[13517-10-7] · BI3 · Boron Triiodide · (MW 391.52)
(strong Lewis acid with labile, nucleophilic iodine atoms; cleaves ethers,2 esters,3 silanes,4 halides,5 and alcohols to alkyl iodides; reductively cleaves sulfonyl6 and sulfinyl7 groups to disulfides)
Physical Data: mp 50.5 °C; d 3.350 g cm-3.
Solubility: sol hydrocarbons, CH2Cl2, CHCl3, CCl4, CS2; reacts with oxygenated solvents.
Form Supplied in: volatile white solid (frequently has a pinkish color due to traces of I2).
Preparative Methods: BI3 of extremely high purity can be prepared in 25% yield by treating I2 in heptane at 80 °C with KBH4 (which has been recrystallized from H2O).8 Recrystallized NaBH4 can also be used; if traces of chloride or bromide can be tolerated then LiBH4 in hexane can be used.9
Purification: BI3 has a vapor pressure similar to that of I2 and can be purified by sublimation from Cu powder at 60 °C (in vacuo). Commercial material is usually contaminated with traces of other halides. While the presence of small amounts of BI2Br and/or BI2Cl does not usually interfere with most organic reactions, these impurities are not easily removed by sublimation or distillation.
Handling, Storage, and Precautions: use in a fume hood; reacts expositively with H2O; traces of moisture result in the liberation of I2 so the formation of any color indicates improper handling. BI3 is also light sensitive; it photolyzes below 360 nm. It should be stored under an inert atmosphere in the dark.
All of the boron trihalides, except the fluorides, will cleave ethers with varying degrees of efficacy.10 However, the nucleophilic character of iodine coupled with the strong Lewis acidity of boron makes boron triiodide the most potent of these reagents. It is a powerful reagent for the cleavage of C-O bonds in ethers, esters, and alcohols, resulting in the formation of alkyl iodides under mild conditions. Aryl alkyl ethers are cleaved to phenols (eq 1). Diaryl ethers are unreactive. BI3 reacts at least an order of magnitude faster than Boron Tribromide in ether cleavages.1 This is especially useful in the cleavage of the ethers of higher alkyl groups (eq 2).
The initial products of ether cleavage are the alkyl halide and a borate ester, (RO)3B. The borate esters are usually inert to further displacement but, because the iodide is more nucleophilic than the other halides, warming the borate esters (60-80 °C) in the presence of BI3 will result in the complete conversion of all the alkyl residues to iodides (eq 3).2
Kabalka3 and co-workers have shown that an attenuated form of BI3, BI3.NEt2Ph, will cleave a variety of compounds containing C-O single bonds at elevated temperatures. Solutions of this reagent are prepared by reacting the commercially available amine-borane complex with I2 in benzene at 80 °C for several hours. The reagent cleaves ethers,11 esters,3 and geminal diacetates.11 Esters3 are cleaved to an activated acyl intermediate RCOX which can be used to prepare acids, other esters, and amides (eq 4).
Sulfinyl and sulfonyl compounds react with BI36 and BI3.NEt2Ph7 to afford disulfides (eq 5).
Sulfoxides are deoxygenated by BI3.NEt2Ph.7 Sulfides are also cleaved by BI3. Methionine reacts to yield a complex mixture of C-S bond cleavage products including homocysteic acid, homoserine, and homoserine lactone.12
As a powerful electrophile, BI3 will effect aromatic and aliphatic substitution reactions on alkyl and aryl halides and silanes (eq 6).4 Alkyl halides (R-Cl and R-Br) also undergo substitutions reactions in CH2Cl2 and CCl4 solution, producing iodides without alkyl rearrangement. In CCl4 the reaction is second order in alkyl halide.5 At elevated temperatures the ipso substitution of aryl iodides occurs readily, resulting in borinic iodides eq 7).13 The direct borylation of benzene is a photochemical process which most likely involves the formation of I&bdot; and I2B&bdot; radicals (eq 8).14
Washington State University, Pullman, WA, USA