Heterocyclic compounds cyclopalladation

These alkenylation products are easily utilized for the formation of heterocyclic compounds by cyclization reactions. For example, ethyl A -methyl-lV-(3,4-methylenedioxy)benzylglycinate is cyclopalladated regiospecifically at C(6) when treated with Li2PdCl4. The product, the di-p-chloro-bis(AOV-dialkylbenzylamine-6-C,iV)-dipalladium(II) complex 7.19, undergoes a substitution reaction via the insertion of methyl vinyl ketone between the palladium metal and the phenyl carbon atom. The resultant p-aryl-a,p-unsaturated ketone 7.20 is cyclized using anhydrous potassium carbonate in ethanol to the corresponding ethyl iV-methyl-1,2,3,4-tetrahydroisoquinolinium-3-carboxylate 7.21, as shown in Eq. (7.19) [76, 77]. 

Cyclopalladated t-butylimine compounds are reacted with styrene and then treated with trifluoroacetic acid to give o-formylstilbenes in high yields. Their A -methylimine derivatives are converted to another heterocyclic compound, 3-aryl-N-methyl-isoquinolones 7.22, by oxidation with mercuric acetate, as shown in Eq. (7.20) [78]. 

Cyclopalladation products react with alkynes to be inserted into the palladium-carbon bond, followed by reductive elimination of Pd(0), which leads to the formation of a C-N bond. Then the heterocyclic compound is yielded as shown in eq. (20.62) [195,213]. 

The insertion of allenes in the Pd-C bond of cyclopalladated 3-arylisoquinoline derivatives 327 afforded compounds 328, derived from the berberinium cation (Scheme 71). This reaction takes place via the formation of an intermediate (r/ -allyOpalladium complex <2003JOM313>. This chemistry has been extended to the preparation of other cationic N-heterocycles, including naphtho[r+/ ]( uinolizinium derivatives <2004EJ01724>. 

In 1999, cyclopalladated tolylphosphine compounds were reported to show highly catalytic activities, not only for Heck reactions of which the turnover numbers (TONs) are up to 1,000,000, for example, but also for all other metal-catalyzed cross-coupling reactions [115, 116]. In 2005, Heck reactions with naphthyl phosphines [117] or 77-heterocyclic carbene phosphapalladacycles [118] were also reported to show highly catalytic activities, as evidenced by their TONs of up to 300,000 and 10,800, respectively. 

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