Synthesis oxidation, palladium catalysis

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. 

Previous preparations by Scolastico were based on the Strecker synthesis of aminonitrile and lacked steroselectivity [74,75]. More recently, two formal syntheses were reported from the same ketone 71. In Rama Rao s synthesis (Scheme 11.19) [76], 71 was condensed with vinyl magnesium bromide to give the tertiary alcohol 72 as a single isomer. This compound was then transformed into the vinyl epoxide 73 that, under palladium catalysis, reacted with 4-methoxyphenyl isocyanate to produce the oxazohdinone 74 with retention of its configuration. The remainder of the synthesis consisted of heterocycle opening and adjustment of the oxidation level to provide the lactone 75. Excision of two carbons was necessary to form the known aldehyde 76, previously transformed into myriocin [74]. 

C-H borylation is a widely used methodology for the synthesis of organoboronates [63-65]. Most of the applications have been presented for the synthesis of aryl-boronates. However, functionalization of alkenes has also attracted much interest [66, 67]. In most applications, iridium catalysis was used. However, in case of alkenes, borohydride forms as a side product of the C-H borylation, which undergoes hydroboration with alkenes. This side reaction can be avoided using palladium catalysis under oxidative conditions. In a practically useful implementation of this reaction, pincer-complex catalysis (Ig) was appHed (Figure 4.17) [51]. The reaction can be carried out under mild reaction conditions at room temperature using the neat aUcene 34 as solvent. In this reaction, hypervalent iodine 36, the TFA analog of 29, was employed. In the absence of 36, borylation reaction did not occur. 

Palladium catalysis has become an important tool in synthesis due in large part to its ability to mediate a number of different fundamental transformations with low reaction barriers. These include oxidative addition and reductive elimination reactions, insertion and de-insertion (often (3-hydride elimination), nucleophilic ligand attack, or cycloaddition (Scheme 6.1). 

The milestone discovery by Cotton and coworkers opened a new horizon not only for fundamental palladium organometallic chemistry but also for palladium catalysis. As the cornerstone of modern organic synthesis, palladium cattilysis was widely accepted to be based on Pd(II)/Pd(IV) monosite two-electron redox process. The two-electron oxidation of dipaUadium(II) 146 into diptJladium(III)... 

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