Proton-Metal Exchange with Acidic Hydrocarbons

Organo-Zn, -Cd and -Hg derivatives can be prepared from other compounds by proton-metal exchange with acidic hydrocarbons by olefin insertion and by metal-metal exchange with other organometallics. In all cases alkyl groups are substituted by other... 

I. by Proton-Metal Exchange with Acidic Hydrocarbons. 

Strong acids are able to donate protons to a reactant and to take them back. Into this class fall the common acids, aluminum hahdes, and boron trifluoride. Also acid in nature are silica, alumina, alumi-nosihcates, metal sulfates and phosphates, and sulfonated ion exchange resins. They can transfer protons to hydrocarbons acting as weak bases. Zeolites are dehydrated aluminosilicates with small pores of narrow size distribution, to which is due their highly selective action since only molecules small enough to enter the pores can reacl . 

Alkenes lacking hydrogen atoms at allylic positions are much less acidic than ordinary unsaturated hydrocarbons. Superbases regioselectively exchange allylic protons in alkenes whenever there is a choice. However, a few examples of metallation of alkenic C-H bonds with superbases are known and a compilation of them is reported in Table 1. Ethylene itself has been deprotonated by the superbasic mixture constituted by butyllithium, potassium tert-butoxide, and TMEDA.41... 

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