Answer to Question #281094 in Organic Chemistry for shre

Question

Iodo acids are not normally prepared by direct iodination of carboxylic acids under conditions of the Hell–Volhard–Zelinsky reaction. Show how you could convert octadecanoic acid to its 2-iodo derivative by an efficient sequence of reactions.

Answer 1

Deoxygenations of oxiranes have been reviewed 〈87H(26)1345〉. Of the later methods for deoxygenation, those utilizing P₂I₄ 〈84TL2601, 85S65〉, NbNaAlH₄ in THFC₆H₆ 〈82CL157〉 and Zn/TMSiCl 〈92TL3367〉 are one-step processes. A mild deoxygenation method under neutral conditions using dimethyl diazomalonate and a catalytic amount of Rh₂(OAc)₄ has been described 〈84TL251〉. The reaction proceeds with retention of configuration. Alkyl and homoalkylmanganese complexes have also been used for oxirane deoxygenations 〈84TL293〉. It appears that Bu₃MnLi is more effective than MeMnCl for this purpose.

Arylselelenocarboxamide can be used for the conversion of mono-, di-, and trisubstituted oxiranes to alkenes with retention of configuration 〈85TL669〉. Trifluoroacetic acid in the presence of sodium iodide likewise furnishes alkenes from oxiranes in a stereospecific manner 〈84CI(L)712〉. Tungsten [WCl₂(CH₂CH₂)₂(PMePh₂)₂] 〈91JA870〉 and vanadium [V₂Cl₃(THF)₆]₂[Zn₂Cl₆] 〈92SL510〉 complexes have been reported to effect deoxygenations successfully with predominant retention of configuration. Silyl oxiranes, upon treatment with excess organolithium reagents, form vinyl silanes 〈91TL2783, 91TL3457〉.

A fragmentation reaction, triggered by the formation of an oxiranylcarbinyl radical resulted in the deoxygenation of a spirocyclic oxirane (Scheme 47) 〈95TL19〉.