Tribromomethyl anion

Tribromomethylation ofallylic bromides. Exposure of primary allylic bromides to bromoform-aqueous NaOH in the presence of TEBA results in substitution of bromide by tribromomethyl anion. 

Tribromomethyl anion is not involved in the thermal decomposition of tribromo-methyl(phenyl)mercury, therefore this method allows the preparation of 1,1-dibromocyclo-propanes even from electrophilic alkenes. As a rule, this method is very efficient, yet the high cost and toxicity of dibromocarbene precursor, restrict its wide (particularly large scale) application. For the preparation of tribromomethyl(phenyl)mercury, see ref 5 and Houben-Weyl, Vol. E19b, p 1532). 

The reaction of dibromocarbene with haloalkenes (vinyl halides, alkenes with a halogen more distant from the double bond) leads to the formation of 1,1-dibromocyclopropanes in poor to good yields. However, reactions with allyl halides, particularly substituted allyl bromides, require comment. These alkenes furnish, apart from 1,1-dibromocyclopropanes 1, the alkylation products of tribromomethyl anion 2 and, occasionally, the products of their further transformations (dibromocarbene adducts 3, products of elimination of hydrogen bromide 4 etc.) if they react with bromoform under phase-transfer catalysis conditions (Houben-Weyl, Vol.E19b, pi620). 

Electrophilic alkenes react with bromoform using base/phase-transfer catalyst to give either Michael adducts of the tribromomethyl anion or adducts of dibromocarbene. The 1,1-di-bromocyclopropanes result via direct addition of carbene to the alkene or via the cyclization of the anion of the Michael adduct. Such discrimination between mechanisms is not always made in the literature, therefore all syntheses of 1,1-dibromocyclopropanes, irrespective of the way in which they are formed, are described below (see Houben-Weyl, Vol. El 9b, pp 1618-1620). 

Acrylonitriles and acrylates substituted in the a-position by an alkyl, aryl or alkoxycarbonyl group, or by bromine, reacted with bromoform/base/phase-transfer catalyst to give 1,1-dibro-mocyclopropanes 3 via addition of dibromocarbene. More detailed investigations showed that at least some of these cyclopropanes are formed by the addition of the tribromomethyl anion and the cyclization of the anion thus formed " (see Houben-Weyl, Vol. E19b, p 1619). 

An alternate interpretation of the effect of alcohol is as follows like chloroform, bromoform is deprotonated at the interface and, because the tribromomethyl anion... 

Similarly, the ratio of dibromocarbene addition to CBr. substitution in the reaction of propyl bromide with tribromomethyl can be varied between 92 1 and 1 91, depending on whether a small and accessible quat or a bulky anion-activating quat is used (Dehmlow and... 

Dibromocarbene undergoes addition to alkenes in a stereospecific manner. The sole case of nonstereospecific dibromocyclopropanation using bromoform/base/phase-transfer catalyst concerns ( )-cyclooctene, and is explained by isomerization of this cycloalkene caused by reversible addition of tribromomethyl or ethoxide anion the latter is formed from the ethanol present in bromoform (see also ref 2 and Houben-Weyl, Vol. El 9b, p 1617 for stereomutation in the reactions of dibromocarbene, generated from organomercury reagents, with low-active alkenes, see Section 1.2.1.4.3.1.5.1. and Vol. E19b, pp 1615 1616). 

Qiloroform yields both the trichloromethyl anion and dichlorocarbene as reactive intermediates under basic phase transfer conditions. The trichloromethyl anion reacts with phenylmercuric chloride under these conditions to yield phenyl(trichloromethyl)-mercury (72%). The product is unstable, however, to the 50% aqueous sodium hydroxide solution usually used in phase transfer catalysis. When 10—15% aqueous sodium hydroxide solution was used, while maintaining the ionic strength by addition of potassium fluoride, the product survived. Reasonable yields of the mercury compound were thus obtained and the reaction was successfully extended to bromodichloromethane [yielding 64% of phenyl(bromodichloromethyl)mercury] and bromoform [yielding phenyl(tribromomethyl)mercury, 54%]. The transformation is illustrated in equation 3.18 [26]. 

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