Phosphorus Iodide elimination reactions

A homo-1,4-elimination occurs on reaction of the anti-bishomoquinone derivatives (624 R = Me or Ar) with phosphorus iodide cf. p. 97). The n.m.r. spectra of the products (625) show temperature dependence due to a degenerate Cope rearrangement With phosphorus tribromide (624 . R = Me) undergoes a double homoallylic rearrangement to give (626). °... 

The next reaction in this sequence is an Appel reaction, which transforms an alcohol to an alkyl iodide. The simplified mechanism is depicted in the margin. The initial step of this reaction is the nucleophilic attack of triphenyl phosphine to iodine with formation of an iodine triphenyl phosphonium cation. The positive charge increases the oxophilicity of phosphorus, which is attacked in the next step by an alcohol. After elimination of hydrogen iodide, the resulting alkoxy triphenyl phosphonium cation is attacked by an iodide anion in an SN2-type reaction. Therefore, the reaction of chiral alcohols typically proceeds with inversion of configuration, although other behavior is also reported. The product of the Appel reaction is alkyl iodide 37 and triphenyl phosphine oxide is formed as byproduct. ... 

The reaction is presumed to occur by initial formation of HjfMenO) PO], which then undergoes direct P-alkylation by nucleophilic attack of phosphorus on carbon with displacement of halide. An alternative alkylation mechanism involving nucleophilic attack of oxygen on carbon, followed by a Michaelis-Arbuzov rearrangement i) of a dialkyl phenylphosphonite (C6H5(MenO)POR) intermediate with the alkyl halide was effectively eliminated by the observation that reaction of methyl phenylphosphinate with a tenfold excess of methyl-c/3 iodide gave the product distribution shown in Eq. (2). 

It has already been indicated that the course of any reaction may depend, to some extent, on the nature of the phosphite (or phosphonite) ester (phosphinite esters yield phosphine oxides). Thus, tris(perfluoroalkyl) phosphites do not undergo a Michaelis-Arbuzov reaction with perfluoroiodoalkanes, although reports on the outcome of any reaction between triethyl phosphite and CF3I, under normal conditions, are conflicting reactions do appear to proceed under photostimulation. A normal reaction does take place at high temperatures between polyfluorinated trialkyl phosphites and methyl iodide, when the product, MeP(0)(0Rf)2, is accompanied by oxidation of the phosphite to phosphate Either elimination or alkylation accompanies the formation of unidentified phosphorus-containing products in the reactions between trialkyl phosphites and the halides Cl3C(CF2) Cl n - 2, 4 or 6) ... 

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