Palladium carbonylation intermediate

The arylation of electron-rich arenes, such as azulene (55)206 and heteroarenes, has been sporadically described. Under similar conditions phenols undergo arylation, which is preferably directed at the ort/zo-positions, probably due to the involvement of palladium phenolate intermediates.188,207 Polysubstitution occurs readily.208 The para-position can be attacked only with the sterically hindered 2,6-di-t-butylphenol.209 Similar ortho-diarylation of arenes bearing carbonyl groups (acetophenone, anthrone, benzanilide, etc.) shows that the or//zo-di reeling effect of the substituent is more important than its other electronic effects.189... 

The palladium-catalyzed carbonylation of 4-amino-2-alkynyl carbonates 40 or 5-hydroxy-2-alkynyl carbonates 41 afforded a-vinylidene-/i-lactams 42 [60] or a-vinyl-idene-y-lactones 43 [61] in good yields (Scheme 3.25). The initially formed (allenyl-carbonyl)palladium(II) intermediates were trapped by the intramolecular amino- or hydroxy-nucleophiles to give 42 or 43. 

Figure 15.8 a simple example is presented of a subsequent insertion of CO and methanolysis of the palladium acyl intermediate [14], This is not a very common reaction, because both the ligand requirements and the redox conditions for Wacker and carbonylation chemistry are not compatible. For insertion reactions one would use cis coordinating diphosphines or diimines, which makes the palladium centre more electron-rich and thus the nucleophilic attack in the Wacker part of the scheme will be slowed down. In addition, the oxidants present may lead to catalytic oxidation of carbon monoxide. 

Thus the iron atom in (Tr-cyclopentadienvl)iron dicarbonyl bromide forms additional bonds with the halide ligands on palladium. Carbonyl substitution on the generated intermediate (XV) then occurs. In this case the cyclopentadienyl group acts as a bridge. In general, such a mechanism corresponds to an A-type substitution, where the halogens in (7T-tetraphenyl-cyclobutadiene)palladium dichloride function as nucleophiles. 

An important development in Heck chemistry arose from the recognition that a-palladium(II) intermediates can be exploited for transformations other than y3-hydride elimination [50]. Examples of potential reaction pathways include insertion of a second olefin (path A), anion capture (path B), or carbonylation (path C) (Scheme 6-22). Due to the... 

Luo etal used both HP-IR and HP-NMR in their studies of CO-ethylene co-polymerization. Using a Pd(ll)/dppp/ CF3COOH catalyst system under GO pressure, they detected three palladium-carbonyl absorptions in the IR spectrum, and linked these to three intermediates in the proposed mechanism. In addition, H NMR revealed the presence of coordinated ethylene and a palladium-methylene species (Pd-CH2-). The co-polymerization of CO with propylene was investigated using the unsymmetrical phosphine-phosphite bidentate ligand, (RSl-BINAPHOS. Two major resting states of the catalyst were observed by HP-NMR, namely alkylpalladium and acylpalladium species which are believed to correspond to [Pd(COR)(BINAPHOS)(CO)]+[BF4]- and [Pd(CH2CHCH3(CO)R)(BINAPHOS)]+[BF4]U... 

Despite all the synthetic developments, relatively little detailed mechanistic work has been performed on Sonogashira carbonylations until the present. The generally accepted mechanism is shown in Scheme 5.25. The typical reaction begins with the oxidative addition of ArX to a palladium(0) complex to form an aryl palladium(II) intermediate. The subsequent insertion of CO leads to the respective palladium acyl complex. Transmetallation, and finally reductive elimination, releases the product and a new catalytic cycle can be started. Notably, all species passing through the cycle are believed to be in a reversible equilibrium. 

While this cyclization method does not allow the incorporation of substituents into the 3-position, as is available with internal alkynes, in analogous chemistry to that in Section 6.2.2, the palladium bound intermediate of cyclization can be intercepted with a range of external reagents to functionalize this site. These include aryl or vinyl halides, allylic substrates, carbonylation, and Heck coupling with alkenes [72]. 

Transmetallation is also a key step in the palladium-catalyzed Buchwald-Hartwig-Miura arylation of carbonyl compounds [171] with its manifold stereoselective variants [172]. In the general catalytic cycle, illustrated in Scheme 2.53, a ketone enolate 179 [173] reacts with the palladium(II) intermediate 178, resulting from oxidative addition, in a transmetallation step to give palladium enolates that are assumed to exist as an equilibrium between C-bond tautomer 180 and O-bound 181. Their reductive elimination under regeneration of the palladium(O) catalyst and concomitant release of the a-arylated ketone 182 closes the catalytic cycle. 

Dihydrouracils (27) can be obtained in one step by a palladium-catalyzed carbonylation of (28). The key intermediate is believed to be 2-trifluoro-methylpropenoyl palladium (82TL4099). 

Other examples that involve intermediate allyl cations are illustrated in Scheme 1.4. The cationic palladium(II) complex [Pd(dppp)(PhCN)2](BF4)2 coordinates the carbonyl oxygen of benzaldehyde and the activated carbonyl carbon attacks the isoprene, forming the allyl cation 10 which then cyclizes to give the 4-methyl-6-phenyl-5,6-dihydro-2H-pyran [22]. 2-Oxopropyl acrylate 11, in the presence of trimethylsilyltrifluoromethane sulfonate (TMSOTf) and methoxytrimethylsilane (MeOSMT), generates the cation 11a which is an efficient dienophile that reacts easily with the cyclohexadiene to give the Diels-Alder adduct in good yield [23]. 

Carbonylation reactions encompass a diverse set of transformations used to synthesise many important high-value fine chemicals, synthetic intermediates and materials such as polycarbonates [36]. Palladium catalysts modified with PRj ligands facilitate these reactions. However, carbonylation often requires harsh conditions, especially for less reactive C-X bonds, thereby promoting catalyst degradation via P-C bond cleavage. The strength of the NHC bond may demonstrate the utility of... 

The allylation of aldehydes can be carried out using stannous chloride and catalytic cupric chloride or copper in aqueous media." In-situ probing provides indirect (NMR, CV) and direct (MS) evidence for the copper(I)-catalyzed formation of an allyltrihalostannane intermediate in very high concentration in water (Scheme 8.6). Hydrophilic palladium complex also efficiently catalyzes the allylation of carbonyl compounds with allyl chlorides or allyl alcohols with SnCl2 under aqueous-organic... 

Palladium catalyzes allylation of carbonyl compounds with various ally lie compounds using In-InCl3 in aqueous media (Eq. 8.66).158 Various allylic compounds can be effectively applied via the formation of TT-allylpalladium(II) intermediates and their transmetalation with indium in the presence of indium trichloride in aqueous media. 

---