanol and further conversion of them under the oxidation conditions are shown in **Scheme 8**, and the amounts are given in **Table 4**.

intermediate, and finally stabilizes the product as ester, preventing its secondary transformations. Based on the unique properties of the PdTe/C-HOAc catalytic systems, the industrial process for the production of 2-butene-1,4-diol diacetate has been developed by Mitsubishi Chemical. BD is oxidized to 2-butene-1,4-diol diacetate with selectivity of 98%. Possible further improvements of the process can be connected with the application of other platinum

If acetic acid is replaced by alcohol, 1,4-dialkoxylation of conjugated dienes was developed

a promoter [58]. The oxidation is suggested to follow mechanism including the formation of

In other case, dialkoxybutenes are prepared by reacting BD in the presence of carbon-supported Group VIII noble metals with Te or Se additives. Similar to diacetates, the formation of ethers in alcohol solvent increased the stability of dioxygenated products against secondary oxidation. However, the formation of 3,4-dimethoxy-1-butene and 1,4-dimethoxy-2-butene in comparable amounts is in contrast with **Scheme 6** and indicates a radical mechanism of BD oxidation, when 2-butene-1,4-diol and 3-butene-1,2-diol are formed as primary products and

We have prepared PdTe/C catalysts by hydrolytic deposition of palladium under the reduc-

described for the synthesis of PdTe catalysts. No evidences for the occurrence of binary Pd–Te

[60]. The absence of the Pd-Te phase and the partially oxidized state of the active metals have also been reported by Takehira [54] for Pd-Te-C catalysts. As assumed, Te is located in the

TeO<sup>6</sup>

. The procedure is similar to one often

, PdO, Te0

, and TeO2

solution. p-Benzoquinone was used as the oxidant and methanesulfonic acid as

metals (Pt, Rh, and Ir) combined with various promotors.

**Scheme 6.** Mechanism of 1,4-oxidative addition to BD [54].

then converted to ethers in the alcohol medium [59].

tive conditions, followed by treatment with H<sup>6</sup>

**Scheme 7.** 1,4-Dialkoxylation of conjugated dienes [38].

the (π-allyl)palladium(benzoquinone) intermediate (**Scheme 7**).

phases have been provided by XRD, and XPS analysis evidences Pd0

in Pd(OAc)2

108 Alkenes

As well as in DMA, nonradical heterolytic oxidation of BD in alcohol medium leads to the formation of crotonaldehyde (1) and methyl vinyl ketone (4). Besides, 1,4-dimethoxy-2-butene (6) is produced analogously to 2-butene-1,4-diol diacetate in acetic acid. The primary products undergo further transformations depending on the reaction conditions. Sulfuric acid promotes oxidation, especially toward 1,4-oxidative addition (comparison of first and second rows in **Table 4**). An increase in Te content lowers the reaction rate but increases proportion of products formed through 1,4-addition (third row in **Table 4**). Composition of oxidation products obtained in the presence of the Pd0.5Te/C catalyst and H2 SO4 is differed from the one in the radical chain oxidation (compare data given in **Tables 1** and **4**). 3,4-Dimethoxy-1-butene and acrolein that indicate nonradical oxidation do not appear. Peroxide compounds were also not detected in the solution after the reaction. The chain process does not develop due to the presence of Te and low concentration of BD used to eliminate the formation of the radical chains. Moreover, the radical products do not appear even at increased concentration of BD (fourth row in **Table 4**). Similarly to acetic acid, methyl alcohol in a mixture with sulfuric acid converts the oxidation products to methyl esters. However, oxidation in the alcohol medium is slower than in acetic acid, and further improvement of the selectivity of the formation of 1,4-addition products is required.

**Scheme 8.** Products of BD oxidation on PdTe/C catalyst in methyl alcohol.


**Table 4.** Products of BD oxidation in solvent CH3 OH (10% H2 O) (30 mL), H2 SO4 (0.1 mmol where indicated).
