Deoxygenation of oxiranes



Although there is no general way to de oxygenate ethers, but oxiranes can be deoxygenated (reduced) to alkenes by certain trivalent phosphorous compounds for example.

In this reaction oxirane derivatives which are originally cis end up trans in the alkene product and vice versa so this reaction could be quite valuable. While per acids epoxidation preserves the stereochemistry of the alkene, deoxygenation of the alkene, deoxygenation by the above path create inversion of configuration




Mechanism



A nucleophilic ring opening of the oxirane at its less hindered carbon, formation of dipolar ion that undergoes bond rotation followed by ring closure to the strained oxaprosphetane (unstable) ultimately gives final product.




Problems
– Find the products of following reaction and give stereochemistry where appropriate.

Answers:



(a) PhOH + ICH₂Ph, S_N
2 cleavage at the alkyl, as opposed to aryl, carbon of the protonated ether.




(b) (H₃C)₂CHOH + BrC(CH₃)₃, S_N
1 cleavage at the more Su.