[75-47-8] · CHI3 · Iodoform · (MW 393.72)
(precursor to both mono- and diiodocarbene for addition to alkenes and arenes; adds to alkenes by a radical chain mechanism when correctly initiated; iodine atom donor for quenching carbon radicals; used in conjunction with chromium(II) chloride for the homologation of aldehydes to (E)-iodoalkenes)
Alternate Name: triiodomethane.
Physical Data: mp 120-123 °C; d 4.008 g cm-3.
Solubility: sol ethanol, acetone, ether, benzene, carbon disulfide, dichloromethane, THF; slightly sol petroleum ether.
Purification: crystallization from ethanol.
Handling, Storage, and Precautions: yellow crystalline solid with a disagreeable odor. Decomposes at high temp with evolution of iodine.
Diiodocarbene, formed by base-induced decomposition of iodoform, with or without phase-transfer catalysis, adds to alkenes giving diiodocyclopropanes which may be rearranged with AgI to iodoalkenes (eq 1).1 Under phase-transfer conditions, reaction with adamantane gives 1-(diiodomethyl)adamantane.2
The triiodomethyl anion, formed from iodoform and 50% Potassium Hydroxide, can be trapped by addition to pyridinium and isoquinolinium salts (eq 2).3
Monoiodocyclopropanes may be prepared by reaction of alkenes with iodoform in the presence of copper, albeit in only low yield.4 The iodocarbenoid formed from iodoform and Diethylzinc adds to alkenes in moderate yield with clean retention of configuration and with high preference for the syn-isomer (eq 3).5
Photolysis of iodoform in the presence of alkenes also leads to the formation of monoiodocyclopropanes, also with retention of configuration (eq 4). With cis-2-butene the syn:anti ratio was 1.6:1 (eq 5).5c,6 Interestingly, with cyclic cis-alkenes such as cyclooctene only the endo-iodocyclopropane was formed both under photolytic conditions and with diethylzinc.5c Flash photolysis experiments indicated that the photochemical reaction proceeds via homolysis of iodoform to iodine atoms and diiodomethyl radicals.7 Accordingly, photolysis in the presence of a more reactive alkene, norbornene, gave a mixture of radical and carbene addition products.8
Arenes also react with the diethylzinc/iodoform couple to give a cyclopropane adduct that suffers further reaction with diethylzinc and rearrangement to cycloheptatrienes (eq 6).9
Triethylborane initiates the radical addition of iodoform to ketene silyl acetals, resulting in the isolation of b-iodo-a,b-unsaturated esters (eq 7).10 With tetraiodomethane the corresponding b,b-diiodide was obtained. Radical addition of iodoform to methyl acrylate is also reported to be initiated with Pentacarbonyliron (eq 8).11
Addition to limonene (eq 9) serves to illustrate that terminal alkenes are more readily attacked than internal alkenes and that the reaction is not limited to electron-deficient substrates.12
Iodoform acts as an iodine atom donor in the Barton version of the Hunsdiecker reaction. Aliphatic, and to some extent aromatic, carboxylic acids undergo this oxidative decarboxylation and optimum yields were obtained when cyclohexene was used as solvent in place of benzene.13 The reaction, which may be initiated thermally,13,16 photolytically,14 or with ultrasound,15 tolerates many functional groups and has been applied to cyclopropanecarboxylic acids (eq 10)16 and notably to electron-rich aromatic acids (eq 11).13
Takai has described a very simple procedure for the homologation of aldehydes to iodoalkenes which involves treatment with iodoform and Chromium(II) Chloride and which proceeds with high selectivity for the (E)-product (eqs 12 and 13).17
Although ketones are less reactive, as demonstrated by competition reactions, good yields of iodoalkenes are nonetheless obtained (eq 14).17
In the course of a synthesis of Macbecin I, Baker and Castro devised a synthesis of b-iodomethacrylic acid that involved alkylation of diethyl methylmalonate with iodoform followed by saponification and decarboxylation (eq 15).18
Deuterated or tritiated methylene diiodide may be prepared from iodoform as reported by Saljoughian (eq 16), providing a convenient method for the incorporation of deuterium or tritium into cyclopropane rings via the Simmons-Smith reaction.19
The Finkelstein reaction of iodoform with Silver(I) Fluoride or Mercury(II) Fluoride provides mixtures of diiodofluoromethane and difluoroiodomethane (eq 17).20
David Crich, Milan Bruncko & Jarmila Brunckova
University of Illinois at Chicago, IL, USA