[22060-04-4]  · C9H8Br2HgO2  · [Dibromo(methoxycarbonyl)methyl]phenylmercury  · (MW 508.56)

(by thermolysis, transfers :CBrCO2Me to alkenes, giving cyclopropanes,2 and to silanes, inserting into Si-H bond2,3)

Physical Data: mp 154-156 °C (dec).

Solubility: sol THF, CHCl3, chlorobenzene, benzene.

Analysis of Reagent Purity: spectroscopic data: IR,3 1H NMR.3

Preparative Method: reaction of HCBr2CO2Me with Phenylmercury(II) Chloride and Potassium t-Butoxide in THF at -60 °C,2,3 according to the general method to prepare phenyl(trihalomethyl)mercurials.4 The yield, though typically in the range of 55%, can be as high as 68%.

Purification: recrystallization from chloroform/hexane (1:1).

Handling, Storage, and Precautions: flame-dried glassware and an atmosphere of dry nitrogen are used for the reactions. Organomercury compounds should be carefully handled, and skin contact and inhalation should be particularly avoided. All reactions must be conducted in a well-ventilated fume hood.

Cyclopropanation of Alkenes.

The observed high thermal stability of PhHgCBr2CO2Me (1) restricts its application in synthesis. An additional drawback is related to the long reaction times and high reaction temperatures required, which results in significant side reactions, as is clear from the disparity between the amounts of phenylmercury(II) bromide recovered and the cyclopropane formed. Nevertheless, the cyclopropanation of several alkenes has been reported.3 For instance, reaction of cyclooctene with (1) in chlorobenzene gives rise to the cyclopropane derivative (eq 1), with greater yields of cyclopropane ring formation as the reaction temperature is raised (50% at reflux, 19% at 130 °C).

Allyltrimethylsilane is also an efficient partner for (1) in the cyclopropanation process (eq 2).3 The cyclopropane product is formed as a mixture of stereoisomers in a 1.5:1 ratio. The major isomer has been tentatively assigned to that having a syn disposition for the Me3SiCH2 and Br groups on the basis of the larger steric bulk of the CO2Me group.

Trisubstituted alkenes gave very modest yields. Thus 2-methyl-2-butene affords 9% of cyclopropane formation, while phenylmercury(II) bromide is recovered in 92% after 24 h at 130 °C in chlorobenzene.3

Insertion into Si-H Bond.

This is another representative reaction for a-haloorganomercurials that is also observed for (1) (eq 3).3

In spite of the limitations found, PhHgCBr2CO2Me is an interesting reagent to transfer the functional divalent CBrCO2Me group to several alkenes.

1. (a) Seyferth, D. PAC 1970, 23, 391. (b) Seyferth, D. ACR 1972, 5, 65. (c) Zeller, K.-P.; Straub, H. MOC 1974, 13/2b, 351. (d) Larock, R. C. Organomercury Compounds in Organic Synthesis; Springer: Berlin, 1985; Chapter X, pp 327-413.
2. Seyferth, D.; Mueller, D. C.; Lambert, Jr., R. L. JACS 1969, 91, 1562.
3. Seyferth, D.; Woodruff, R. A.; Mueller, D. C.; Lambert, Jr., R. L. JOM 1972, 43, 55.
4. Seyferth, D.; Lambert, Jr., R. L. JOM 1969, 16, 21.

José Barluenga, Miguel Tomás & José M. González

Universidad de Oviedo, Spain

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