Organolead compound

Going down the carbon group the C–X (X = C, Si. Ge, Sn, Pb) bond becomes weaker and the bond length larger. The C–Pb bond in tetramethyllead is 222 pm long with a dissociation energy of 204 kcal/mol (854 kJ/mol). For comparison the C–Sn bond in tetramethyltin is 214 pm long with dissociation energy 297 kcal/mol (1.24 MJ/mol).

By far the most important organolead compound is tetraethyl lead used as an anti-knocking agent. The most important lead reagents for introducing lead are lead tetraacetate and lead chloride.

The use of organoleads is limited partly due to their toxicity although it is remarked^[1] that the toxicity is only 10% of that of palladium compounds.

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Synthesis

Organolead compounds can be derived from Grignard reagents and lead chloride. For example methylmagnesium chloride reacts with lead chloride to tetramethyllead, a water-clear liquid with boiling point 110 °C and density 1.995 g/cm³. Reaction of lead chloride with the lithium salt of pentamethylcyclopentadiene gives the lead metallocene.

Certain arene compounds react directly with lead tetraacetate to aryl lead compounds in an electrophilic aromatic substitution. For instance anisole with lead tetraacetate forms p-methoxyphenyllead triacetate in chloroform and dichloroacetic acid:^[2]

Reactions

The C–Pb bond is weak and for this reason homolytic cleavage of organolead compounds to free radicals is easy. In its anti-knocking capacity, its purpose is that of a radical initiator. General reaction types of aryl and vinyl organoleads are transmetalation for instance with boronic acids and acid-catalyzed heterocyclic cleavage. Organoleads find use in coupling reactions between arene compounds. They are more reactive than the likewise organotins and can therefore be used to synthesise sterically crowded biaryls.

Aryllead triacetates

The lead substituent in p-methoxyphenyllead triacetate is displaced by carbon nucleophiles such as the phenol 2,4,6-trimethylphenol (mesitol) exclusively at the aromatic ortho position:^[3]

The reaction requires the presence of a large excess of a coordinating amine such as pyridine which presumably binds to lead in the course of the reaction. The reaction is insensitive to radical scavengers and therefore a free radical mechanism can be ruled out. The reaction mechanism is likely to involve nucleophilic displacement of an acetate group by the phenolic group to a diorganolead intermediate which in some related reactions can be isolated. The second step is then akin to a Claisen rearrangement except that the reaction depends on the electrophilicity (hence the ortho preference) of the phenol.

The nucleophile can also be the carbanion of a β-dicarbonyl compound (please fix my picture):^[2]

The carbanion forms by proton abstraction of the acidic α-proton by pyridine (now serving a double role) akin to the Knoevenagel condensation. This intermediate displaces an acetate ligand to a diorganolead compound and again these intermediates can be isolated with suitable reactants as unstable intermediates. The second step is reductive elimination with formation of a new C–C bond and lead(II) acetate.

See also

References

1. ^ ^{a b} Main Group Metals in Organic Synthesis Yamamoto, Hisashi / Oshima, Koichiro (eds.) 2004 ISBN 3-527-30508-4
2. ^ ^{a b} Organic Syntheses, Coll. Vol. 7, p.229 (1990); Vol. 62, p.24 (1984). Article
3. ^ Organolead(iv) triacetates in organic synthesis John T. Pinhey Pure & Appl. Chem., Vol. 68, No. 4, pp. 819-824, 1996. Abstract