Methylene groups, activated iodination

In a rather remarkable reaction, methylene groups activated by two electron-withdrawing substituents react with non-activated alkenes under soliddiquid phase-transfer conditions in the presence of a molar equivalent of iodine to yield cyclopropane derivatives (Scheme 6.29) [62, 63], The reaction fails, when the catalyst is omitted or if iodine is replaced by bromine or chlorine. The intermediate iodomethylene systems are unstable in the absence of the reactive alkene and dimerize to produce, for example, ethane-1,1,2,2-tetracarboxylie esters and ethene-1,1,2,2-tetracarboxy lie esters. 

Oxidation. DMSO in combination with catalytic amounts of iodine and a strong acid oxidizes methylene groups activated by an adjacent benzoyl group in good yield. The method is only suitable for preparation of diaryl di- and triketones. Examples ... 

The phase-transfer catalysed oxidative iodination of a,p-unsaturated carbonyl compounds (Table 10.23) using iodine/persulphate to produce the a-iodo derivatives has been described [16]. Activated methylene groups are also iodinated under similar conditions. 

Selected examples of a-iodination of a.ji-unsaturated carbonyl compounds and activated methylene groups... 

The standard method for the preparation of many phenyliodonium ylides is reaction of compounds having an active methylene group with an iodine(III) species, usually in aqueous alkali, to give iodonium ylides (Scheme 50) [151]. 

The synthetic utility of this base (1) was demonstrated in the preparation of vinyl iodides in high yields from simple ketohydrazones and iodine (Table), a process that normally gives mixtures of vinyl iodides and geminal diiodides if less hindered bases are employed.6 This base has also been used in the elimination of sulfonic acids from the corresponding sulfonates, the alkylation of compounds containing active methylene groups, the conversion of hydrazones to vinyl selenides, and the preparation of esters from sterically hindered acids.4 5... 

Bromination of 2,3-dibenzylquinoxaline with excess of N-bromosuccinimide in carbon tetrachloride yields a mixture of meso- and d/-2,3-bis(a-bromobenzyl)quinoxalines. When 2,3-dimethylquinoxaline is treated in pyridine solution with powdered iodine, the bispyridinium iodide 37 is formed. Reaction with p-nitroso-iV,N-dimethylaniline gives the bisnitrone 38 which on acid hydrolysis yields quinoxaline-2,3-dicarboxaldehyde (40). The activated methylene groups of compound 37 condense with diacetyl in the presence of piperidine to give the phenazine derivative 39. 

RfS02I was also able to iodinate spontaneously compounds containing an active methylene group as well as simple carbonyl compounds containing an a-hydrogen atom, such as acetone18,64. 

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