[141-51-5] · CH2HgI2 · Iodo(iodomethyl)mercury · (MW 468.42)
(reagent for the methylenation of alkenes1)
Physical Data: pale yellow crystals; mp 113-116 °C.
Solubility: sol THF, hot benzene.
Preparative Method: treatment of Mercury(0) with 5 equiv of Diiodomethane with mechanical stirring and irradiation with a sunlamp resulted in the disappearance of the liquid mercury. Upon cooling and addition of benzene, a precipitate was obtained, which after recrystallization from hot (93 °C) diiodomethane gave pale yellow crystals of product in 48% yield.2
Handling, Storage, and Precautions: little information is available about iodo(iodomethyl)mercury, but as with all mercury reagents, care should be taken when handling these toxic reagents. Use in a fume hood.
Iodo(iodomethyl)mercury was first prepared in 1880.3 However, it was not until 1964 that its use as a methylene transfer reagent was reported.1 The reaction of iodo(iodomethyl)mercury with cyclohexene affords the corresponding cyclopropane, norcarnene, in 24% yield after 8 d (eq 1). The reaction benefits from the addition of an equimolar quantity of Diphenylmercury (eq 2). The reaction is stereospecific, and the alkene geometry is retained during the cyclopropanation reaction (eqs 3 and 4). There is also sensitivity to the electronic nature of the alkene, since tetrachloroethylene was found to be inert to cyclopropanation (eq 5).
More recently, the use of iodo(iodomethyl)mercury for the methylenation of bis(trimethylsilyl)acetylene has been reported.4 The unstable cyclopropene could be isolated in 10% yield, but by repeating the reaction in the presence of 1,3-diphenylisobenzofuran the Diels-Alder adduct could be isolated in 35% yield (eq 6).
It is believed that there is an exchange process between iodo(iodomethyl)mercury and diphenylmercury which affords the more reactive methylenating reagent bis(iodomethyl)mercury (eq 7).5 Other mercury-containing reagents, including bis(bromomethyl)mercury and Phenyl(trichloromethyl)mercury, have been reported to cyclopropanate alkenes.6
For many applications it would seem that the related Simmons-Smith cyclopropanation7 using Diiodomethane and a Zinc/Copper Couple is more effective than the use of mercury-containing reagents. However, to date, stereoselective reactions have not been exploited with the mercury reagents.
I. Craig Baldwin & Jonathan M. J. Williams
Loughborough University of Technology, UK