Conversion+of+d-lactone



δ-lactone can be converted to various important chemicals through the addition of different compounds. Its ability to undergo different reactions can be explained by the different functional groups that is attached to it. Table 1 shows a list of industrial chemicals produced from δ-lactone. This table demonstrates that CO 2 can be efficiently integrated as building block for many organic substances.

**__2.1 Hydrogenation to form Carboxylic acids__**
The resulting carboxylic acids are often used for the production of plasticisers for polymers. Hydrogenation of δ-lactone can be carried out using palladium as a heterogenic catalyst using methanol as a solvent. When δ-lactone is hydrogenated, 2-ethylhaptanoic acid(11) and saturated lactone(12) is formed. However, the hydrogenation of the saturated lactone greatly reduces the production of 2-ethylheptanoic acid(11). Therefore, using a heterogenic catalyst is economically inefficient.



On the other hand, using the homogenous catalyst rhodium triphenylphosphinetrisulfonate(TPPTS) yields 2-ethylheptanoic acid (11) as the main product (63% after 4 h) (1). Under a homogeneous catalyst and two-phase reaction conditions, the cleavage of the lactone ring is faster than the hydrogenation of the double bond. Saturated lactones are not cleaved nor hydrogenated under the same reaction conditions. This is due to the lacking coordination of the double bonds to the catalytic active metal atom. Therefore, all products contain the carboxylic functional group.



As shown in the figure above, the first step involves the cleavage of δ-lactone into three isomeric ethylidene heptanoic acids. The mechanism is explained via a ring opening under the formation of a h3-allyl-carboxylate-rhodium complex which is able to insert the hydrogen in two different positions of the molecule. The isomeric ethylidene heptanoic acids then undergo another hydrogenation of the carbon double bond under the same conditions to form 2-ethylheptanoic acid (11) as the main product.

__2.2 Hydroformylation to form Aldehydo-carboxylic acids__
The resulting aldehydes are important intermediates for several industrial products like plasticisers, alcohols and polymers. The hydroformylation of δ-lactone is carried out using a phosphate modified rhodium catalyst in tetrahydrofuran as solvent. The reaction is performed at mild conditions of 90C, 5 bars of 1:1 ratio syngas (1). The reaction is held at these conditions because only the terminal carbon carbon double bond shows activity. As such, this process yields a high amount of linear aldehydes (95%)(17).



__2.3 Hydroaminomethylation to form Amino-lactones__
Amines are widely used for agro chemicals, surfactants corrosion inhibitors and solvents. However, classical production of amines require high costs for the starting materials as well as the by-products. This can be optimized by performing hydroaminomethlation of δ-lactone as a substitute to produce amines. δ-lactone is subjected to hydroformylation using the catalyst system Rh/BIPHEphos (1) to produce the linear intermediate aldehyde which is then directly condensed with morpholine. The enamine is hydrogenated yielding the linear tertiary amine (22).

2.3.1 Upper pathway illustrated by figure 8:
The internal carbon-carbon double bond(22) undergoes hydroformylation and the aldehyic amine(23) is formed. This aldehyde then undergoes hydrogenation to its corresponding alcohol(24). Subsequent hydrogenation of the lactone ring cleaves it to produce hydroxyl amino acid(25).

2.3.2 Lower pathway illustrated by figure 8:
The lactone ring (22) undergoes hydrogenation to from an unsaturated amino acid( 26). The carbon-carbon double bond of this compound is then hydroformylated to the corresponding aldehyde (27). Hydrogenation of the aldehyde then occurs to produce a hydroxyl amino acid(25).



** __2.4 Hydroamination to form Amino-carboxylic acids__ **
δ-lactone is subjected to hydroamination by the selective addition of amine morpholine to the a-double bond of the δ-lactone. This reaction results in the production of z-amino acid (28). Various catalyst such as aluminium, iron, zirconium, platinum, palladium, rhodium can be utilized to perform the reaction.



Morpholine can be substituted by different nitrogen-compounds to yield various amino-carboxylic acids.



** __2.5 Alcoholysis to form Aloxy, Hydroxy Esters__ **
Figure 10 illustrates the formation of hydroxy ester(29),and 2 methoxy esters(30,31) from δ-lactone. The reaction below was prepared using a catalyst concentration of 0.4mol% Pd(OAc)2, 35 bar carbon monoxide at 75C for two hours with a primary alcohol as the solvent (1). The use of primary alcohol yielded a 95% conversion rate where as secondary alcohols only yielded a 5% conversion rate. It should be noted that the process did not undergo hydrocarbalkoxylation of δ-lactone’s carbon-carbon double bond but rather alcoholysis of the lactone ring.



The methoxy esters(30,31) can be hydroformylated to form an aldehyde at a 99% conversion rate.

** __2.6 Hydration to form Hydroxy-carboxylic acids__ **
The hydroxyl group is incorporated into the molecule through hydration. Hydration of δ-lactone is carried out using Amberlyst (1) as the catalyst at 100C for 24 hours. This process causes the lactone ring to open to produce 2-ethylidene-5-hydroxy-hept-6-enoic acid (33) at 98% conversion rate. The desired product is easily separated due to its immiscibility with water.



__2.7 Oxidation to form Lactone Epoxides__
For the epoxidation of δ-lactone, only methyltrioxorhenium (MTO) (1) with H 2 O 2 as the oxidation agent and pyridine as the auxiliary would lead to a 9% yield of a mono-epoxide mixture



__ 2.8 Hydrosilylation to form Silano-Carboxylic Acids __
Hydrosilylation is the addition of silane to the terminal carbon-carbon double bond of δ-lactone. Similar to hydroamination, alcoholysis and hydration, hydrosilylation also leads to ring cleavage instead of a 1,2-addition. When δ-lactone undergoes hydrosilylation with triethoxysilane, aliphatic silane (34) forms as the main product.

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