PaUadium complexes

The next step involves the generation of the new aUcene by P-hydride elimination, throngh an agostic interaction, and evolution to a hydride-paUadium complex. The calculated potential surfaces for the overall insertion-elimination process are quite flat and globally exothermic [11,15], Finally, the reductive elimination of the hydride-Pd(ll) complex, which is favoured by steric factors related to the buUdness of the iV-substituents on the carbene [13], provides the active species that can enter into a new catalytic cycle. 

Coupling of n- allylic)paUadium complexes with an alkenylzirconium(IV) complex l,4-< enes.2 A recent stereospecific synthesis of natural (20R)-cholestanone-3 (4) involves coupling of the alkenylzirconium complex 2 with the jr-(allylic)palladium chloride dimer (1) of a (Z)-17(2U)-pregnene. The major product is a 1,4-diene (3), formed by regioselective attack of 2 at C20, the less hindered terminus of the allylic unit, and with inversion at C20. Coupling of 2 with the 7i-(allylic)palladium chloride dimer of the (E)-isomer of 1 results in a 1,4-diene epimeric at C20 with 3. Hydrogenation of the diene completes the synthesis of the desired natural cholestanone-3 (4). 

In acyclic systems the 1,4-relative stereoselectivity was controlled by the stereochemistry of the diene. Thus, oxidation of (E,E)- and ( ,Z)-2,4-hexadienes to their corresponding diacetates affords dl (>88% dl) and meso (>95% meso) 2,5-diacetoxy-3-hexene, respectively. A mechanism involving a trani-acetoxypalladation of the conjugated diene to give an intermediate ( 7r-allyl)paUadium complex, followed by either a cis or tram attack by acetate on the allyl group, has been suggested. The cis attack is explained by a cis migration from a (cr-allyl)palladium intermediate. The diacetoxylation reaction was applied to the preparation of a key intermediate for the synthesis of d/-shikimic acid, 3,... 

Polystyrene is the most frequently used polymeric support for the immobiUzation of paUadium complexes [15]. The PS resins are weU developed in the field of soUd-phase peptide synthesis. Therefore resins bearing various chemical functional groups, diameter size, loading value, degree of cross-linking, for example, are now commerciaUy readily avaUable to permit investigation of these variables on the catalytic property. The most widely used polystyrene in... 

Hydridostannyl-palladium complexes have been prepared by an addition of RjSnH to palladium-ethylene complex. The use of distannane in place of hydrostannane affords distannyl-paUadium complex by oxidative addition of distannane onto Pd(0) species. 

Battace, A., Feuerstein, M., Lemhadri, M. et al. (2007) Heck reactions of a- or j8-substituted enol ethers with aryl bromides catalysed by a tetraphosphane/paUadium complex - direct access to acetophenone or 1-arylpropanone derivatives. Eur. J. Org. Chem., 3122-32. 

Berthiol, F., Doucet, H. and Santelli, M. (2004) Heck reactions of aryl bromides with alk-1-en-3-ol derivatives catalysed by a tetraphosphine/paUadium complex. Tetrahedron Lett., 45, 5633-6. 

Uozumi, Y. and Watanabe, T. (1999) Green catalysis Hydroxycarbonylation of aryl halides in water catalyzed by an amphiphilic resin-supported phosphine-paUadium complex. J. Org. Chem., 64, 6921-3. 

The antifungal, antiviral agent (+)-brefeldin A, whose recent synthesis using rr-allyl-paUadium complexes has previously been described (Scheme 24), was prepared by Trost et using the [3 + 2] cycloaddition to form the cyclopentyl ring (Scheme 41). Chiral acrylate 190, prepared from mannitol, was subjected to cyclizalion conditions to give cycloadduct 191 in 87% yield and an excellent selectivity. Diastereoselectivity in the cycloaddition extended the stereochemistry of 190 into the absolute stereochemistry of 191, generating three of the live stereocenters of brefeldin A. After ozonolysis, reduction, and protection, the lower side chain was attached to prepare the sulfone 192. The upper side chain was then introduced, and further functionalization led to the key intermediate 194, which had already been transformed to (+)-brefeldin A. 

Cyclooctadiene was reported to undergo oxidative cyclocarbonylation in the presence of PdCL (0.018 equiv), AcONa (0.84 equiv), and CuCL (0.84 equiv) in CH2Cl2/MeOH (17 1, v/v) at 100 °C and 4.17 atm of CO for 24 h to afford 2-(methoxycarbonyl)bicy-clo[3.3.1]non-6-en-9-one as the major product in moderate yield (30%, Scheme Even though the authors invoke an H— Pd—Cl species as the key intermediate for this process, the possibility that the reaction is initiated by insertion of a (methoxy-carbonyl)paUadium complex on the double bond followed by CO insertion, ring closure, and j8-hydrogen elimination cannot be ruled out. 

Efforts to increase the catalyst turnover number are indeed another major area where further improvements can be expected. Snch improvanents have recently been achieved using preformed palladacycles as catalysts or by using a macrocyclic tetraphole as ligand. Dendritic diphosphine-paUadium complexes as catalysts for Heck reactions have also been reported to possess superior stability compared to the monomeric parent compounds.f In addition, the same research group could considerably activate the rate of the Heck reaction of chlorobenzene and styrene by addition of tetraphenylphosphonium salts and A,A-dimethylglycine.f Transferring such innovations to the AHR ranains an important goal. 

Fig. 6.7 Gibbs energy profile in toluene (AGsoiv, kcal mol ) at 298 K for the two possible oxidative addition reactions of 2 to the paUadium complex 1. Approach A is shown in blMe, whereas approach B is shown in red...

Scheme 4.2 Rotation of the 2 1 rra/w-paUadium complex 16 about the palladium center...

N. Batail, A. Bendjeriou, L. Djakovitch, V. EHtfaud, Larock indole synthesis using paUadium complexes immobilized onto mesoporous siUca, Appl. Catal. A Gen. 388 (2010) 179-187. 

Guo T, Dechert S, Meyer F. Dinuclear paUadium complexes of pyrazole-bridged bis(NHC) Hgands a delicate balance between normal and abnormal carbene coordination. Organometallics. 2014 33 5145-5155. 

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