[10034-85-2] · HI · Hydrogen Iodide · (MW 127.91)
(electrophilic hydriodination of alkenes and alkynes;3-10 cleavage of epoxides,1b,11,12 ethers, and acetals;13 conversion of alcohols to iodides;14-17 reducing agent for many groups including quinones,24,25 a-diketones,23 a-ketols,23 a-halo ketones,19 a-diazo ketones,22 and sulfoxides;30 reductive cyclization of keto acids27,28)
Alternate Name: hydriodic acid.
Physical Data: 57% aqueous solution: bp 127 °C; d 1.70 g cm-3.
Solubility: sol most common organic solvents.
Form Supplied in: compressed gas; colorless 57% aq solution; widely available.
Preparative Methods: from the reaction of tetrahydronaphthalene with I2;2 can be generated in situ (a) from Me3SiCl and NaI in the presence of water,3 (b) from I2 and activated alumina,4 (c) from KI and H3PO4,5 and (d) from Et2PhN.BI3 and AcOH.6
Purification: distillation of the aqueous azeotrope; concentrated solutions can be regenerated after long storage by treatment with hypophosphorous acid.
Handling, Storage, and Precautions: store protected from air and light at or below rt. Highly corrosive and toxic. This reagent should be handled in a fume hood.
Being a stronger acid, HI undergoes addition more readily than Hydrogen Chloride or Hydrogen Bromide to most alkenes and alkynes.1a Moreover, there is no competing radical addition as with HBr. However, because of the difficulty in generating and transferring anhydrous HI, addition of HI has received less attention than addition of HCl and HBr. As mentioned above, several techniques have been developed for generating HI in situ. These include the use of KI and H3PO4 (eq 1);5 Me3SiCl and NaI in the presence of water (eq 2);3 I2 and activated Al2O3 (eq 2);4 and the Et2PhN.BI3 complex and AcOH (eq 3).6 Alternatively, I(py)2BF4 has been used with the hydride donor Et3SiH (eq 1).7
Aqueous HI has been used with a phase-transfer catalyst to hydriodinate alkenes (eq 2).8 Similarly, it has been used to convert dialkylalkynes to the corresponding (Z)-vinyl iodides (eq 4).9
A particularly convenient method for generating HI in situ involves the use of various inorganic and organic iodides in the presence of appropriately prepared silica gel or alumina (eq 2).10 These adsorbents also facilitate the addition process. Surface-mediated hydriodination of phenylalkynes affords the (E) isomers, resulting from syn addition (eq 5).10 The regiochemical course of these hydroiodinations follows Markovnikov's rule.
The addition of HI to epoxides to give iodohydrins proceeds readily using either aqueous HI or anhydrous HI in organic solvents.1b,11 Because of the difficulty of preparing anhydrous HI, aqueous solutions have most often been used for this transformation. The stereo- and regioselectivity of the addition process is similar to the general trends discussed for the corresponding additions with HCl (see Hydrogen Chloride). Trimethylsilyl-substituted epoxides give the corresponding iodohydrins with particularly high stereo- and regiospecificity (eq 6).12
HI readily cleaves ethers to alcohols and/or iodides13 and is an attractive reagent for this transformation from the standpoint of economy and convenience. Primary and secondary alkyl methyl ethers are cleaved to afford alcohols (or derivatives), while benzyl and tertiary alkyl ethers often yield iodides. Acetals react in a similar fashion to produce ketones, although this deprotection method rarely offers advantages over more common procedures.
The reaction of 57% aqueous HI with saturated primary and secondary alcohols at elevated temperatures leads in fair to high yield to the corresponding iodides.14 Tertiary iodides have been synthesized in good to high yields from the corresponding alcohols under especially mild conditions by 55% aqueous HI in the presence of Lithium Iodide.15 Allylic alcohols are transformed to allylic iodides by HI generated in situ from Me3SiCl/NaI.16 Benzylic alcohols are subject to conversion to the saturated system, presumably via iodine substitution and ensuing reduction (eq 7).17
Alkyl iodides can also be synthesized via treatment of secondary and tertiary alkyl chlorides with anhydrous HI in the presence of catalytic amounts of FeI3.18
Treatment of various a-substituted ketones with HI leads to reductive scission of the a-substituent. Reductive dehalogenation of a-halo ketones can thus be accomplished with HI to furnish the corresponding ketones in high yield (eq 8).19 Reaction occurs readily even with sterically hindered substrates. Related procedures employing cat. NaI or 57% aq HI and phosphorous acid in acetonitrile20 or NaI in concd H2SO421 require long reaction times, high temperatures, and/or reactive substrates and are less satisfactory.
a-Diazo ketones are reduced to methyl ketones by 47% aqueous HI in CHCl3 (eq 9).22 The reaction with HI differs from that of HBr and HCl, which give halomethyl ketones as products. Presumably, the initially formed iodomethyl ketone is reduced to the saturated ketone under the reaction conditions.
a-Diketones and a-ketols are reduced to the corresponding saturated ketones in good yields by aqueous HI in acetic acid at reflux (eq 10).23
Polycyclic quinones may be reduced to polyarenes by HI in HOAc at reflux (eq 11).24,25 In resistant cases, concentrated aqueous HI may be employed; addition of phosphorus often results in cleaner reaction by removing the I2 formed. Large excess of HI or prolonged reaction time may lead to overreduction. Since hydroquinones and phenols are intermediates in these reactions, they are also readily reducible with this reagent. Reductive methylation of quinones can be accomplished in high yield by reaction of polycyclic quinones with excess Methyllithium followed by reduction with HI (eq 12).26
A method for the construction of fused polyarenes entails reaction of a smaller aromatic ring system with phthalic anhydride followed by reductive cyclization of the keto acid product with HI in acetic acid to form a polyarene with two additional rings (eq 13).27,28 This method conveniently combines three steps (reduction of the carbonyl group, cyclodehydration, and reduction) into one step.
The combination HI/P/HOAc effectively deoxygenates aryl ketones (eq 14).27-29 While this method is utilized infrequently, it represents a useful alternative to the better known Wolff-Kishner and Clemmensen reduction methods.
Sulfoxides are readily deoxygenated by HI without the complicating halogenation that often accompanies reduction using HBr or HCl.30
Hydriodic acid reacts with vinylsilanes with replacement of R3Si by hydrogen (eq 15).31 A small amount of I2 and water (or D2O) is also effective. These reactions usually occur with retention of configuration.
Gary W. Breton & Paul J. Kropp
University of North Carolina, Chapel Hill, NC, USA
Ronald G. Harvey
University of Chicago, IL, USA