[3294-60-8] · C7H5Br3Hg · Phenyl(tribromomethyl)mercury · (MW 529.41)
(precursor for the generation of dibromocarbene under neutral conditions)
Physical Data: mp 119-120 °C (dec).
Form Supplied in: commercially available.
Preparative Method: readily prepared from Phenylmercury(II) Chloride and Bromoform using Potassium t-Butoxide in THF.2
Handling, Storage, and Precautions: highly toxic; should be handled in an efficient fume cupboard with caution. Best stored as a solid below 5 °C.
Phenyl(tribromomethyl)mercury is a source of dibromocarbene and as such has been employed in cyclopropanation and insertion reactions.1 The carbene is generated by gently heating phenyl(tribromomethyl)mercury in a hydrocarbon solvent (often benzene).3 The mild reaction conditions make this a good method for the generation of dibromocarbene under neutral conditions.
The most widespread application of phenyl(tribromomethyl)mercury has been in the dibromocyclopropanation of alkenes. The mechanism of this and related reactions has been studied in some detail and it has been proposed that they proceed via a free carbene intermediate.4 The reagent has proven particularly useful for the dibromocyclopropanation of base-sensitive systems and of relatively unreactive alkenes.1 For example, vinyl chlorides5 and monosubstituted alkenes6 can be successfully transformed to the corresponding cyclopropanes using this methodology (eqs 1 and 2). Cyclopropane (1) was produced in only 7% yield using a more conventional method (HCBr3/t-BuOK) to generate the dibromocarbene.
This method for preparing cyclopropanes is highly chemoselective and tolerates a wide range of functional groups. Notable exceptions are alcohols, amines, and carboxylic acids, which undergo competitive insertion reactions.1 The mildness of this reagent is well illustrated by the conversion of tricyclic ketone (2) into (3) in near quantitative yield (eq 3).7,8 This reaction is also stereospecific, with attack of the carbene occurring on the least hindered side of the double bond.
Systems containing several double bonds can be regioselectively cyclopropanated at the most nucleophilic position. For example, treatment of costunolide (4) with phenyl(tribromomethyl)mercury in refluxing benzene produces (5) as the sole carbene addition product (eq 4).9 It has been speculated that the Lewis acidity of the organomercury reagent is responsible for the competitive formation of the undesired cyclocostunolide (6).
Phenyl(bromodichloromethyl)mercury has been prepared in a fashion similar to phenyl(tribromomethyl)mercury.2 Thermal decomposition of this reagent occurs via loss of phenylmercury(II) bromide, producing dichlorocarbene as the only carbene intermediate.10 Phenyl(bromodichloromethyl)mercury has been utilized in the dichlorocyclopropanation of unreactive11 and base-sensitive alkenes (eqs 5 and 6).12 Dichloromethyl esters can readily be prepared from the corresponding carboxylic acids using this reagent by insertion of the carbene into the OH bond.13
Loughborough University of Technology, UK