Plumbylation

Reaction, of aromatics with lead tetracarboxylates leads to substitution of a tri(acyloxy)lead group into the aromatic. This reaction takes place more readily if the acyl group contains electron-withdrawing groups such as halogen. The reaction with thiophene has been examined [58DOK (123)295 74TL853]. 

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Germyl, Stannyl, and Plumbyl Anions The preparative methods for the synthesis of the germyl, stannyl, and plumbyl anions are essentially the same as those mentioned above for the silyl anions. The most widely used methods are (1) reduction of halides R3EX (R = alkyl, aryl E = Ge, Sn, Pb X = Cl, Br) with alkali metals and (2) reductive cleavage of the E-E bond of R3E-ER3 (R = alkyl, aryl E = Ge, Sn, Pb) with alkali metals or organolithium reagents. Due to the favorable polarization of the (E = Ge, Sn, Pb) bond, the direct metalation... 

Bis(triorganostannyl)acetylenes are also isolated after heating either bis(triorganostannyl) acetylenedicarboxylates, R3Sn02CC=CC02SnR3 (R = Et, Pr, or Bu), at 160-180°C [Eq. (40)] (53) or O-triethylstannyl triethylstannylpropiolate above 100°C [Eq. (41)] (56). Bis(triphenyl-plumbyl) acetylenedicarboxylate was also reported to decarboxylate on... 

Plumbyl halides with a lead-manganese bond have been prepared by the following dephenylation or demethylation reactions191 ... 

FIGURE 56. Preparation of silyl plumbyl and silyl stannyl complexes of iron. Reproduced from Reference 168 by permission of Elsevier Sequoia S.A. 

Nucleophilic cleavage of hexaphenyldiplumbane by potassium tert-butoxide results in the formation of the plumbyl anion K+PbPh3, which is principally characterized by the high field shift of the C-l nuclei to 191.11 ppm in the NMR spectrum hexaphenyldigermane and hexaphenyldistanne behave analogously.30... 

Similarly, trityl cation in aromatic hydrocarbons initiates the fragmentation of simple tetraalkyl plumbanes and stannanes yielding the plumbyl or stannyl cationic species, e.g. 11, and alkenes. The reaction is thought to proceed via plumbyl-or stannyl-substituted carbocations 12, which in a second step eliminate the al-kene. This approach was used in the synthesis of norbornyl cations of the elements tin and lead, e.g. 13, (Scheme 5). ... 

The closely related lead analogue 81 shows essentially the same structural features, but the lead atom in 81 is less pyramidalized (355.7°, see Table XIV), which indicates weaker coordination. The NMR spectroscopic parameters for 81, such as " Pb NMR chemical shift and scalar Pb-C(sp) coupling constants (see Section III.B.3) clearly demonstrate however the side-on complexation of the plumbyl cation by the C = C triple bond in zwitterions 81. ... 

In recent years, there has been much interest in the use of aryllead compounds in organic synthesis, because of the ability of compounds of the type ArPb(02CR)3 to smoothly arylate a variety of nucleophiles [34—37]. Aryllead tricarboxylates can be prepared by direct plumbylation of aromatic compounds (Scheme 5), a reaction that has been known for a long time [38],... 

Lead(IV) acetate is most commonly employed to plumbylate arenes, often using chloroform as the solvent. Being a relatively weak electrophile, Pb(OAc)4 can react only with electron-rich aromatic compounds, for example anisole and polyalkoxy-benzenes. The electrophilicty of the lead reagent is, however, enhanced substan-... 

Scheme 5. Plumbylation of a substituted benzene with lead(IV) acetate.

Lead(IV) trifluoroacetate in TFAH is a very reactive electrophile that is capable of plumbylating less electron-rich arenes. Nonetheless, the use of trifluoroacetate anions in the plumbylation reactions should be avoided, because aryllead(IV) tri-fluoroacetates are unstable compounds that readily decompose to the corresponding aryl trifluoroacetates and biaryls [34—37, 40, 41]. It has been reported [41] that 4-FC6H4ArPb(TFA)3 is reasonably stable and can be isolated from the reaction of Pb(TFA)4/TFAH with fluorobenzene. A mechanistic study [41] indicated an electrophilic substitution path for the plumbylation reaction, which seemed to be substantially more para-selective than mercuration and thallation. For example, the plumbylation of toluene with Pb(OAc)4 in dichloroacetic acid has been reported [41] to occur with >90 % para-selectivity. 

The products of direct aromatic plumbylation, ArPb(OAc)3, are attractive reagents, because of their accessibility and ability to arylate various nucleophiles under mild conditions [36]. These arylation reactions might involve ligand exchange at the Pb(IV) center, followed by reductive elimination [35], Among nu-... 

In 1958, Panov and Kocheshkov1216 found another route to the formation of the C—Pb bond, namely the interaction of tetraacyloxyplumbanes with aromatic and heteroaromatic compounds (the plumbylation reaction). They showed that the reaction of thiophene with Pb(OCOPr-i)4 at room temperature during 10 days led to unstable RPb(OCOR/)3 (R = 2-thienyl R = i-Pr), which was disproportionated to R2Pb(OCOR/)2 and Pb(OCOR/)4-... 

While germyl cations are not known in aqueous solutions, reports on the formation of stannyl cations appeared as early as 192324. Since then, numerous investigations25-27 established that ions of the type R3M(OH2)n+ (M = Sn, Pb) can be prepared in aqueous solutions. The early literature on this subject was reviewed in 1966 by Tobias28. Among earlier studies one should mention a series of publications by the group of Rabenstein29,30 who studied plumbyl cation complexes in aqueous solutions by H NMR. 

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