Subject palladium-mediated

Palladium-mediated substitutions of silylated allylic compounds are not subject to steric hindrances. The silicon atom exerts control over the reaction site snch thaty-substitution results [64]. While the electrophiles are unavoidably disjoint with respect to the acceptor silicon, one of the two limiting forms, i.e., the a-silylcarbenium ion, is much more unfavorable (a-a arrangement) and therefore its population is expectedly low. 

The application of resin 13 to the sohd-phase synthesis of other useful target compounds was also explored and an example of this is the multistep synthesis of Meclizine (Fig. 10).26 The starting material, 3-methyl-4-hydro-xybenzaldehyde, is attached to the PFS hnker, and a polymer-bound amine intermediate is prepared by a reductive amination of resin 23 with amine 24. The resulting resin 25 is subjected to a palladium-mediated reductive cleavage to give Meclizine 26 in 80% yield, based on the original resin loading. 

The phenanthrene nucleus required for ( )-hasubanonine 79 (Scheme 17.16) was also assembled using RCM [35], in a similar way to that illustrated by luliano [26]. Initially, the biaryl compound 83 had to be assembled. This was done by synthesizing the aromatic iodide 84 and the aromatic pinacol ester 85. These compounds were then subjected to a palladium-mediated coupling, followed by conversion of the aldehydes to aUcenes, which proceeded smoothly to furnish 83. Compound 83 was then subjected to 2-Ru catalyst affording the required phenanthrene 86, which was then converted into ( )-hasubanonine 79 over a number of further steps. 

To investigate the feasibility of employing 3-oxidopyridinium betaines as stabilized 1,3-dipoles in an intramolecular dipolar cycloaddition to construct the hetisine alkaloid core (Scheme 1.8, 77 78), a series of model cycloaddition substrates were prepared. In the first (Scheme 1.9a), an ene-nitrile substrate (i.e., 83) was selected as an activated dipolarophile functionality. Nitrile 66 was subjected to reduction with DIBAL-H, affording aldehyde 79 in 79 % yield. This was followed by reductive amination of aldehyde x with furfurylamine (80) to afford the furan amine 81 in 80 % yield. The ene-nitrile was then readily accessed via palladium-catalyzed cyanation of the enol triflate with KCN, 18-crown-6, and Pd(PPh3)4 in refluxing benzene to provide ene-nitrile 82 in 75 % yield. Finally, bromine-mediated aza-Achmatowicz reaction [44] of 82 then delivered oxidopyridinium betaine 83 in 65 % yield. 

The stereogenic centers were then introduced by palladium-catalyzed dynamic kinetic asymmetric transformatitHi. ITierefore, 41 was coupled with lactone 42 in the presence of chiral ligand (RJt)-43 and gave 44 in 89% yield. The synthesis of 42 is shown below in Scheme 2.6. Compound 44 was subjected to an intramolecular Heck reaction followed by acidic cleavage of the ester function 45). The intramolecular Heck reaction only produced one diastereomer, because the c/s-annelated rings are favored. Scandium(lll)-mediated cyclizatitMi and reduction of the lactone with DIB ALII yielded (-)-aflatoxin Baa (46). It was acetoxylated and then pyiolyzed to give (-)-aflatoxin Bi (1) in 1.6% overall yield and nine linear steps from catechol (40). 

The authors further explored the chemistry on solid support by decorating the obtained resin-bound 2-aminobenzothiazoles using different palladium-catalyzed cross-coupling reactions (Scheme 8.24). Thus, the reaction product of 3-bromoaniline was subjected to bromine-mediated cyclization in AcOH at rt for 16 h, furnishing a 1 1 mixture of 5- and... 

The incorporation of metals in polymers will not be covered in detail because only a limited number of reports exist on this subject. The major interactions that have been used for the incorporation of metals in polymers are those of ftmctional groups that form polymer-metal oxide-based composites. The incorporation of metals leads to dramatic changes in the properties of polymers, such as an increase of the glass-transition temperature Polybutadiene or polyisoprene incorporating palladium salts can imdergo crosslinking, mediated by palladium, between polymer... 

Substituted alkynes play a pivotal role in modem organic synthesis. Although many methods are viable for their constmction, metal-catalyzed processes belong to the most efficient ones. The copper-mediated (Palladium free) alkynylation is known under the name Castro-Stephens reaction, j e palladium-catalyzed counterpart was discovered by Heck and Cassar (see Scheme 5-122), which is subject of the next paragraph. Nowadays the Sonogashira reaction, j ich uses copper co-catalysts, can be found as the method of choice. 

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