Question

2.6. a. A stereospecific method for deoxygenating epoxides to alkenes involves reaction of the epoxide with the diphenylphosphide ion, followed by methyl iodide. The method results in overall inversion of alkene stereochemistry. Thus, cis-cyclooctene epoxide gives trans-cyclooctene. Propose a mechanism for this reaction and discuss its relationship to the Wittig reaction.

Answer 1

Phosphine has very high affinity towards oxygen. Once phosphorous will undergo nucleophilic addition to the epoxide the oxygen and phosphorous will go to the opposite orientation. The single bond in between this two atom will rotate and bring the oxygen and phosphorous atom close to each oher and make bond in betwwen. Finally this phosphorous oxide will be eliminated and alkene will changed to trans from cis or cis from trans because of the rotation took place in the mid way of the reaction. Here Methyl Iodide has no role except to quaternize phophorous atom (see reaction mechanism below)

In wittig reaction a mixture of cis/trans isomer will be observed. Here also phosphonium oxide will be elliminated.

For the reaction mechanism of second part see below:

cm.Rewe. . RC H₂ PPhz B8 base [RCH= PPh 3 Retto ...................... R + Ph.

meron Geisteswete lis opha Үle Ph. (Me) P=0 Lit