Like other strong nucleophiles, triphenylphosphine attacks and opens epoxides. The initial product (a betaine) quickly cyclizes to an oxaphosphetane that collapses alkene and triphenylphospine oxide.

1. Show each step in the reaction of trans-2,3-epoxybutane with triphenylphospine to give but-2-ene. What is the stereochemistry of the double bond in the product?
2. Show how this sequence might be used to convert cis-cyclooctene to trans-cyclooctene.

A

Trans-2,3-epoxybutane on reaction with triphenylphosphine undergoes nucleophilic attack of triphenylphosphine onto the epoxide ring in the first step, and as a result strained epoxide ring opens up and strain is released. In the next step, for obtaining the correct orientation for attack of oxygen anion onto the phosphorous which is positively charged and for effective back bonding, the rotation of the molecule takes place. After the attack, four-membered ring is formed which is also strained, thus opens up in a simultaneous bond-breaking and forming fashion, giving the alkene product which is cis in nature as both the methyl groups are cis to each other.

The stereochemistry of the final product is inverted as nucleophile attacked at the epoxide ring in an anti-fashion that is, from the opoosite side of the epoxide plane.

Mechanism of formation of cis-alkene from trans reactant

Cis-cyclooctene undergoes epoxidation in the first step in the presence of a peroxy acid which forms epoxide at the double bond. Then, triphenylphosphine acts as nucleophile and attacks from the opposite plane of the epoxide in an anti-manner, hence, giving trans-product. Then, after roation of the molecule to bring groups in proper orientation to attack and elimination of leaving group that is, triphenylphospine oxide, we get the final product which is trans-alkene.

Mechanism of formation of trans alkene from cis-reactant