Cadmium trifluoromethylation reagents

One of the best known reactions of dialkyl cadmium compounds is that with acyl halides which results in the formation of ketones. The reactions of (CF3)2Cd glyme with CH, COBr and C6HhCOC1 were the third class of reactions of the trifluoromethylating reagent examined. However, the mixed trifluoromethylalkyl or -aryl ketones were not observed. The organic products obtained were the acyl fluorides (95% yield) along with difluorocarbene. The latter could be stereospecifically trapped at temperatures as low as -78°C. The addition of either lithium or magnesium halides to the reaction mixture did not affect the outcome (106). 

Once Cd(CF3)2DME had been isolated, the first experiments with the reagent were devoted to comparing the reactivity of the new compound with that of Hg(CF3)2 in ligand substitution reactions of main group metalloids. The results shown in Eqs. (5)—(8) clearly indicated that at least with Ge and Sn halides, the cadmium-based trifluoromethylating reagent... 

Dialkylcadmium reagents are often useful alternatives to the more reactive Gngnard reagents in the preparation of ketones from acyl halides However, bis(trifluotomethyl)cadmium glyme is decomposed by acyl halides and does not give trifluoromethyl ketones [, 124] Nevertheless, this reaction can be used as a low-temperature source of difluorocarbene [S, 124] (equation 102)... 

Dihalodifluoromethanes react readily with acid-washed cadmium or zinc powder in DMF at room temperature to produce stable solutions of trifluoromethyl cadmium and zinc reagent, respectively in high yields [42], which undergo a metathesis reaction with copper (I) salts to give a trifluoromethylcopper solution [92] (Scheme 30). 

Bis(trifluoromethyl)cadmium reagent undergoes a related ligand-exchange process [5] (equation 6)... 

Although no trifluoromethyl derivatives of cadmium had ever been isolated, their attempted synthesis was particularly appealing from several viewpoints. First, alkyl cadmium reagents are typically more... 

Of the bis(trifluoromethyl)cadmium derivatives obtained to date, the glyme adduct, (CF3)2Cd glyme, is the best compromise between chemical reactivity and thermal stability this compound appears to be almost a classic example of a lightly stabilized species, one which is stabilized enough to be isolatable but which is still quite reactive (107). That this material is a much more effective reagent than, for example, (CF3)2Cd 2 pyridine, can be most easily seen from the results of the reactions of these compounds with a second class of substrates, the halides of germanium and tin. Within 15 min at ambient temperature... 

The reaction of Cd(CF3)2DME with Mn(CO)5Br is very slow at ambient temperature, but if AgN03 is added in a 1 1 mole ratio with the cadmium reagent, the silver-assisted reaction is complete within 15 min. The addition of CuBr to Cd(CF3)2DME trifluoromethylations of Mn(CO)jBr, however, is ineffective (36). The results of the AgN03 comparison are... 

The role of the silver salt is not known with certainty, but two possibilities are most plausible. One is that first the AgN03 is trifluoromethylated by the cadmium reagent and then the more reactive, but much less thermally stable, AgCF3 (see below) trifluoromethylates the organometallic halide. An alternative possibility is that the silver may remove halide from the substrate, leaving a more reactive, coordinatively unsaturated metal that is then trifluoromethylated by Cd(CF3)2DME. 

Very soon after the reactivity of Cd(CF3)2DME toward main group halides (31) and group 10 dihalides (32) had been established, we set out to determine if the cadmium reagent was effective in trifluoromethylating representative organometallic compounds. One of the first substrates chosen was CpCo(CO)I2, which was selected because the mono-trifluoro-methyl derivative was known, but the disubstituted compound was not. The results of this reaction are (60)... 

In the reactions of the organic substrates, the trifluoromethyl cadmium reagents are generally prepared in situ. See, for example, P. L. Heinze, and D. J. Burton, J. Fluorine Chem. 29, 359 (1985). 

Scheme 2.118 Mechanism of the synthesis of trifluoromethyl zinc and cadmium reagents (top X, Y = Br, Cl M = Zn, Cd), and the preparation of trifluoromethyl copper(l) by in situ transmetalation (bottom).

Burton DJ, Wiemers DM. A remarkahly simple preparation of (trifluoromethyl)cadmium and zinc reagents directly from difluorodihalomethanes. J. Am. Chem. Soc. 1985 107 5014-5015. 

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