[13760-02-6] · H2I2Si · Diiodosilane · (MW 283.91)
Alternate Name: DIS.
Physical Data: colorless liquid; mp -1 °C; bp 149.5 °C; bp 58 °C/25 mmHg; bp 0 °C/3.8 mmHg; fp 38 °C; d 2.7943 g cm-3.
Solubility: highly sol hydrocarbons and chlorinated solvents; reacts with oxygen- and nitrogen-containing solvents.
Form Supplied in: pale yellow liquid packed under nitrogen in ampules.
Preparative Method: phenylsilane and iodine are mixed in a 1:1 molar ratio without solvent at -20 °C in the presence of catalytic amounts of ethyl acetate; fractional distillation removes volatile side products (benzene and HI) and affords the reagent in the form of a colorless oil.
Handling, Storage, and Precautions: is sensitive to moisture; handle and store under nitrogen; stabilized by metallic copper; refrigeration and absence of light recommended for storage over long periods of time.
All transformations described below are conveniently carried out in either chloroform or dichloromethane at temperatures between -40 and 50 °C. Aqueous work-up with sodium hydrogencarbonate (in which all silicon compounds, including excess reagent, form insoluble polymers) is applicable in most cases. Water-sensitive products are isolated by direct fractional distillation of the reaction mixture.
Diiodosilane (DIS) exhibits properties and reactivities that are either similar or complementary to those of Iodotrimethylsilane (TMSI). These reagents share two dominant characteristics: both are strong,
hard, oxophilic Lewis acids and effective sources of nucleophilic,
soft iodide ions. The differences between them arise from the facts that DIS is a stronger Lewis acid than TMSI, it is less sensitive to steric effects, and it can also behave as a hydridic reducing agent.
Although both TMSI and DIS react with alcohols and alkyl ethers to produce the corresponding iodoalkanes (with inversion of configuration at carbon), they exhibit complementary selectivities. While TMSI reacts with primary oxygen functions more readily than with secondary ones, DIS exhibits the opposite preference, reacting with secondary alcohols and ethers faster than with the primary functions. As illustrated by the case of butane-1,3-diol (eq 1), this preference of DIS is applicable for regioselective transformations of polyols.
Various carboxylic acid derivatives, including carboxylic acids, esters, lactones, anhydrides, and acyl chlorides are converted by DIS to acyl iodides. These transformations are accelerated by Iodine. These reactions, when followed by addition of an appropriate alcohol, represent an overall esterification or transesterification method which is particularly useful for sterically hindered and/or poorly nucleophilic alcohols. In the absence of iodine, DIS reacts with carboxylic acids and esters, much as does TMSI, to form the corresponding silyl carboxylates as the final product. Lactones react with DIS to produce either silyl o-iodocarboxylates or o-iodoacyl iodides (eq 2), depending on the reaction conditions. The reaction between DIS and 1 equiv of a carboxylic anhydride affords, in the presence of iodine, 2 equiv of acyl iodide.
At low temperatures (-42 °C) and short reaction times (few minutes), DIS (5-10 mol %) catalyzes clean deprotection of various acetals, affording the parent ketones and aldehydes with no apparent reduction of the latter (eq 3). These reactions proceed much more rapidly with DIS than with TMSI, probably due to the higher Lewis acidity and lower steric demands of DIS relative to TMSI. The reactivity difference between the two reagents is particularly apparent with sterically congested acetals, e.g. 4,5-dimethyldioxolane. While these compounds react with TMSI quite sluggishly, their reactions with DIS proceed almost as rapidly as observed with sterically nonhindered acetals.
At temperatures above 0 °C, DIS effectively reduces acetals as well as unprotected ketones and aldehydes to iodoalkanes (eqs 3-6). Reaction rates are highly dependent on the substrate, with the following tendencies:
This reactivity pattern represents inversion of the normal order of reactivity of
unprotected carbonyl compounds. In polyfunctional molecules containing both free and protected carbonyls, acetals are chemoselectively reduced by DIS to iodoalkanes in preference to unprotected ketones and aldehydes (eqs 7 and 8).4
The reaction of DIS with tetrahydropyranyl (THP) derivatives can be controlled by the amount of reagent used (eqs 9 and 10).3 While 1 equiv of DIS yields 2-iodo-THP exclusively, excess reagent produces 1,5-diiodopentane.
a-Halo ketones react readily with DIS between 0 °C and rt to produce the parent ketones. Reactions are faster with DIS than with TMSI but the same order of substrate reactivity, i.e. primary > secondary > tertiary halides, is observed with both reagents. The same transformation can be achieved with phenylsilane (the precursor of DIS) and catalytic amounts of iodine.
Cyclopropyl ketones react with either DIS or TMSI to produce g-iodo ketones. Again, reaction occurs much faster with DIS than with TMSI (eq 11). Sulfoxides are reduced readily with both reagents to sulfides. Phosphine oxides may be reduced to phosphines provided that an iodine scavenger is present.
Technion-Israel Institute of Technology, Haifa, Israel
Cynthia K. McClure & Pranab K. Mishra
Montana State University, Bozeman, MT, USA