Olefins aminopalladation

Scheme 1. Proposed Mechanistic Steps Leading to Olefin Aminopalladation...

Unlike the intermolecular reaction, the intramolecular aminopalladation proceeds more easily[13,14,166], Methylindole (164) is obtained by the intramolecular exo amination of 2-allylaniline (163). If there is another olefinic bond in the same molecule, the aminopalladation product 165 undergoes intramolecular alkene insertion to give the tricyclic compound 166[178]. 2,2-Dimethyl-l,2-dihydroquinoline (168) is obtained by endo cyclization of 2-(3,3-dimethyiallyl)aniline (167). The oxidative amination proceeds smoothly... 

The Hegedus indole synthesis involves one of the earlier (formal) examples of olefin hydroamination. An ortho-vinyl or ortho-nllyl aniline derivative 1 is treated with palladium(II) to deliver an intermediate resulting from alkene aminopalladation. Subsequent reduction and/or isomerization steps then provide the indoline or indole unit 2, respectively. 

Narasaka has utilized a Heck-type intramolecular aminopalladation of the olefinic moiety of r,5-unsaturated ketone 0-pentafluorobenzoyloximes 9 to afford 2,5-disubstituted pyrroles 10 (R = CH3, C02Et) <99CL45>. The use of the O-pentafluorobenzoyl group was found necessary to preclude competing Beckmann rearrangements. 

Hegedus proposed that the mechanism of this transformation proceeds through a Wacker-type reaction mechanism that is promoted by Pd(II). As shown below, coordination of the olefin to Pd(II) results in precipitate 121, which upon treatment with Et,N undergoes intramolecular trany-aminopalladation to afford intermediate 122. As expected, the nitrogen atom attack occurs in a 5-exo-trig fashion to afford 123. [l-Hydride elimination of 123 gives rise to exocyclic olefin 124, which rearranges to indole 120. The final step of this mechanism leads to the formation of catalytically inactive Pd(0). However, addition of oxidants such as benzoquinone allows for catalytic turnover. 

When an aliphatic amine is used as nucleophile, aminopalladation of olefins takes place (eq (96)) [129], The resulting (i-aminoalkylpalladium complex undergoes CO insertion into the Pd—C bond to gi e a P-aminoacyl complex, which has been isolated as a stable solid. 

The oxidative addition reactions to alkenes promoted or catalyzed by PdCl2(CH3CN)2 have been classified based on the nature of the attacking species. Oxygen nucleophiles such as water, alcohols and carboxylic acids undergo oxypalladation, while ammonia, amines and their derivatives are typical nucleophiles for aminopalladation. Carbopalladation with active methylene compounds is also discussed The palladium-catalyzed intramolecular hetero- and carbopalladation of olefins is extensively used as the ring-forming step in the synthesis of a variety of heterocyclic and carbocyclic systems, and representative examples are provided. 

A large number of nitrogen heterocycles can be synthesized by Pd -catalyzed cyclization of aminoalkenes. The first reported intramolecular aminopalladation of olefins mediated by Pd consisted of the cyclization of o-allylic anilines to 2-methylindoles. Both catalytic and stoichiometric procedures were developed using PdCl2(CH3CN)2 as catalyst (eq 14). This reaction features high yields, and tolerance toward functional groups and substitution on the allyl side chain. When the aniline substrate bears a methallyl side chain, palladium 8-hydride elimination is prevented and the cyclized alkylpalladium intermediate can be trapped by olefins in a Heck-type process (eq 15). Similar conditions have been used to convert o-amino styrenes into indoles. ... 

Olefins can be transformed stereospecifically into vicinal diamines (23) by an aminopalladation-oxidation sequence (Scheme 16) using oxidants such as bromine, m-chloroperbenzoic acid, and iV-bromosuccinimide. Although this diamination procedure has limitations, the results are nevertheless useful since vicinal diamines are not easily prepared from olefins by other methods. 

Carboamination reactions have been classified into two types depending on the mode of reaction in the first case, the reaction proceeds via the formation of 1) an outersphere aminopalladation complex like the PdX2-aIkene complex via oxidative addition, which in the next step reacts with the amine and an external electrophile to afford the cyclized product. In a type II mechanism, the reaction takes place through the formation of 2) an innersphere complex where the Pd(0) catalyst forms the complex with an electrophile, e.g., aryl haUde via oxidative addition. Then, it reacts with the amine to form the Pd(Ar) amide complex, which eventually gets converted into the product via the alkene insertion and C—C bond-forming reductive elimination. Here, it is noteworthy that, in a type I reaction, the addition across the olefinic bond took place in rarri-fashion, whereas the 5y -addition mode was observed in case of the type It mechanism (Scheme 40.1). ... 

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