Abstract
The mechanisms of many of the typical reactions of organometallic compounds have been reviewed, and an attempt has been made to deduce the mechanisms of reaction of some common, but hitherto little investigated, reactions of organometallic compounds. The organometallic compounds chosen for study were the dialkyl zincs, as they are readily obtained and purified, they are known to be stable, monomeric liquids unassociated in organic solvents, and they undergo the typical reactions of organometallic compounds. In the present work, a study has been made of the reactions of di-n-alkyl zincs from dimethyl- to di-n-butyl-zinc with oxygen,nitric oxide, iodine and p-toluidine (which functions as a monobasic acid). The relative rates of cleavage of the different alkyl groups by the various reagents, in solution, were deduced by means of competition experiments in which (a) mixtures of two symmetrical dialkyls R[2]Zn and R[2]'Zn (b) an unsymmetrical dialkyl RZnR' were allowed to react with a deficiency of the reagent. Studies on the autoxidation of dialkyl zincs in diethyl ether at room temperature showed that the formation of peroxide accompanied the uptake of oxygen very closely. Competition studies were unsuccessful. The dialkyl zincs were shown to react with two moles of nitric oxide in solvents such as benzene, cyclohexane, and diethyl ether, at room temperature, to give a derivative of an N-alkyl-N-nitrosohydroxylamine: R[2]Zn + 2NO ? R-Zn-O-N-R This derivative evolved one mole of alkane on aqueous hydrolysis, and a salt Zn(O[2]N[2]R)[2] was isolated in quantitative yield from the hydrolysis product. The order of cleavage from unsymmetrical dialkyls in both cyclohexane and diethyl ether was Et > Pr[n] < Bu[n], and a mechanism involving initial coordination of nitric oxide to the zinc has been proposed. The rate of acidolysis by p-toluidine in refluxing di-iso-propyl ether or diethyl ether was in the order : Me[2]Zn Pr[2n]Zn. For the unsymmetrical dialkyls, the alkyl groups were cleaved in the order Et > Pr[n] > Bu[n]. The mechanism of electrophilic substitution at saturated carbon (S[E]2 or S[E]i) has been proposed to account for our observations. The results of the competition studies with iodine were indefinite, but were not incompatible with a free radical mechanism, accompanied, in part, perhaps by a coordination mechanism.