Synthesizing+Lactone

__1.1 Reaction mechanism of the δ-lactone synthesis__
δ-lactone can be prepared by radical polymerization of butadiene with a palladium catalysis as shown in the figure below. It was known for decades that the d-lactone could be synthesized using butadiene and carbon dioxide as a feedstock, but the specific pathways for the reaction was not known until recently by Dr. Behr and his colleagues.

The suggested reaction mechanisms are as follows(Fig 3 ):
 * 1) A palladium(0) phosphine complex is formed //in situ// from a palladium(II) compound and a tertiary phosphine.
 * 2) A butadiene molecule is added to the complex- PdLn and the bis-η3-allyl complex, 7 is formed. In adding to the palladium complexes, butadiene act as a Lewis base and donate its pi-electron to the transition metal.
 * 3) The allyl complex 7 reaches chemical equilibrium with complex 8 by rearranging its organ-metallic bond.
 * 4) Complex 8 either transform itself further into this 1,3,7-octatriene or insert carbon dioxide into its(8) allylic bonds and form allylic carbonate complex 9.
 * 5) From complex 9, δ-lactone is formed by ring closure. Also γ-lactone and its conjugate isomer, 2-ethyl-2,4-heptadiene-5-olide are formed. This step involves reductive elimination, and the coordination of the complex reduces from 3 to 0, also the oxidate state reduces of the transition metal reduces.
 * 6) A byproduct also forms from complex 9, which are aliphatic esters 4 and 5.
 * 7) Complex 9 re-enters the catalytic cycle and palladium complex is re-activated.

__1.2 Miniplant implication of δ-lactone synthesis__
Dr. Behr also implemented the miniplant for the reaction, and as shown in the figure, all solvent and catalyst loops could be closed, yielding a stable process that would be ready to be scaled up industrially (Fig 4). Process steps:(2)
 * 1) Butadiene and carbon dioxide are mixed in a continuously stirred tank reactor (CSTR) with the selectivity enhancing solvent acetonitrile and the catalyst palladium(II) acetylacetonate/triphenylphosphine(PPh3).
 * 2) The mixture is constantly fed into a thermal separation unit where unreacted carbon dioxide, butadiene and the solvent are removed by gaseous phase and recycled to the mixer.
 * 3) At the same time, the δ-lactone/catalyst mixture from the reactor is fed into the second thermal separator. δ-lactone is separated in a vacuum as gaseous product while the catalyst is recycled into the mixer by distillation.

Behr //et al// investigated this reaction intensively and found out that the selectivity of δ-lact one could be increased to above 95% by recycling its by-products. He also concluded that the overall butadiene conversion rate was 45%(2).

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