Palladium-catalyzed continued

In their experiments, an unsubstituted dppf-complex was compared with the analogous dendritic complex (Figure 4.23). After 35 exchanged reactor volumes the dendritic catalyst still showed a conversion of 77% (maximum 85% after 10 reactor volumes) while the unsubstituted catalyst deactivated from 70% to 15% at the end of the catalytic run. The drop in activity for both systems can be completely explained by their retention (97% and 99.8% for the unsubstituted and the dendritic complex, respectively). This means, no deactivation of the catalyst occurred during catalysis, which is often the case for palladium-catalyzed continuous catalysis. 

The late Professor J. K. Stille pioneered the development of a very effective and versatile palladium-mediated C-C bond forming method - the palladium-catalyzed cross-coupling of organic electrophiles with organostannanes.48 This process continues to enjoy much success in organic synthesis because it proceeds in high yields under mild reaction conditions and because it tolerates a... 

Catalysis and Synthesis in the Laboratory. Research on the practical applications of catalysis was not matched in the laboratory. We began a study of metal and non-metal catalyzed reactions early in our sonochemistry program. Our first project was to develop a convenient method of hydrogenating a wide range of olefins. We chose formic acid as our hydrogen source and found it to be effective. For example, with continuous irradiation, palladium catalyzed hydrogenations of olefins are complete in one hour(44). 

Palladium-catalyzed allylic substitution reactions are popular in the chemical community and the number of applications of the reaction, perhaps in particular for asymmetric procedures, continues to grow [53]. The efficiency of asymmetric chemistry is best described in terms of the enantiomeric excess (ee) of the reaction, and it... 

Another example is the palladium catalyzed allylic substitution of 3-phenyl-2-propenyl-carbonic acid methyl ester to yield iV-(3-phenyl-2-propenyl)morpho-line reported by Reetz, Kragl and co-workers. This reaction was performed in the presence of phosphino-terminated amine dendrimers [17, 18] loaded with Pd11 cations as shown in Scheme 10. For this particular dendrimer with a molecular weight of 10 212 g/mol, a retention of 0.999 per residence time [35] was estimated in a membrane reactor with a SELRO MPF-50 membrane. It must be noted that a very high retention is a prerequisite for a continuous operating system, since a small leaching of the dendrimer leads to an exponential decrease in the amount... 

Palladium-catalyzed transformations greatly enhance the scope of solid-phase synthetic chemistry. A number of fundamental pharmacophores are accessible through a variety of reliable manipulations that may be performed in high yield under mild conditions. This area continues to grow, as solution-phase chemistry is adapted to provide better methods for carbon-carbon bond formation in combinatorial chemistry. We view these advances as central to the field and look forward to future developments. 

Palladium-catalyzed cyclization of o-allylphenols to benzofurans has been extensively studied.11 As is usual, early systems were catalytically inefficient but continued studies led to substantial improvement. A wide range of catalyst systems work for this process. One of the most efficient, from a standpoint of catalyst turnover and chemical yield, was based on chiral TT-allylpalladium catalysts (equation 6), although the optical yields were low. y-Pyrones can also be efficiently synthesized by palladium(II)-cata-lyzed addition of phenolic OH groups to conjugated enones (equation 7).12... 

The field of homogeneous palladium catalysis traces its origin to the development of the Wacker process in the late 1950s (Eq. 7) [83]. Since this discovery, palladium-catalyzed reactions have evolved into some of the most versatile reactions for the synthesis of organic molecules [84,85]. Palladium-catalyzed Wacker-type oxidation of alkenes continues to be an active field of research [86-88], and several recent applications of NHC-coordinated Pd catalysts have been reported for such reactions. 

In general for carbonylations, palladium as catalyst metal is preferable to nickel with respect of catalyst efficiency. Thus, Okano, Kiji, and co-workers described some other efficient palladium-catalyzed carbonylations of allyl chloride and substituted allyl halides (Eqs. 5-10). In greater detail, the water-soluble palladium complex PdCl2[Ph2P(w-C6H4S03Na)]2 has been used in a two-phase system (e.g., aqueous NaOH/benzene medium) at atmospheric carbon monoxide pressure, giving 3-butenoic acids [20], In the carbonylation of allyl chloride a mixture of 2-bute-noic acid, which was formed by base-catalyzed isomerization, and 3-butenoic acid was obtained in up to 90% yield (TON = 135), albeit at moderate selectivity (24 76). Clearly, the isomerization depends on the concentration of the base and was therefore suppressed by a method of continuous addition to the aqueous medium. 

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