Elimination haloform

The most familiar, and most studied, example of 1,1-elimination occurs where no / -H atoms are available—the operation of factor (c) above—in the hydrolysis of haloforms, e.g. CHC13 (77), with strong bases. This involves an initial 1,1-elimination, probably via a two-step, i.e. 1,1-ElcB, pathway, to yield a dichlorocarbene intermediate (78) ... 

A new and convenient method of preparation of trichloro- and trifluoro-methyl sulfones has found application in /3-elimination of haloform via an unusually facile Ramberg-Backlund rearrangement under extremely mild and non-aqueous conditions. Thus, 9-fluorenyl trichloromethyl sulfone in CHCI3 affords... 

A second type of elimination reaction for some halocarbons is the 1,1 elimination. For example, hydrolysis of the halogenated methanes or haloforms is thought to occur by proton abstraction and subsequent formation of a carbene that reacts with water or hydroxide to form carbon monoxide and water (Equations (9)-(ll)). 

A reaction of haloforms with a base, which generates dihalocarbenes (a-elimination) and their addition to alkenes is an efficient method for the preparation of 1,1-dihalocyclopropanes, with the exception of 1,1-difluoro derivatives (Houben-Weyl, Vol.E19b, pp 1464-1466). When chlorodifluoromethane and an alkene are treated with methyllithium, potassium tcrt-butox-ide, powdered sodium hydroxide in tetraglyme or a concentrated aqueous solution of alkali metal hydroxide and a phase-transfer catalyst, the expected 1,1-difluorocyclopropanes are formed in low yields. Comparable low yields of these products result, if dichlorodi-fluoromethane and an alkene are treated with methyllithium. " The main products formed are those that result from reaction of difluorocarbene (carbenoid), and its precursor, with the base or the solvent present in the system (for examples, see refs 10-12). Therefore, the reaction of chlorodifluoromethane with base and an alkene lacks preparative value. The difficulties mentioned above are circumvented in the method using chlorodifluoromethane, oxirane (or chloromethyloxirane), with tetraalkylammonium halide as a catalyst and an alkene (Houben-eyl, Vol. 4/3, p 380 and Vol. E19b, pp 1468-1469). 

Addition of bromochlorocarbene (carbenoid) to an alkene results in the formation of 1-bromo-1-chlorocyclopropanes. The preparative value of this process depends on the structure of the carbene source, the type of base used for a-elimination of hydrogen bromide from the haloform, reaction conditions etc. 

Oxidation of conanine (25) to the lactam (28) required four moles of bromine. In this case, by analogy with the reactions of a methyl ketone, it may be assumed that the iminium salt (26), formed in the first step, was perbrominated at position 21 to give (27), and that tribromomethane was eliminated under the action of alkali (haloform reaction). 

The basic hydrolysis of haloforms is first-order with respect to both haloform and hydroxide ion . Several arguments eliminate nucleophilic attack of hydroxide ion on haloform, Sn2 displacement (22), as a possible mechanism. 

It should be stressed that this methodology is of general use for generation of other dihalocarbenes via oc-elimination of hydrogen halide from haloforms, and many other carbenes which can be generated via base-induced a-eliminations ... 

Elimination Reactions. t-BuOK is a widely used base for both a- and /3-elimination reactions. It is the most effective base in the conventional alkoxide-haloform reaction for the generation of dihalocarbenes. This procedure still finds general use (eq 19), but since it requires anhydrous conditions, it has been replaced to a degree by use of phase-transfer catalysts. Vinylidene carbenes have also been produced from the reaction of a-halo allenes with f-BuOK. ... 

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