Plumbylenes insertion reactions

Recently, however, Tokitoh and co-workers " °° reported a versatile reactivity of Tbt-substituted plumbylenes 182a-d, that is, insertion reactions with alkyl halides, diphenyl dichalcogenides, bromine abstraction from carbon tetrabro-mide, and sulfurization with elemental sulfur as shown in Scheme 14.83. 

There are examples of electron-deficient divalent metal derivatives called heavy analogs of carbene with the general formula R2M , including silylenes, "" germylenes, "" " stannylenes, "" "" and plumbylenes. When compared to carbenes, however, these compounds exhibit a significantly decreased reactivity in C-H insertion reactions. Theoretical studies have indicated that the heavier the metal, the larger the activation barrier and the less exothermic the reaction. Electropositive substituents, however, may increase reactivities. " ... 

Note also the reactivity of the heteroleptic plumbylene 174 toward carbon disulfide. Treatment of 174 with excess amount of carbon disulfide resulted in a formation of unexpected product, lead(II) bis(aryl trithiocarbonate) (192), as a yellow, air- and moisture-stable solid (Scheme 14.85). The reaction must involve not only the insertion of carbon disulfide into the Pb—S bond, but also the formal insertion of a sulfur atom into the Pb C(Tbt) bond and subsequent insertion of another... 

Quantum-chemical calculations have been used to probe all the characteristic chemical reactions of CAs (at least in the case of silylenes and germylenes). The theoretical studies cover intramolecular rearrangements, insertions into 0-bonds, additions to double and triple bonds and dimerizations. Note that experimental data on the mechanisms of these reactions are still scarce and the results of theoretical studies are needed to understand the main trends in the reactivity of germylenes, stannylenes and plumbylenes. 

In conclusion, for both the insertion and addition reactions of MH2, the reaction enthalpy and the reaction activation barrier correlate with the metallylene s AZ st> pointing to a decrease in the metallylene reactivity as M becomes heavier and when the metallylene is substituted by electronegative substituents which increase AZ st (Table 37). According to these trends, plumbylenes and especially plumbylenes substituted with electronegative... 

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