Phosphorus(III) Iodide

PI3

[13455-01-1]  · I3P  · Phosphorus(III) Iodide  · (MW 411.67)

(conversion of alcohols to iodides; deoxygenation of epoxides to alkenes; reduction of sulfoxides, selenoxides, selenones, ozonides, and a-halo ketones; elimination of b-hydroxy sulfides, dithioacetals, and orthothioesters or their selenium counterparts to form alkenes; conversion of aldoximes and nitro compounds to nitriles; cleavage of dimethyl acetals; water-soluble phosphorus byproducts are usually formed)

Alternate Name: phosphorus triiodide.

Physical Data: mp 61 °C.

Solubility: very sol CS2; reacts with protic solvents.

Form Supplied in: red solid; widely available commercially.

Handling, Storage, and Precautions: commercial samples are generally suitable for use without purification; store under an inert atmosphere.

Conversion of Alcohols to Iodides.

PI3 is a classic reagent for the conversion of iodides to alcohols. For example, cetyl alcohol is converted to 1-iodohexadecane in 85% yield by treatment with PI3 (neat).1 More recently, Krief and co-workers2 found that a cyclopropyl carbinol (eq 1) is cleanly converted to the iodide with ring opening upon treatment with PI3 (Et3N, CH2Cl2, 0 °C, 0.5 h, 73%). Diphosphorus Tetraiodide, a commercially available phosphorus halide which is in equilibrium with PI3 in solution, is a more commonly employed reagent for the conversion of alcohols to iodides and effects many of the same transformations as PI3.3 See also Triphenylphosphine-Iodine for the conversion of alcohols to iodides.

Deoxygenations and Reductions.

PI3 has found use as a deoxygenating and reducing agent. Epoxides are stereospecifically deoxygenated to the corresponding alkenes in good yield and with retention of configuration. For example, the cis- and trans-epoxides shown in eq 2 are converted (CS2, 20 °C, 6-8 h) to cis- and trans-9-octadecene in 83 and 90% yields, respectively.4 Diphosphorus tetraiodide (P2I4) and Iodotrimethylsilane (TMSI) give similar results. A number of other methods for epoxide deoxygenation have been developed.5

A variety of methods for the reduction of sulfoxides to sulfides are available.6 PI3 rapidly reduces aryl alkyl and dialkyl sulfoxides and selenoxides, usually at -78 °C (eq 3).7 For example, treatment of ethyl phenyl sulfoxide with 1 equiv of PI3 (CH2Cl2, -78 °C, 15 min) affords ethyl phenyl sulfide in 91% yield. Others have successfully employed this procedure.8,9 Dialkyl sulfoxides generally react in somewhat lower yield. A phenyl vinyl sulfoxide (eq 3, entry c) requires ambient temperatures to react. Selenoxides behave similarly, and P2I4 can usually be used in place of PI3.10 Treatment of decyl phenyl selenone with PI3 (CH2Cl2, 0 °C, 30 min) affords a mixture of the reduced product, decyl phenyl selenide (69%), and the substitution product, n-decyl iodide.11

PI3 can be used for the efficient reduction of an ozonide, as illustrated by the formation of the cyclopropanecarbaldehyde in eq 4.12,13 An advantage over the traditional Triphenylphosphine reduction is the formation of water-soluble phosphorus byproducts from PI3. It should be noted, however, that PI3 is reported to react with certain aldehydes.14

PI3 is reported to dehalogenate a-bromo and a-iodo ketones.15 For example, as shown in eq 5, 1-iodo-2-dodecanone is reduced to 2-dodecanone by PI3 in CH2Cl2 (25 °C, 1 h, 89%). 1-Bromo-2-octanone affords 2-octanone in 75% yield (25 °C, 4.5 h). This PI3 reduction can be coupled with the regioselective opening of terminal epoxides by TMSI and alcohol oxidation for a three-step transformation of a terminal epoxide to a methyl ketone.15,16 P2I4,15 triphenylphosphine,17 1,3-Dimethyl-2-phenylbenzimidazoline,18 PhSiH3/Mo0,19 Samarium(II) Iodide,20 Zinc-Acetic Acid,21 Iron-Graphite,22 Tri-n-butylstannane,23 Sodium O,O-Diethyl Phosphorotelluroate,24 lithium 2-thiophenetellurolates,25 and a wide variety of other reagents26,27 can also be used for the reduction of a-halo carbonyl compounds.

Elimination Reactions.

PI3 or P2I4 effects the stereospecific anti-elimination of b-hydroxy sulfides to alkenes.28 For example (eq 6), treatment of the anti-b-hydroxy sulfide with PI3 (CH2Cl2-Et3N, 50 °C, 1.5 h) affords (E)-9-octadecene (93% yield). Treatment of the syn-isomer affords (Z)-9-octadecene (93% yield, Z:E = 94:6). Trisubstituted, but apparently not tetrasubstituted, alkenes can also be formed from the appropriately substituted b-hydroxy sulfides using these reagents. Similarly, b-hydroxy selenides undergo elimination to the corresponding alkenes. With these latter substrates, certain tetrasubstituted alkenes are accessible.29 Methanesulfonyl Chloride/Et3N30 and N-ethyl-2-fluoropyridinium tetrafluoroborate/lithium iodide31 are alternative reagents.

Vinyl sulfides and ketene dithioacetals32 are available via treatment of b-hydroxy thioacetals and b-hydroxy orthothioesters with PI3 or P2I4.33 For example (eq 7), treatment of the b-hydroxy orthothioester (entry a) with PI3 (CH2Cl2-Et3N, 0 °C, 0.5 h) affords the thioketene acetal (1) in 69% yield. With certain substitution patterns the rearranged product (2) competes (e.g. entry b) or is the predominant product (e.g. entry c). The rearrangement is less of a problem with the corresponding b-hydroxy orthoselenoester (i.e. entry c vs. entry d). Thionyl Chloride is also a useful reagent in some cases.

Vinyl sulfides and ketene thioacetals32 are also available via treatment of thioacetals and orthothioesters with PI3 or P2I4 (eqs 8 and 9).34 The corresponding seleno analogs are similarly available. For example (eq 8), treatment of the trimethyl orthoselenoester with PI3 (CH2Cl2-Et3N, 20 °C, 1 h) affords the ketene selenoacetal in 90% yield. Tin(IV) Chloride or Titanium(IV) Chloride in combination with Diisopropylamine is also effective for the transformation of orthothioesters and orthoselenoesters to ketene acetal derivatives.35 Copper(I) Trifluoromethanesulfonate,36 Mercury(II) Trifluoroacetate/Lithium Carbonate,37 and Benzenesulfenyl Chloride38 are useful reagents for the conversion of dithioacetals to vinyl sulfides.

In the presence of Triethylamine, PI37 or P2I410 effects the conversion of aldehyde oximes or primary nitro compounds to nitriles. For example, treatment of the aldoxime derived from phenylacetaldehyde with 1 equiv of PI3 (CH2Cl2-Et3N, 25 °C, 15 min) affords benzyl cyanide in 83% yield (eq 10). Similarly, treatment of 1-nitrodecane with 2 equiv of PI3 (CH2Cl2-Et3N, 25 °C, 15 min) affords 1-cyanononane (82%) (eq 11). Other reagents for the direct conversion of primary nitro compounds to nitriles include Sn(SPh)4/Ph3P/DEAD,39 TMSI,40 Sulfur Dioxide/Et3N,41 and Phosphorus(III) Chloride.42

Miscellaneous Transformations.

PI3 has been used for the cleavage of dimethyl acetals under nonaqueous conditions.43 For example (eq 12), treatment of the dimethyl acetal derived from 2-decanone with PI3 (CH2Cl2, 20 °C, 15 min; aq workup) affords the ketone in 85% yield. Alternative nonaqueous acetal cleavage reagents include P2I4,43 TMSI,44,45 Diiodosilane,46 TiCl4/Lithium Iodide,47 Bromodimethylborane or bromodiphenylborane,48 and Samarium(III) Chloride/Chlorotrimethylsilane.49

PI3 is a useful Hydrogen Iodide precursor in the silica- and alumina-mediated additions of HI to alkenes.50,51 PI3 has also found use as a source of electrophilic phosphorus in the synthesis of novel organophosphorus compounds (e.g. (CF3)3P52) and novel phosphorus-containing organometallic compounds.53


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James M. Takacs

University of Nebraska-Lincoln, NE, USA



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