Thallation-carbonylation

Thallation-carbonylation.2 Catalytic amounts of PdCl2 permit carbonylation of arylthallium ditrifluoroacetates (3, 287) at room temperature and atmospheric pressure. This sequence permits synthesis of esters, lactones, anhydrides, and various heterocycles. 

Thallation-carbonylation of aromatics.1 ortho-Thallation of benzoic acids, ben-zylic and (3-phenethyl alcohols, and benzamides followed by Pd(II)-catalyzed carbonyl-ation provides a route to aromatic carbonyl compounds such as phthalides, anhydrides, and imides. 

Larock and Fellows reported on the thallation-carbonylation of benzyl alcohols in 1982 [241]. Thallium (III) trifluoroacetate was used for the ortho-thallation of... 

Synthesis of Phthalides.—Substituted phthalides are produced by the thallation-carbonylation sequence (194)- (195) (Scheme 106) and phthalide itself (197)... 

Compared with the synthesis of five-membered rings relatively little has been done on the synthesis of 8-lactones. Homoallylic alcohols can be converted into 8-lactones by rhodium-catalyzed hydroformyla-tion followed by oxidation (equation 48). The thallation and subsequent palladium-catalyzed carbonylation described earlier can also be used for the synthesis of six-membered rings (equation 49). ... 

The major advantage of thallation is orientational specificity. While thallation is essentially all para to alkyl groups, halides and alkoxy substituents, orientation is almost all ortho to a carboxyl or an ester substituent. We see that such orientation to ester and carboxyl substituents is unusual in that these groups are typically meta-directing in electrophilic aromatic substitution. This behavior can be attributed to an intramolecular interaction involving the electrophilic thallium and the carbonyl group which allows for facile ortho substitution. 

Phthalides.—Substituted bromobenzenes (96 R = Br) can be carbonylated in the presence of catalytic amounts of Pd(OAc)2 to give phthalides (97 n = 1) and other lactones (97 n = 2, 3) in yields ranging from 42— 70%. (For much the same reaction, see ref. 94). Alternatively, a sequence of thallation [T1(02CCF3)3] and Pd"-catalysed carbonylation can be used to convert benzyl alcohols (96 R = H) into lactones (97). ° The latter method can also be used to prepare phthalic anhydrides from benzoic acids. Presumably both methods will be largely restricted to symmetrical substrates. The dianions (98) can be obtained from the parent benzyl alcohols using Bu"Li in hexane at 0 C they react with CO2 to give good yields of 7-methoxyphthalides. ... 

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