Palladacycles Subject

In order to gain more insight into the intervention of the palladacycle, the following two substrates were subjected to the carbonylation with an icreased pressure of CO (15 atm). As a result, the starting materials were recovered intact without giving the cyclized products, which led to the definite conclusion that the 2-vinyl-2,3-dienyl ester is undoubtedly a precursor of the palladacycle (Scheme 16.37). 

The key step is the selective C—H bond activation of two methyl groups of an ortho-tert-hutyl in the Schiff base 434. Treatment of 434 with Pd(OAc)2 afforded the palladacycle 435 in 75 % yield by the help of rather strong coordination to N and O functions. The first functionalization was achieved by the reaction with the alkenylboronic acid to yield the alkylated product 436 in 86 % yield, which was converted to 437 by the Friedel-Crafts reaction. Then the second palladacycle formation from 437 provided two diastereomers 438, which were, without isolation, subjected to carbonylation (40 atm) at room temperature. Treatment of crude reaction mixture with silica gel cleaved the Schiff base and spontaneous lactonization occurred to give a mixture of the lactones 439 and 440 (6 1). The main product was N-alkylated to yield 441. Finally, the fourth ring was constructed by a Heck-type reaction on the aromatic ring to give the desired compound. 

A large number of palladacycles have been applied as (pre)catalysts in G-G coupling reactions such as Heck and Suzuki reactions.The subject has been recently reviewed." " ... 

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