Functional group transformations palladium complexes

Jt-allyl complex can be generated after cyclization, as suggested by Takacs in a Fe(0)-catalyzed cyclization of polyenes. It also can be preformed if an active functional group is present in the allylic position. The palladium-catalyzed intramolecular cycloisomerization reaction of allylic acetates is an efficient method for constructing five- or six-membered rings [56, 57]. An asymmetric approach to this transformation has been studied and so far only poor enantioselectivity has been achieved (0-20% ee) [58]. Very recently, Zhang et al. also reported a Rh-catalyzed cycloisomerization involving a Jt-allylrhodium intermediate formed from an allylic halide [59]. 

Because of the creative minds contributing to the field, the tools of C-H bond transformation available to synthetic chemists are actively expanding [1], Among these, coordination-directed C-H bond-activation has long preserved its appeal, because it enables selective functionalization of a particular C-H bond in the presence of other functional groups. This can be achieved by using a heteroatom (FG = functional group shown in Scheme 1) in the substrate structure to direct the metal complex to the proximity of the specific C-H bond. Even unactivated sp3-centered C-H bonds tend to react in a cyclometalation step with palladium, platinum [2], and ruthenium catalysts [3]. 

Amino sulfides 13 were inactive, but transformation of the amino function into an imine produced ligands that made good chiral catalysts with palladium, since amine ligands afford palladium complexes of low reactivity compared to ligands containing rr-accepting donor groups. 

Direct sp C-H bond functionalization has attracted much attention in the past few decades, which presents high efficient, atom-economical pathways to construct new functional groups from easily available chemicals. Most of present C-H bond transformations need to use expensive metals, such as palladium, rhodium, iridium, etc. [13]. Compared to these metals, silver is relatively economically attractive and has been proved highly efficient for C-H activation in recent years, in particular, silver-mediated sp C-H bond transformation. Diverse Ag complexes exhibited... 

Oxidative vicinal functionalization of alkenes represents an important reaction among alkene transformations as it allows the introduction of two functional groups within a single transformation. In the area of palladium catalysis, this type of transformation usually consists of a two-step procedure, during which the two heteroatoms are introduced into the carbon framework. Despite this simplicity at first sight, the actual reaction mechanism may be very complex and, in principle, several mechanistic pathways are possible. 

By altering the palladium precatalyst to Pd ldbalj and using other inorganic bases and solvents, it was possible to synthesize diarylketones with moderate to excellent yields under air. Since the aryl aldehydes can be easily transformed into several functional groups such as methyl, hydroxymethyl, carboxyl, cyano, and ester (see Scheme 7.29), this methodology appears to be versatile and may provide potential opportunities for the synthesis of complex organic compounds. 

In the twentieth century, palladium was the most important metal catalyst in transition metal-catalyzed organic transformations. First, many types of transformations can be catalyzed by a palladium catalyst, including the Heck reaction, the cross-coupling reaction, and the Tsuji-Trost reaction. Second, palladium is extraordinarily tolerant of nearly any type of organic functional group and its high chemoselectivity makes it feasible for use in functionalized or complex systems. Due to these characteristics, palladium is an ideal catalyst in cascade reactions and the total synthesis of natural products. 

The applications of palladium-catalyzed allylation reactions in the realm of total synthesis have been numerous, owing to the general functional group tolerance and high stereoselectivity observed in these transformations [8,17, 25, 27, 30]. In pioneering studies, Trost reported that alkene 1 was converted into allylpalladium complex 2 on treatment with a stoichiometric amounts of palladium dichloride (Scheme 14.1) [42]. Nucleophilic substitu-... 

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