Metal—hydrogen exchange reactions

Under the action of strong organometallic bases, thiazole undergoes hydrogen-metal interconversion. Ethylmagnesium bromide reacts at 0°C with thiazole in ether to form an insoluble adduct that upon heating evolves ethane almost quantitatively and affords an etheral solution of thiazol-2-ylmagnesium bromide (155) (12). Proof of the structure of this 

Grignard reagent comes from the substitution products it gives with various reactive substrates. When the low-temperature adduct is heated in an autoclave at 90 to 170 C for 3 to 6 hr, it does not rearrange to 2-ethylthiazole (12) as is the case in the pyridine series (436). 

Similarly, thiazole reacts at —60°C with phenyllithium affording thiazol-2-yllithium (156) (13, 437). As in the case of the Grignard derivative, thiazolyllithium does not rearrange under heating as does the adduct of pyridine and butyllithium (438). 

The 2-metalated thiazoles react with a variety of electrophilic substrates in a standard way, leading to addition products with aldehydes, ketones, carbon dioxide, epoxides, nitriles, Schiff bases, and to substitution products with alkyl iodides (12, 13, 437, 440). 

From Table 1-58, the practical constancy of the sum of 157 having reacted and the lithio salt being decomposed can be seen. 

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Two types of hydrogen replacement are discussed here (1) the base-induced hydrogen-deuterium exchange reactions and (2) the hydrogen-metal exchange reactions. 

In conclusion, it appears that in neutral or weakly acidic conditions only the methyl in the 2-position shows pseudoacidic behavior. The same conclusion can be drawn from the base-induced hydrogen-metal exchange reactions discussed in Section III.5.B. 

The w-cyclopentadienyl rings of ferrocene are readily metalated for example by treatment with butyl-lithium. This metalation is similar to the hydrogen/metal exchange reactions typical of, for example, certain benzenoid hydrocarbons, (p 48). Mixtures of mono- and di-metalated products are frequently obtained. A suitable method of preparing 1,1 -dilithioferrocene is given on p 49. 

In discussing the mechanism of the para hydrogen conversion or the hydrogen deuterium exchange reaction, Eley (117) suggested that it could occur at a single metal atom. Two orbitals are needed to bind the activated complex and these can be provided easily by the d orbitals of... 

A number of reactivity studies have been performed on 6 and 8 and indicate a strongly polar (if not ionic) Mn—E bond Mn "—E,+ (E = In, Tl). Thus heterolytic bond dissociation occurs in polar ligating solvents such as MeCN or DMF, and halogens, hydrogen halides, and alkyl halides readily add across the metal-metal bond in a manner consistent with the polarity described above (13,13a,18). In the thallium example, however, the reactions are generally more complicated and result in T1(I) salts [e.g., Eq. (3)], and metal exchange reactions are also more facile, e.g., the synthesis of 6 from 8 and indium metal. In general, therefore, the chemistry of 6 and 8 is consistent with predominantly ionic behavior. 

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